Category Archives: Operational space control

Full body obstacle collision avoidance

Previously I’ve discussed how to avoid obstacles using DMPs in the end-effector trajectory. This is good when you’re controlling a single disconnected point-mass, like a mobile robot navigating around an environment. But if you want to use this system to control a robotic manipulator, then pretty quickly you run into the problem that your end-effector is not a disconnected point-mass moving around the environment. It’s attached to the rest of the arm, and moving such that the arm segments and joints also avoid the obstacle is a whole other deal.

I was doing a quick lit scan on methods for control methods for avoiding joint collision with obstacles, and was kind of surprised that there wasn’t really much in the realm of recent discussions about it. There is, however, a 1986 paper from Dr. Oussama Khatib titled Real-time obstacle avoidance for manipulators and mobile robots that pretty much solves this problem short of getting into explicit path planning methods. Which could be why there aren’t papers since then about it. All the same, I couldn’t find any implementations of it online, and there are a few tricky parts, so in this post I’m going to work through the implementation.

Note: The implementation that I’ve worked out here uses spheres to represent the obstacles. This works pretty well in general by just making the sphere large enough to cover whatever obstacle you’re avoiding. But if you want a more precise control around other shapes, you can check out the appendix of Dr. Khatib’s paper, where he works provides the math for cubes and cones as well.

Note: You can find all of the code I use here and everything you need for the VREP implementation up on my GitHub. I’ve precalculated the functions that are required, because generating them is quite computationally intensive. Hopefully the file saving doesn’t get weird between different operating systems, but if it does, try deleting all of the files in the ur5_config folder and running the code again. This will regenerate those files (on my laptop this takes ~4 hours, though, so beware).

The general idea

Since it’s from Dr. Khatib, you might expect that this approach involves task space. And indeed, your possible suspicions are correct! The algorithm is going to work by identifying forces in Cartesian coordinates that will move any point of the arm that’s too close to an obstacle away from it. The algorithm follows the form:

Setup

  • Specify obstacle location and size
  • Specify a threshold distance to the obstacle

While running

  • Find the closest point of each arm segment to obstacles
  • If within threshold of obstacle, generate force to apply to that point
  • Transform this force into joint torques
  • Add directly to the outgoing control signal

Something that you might notice about this is that it’s similar to the addition of the null space controller that we’ve seen before in operational space control. There’s a distinct difference here though, in that the control signal for avoiding obstacles is added directly to the outgoing control signal, and that it’s not filtered (like the null space control signal) such that there’s no guarantee that it won’t affect the movement of the end-effector. In fact, it’s very likely to affect the movement of the end-effector, but that’s also desirable, as not ramming the arm through an obstacle is as important as getting to the target.

OK, let’s walk through these steps one at a time.

Setup

I mentioned that we’re going to treat all of our obstacles as spheres. It’s actually not much harder to do these calculations for cubes too, but this is already going to be long enough so I’m only addressing sphere’s here. This algorithm assumes we have a list of every obstacle’s centre-point and radius.

We want the avoidance response of the system to be a function of the distance to the obstacle, such that the closer the arm is to the obstacle the stronger the response. The function that Dr. Khatib provides is of the following form:

\textbf{F}_{psp} = \left\{ \begin{array}{cc}\eta (\frac{1.0}{\rho} - \frac{1}{\rho_0}) \frac{1}{\rho^2} \frac{\partial \rho}{\partial \textbf{x}} & \rho \leq \rho_0 \\ \textbf{0} & \rho > \rho_0 \end{array} \right. ,

where \rho is the distance to the target, \rho_0 is the threshold distance to the target at which point the avoidance function activates, \frac{\partial \rho}{\partial \textbf{x}} is the partial derivative of the distance to the target with respect to a given point on the arm, and \eta is a gain term.

This function looks complicated, but it’s actually pretty intuitive. The partial derivative term in the function works simply to point in the opposite direction of the obstacle, in Cartesian coordinates, i.e. tells the system how to get away from the obstacle. The rest of the term is just a gain that starts out at zero when \rho = \rho_0, and gets huge as the obstacle nears the object (as \rho \to 0 \Rightarrow \frac{1}{\rho} \to \infty). Using \eta = .2 and \rho_0 = .2 gives us a function that looks like

gain

So you can see that very quickly a very, very strong push away from this obstacle is going to be generated once we enter the threshold distance. But how do we know exactly when we’ve entered the threshold distance?

Find the closest point

We want to avoid the obstacle with our whole body, but it turns out we can reduce the problem to only worrying about the closest point of each arm segment to the obstacle, and move that one point away from the obstacle if threshold distance is hit.

To find the closest point on a given arm segment to the obstacle we’ll do some pretty neat trig. I’ll post the code for it and then discuss it below. In this snippet, p1 and p2 are the beginning and ending (x,y,z) locations of arm segment (which we are assuming is a straight line), and v is the center of the obstacle.

# the vector of our line
vec_line = p2 - p1
# the vector from the obstacle to the first line point
vec_ob_line = v - p1
# calculate the projection normalized by length of arm segment
projection = (np.dot(vec_ob_line, vec_line) /
              np.dot(vec_line, vec_line))
if projection < 0:               
    # then closest point is the start of the segment
    closest = p1  
elif projection > 1:
    # then closest point is the end of the segment
    closest = p2
else:
    closest = p1 + projection * vec_line

The first thing we do is find the arm segment line, and then line from the obstacle center to the start point of the arm segment. Once we have these, we do:

\frac{\textbf{v}_\textrm{ob\_line} \; \cdot \; \textbf{v}_\textrm{line}}{\textbf{v}_\textrm{line} \; \cdot \; \textbf{v}_\textrm{line}},

using the geometric definition of the dot product two vectors, we can rewrite the above as

\frac{||\textbf{v}_\textrm{ob\_line}|| \; || \textbf{v}_\textrm{line} || \; \textrm{cos}(\theta)}{||\textbf{v}_\textrm{line}||^2} = \frac{||\textbf{v}_\textrm{ob\_line}||} {||\textbf{v}_\textrm{line}||} \textrm{cos}(\theta)

which reads as the magnitude of vec_ob_line divided by the magnitude of vec_line (I know, these are terrible names, sorry) multiplied by the angle between the two vectors. If the angle between the vectors is < 0 (projection will also be < 0), then right off the bat we know that the start of the arm segment, p1, is the closest point. If the projection value is > 1, then we know that 1) the length from the start of the arm segment to the obstacle is longer than the length of the arm, and 2) the angle is such that the end of the arm segment, p2, is the closest point to the obstacle.

Finally, in the last case we know that the closest point is somewhere along the arm segment. To find where exactly, we do the following

\textbf{p}_1 + \textrm{projection} \; \textbf{v}_\textrm{line},

which can be rewritten

\textbf{p}_1 + \frac{||\textbf{v}_\textrm{ob\_line}||} {||\textbf{v}_\textrm{line}||} \textrm{cos}(\theta) \; \textbf{v}_\textrm{line},

I find it more intuitive if we rearrange the second term to be

\textbf{p}_1 + \frac{\textbf{v}_\textrm{line}} {||\textbf{v}_\textrm{line}||} \; ||\textbf{v}_\textrm{ob\_line} || \; \textrm{cos}(\theta).

So then what this is all doing is starting us off at the beginning of the arm segment, p1, and to that we add this other fun term. The first part of this fun term provides direction normalized to magnitude 1. The second part of this term provides magnitude, specifically the exact distance along vec_line we need to traverse to form reach a right angle (giving us the shortest distance) pointing at the obstacle. This handy figure from the Wikipedia page helps illustrate exactly what’s going on with the second part, where B is be vec_line and A is vec_ob_line:

dot_product
Armed with this information, we understand how to find the closest point of each arm segment to the obstacle, and we are now ready to generate a force to move the arm in the opposite direction!

Check distance, generate force

To calculate the distance, all we have to do is calculate the Euclidean distance from the closest point of the arm segment to the center of the sphere, and then subtract out the radius of the sphere:

# calculate distance from obstacle vertex to the closest point
dist = np.sqrt(np.sum((v - closest)**2))
# account for size of obstacle
rho = dist - obstacle[3]

Once we have this, we can check it and generate F_{psp} using the equation we defined above. The one part of that equation that wasn’t specified was exactly what \frac{\partial \rho}{\partial \textbf{x}} was. Since it’s just the partial derivative of the distance to the target with respect to the closest point, we can calculate it as the normalized difference between the two points:

drhodx = (v - closest) / rho

Alright! Now we’ve found the closest point, and know the force we want to apply, from here it’s standard operational space procedure.

Transform the force into torques

As we all remember, the equation for transforming a control signal from operational space to involves two terms aside from the desired force. Namely, the Jacobian and the operational space inertia matrix:

\textbf{u}_\textrm{psp} = \textbf{J}^T_{psp} \textbf{M}_{psp} \textbf{F}_{psp},

where \textbf{J}_{psp} is the Jacobian for the point of interest, \textbf{M}_{psp} is the operational space inertia matrix for the point of interest, and \textbf{F}_{psp} is the force we defined above.

Calculating the Jacobian for an unspecified point

So the first thing we need to calculate is the Jacobian for this point on the arm. There are a bunch of ways you could go about this, but the way I’m going to do it here is by building on the post where I used SymPy to automate the Jacobian derivation. The way we did that was by defining the transforms from the origin reference frame to the first link, from the first link to the second, etc, until we reached the end-effector. Then, whenever we needed a Jacobian we could string together the transforms to get the transform from the origin to that point on the arm, and take the partial derivative with respect to the joints (using SymPy’s derivative method).

As an example, say we wanted to calculate the Jacobian for the third joint, we would first calculate:

^3_{\textrm{org}}\textbf{T} = ^0_{\textrm{org}}\textbf{T} \; ^1_0\textbf{T} \; ^2_1\textbf{T} \; ^3_2\textbf{T},

where ^m_n\textbf{T} reads the transform from reference frame m to reference frame n.

Once we have this transformation matrix, ^3_\textrm{org}\textbf{T}, we multiply it by the point of interest in reference frame 3, which, previously, has usually been \textbf{x} = [0, 0, 0]. In other words, usually we’re just interested in the origin of reference frame 3. So the Jacobian is just

\frac{\partial \; ^3_\textrm{org}\textbf{T} \textbf{x}}{\partial \textbf{q}}.

what if we’re interested in some non-specified point along link 3, though? Well, using SymPy we set make \textbf{x} = [x_0, x_1, x_2, 1] instead of \textbf{x} = [0, 0, 0, 1] (recall the 1 at the end in these vectors is just to make the math work), and make the Jacobian function SymPy generates for us dependent on both \textbf{q} and \textbf{x}, rather than just \textbf{q}. In code this looks like:

Torg3 = self._calc_T(name="3")
# transform x into world coordinates
Torg3x = sp.simplify(Torg3 * sp.Matrix(x))
J3_func = sp.lambdify(q + x, Torg3)

Now it’s possible to calculate the Jacobian for any point along link 3 just by changing the parameters that we pass into J3_func! Most excellent.

We are getting closer.

NOTE: This parameterization can significantly increase the build time of the function, it took my laptop about 4 hours. To decrease build time you can try commenting out the simplify calls from the code, which might slow down run-time a bit but significantly drops the generation time.

Where is the closest point in that link’s reference frame?

A sneaky problem comes up when calculating the closest point of each arm segment to the object: We’ve calculated the closest point of each arm segment in the origin’s frame of reference, and we need thew relative to each link’s own frame of reference. Fortunately, all we have to do is calculate the inverse transform for the link of interest. For example, the inverse transform of ^3_\textrm{org}\textbf{T} transforms a point from the origin’s frame of reference to the reference frame of the 3rd joint.

I go over how to calculate the inverse transform at the end of my post on forward transformation matrices, but to save you from having to go back and look through that, here’s the code to do it:

Torg3 = self._calc_T(name="3")
rotation_inv = Torg3[:3, :3].T
translation_inv = -rotation_inv * Torg3[:3, 3]
Torg3_inv = rotation_inv.row_join(translation_inv).col_join(
    sp.Matrix([[0, 0, 0, 1]]))

And now to find the closest point in the coordinates of reference frame 3 we simply

x = np.dot(Torg3_inv, closest)

This x value is what we’re going to plug in as parameters to our J3_func above to find the Jacobian for the closest point on link 3.

Calculate the operational space inertia matrix for the closest point

OK. With the Jacobian for the point of interest we are now able to calculate the operational space inertia matrix. This code I’ve explicitly worked through before, and I’ll show it in the full code below, so I won’t go over it again here.

The whole implementation

You can run an example of all this code controlling the UR5 arm to avoid obstacles in VREP using this code up on my GitHub. The specific code added to implement obstacle avoidance looks like this:

# find the closest point of each link to the obstacle
for ii in range(robot_config.num_joints):
    # get the start and end-points of the arm segment
    p1 = robot_config.Tx('joint%i' % ii, q=q)
    if ii == robot_config.num_joints - 1:
        p2 = robot_config.Tx('EE', q=q)
    else:
        p2 = robot_config.Tx('joint%i' % (ii + 1), q=q)

    # calculate minimum distance from arm segment to obstacle
    # the vector of our line
    vec_line = p2 - p1
    # the vector from the obstacle to the first line point
    vec_ob_line = v - p1
    # calculate the projection normalized by length of arm segment
    projection = (np.dot(vec_ob_line, vec_line) /
                  np.sum((vec_line)**2))
    if projection < 0:         
        # then closest point is the start of the segment
        closest = p1
    elif projection > 1:
        # then closest point is the end of the segment
        closest = p2
    else:
        closest = p1 + projection * vec_line
    # calculate distance from obstacle vertex to the closest point
    dist = np.sqrt(np.sum((v - closest)**2))
    # account for size of obstacle
    rho = dist - obstacle_radius

    if rho < threshold:
        eta = .02
        drhodx = (v - closest) / rho
        Fpsp = (eta * (1.0/rho - 1.0/threshold) *
                1.0/rho**2 * drhodx)

        # get offset of closest point from link's reference frame
        T_inv = robot_config.T_inv('link%i' % ii, q=q)
        m = np.dot(T_inv, np.hstack([closest, [1]]))[:-1]
        # calculate the Jacobian for this point
        Jpsp = robot_config.J('link%i' % ii, x=m, q=q)[:3]

        # calculate the inertia matrix for the
        # point subjected to the potential space
        Mxpsp_inv = np.dot(Jpsp,
                        np.dot(np.linalg.pinv(Mq), Jpsp.T))
        svd_u, svd_s, svd_v = np.linalg.svd(Mxpsp_inv)
        # cut off singular values that could cause problems
        singularity_thresh = .00025
        for ii in range(len(svd_s)):
            svd_s[ii] = 0 if svd_s[ii] < singularity_thresh else \
                1./float(svd_s[ii])
        # numpy returns U,S,V.T, so have to transpose both here
        Mxpsp = np.dot(svd_v.T, np.dot(np.diag(svd_s), svd_u.T))

        u_psp = -np.dot(Jpsp.T, np.dot(Mxpsp, Fpsp))
        if rho < .01:
            u = u_psp
        else:
            u += u_psp

The one thing in this code I didn’t talk about is that you can see that if rho < .01 then I set u = u_psp instead of just adding u_psp to u. What this does is basically add in a fail safe take over of the robotic control saying that “if we are about to hit the obstacle forget about everything else and get out of the way!”.

Results

And that’s it! I really enjoy how this looks when it’s running, it’s a really effective algorithm. Let’s look at some samples of it in action.

First, in a 2D environment, where it’s real easy to move around the obstacle and see how it changes in response to the new obstacle position. The red circle is the target and the blue circle is the obstacle:
avoid2d

And in 3D in VREP, running the code example that I’ve put up on my GitHub implementing this. The example of it running without obstacle avoidance code is on the left, and running with obstacle avoidance is on the right. It’s kind of hard to see but on the left the robot moves through the far side of the obstacle (the gold sphere) on its way to the target (the red sphere):

And one last example, the arm dodging a moving obstacle on its way to the target.

movingavoid3d

The implementation is a ton of fun to play around with. It’s a really nifty algorithm, that works quite well, and I haven’t found many alternatives in papers that don’t go into path planning (if you know of some and can share that’d be great!). This post was a bit of a journey, but hopefully you found it useful! I continue to find it impressive how many different neat features like this can come about once you have the operational space control framework in place.

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Velocity limiting in operational space control

Recently, I was reading through an older paper on effective operational space control, talking specifically point to point control in operational space. The paper mentioned that even if you have a perfect model of the system, you’re going to run into trouble if you use just a basic PD formula to define your control signal in operational space:

u_x = k_p (\textbf{x}^* - \textbf{x}) - k_v \dot{\textbf{x}},

where \textbf{x} and \dot{\textbf{x}} are the system position and velocity in operational space, \textbf{x}^* is the target position, and k_p and k_v are gains.

If you define your operational space control signal like this, and then translate this signal into joint torques (using, for example, methods discussed in other posts), you’re going to see a very non-straight trajectory emerge in larger movements as a result of “actuator saturation, and bandwidth and velocity limitations”. In the example of a standard robot, you might run into issues with your motors not being able to actually generate the torques that have been specified, the frequency of control and feedback might not be sufficient, and you could hit hard constraints on system velocity. The solution to this problem was presented in this 1987 paper by Dr. Oussama Khatib, and is pretty slick and very useful, so I thought I’d write it up here for any other unfortunate souls wandering around in ignorance. First though, here’s what it looks like to move large point to point distances without velocity limiting:

As you can see, the system isn’t moving in a straight line, which can be very aggravating if you’ve worked and reworked out the equations and double checked all your parameters, etc etc. A few things, first, when working with simulations it’s easy to forget how ridiculously fast this actually would be in real-time. Even though it takes a minute to simulate the above movement, in real-time, is happening over the course of 200ms. Taking that into account, this is pretty good. Also, there’s an obvious solution here, slow down your movement. The source of this problem is largely that all of the motors are not able to apply the torques specified and move at the required speed. Some of the motors have a lot less mass to throw around and will be able to move at the specified torques, but not all. Hence the not straight trajectory.

You can of course drop the gains on your PD signal, but that’s not really a great methodical solution. So, what can we do?

Well, if we rearrange the PD control signal specified above into

u_x = k_v (\dot{\textbf{x}}^* - \dot{\textbf{x}}),

where \dot{\textbf{x}}^* is the desired velocity, we see that this signal can be interpreted as a pure velocity servo-control signal, with velocity gain k_v and a desired velocity

\dot{\textbf{x}}^* = \frac{k_p}{k_v}(\textbf{x}^* - \textbf{x}).

When things are in this form, it becomes a bit more clear what we have to do: limit the desired velocity to be at most some specified maximum velocity of the end-effector, V_\textrm{max}. This value should be low enough that the transformation into joint torques doesn’t result in anything larger than the actuators can generate.

Taking V_\textrm{max}, what we want is to clip the magnitude of the control signal to be V_\textrm{max} if it’s ever larger (in positive or negative directions), and to be equal to \frac{kp}{kv}(\textbf{x}^* - \textbf{x}) otherwise. The math for this works out such that we can accomplish this with a control signal of the form:

\textbf{u}_\textbf{x} = -k_v (\dot{\textbf{x}} + \textrm{sat}\left(\frac{V_\textrm{max}}{\lambda |\tilde{\textbf{x}}|} \right) \lambda \tilde{\textbf{x}}),

where \lambda = \frac{k_p}{k_v} , \tilde{\textbf{x}} = \textbf{x} - \textbf{x}^*, and \textrm{sat} is the saturation function, such that

\textrm{sat}(y) = \left\{ \begin{array}{cc} |y| \leq 1 & \Rightarrow y \\ |y| > 1 & \Rightarrow 1 \end{array} \right.

where |y| is the absolute value of y, and is applied element wise to the vector \tilde{\textbf{x}} in the control signal.

As a result of using this saturation function, the control signal behaves differently depending on whether or not \dot{\textbf{x}}^* > V_\textrm{max}:

\textbf{u}_\textbf{x} = \left\{ \begin{array}{cc} \dot{\textbf{x}}^* \geq V_\textrm{max} & \Rightarrow -k_v (\dot{\textbf{x}} + V_\textbf{max} \textrm{sgn}(\tilde{\textbf{x}})) \\ \dot{\textbf{x}}^* < V_\textrm{max} & \Rightarrow -k_v \dot{\textbf{x}} + k_p \tilde{\textbf{x}} \end{array} \right.

where \textrm{sgn}(y) is a function that returns -1 if y < 0 and 1 if y \geq 0, and is again applied element-wise to vectors. Note that the control signal in the second condition is equivalent to our original PD control signal k_p(\textbf{x}^* - \textbf{x}) - k_v \dot{\textbf{x}}. If you’re wondering about negative signs make sure you note that \tilde{\textbf{x}} = \textbf{x} - \textbf{x}^* and not \textbf{x}^* - \textbf{x}, as you might assume.

So now the control signal is behaving exactly as desired! Moves the system towards the target, but no faster than the specified maximum velocity. Now our trajectories look like this:

So those are starting to look a lot better! The first thing you’ll notice is that this is a fair bit slower of a movement. Well, actually, it’s waaaayyyy slower because the playback speed here is 4x faster than in that first animation, and this is a movement over 2s. Which has pros and cons, con: it’s slower, pro: it’s straighter, and you’re less likely to be murdered by it. When you move from simulations to real systems that latter point really moves way up the priority list.

Second thing to notice, the system seems to be minimising the error along one dimension, and then along the next, and then arrives at the target. What’s going on?  Because the error along each of the (x,y,z) dimensions isn’t the same, when speed gets clipped along one of the dimensions you’re no longer going to be moving in a straight line directly to the target. To address this, we’re going to add a scaling term whenever clipping happens, such that you reduce the speed you move along each dimension by the same ratio, so that you’re still moving in a straight line.

It’s a liiiiittle bit more complicated than that, but not much. First, we’ll calculate the values being passed in to the saturation function for each (x,y,z) dimension. We’ll then check to see if any of them are going to get clipped, and if there’s more than one that saturates we’ll find the one that is affected the most. After we’ve identified which dimension it is, we go through and calculate what the control signal would have been without velocity limiting, and what it will be now with velocity limiting. This scaling term tells us how much the control signal was reduced, and we can then use it to reduce the control signals of the other dimensions by the same amount. These other dimensions might still saturate, though, so we have to recalculate the saturation function for them once they’ve been scaled. Here’s what this all looks like in code:

# implement velocity limiting
lamb = kp / kv
x_tilde = xyz - target_xyz
sat = vmax / (lamb * np.abs(x_tilde))
scale = np.ones(3)
if np.any(sat < 1):
    index = np.argmin(sat)
    unclipped = kp * x_tilde[index]
    clipped = kv * vmax * np.sign(x_tilde[index])
    scale = np.ones(3) * clipped / unclipped
    scale[index] = 1
u_xyz = -kv * (dx + np.clip(sat / scale, 0, 1) *
               scale * lamb * x_tilde)
 

And now, finally, we start getting the trajectories that we’ve been wanting the whole time:

And finally we can rest easy, knowing that our robot is moving at a reasonable speed along a direct path to its goals. Wherever you’d like to use this neato ‘ish you should be able to just paste in the above code, define your vmax, kp, and kv values and be good to go!

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Using VREP for simulation of force-controlled models

I’ve been playing around a bit with different simulators, and one that we’re a big fan of in the lab is VREP. It’s free for academics and you can talk to them about licences if you’re looking for commercial use. I haven’t actually had much experience with it before myself, so I decided to make a simple force controlled arm model to get experience using it. All in all, there were only a few weird API things that I had to get through, and once you have them down it’s pretty straight forward. This post is going to be about the steps that I needed to take to get things all set up. For a more general start-up on VREP check out All the code in this post and the model I use can be found up on my GitHub.

Getting the right files where you need them

As discussed in the remote API overview, you’ll need three files in whatever folder you want to run your Python script from to be able to hook into VREP remotely:

  • remoteApi.dll, remoteApi.dylib or remoteApi.so (depending on what OS you’re using)
  • vrep.py
  • vrepConstants.py

You can find these files inside your VREP_HOME/programming/remoteApiBindings/python/python and VREP_HOME/programming/remoteApiBindings/lib/lib folders. Make sure that these files are in whatever folder you’re running your Python scripts from.

The model

It’s easy to create a new model to mess around with in VREP, so that’s the route I went, rather than importing one of their pre-made models and having some sneaky parameter setting cause me a bunch of grief. You can just right click->add then go at it. There are a bunch of tutorials so I’m not going to go into detail here. The main things are:

  • Make sure joints are in ‘Torque/force’ mode.
  • Make sure that joint ‘Motor enabled’ property is checked. The motor enabled property is in the ‘Show dynamic properties dialogue’ menu, which you find when you double click on the joint in the Scene Hierarchy.
  • Know the names of the joints as shown in the Scene Hierarchy.

So here’s a picture:

VREP
where you can see the names of the objects in the model highlighted in red, the Torque/force selection highlighted in blue, and the Motor enabled option highlighted in green. And of course my beautiful arm model in the background.

Setting up the continuous server

The goal is to connect VREP to Python so that we can send torques to the arm from our Python script and get the feedback necessary to calculate those torques. There are a few ways to set up a remote connection in VREP.

The basic one is they have you add a child script in VREP and attach it to an object in the world that sets up a port and then you hit go on your simulation and can then run your Python script to connect in. This gets old real fast. Fortunately, it’s easy to automate everything from Python so that you can connect in, start the simulation, run it for however long, and then stop the simulation without having to click back and forth.

The first step is to make sure that your remoteApiConnections.txt file in you VREP home directory is set up properly. A continuous server should be set up by default, but doesn’t hurt to double check. And you can take the chance to turn on debugging, which can be pretty helpful. So open up that file and make sure it looks like this:

portIndex1_port                 = 19997
portIndex1_debug                = true
portIndex1_syncSimTrigger       = true

Once that’s set up, when VREP starts we can connect in from Python. In our Python script, first we’ll close any open connections that might be around, and then we’ll set up a new connection in:

import vrep 

# close any open connections
vrep.simxFinish(-1)
# Connect to the V-REP continuous server
clientID = vrep.simxStart('127.0.0.1', 19997, True, True, 500, 5) 

if clientID != -1: # if we connected successfully
    print ('Connected to remote API server')

So once the connection is made, we check the clientID value to make sure that it didn’t fail, and then we carry on with our script.

Synchronizing

By default, VREP will run its simulation in its own thread, and both the simulation and the controller using the remote API will be running simultaneously. This can lead to some weird behaviour as things fall out of synch etc etc, so what we want instead is for the VREP sim to only run one time step for each time step the controller runs. To do that, we need to set VREP to synchronous mode. So the next few lines of our Python script look like:

    # --------------------- Setup the simulation 

    vrep.simxSynchronous(clientID,True)

and then later, once we’ve calculated our control signal, sent it over, and want the simulation to run one time step forward, we do that by calling

    # move simulation ahead one time step
    vrep.simxSynchronousTrigger(clientID)

Get the handles to objects of interest

OK the next chunk of code in our script uses the names of our objects (as specified in the Scene Hierarchy in VREP) to get an ID for each which to identify which object we want to send a command to or receive feedback from:

    joint_names = ['shoulder', 'elbow']
    # joint target velocities discussed below
    joint_target_velocities = np.ones(len(joint_names)) * 10000.0

    # get the handles for each joint and set up streaming
    joint_handles = [vrep.simxGetObjectHandle(clientID,
        name, vrep.simx_opmode_blocking)[1] for name in joint_names]

    # get handle for target and set up streaming
    _, target_handle = vrep.simxGetObjectHandle(clientID,
                    'target', vrep.simx_opmode_blocking)

Set dt and start the simulation

And the final thing that we’re going to do in our simulation set up is specify the timestep that we want to use, and then start the simulation. I found this in a forum post, and I must say whatever VREP lacks in intuitive API their forum moderators are on the ball. NOTE: To use a custom time step you have to also set the dt option in the VREP simulator to ‘custom’. The drop down is to the left of the ‘play’ arrow, and if you don’t have it set to ‘custom’ you won’t be able to set the time step through Python.

    dt = .01
    vrep.simxSetFloatingParameter(clientID,
            vrep.sim_floatparam_simulation_time_step,
            dt, # specify a simulation time step
            vrep.simx_opmode_oneshot)

    # --------------------- Start the simulation

    # start our simulation in lockstep with our code
    vrep.simxStartSimulation(clientID,
            vrep.simx_opmode_blocking)

Main loop

For this next chunk I’m going to cut out everything that’s not VREP, since I have a bunch of posts explaining the control signal derivation and forward transformation matrices.

So, once we’ve started the simulation, I’ve set things up for the arm to be controlled for 1 second and then for the simulation to stop and everything shut down and disconnect.

    while count < 1: # run for 1 simulated second

        # get the (x,y,z) position of the target
        _, target_xyz = vrep.simxGetObjectPosition(clientID,
                target_handle,
                -1, # retrieve absolute, not relative, position
                vrep.simx_opmode_blocking)
        if _ !=0 : raise Exception()
        track_target.append(np.copy(target_xyz)) # store for plotting
        target_xyz = np.asarray(target_xyz)

        q = np.zeros(len(joint_handles))
        dq = np.zeros(len(joint_handles))
        for ii,joint_handle in enumerate(joint_handles):
            # get the joint angles
            _, q[ii] = vrep.simxGetJointPosition(clientID,
                    joint_handle,
                    vrep.simx_opmode_blocking)
            if _ !=0 : raise Exception()
            # get the joint velocity
            _, dq[ii] = vrep.simxGetObjectFloatParameter(clientID,
                    joint_handle,
                    2012, # ID for angular velocity of the joint
                    vrep.simx_opmode_blocking)
            if _ !=0 : raise Exception()

In the above chunk of code, I think the big thing to point out is that I’m using vrep.simx_opmode_blocking in each call, instead of vrep.simx_opmode_buffer. The difference is that you for sure get the current values of the simulation when you use blocking, and you can be behind a time step using buffer.

Aside from that the other notable things are I raise an exception if the first parameter (which is the return code) is ever not 0, and that I use simxGetObjectFloatParameter to get the joint velocity instead of simxGetObjectVelocity, which has a rotational velocity component. Zero is the return code for ‘everything worked’, and if you don’t check it and have some silly things going on you can be veeerrrrryyy mystified when things don’t work. And what simxGetObjectVelocity returns is the rotational velocity of the joint relative to the world frame, and not the angular velocity of the joint in its own coordinates. That was also a briefly confusing.

So the next thing I do is calculate u, which we’ll skip, and then we need to set the forces for the joint. This part of the API is real screwy. You can’t set the force applied to the joint directly. Instead, you have to set the target velocity of the joint to some really high value (hence that array we made before), and then modulate the maximum force that can be applied to that joint. Also important: When you want to apply a force in the other direction, you change the sign on the target velocity, but keep the force sign positive.

        # ... calculate u ... 

        for ii,joint_handle in enumerate(joint_handles):
            # get the current joint torque
            _, torque = \
                vrep.simxGetJointForce(clientID,
                        joint_handle,
                        vrep.simx_opmode_blocking)
            if _ !=0 : raise Exception()

            # if force has changed signs,
            # we need to change the target velocity sign
            if np.sign(torque) * np.sign(u[ii]) < 0:
                joint_target_velocities[ii] = \
                        joint_target_velocities[ii] * -1
                vrep.simxSetJointTargetVelocity(clientID,
                        joint_handle,
                        joint_target_velocities[ii], # target velocity
                        vrep.simx_opmode_blocking)
            if _ !=0 : raise Exception()

            # and now modulate the force
            vrep.simxSetJointForce(clientID,
                    joint_handle,
                    abs(u[ii]), # force to apply
                    vrep.simx_opmode_blocking)
            if _ !=0 : raise Exception()

        # move simulation ahead one time step
        vrep.simxSynchronousTrigger(clientID)
        count += dt

So as you can see we check the current torque, see if we need to change the sign on the target velocity, modulate the maximum allowed force, and then finally step the VREP simulation forward.

Conclusions

And there you go! Here’s an animation of it in action (note this is a super low quality gif and it looks way better / smoother when actually running it yourself):

reach_to_target

All in all, VREP has been enjoyable to work with so far. It didn’t take long to get things moving and off the ground, the visualization is great, and I haven’t even scratched the surface of what you can do with it. Best of all (so far) you can fully automate everything from Python. Hopefully this is enough to help some people get their own models going and save a few hours and headaches! Again, the full code and the model are up on my GitHub.

Nits

  • When you’re applying your control signal, make sure you test each joint in isolation, to make sure your torques push things in the direction you think they do. I had checked the rotation direction in VREP, but the control signal for both joints ended up needing to be multiplied by -1.
  • Another nit when you’re building your model, if you use the rotate button from the VREP toolbar on your model, wherever that joint rotates to is now 0 degrees. If you want to set the joint to start at 45 degrees, instead double click and change Pos[deg] option inside ‘Joint’ in Scene Object Properties.
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Operational space control of 6DOF robot arm with spiking cameras part 3: Tracking a target using spiking cameras

Alright. Previously we got our arm all set up to perform operational space control, accepting commands through Python. In this post we’re going to set it up with a set of spiking cameras for eyes, train it to learn the mapping between camera coordinates and end-effector coordinates, and have it track an LED target.

What is a spiking camera?

Good question! Spiking cameras are awesome, and they come from Dr. Jorg Conradt’s lab. Basically what they do is return you information about movement from the environment. They’re event-driven, instead of clock-driven like most hardware, which means that they have no internal clock that’s dictating when they send information (i.e. they’re asynchronous). They send information out as soon as they receive it. Additionally, they only send out information about the part of the image that has changed. This means that they have super fast response times and their output bandwidth is really low. Dr. Terry Stewart of our lab has written a bunch of code that can be used for interfacing with spiking cameras, which can all be found up on his GitHub.

Let’s use his code to see through a spiking camera’s eye. After cloning his repo and running python setup.py you can plug in a spiking camera through USB, and with the following code have a Matplotlib figure pop-up with the camera output:

import nstbot
import nstbot.connection
import time

eye = nstbot.RetinaBot()
eye.connect(nstbot.connection.Serial('/dev/ttyUSB0', baud=4000000))

time.sleep(1)

eye.retina(True)
eye.show_image()

while True:
    time.sleep(1)

The important parts here are the creation of an instance of the RetinaBot, connecting it to the proper USB port, and calling the show_image function. Pretty easy, right? Here’s some example output, this is me waving my hand and snapping my fingers:
spikingcameratest

How cool is that? Now, you may be wondering how or why we’re going to use a spiking camera instead of a regular camera. The main reason that I’m using it here is because it makes tracking targets super easy. We just set up an LED that blinks at say 100Hz, and then we look for that frequency in the spiking camera output by recording the rate of change of each of the pixels and averaging over all pixel locations changing at the target frequency. So, to do this with the above code we simply add

eye.track_frequencies(freqs=[100])

And now we can track the location of an LED blinking at 100Hz! The visualization code place a blue dot at the estimated target location, and this all looks like:
tracking-100hz
Alright! Easily decoded target location complete.

Transforming between camera coordinates and end-effector coordinates

Now that we have a system that can track a target location, we need to transform that position information into end-effector coordinates for the arm to move to. There are a few ways to go about this. One is by very carefully positioning the camera and measuring the distances between the robot’s origin reference frame and working through the trig etc etc. Another, much less pain-in-the-neck way is to instead record some sample points of the robot end-effector at different positions in both end-effector and camera coordinates, and then use a function approximator to generalize over the rest of space.

We’ll do the latter, because it’s exactly the kind of thing that neurons are great for. We have some weird function, and we want to learn to approximate it. Populations of neurons are awesome function approximators. Think of all the crazy mappings your brain learns. To perform function approximation with neurons we’re going to use the Neural Engineering Framework (NEF). If you’re not familiar with the NEF, the basic idea is to using the response curves of neurons as a big set of basis function to decode some signal in some vector space. So we look at the responses of the neurons in the population as we vary our input signal, and then determine a set of decoders (using least-squares or somesuch) that specify the contribution of each neuron to the different dimensions of the function we want to approximate.

Here’s how this is going to work.

  1. We’re going to attach the LED to the head of the robot,
  2. we specify a set of (x,y,z) coordinates that we send to the robot’s controller,
  3. when the robot moves to each point, record the LED location from the camera as well as the end-effector’s (x,y,z) coordinate,
  4. create a population of neurons that we train up to learn the mapping from camera locations to end-effector (x,y,z) locations
  5. use this information to tell the robot where to move.

A detail that should be mentioned here is that a single camera only provides 2D output. To get a 3D location we’re going to use two separate cameras. One will provide (x,z) information, and the other will provide (y,z) information.

Once we’ve taped (expertly) the LED onto the robot arm, the following script to generate the information we to approximate the function transforming from camera to end-effector space:

import robot
from eye import Eye # this is just a spiking camera wrapper class

import numpy as np
import time

# connect to the spiking cameras
eye0 = Eye(port='/dev/ttyUSB2')
eye1 = Eye(port='/dev/ttyUSB1')
eyes = [eye0, eye1]
# connect to the robot
rob = robot.robotArm()

# define the range of values to test
min_x = -10.0
max_x = 10.0
x_interval = 5.0
min_y = -15.0
max_y = -5.0
y_interval = 5.0
min_z = 10.0
max_z = 20.0
z_interval = 5.0

x_space = np.arange(min_x, max_x, x_interval)
y_space = np.arange(min_y, max_y, y_interval)
z_space = np.arange(min_z, max_z, z_interval)

num_samples = 10 # how many camera samples to average over

try: 
    out_file0 = open('eye_map_0.csv', 'w')
    out_file1 = open('eye_map_1.csv', 'w')

    for i, x_val in enumerate(x_space):
        for j, y_val in enumerate(y_space):
            for k, z_val in enumerate(z_space):

                rob.move_to_xyz(target)
                time.sleep(2) # time for the robot to move

                # take a bunch of samples and average the input to get 
                # the approximation of the LED in camera coordinates
                eye_data0 = np.zeros(2)
                for k in range(num_samples):
                    eye_data0 += eye0.position(0)[:2]
                eye_data0 /= num_samples
                out_file0.write('%0.2f, %0.2f, %0.2f, %0.2f\n' %
                                (y_val, z_val, eye_data0[0], eye_data0[1]))

                eye_data1 = np.zeros(2)
                for k in range(num_samples):
                    eye_data1 += eye1.position(0)[:2]
                eye_data1 /= num_samples
                out_file1.write('%0.2f, %0.2f, %0.2f, %0.2f\n' %
                                (x_val, z_val, eye_data1[0], eye_data1[1]))

    out_file0.close()
    out_file1.close()
except: 
    import sys
    import traceback
    print traceback.print_exc(file=sys.stdout)
finally:
    rob.robot.disconnect()

This script connects to the cameras, defines some rectangle in end-effector space to sample, and then works through each of the points writing the data to file. The results of this code can be seen in the animation posted in part 2 of this series.

OK! So now we have all the information we need to train up our neural population. It’s worth noting that we’re only using 36 sample points to train up our neurons, I did this mostly because I didn’t want to wait around. You can of course use more, though, and the more sample points you have the more accurate your function approximation will be.

Implementing a controller using Nengo

The neural simulation software (which implements the NEF) that we’re going to be using to generate and train our neural population is called Nengo. It’s free to use for non-commercial use, and I highly recommend checking out the introduction and tutorials if you have any interest in neural modeling.

What we need to do now is generate two neural populations, one for each camera, that will receives input from the spiking camera and transform the target’s location information into end-effector coordinates. We will then combine the estimates from the two populations, and send that information out to the robot to tell it where to move. I’ll paste the code in here, and then we’ll step through it below.

from eye import Eye
import nengo
from nengo.utils.connection import target_function
import robot 

import numpy as np
import sys
import traceback

# connect to robot
rob = robot.robotArm()

model = nengo.Network()

try: 
    def eyeNet(port='/dev/ttyUSB0', filename='eye_map.csv', n_neurons=1000, 
               label='eye'):

        # connect to eye
        spiking_cam = Eye(port=port)

        # read in eval points and target output
        eval_points = []
        targets = []

        file_obj = open(filename, 'r')
        file_data = file_obj.readlines()
        for line in file_data:
            line_data = map(float, line.strip().split(','))
            targets.append(line_data[:2])
            eval_points.append(line_data[2:])
        file_obj.close()

        eval_points = np.array(eval_points)
        targets = np.array(targets)

        # create subnetwork for eye
        net = nengo.Network(label=label)
        with net:
            def eye_input(t): 
                return spiking_cam.position(0)[:2]
            net.input = nengo.Node(output=eye_input, size_out=2)
            net.map_ens = nengo.Ensemble(n_neurons, dimensions=2)
            net.output = nengo.Node(size_in=2)

            nengo.Connection(net.input, net.map_ens, synapse=None)
            nengo.Connection(net.map_ens, net.output, synapse=None,
                                    **target_function(eval_points, targets))

        return net

    with model: 
        # create network for spiking camera 0
        eye0 = eyeNet(port='/dev/ttyUSB2', filename='eye_map_0.csv', label='eye0')
        # create network for spiking camera 1 
        eye1 = eyeNet(port='/dev/ttyUSB1', filename='eye_map_1.csv', label='eye1')

        def eyes_func(t, yzxz):
            x = yzxz[2] # x coordinate coded from eye1
            y = yzxz[0] # y coordinate coded from eye0
            z = (yzxz[1] + yzxz[3]) / 2.0 # z coordinate average from eye0 and eye1
            return [x,y,z]
        eyes = nengo.Node(output=eyes_func, size_in=4)
        nengo.Connection(eye0.output, eyes[:2])
        nengo.Connection(eye1.output, eyes[2:])

        # create output node for sending instructions to arm
        def arm_func(t, x):
            if t < .05: return # don't move arm during startup (avoid transients)
            rob.move_to_xyz(np.array(x, dtype='float32'))
        armNode = nengo.Node(output=arm_func, size_in=3, size_out=0)
        nengo.Connection(eyes, armNode)

    sim = nengo.Simulator(model)
    sim.run(10, progress_bar=False)

except: 
    print traceback.print_exc(file=sys.stdout)
finally:
    print 'disconnecting'
    rob.robot.disconnect()

The first thing we’re doing is defining a function (eyeNet) to create our neural population that takes input from a spiking camera, and decodes out an end-effector location. In here, we read in from the file the information we just recorded about the camera positions that will serve as the input signal to the neurons (eval_points) and the corresponding set of function output (targets). We create a Nengo network, net, and then a couple of nodes for connecting the input (net.input) and projecting the output (net.output). The population of neurons that we’ll use to approximate our function is called net.map_ens. To specify the function we want to approximate using the eval_points and targets arrays, we create a connection from net.map_ens to net.output and use **target_function(eval_points, targets). So this is probably a little weird to parse if you haven’t used Nengo before, but hopefully it’s clear enough that you can get the gist of what’s going on.

In the main part of the code, we create another Nengo network. We call this one model because that’s convention for the top-level network in Nengo. We then create two networks using the eyeNet function to hook up to the two cameras. At this point we create a node called eyes, and the role of this node is simply to amalgamate the information from the two cameras from (x,z) and (y,z) into (x,y,z). This node is then hooked up to another node called armNode, and all armNode does is call the robot arm’s move_to_xyz function, which we defined in the last post.

Finally, we create a Simulation from model, which compiles the neural network we just specified above, and we run the simulation. The result of all of this then looks something like the following:

animation

And there we go! Project complete! We have a controller for a 6DOF arm that uses spiking cameras to train up a neural population and track an LED, that requires almost no set up time. I gave a demo of this at the end of the summer school and there’s no real positioning of the cameras relative to the arm required, just have to tape the cameras up somewhere, run the training script, and go!

Future work

From here there are a bunch of fun ways to go about extending this. We could add another LED blinking at a different frequency that the arm needs to avoid, using an obstacle avoidance algorithm like the one in this post, add in another dimension of the task involving the gripper, implement a null-space controller to keep the arm near resting joint angles as it tracks the target, and on and on!

Another thing that I’ve looked at is including learning on the system to fine tune our function approximation online. As is, the controller is able to extrapolate and move the arm to target locations that are outside of the range of space sampled during training, but it’s not super accurate. It would be much better to be constantly refining the estimate using learning. I was able to implement a basic version that works, but getting the learning and the tracking running at the same time turns out to be a bit trickier, so I haven’t had the chance to get it all running yet. Hopefully there will be some more down-time in the near future, however, and be able to finish implementing it.

For now, though, we still have a pretty neat target tracker for our robot arm!

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Operational space control of 6DOF robot arm with spiking cameras part 2: Deriving the Jacobian

In the previous exciting post in this series I outlined the project, which is in the title, and we worked through getting access to the arm through Python. The next step was deriving the Jacobian, and that’s what we’re going to be talking about in this post!

Alright.
This was a time I was very glad to have a previous post talking about generating transformation matrices, because deriving the Jacobian for a 6DOF arm in 3D space comes off as a little daunting when you’re used to 3DOF in 2D space, and I needed a reminder of the derivation process. The first step here was finding out which motors were what, so I went through and found out how each motor moved with something like the following code:

for ii in range(7):
    target_angles = np.zeros(7, dtype='float32')
    target_angles[ii] = np.pi / 4.0
    rob.move(target_angles)
    time.sleep(1)

and I found that the robot is setup in the figures below

armangles
armlengths
this is me trying my hand at making things clearer using Inkscape, hopefully it’s worked. Displayed are the first 6 joints and their angles of rotation, q_0 through q_5. The 7th joint, q_6, opens and closes the gripper, so we’re safe to ignore it in deriving our Jacobian. The arm segment lengths l_1, l_3, and l_5 are named based on the nearest joint angles (makes easier reading in the Jacobian derivation).

Find the transformation matrix from end-effector to origin

So first thing’s first, let’s find the transformation matrices. Our first joint, q_0, rotates around the z axis, so the rotational part of our transformation matrix ^0_\textrm{org}\textbf{T} is

^0_\textrm{org}\textbf{R} = \left[ \begin{array}{ccc} \textrm{cos}(q_0) & -\textrm{sin}(q_0) & 0 \\ \textrm{sin}(q_0) & \textrm{cos}(q_0) & 0 \\ 0 & 0 & 1 \end{array} \right],

and q_0 and our origin frame of reference are on top of each other so we don’t need to account for translation, so our translation component of ^0_\textrm{org}\textbf{T} is

^0_\textrm{org}\textbf{D} = \left[ \begin{array}{c} 0 \\ 0 \\ 0 \end{array} \right].

Stacking these together to form our first transformation matrix we have

^0_\textrm{org}\textbf{T} = \left[ \begin{array}{cc} ^0_\textrm{org}\textbf{R} & ^0_\textrm{org}\textbf{D} \\ 0 & 1 \end{array} \right] = \left[ \begin{array}{cccc} \textrm{cos}(q_0) & -\textrm{sin}(q_0) & 0 & 0\\ \textrm{sin}(q_0) & \textrm{cos}(q_0) & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 1 \end{array} \right] .

So now we are able to convert a position in 3D space from to the reference frame of joint q_0 back to our origin frame of reference. Let’s keep going.

Joint q_1 rotates around the x axis, and there is a translation along the arm segment l_1. Our transformation matrix looks like

^1_0\textbf{T} = \left[ \begin{array}{cccc} 1 & 0 & 0 & 0 \\ 0 & \textrm{cos}(q_1) & -\textrm{sin}(q_1) & l_1 \textrm{cos}(q_1) \\ 0 & \textrm{sin}(q_1) & \textrm{cos}(q_1) & l_1 \textrm{sin}(q_1) \\ 0 & 0 & 0 & 1 \end{array} \right] .

Joint q_2 also rotates around the x axis, but there is no translation from q_2 to q_3. So our transformation matrix looks like

^2_1\textbf{T} = \left[ \begin{array}{cccc} 1 & 0 & 0 & 0 \\ 0 & \textrm{cos}(q_2) & -\textrm{sin}(q_2) & 0 \\ 0 & \textrm{sin}(q_2) & \textrm{cos}(q_2) & 0 \\ 0 & 0 & 0 & 1 \end{array} \right] .

The next transformation matrix is a little tricky, because you might be tempted to say that it’s rotating around the z axis, but actually it’s rotating around the y axis. This is determined by where q_3 is mounted relative to q_2. If it was mounted at 90 degrees from q_2 then it would be rotating around the z axis, but it’s not. For translation, there’s a translation along the y axis up to the next joint, so all in all the transformation matrix looks like:

^3_2\textbf{T} = \left[ \begin{array}{cccc} \textrm{cos}(q_3) & 0 & -\textrm{sin}(q_3) & 0\\ 0 & 1 & 0 & l_3 \\ \textrm{sin}(q_3) & 0 & \textrm{cos}(q_3) & 0\\ 0 & 0 & 0 & 1 \end{array} \right] .

And then the transformation matrices for coming from q_4 to q_3 and q_5 to q_4 are the same as the previous set, so we have

^4_3\textbf{T} = \left[ \begin{array}{cccc} 1 & 0 & 0 & 0 \\ 0 & \textrm{cos}(q_4) & -\textrm{sin}(q_4) & 0 \\ 0 & \textrm{sin}(q_4) & \textrm{cos}(q_4) & 0 \\ 0 & 0 & 0 & 1 \end{array} \right] .

and

^5_4\textbf{T} = \left[ \begin{array}{cccc} \textrm{cos}(q_5) & 0 & -\textrm{sin}(q_5) & 0 \\ 0 & 1 & 0 & l_5 \\ \textrm{sin}(q_5) & 0 & \textrm{cos}(q_5) & 0\\ 0 & 0 & 0 & 1 \end{array} \right] .

Alright! Now that we have all of the transformation matrices, we can put them together to get the transformation from end-effector coordinates to our reference frame coordinates!

^\textrm{ee}_\textrm{org}\textbf{T} = ^0_\textrm{org}\textbf{T} \; ^1_0\textbf{T} \; ^2_1\textbf{T} \; ^3_2\textbf{T} \; ^4_3\textbf{T} \; ^5_4\textbf{T}.

At this point I went and tested this with some sample points to make sure that everything seemed to be being transformed properly, but we won’t go through that here.

Calculate the derivative of the transform with respect to each joint

The next step in calculating the Jacobian is getting the derivative of ^\textrm{ee}_\textrm{org}T. This could be a big ol’ headache to do it by hand, OR we could use SymPy, the symbolic computation package for Python. Which is exactly what we’ll do. So after a quick

sudo pip install sympy

I wrote up the following script to perform the derivation for us

import sympy as sp

def calc_transform():
    # set up our joint angle symbols (6th angle doesn't affect any kinematics)
    q = [sp.Symbol('q0'), sp.Symbol('q1'), sp.Symbol('q2'), sp.Symbol('q3'),
            sp.Symbol('q4'), sp.Symbol('q5')]
    # set up our arm segment length symbols
    l1 = sp.Symbol('l1')
    l3 = sp.Symbol('l3')
    l5 = sp.Symbol('l5')

    Torg0 = sp.Matrix([[sp.cos(q[0]), -sp.sin(q[0]), 0, 0,],
                       [sp.sin(q[0]), sp.cos(q[0]), 0, 0],
                       [0, 0, 1, 0],
                       [0, 0, 0, 1]])

    T01 = sp.Matrix([[1, 0, 0, 0],
                     [0, sp.cos(q[1]), -sp.sin(q[1]), l1*sp.cos(q[1])],
                     [0, sp.sin(q[1]), sp.cos(q[1]), l1*sp.sin(q[1])],
                     [0, 0, 0, 1]])

    T12 = sp.Matrix([[1, 0, 0, 0],
                     [0, sp.cos(q[2]), -sp.sin(q[2]), 0],
                     [0, sp.sin(q[2]), sp.cos(q[2]), 0],
                     [0, 0, 0, 1]])

    T23 = sp.Matrix([[sp.cos(q[3]), 0, sp.sin(q[3]), 0],
                     [0, 1, 0, l3],
                     [-sp.sin(q[3]), 0, sp.cos(q[3]), 0],
                     [0, 0, 0, 1]])

    T34 = sp.Matrix([[1, 0, 0, 0],
                     [0, sp.cos(q[4]), -sp.sin(q[4]), 0],
                     [0, sp.sin(q[4]), sp.cos(q[4]), 0],
                     [0, 0, 0, 1]])

    T45 = sp.Matrix([[sp.cos(q[5]), 0, sp.sin(q[5]), 0],
                     [0, 1, 0, l5],
                     [-sp.sin(q[5]), 0, sp.cos(q[5]), 0],
                     [0, 0, 0, 1]])

    T = Torg0 * T01 * T12 * T23 * T34 * T45

    # position of the end-effector relative to joint axes 6 (right at the origin)
    x = sp.Matrix([0,0,0,1])

    Tx = T * x

    for ii in range(6):
        print q[ii]
        print sp.simplify(Tx[0].diff(q[ii]))
        print sp.simplify(Tx[1].diff(q[ii]))
        print sp.simplify(Tx[2].diff(q[ii]))

And then consolidated the output using some variable shorthand to write a function that accepts in joint angles and generates the Jacobian:

def calc_jacobian(q):
    J = np.zeros((3, 7))

    c0 = np.cos(q[0])
    s0 = np.sin(q[0])
    c1 = np.cos(q[1])
    s1 = np.sin(q[1])
    c3 = np.cos(q[3])
    s3 = np.sin(q[3])
    c4 = np.cos(q[4])
    s4 = np.sin(q[4])

    c12 = np.cos(q[1] + q[2])
    s12 = np.sin(q[1] + q[2])

    l1 = self.l1
    l3 = self.l3
    l5 = self.l5

    J[0,0] = -l1*c0*c1 - l3*c0*c12 - l5*((s0*s3 - s12*c0*c3)*s4 + c0*c4*c12)
    J[1,0] = -l1*s0*c1 - l3*s0*c12 + l5*((s0*s12*c3 + s3*c0)*s4 - s0*c4*c12)
    J[2,0] = 0

    J[0,1] = (l1*s1 + l3*s12 + l5*(s4*c3*c12 + s12*c4))*s0
    J[1,1] = -(l1*s1 + l3*s12 + l5*s4*c3*c12 + l5*s12*c4)*c0
    J[2,1] = l1*c1 + l3*c12 - l5*(s4*s12*c3 - c4*c12)

    J[0,2] = (l3*s12 + l5*(s4*c3*c12 + s12*c4))*s0
    J[1,2] = -(l3*s12 + l5*s4*c3*c12 + l5*s12*c4)*c0
    J[2,2] = l3*c12 - l5*(s4*s12*c3 - c4*c12)

    J[0,3] = -l5*(s0*s3*s12 - c0*c3)*s4
    J[1,3] = l5*(s0*c3 + s3*s12*c0)*s4
    J[2,3] = -l5*s3*s4*c12

    J[0,4] = l5*((s0*s12*c3 + s3*c0)*c4 + s0*s4*c12)
    J[1,4] = l5*((s0*s3 - s12*c0*c3)*c4 - s4*c0*c12)
    J[2,4] = -l5*(s4*s12 - c3*c4*c12)

    return J

Alright! Now we have our Jacobian! Really the only time consuming part here was calculating our end-effector to origin transformation matrix, generating the Jacobian was super easy using SymPy once we had that.

Hack position control using the Jacobian

Great! So now that we have our Jacobian we’ll be able to translate forces that we want to apply to the end-effector into joint torques that we want to apply to the arm motors. Since we can’t control applied force to the motors though, and have to pass in desired angle positions, we’re going to do a hack approximation. Let’s first transform our forces from end-effector space into a set of joint angle torques:

\textbf{u} = \textbf{J}^T \; \textbf{u}_\textbf{x}.

To approximate the control then we’re simply going to take the current set of joint angles (which we know because it’s whatever angles we last told the system to move to) and add a scaled down version of \textbf{u} to approximate applying torque that affects acceleration and then velocity.

\textbf{q}_\textrm{des} = \textbf{q} + \alpha \; \textbf{u},

where \alpha is the gain term, I used .001 here because it was nice and slow, so no crazy commands that could break the servos would be sent out before I could react and hit the cancel button.

What we want to do then to implement operational space control here then is find the current (x,y,z) position of the end-effector, calculate the difference between it and the target end-effector position, use that to generate the end-effector control signal u_x, get the Jacobian for the current state of the arm using the function above, find the set of joint torques to apply, approximate this control by generating a set of target joint angles to move to, and then repeat this whole loop until we’re within some threshold of the target position. Whew.

So, a lot of steps, but pretty straight forward to implement. The method I wrote to do it looks something like:

def move_to_xyz(self, xyz_d):
    """
    np.array xyz_d: 3D target (x_d, y_d, z_d)
    """
    count = 0
    while (1):
        count += 1
        # get control signal in 3D space
        xyz = self.calc_xyz()
        delta_xyz = xyz_d - xyz
        ux = self.kp * delta_xyz

        # transform to joint space
        J = self.calc_jacobian()
        u = np.dot(J.T, ux)

        # target joint angles are current + uq (scaled)
        self.q[...] += u * .001
        self.robot.move(np.asarray(self.q.copy(), 'float32'))

        if np.sqrt(np.sum(delta_xyz**2)) < .1 or count > 1e4:
            break

And that is it! We have successfully hacked together a system that can perform operational space control of a 6DOF robot arm. Here is a very choppy video of it moving around to some target points in a grid on a cube.
6dof-operational-space
So, granted I had to drop a lot of frames from the video to bring it’s size down to something close to reasonable, but still you can see that it moves to target locations super fast!

Alright this is sweet, but we’re not done yet. We don’t want to have to tell the arm where to move ourselves. Instead we’d like the robot to perform target tracking for some target LED we’re moving around, because that’s way more fun and interactive. To do this, we’re going to use spiking cameras! So stay tuned, we’ll talk about what the hell spiking cameras are and how to use them for a super quick-to-setup and foolproof target tracking system in the next exciting post!

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Operational space control of 6DOF robot arm with spiking cameras part 1: Getting access to the arm through Python

From June 9th to the 19th we ran a summer school (brain camp) in our lab with people from all over to come and learn how to use our neural modeling software, Nengo, and then work on a project with others in the school and people from the lab. We had a bunch of fun hardware available, from neuromorphic hardware from the SpiNNaker group over at Cambridge to lego robots on omni-wheels to spiking cameras (i’ll discuss what a spiking camera is exactly in part 3 of this series) and little robot arms. There were a bunch of awesome projects, people built neural models capable of performing a simplified version of the Wisconsin card sorting task that they then got running on the SpiNNaker boards, a neural controller built to move a robot leech, a spiking neurons reinforcement-learning system implemented on SpiNNaker with spiking cameras to control the lego robot that learned to move towards green LEDs and avoid red LEDs, and a bunch of others. If you’re interested in participating in these kinds of projects and learning more about this I highly suggest you apply to the summer school for next year!

I took the summer school as a chance to break from other projects and hack together a project. The idea was to take the robot arm, implement an operational space controller (i.e. find the Jacobian), and then used spiking cameras to track an LED and have the robot arm learn how to move to the target, no matter where the cameras were placed, by learning the relationship between where the target is in camera-centric coordinates and arm-centric coordinates. This broke down into several steps: 1) Get access to the arm through Python, 2) derive the Jacobian to implement operational space control, 3) sample state space to get an approximation of the camera-centric to arm-centric function, 4) implement the control system to track the target LED.

Here’s a picture of the set-up during step 3, training.
spiking-cameras-6DOF-arm
So in the center is the 6DOF robot arm with a little LED attached to the head, and highlighted in orange circles are the two spiking cameras, expertly taped to the wall with office-grade scotch tape. You can also see the SpiNNaker board in the top left as a bonus, but I didn’t have enough time to involve it in this project.

I was originally going to write this all up as one post, because the first two parts are re-implementing previous posts, but even skimming through those steps it was getting long and I’m sure no one minds having a few different examples to look through for generating Cython code or deriving a Jacobian. So I’m going to break this into a few parts. Here in this post we’ll work through the first step (Cython) of our journey.

Get access to the arm through Python

The arm we had was the VE026A Denso training robot, on loan from Dr. Bryan Tripp of the neuromorphic robotics lab at UW. Previously an interface had been built up by one of Dr. Tripp’s summer students, written in C. C is great and all but Python is much easier to work with, and the rest of the software I wanted to work you know what I’m done justifying it Python is just great so Python is what I wanted to use. The end.

So to get access to the arm in Python I used the great ol’ C++ wrapper hack described in a previous post. Looking at Murphy-the-summer-student’s C code there were basically three functions I needed access to:

// initialize threads, connect to robot
void start_robot(int *semid, int32_t *ctrlid, int32_t *robotid, float **angles)
// send a set of joint angles to the robot servos
void Robot_Execute_slvMove(int32_t robotid, float j1, float j2, float j3, float j4, float j5, float j6, float j7, float j8)
// kill threads, disconnect from robot
void stop_robot(int semid, int32_t ctrlid, int32_t robotid)

So I changed the extension of the file to ‘.cpp’ (I know, I know, I said this was a hack!), fixed some compiler errors that popped up, and then appended the following to the end of the file:

/* A class to contain all the information that needs to
be passed around between these functions, and can
encapsulate it and hide it from the Python interface.

Written by Travis DeWolf (June, 2015)
*/

class Robot {
/* Very simple class, just stores the variables
* needed for talking to the robot, and a gives access
* to the functions for moving it around */

int semid;
int32_t ctrlid;
int32_t robotid;
float* angles;

public:
Robot();
~Robot();
void connect();
void move(float* angles_d);
void disconnect();
};

Robot::Robot() { }

Robot::~Robot() { free(angles); }

/* Connect to the robot, get the ids and current joint angles */
/* char* usb_port: the name of the port the robot is connected to */
void Robot::connect()
{
start_robot(&amp;semid, &amp;ctrlid, &amp;robotid, &amp;angles);
printf("%i %i %i", robotid, ctrlid, semid);
}

/* Move the robot to the specified angles */
/* float* angles: the target joint angles */
void Robot::move(float* angles)
{
// convert from radians to degrees
Robot_Execute_slvMove(robotid,
angles[0] * 180.0 / 3.14,
angles[1] * 180.0 / 3.14,
angles[2] * 180.0 / 3.14,
angles[3] * 180.0 / 3.14,
angles[4] * 180.0 / 3.14,
angles[5] * 180.0 / 3.14,
angles[6] * 180.0 / 3.14,
angles[7] * 180.0 / 3.14);
}

/* Disconnect from the robot */
void Robot::disconnect()
{
stop_robot(semid, ctrlid, robotid);
}

int main()
{
Robot robot = Robot();
// connect to robot
robot.connect();

// move robot to some random target angles
float target_angles[7] = {0, np.pi / 2.0, 0.0, 0, 0, 0, 0};
robot.move(target_angles);

sleep(1);

// disconnect
robot.disconnect();

return 0;
}

So very simple class. Basically just wanted to create a set of functions to hide some of the unnecessary parameters from Python, do the conversion from radians to degrees (who works in degrees? honestly), and have a short little main function to test the creation of the class, and connection, movement, and disconnection of the robot. Like I said, there were a few compiler errors when switching from C to C++, but really that was the only thing that took any time on this part. A few casts and everything was gravy.

The next part was writing the Cython pyRobot.pyx file (I describe the steps involved in this in more detail in this post):

import numpy as np
cimport numpy as np

cdef extern from "bcap.cpp":
cdef cppclass Robot:
Robot()
void connect()
void move(float* angles)
void disconnect()

cdef class pyRobot:
cdef Robot* thisptr

def __cinit__(self):
self.thisptr = new Robot()

def __dealloc__(self):
del self.thisptr

def connect(self):
"""
Set up the connection to the robot, pass in a vector,
get back the current joint angles of the arm.
param np.ndarray angles: a vector to store current joint angles
"""
self.thisptr.connect()

def move(self, np.ndarray[float, mode="c"] angles):
"""
Step the simulation forward one timestep. Pass in target angles,
get back the current arm joint angles.
param np.ndarray angles: 7D target angle vector
"""
self.thisptr.move(&amp;angles[0])

def disconnect(self):
"""
Disconnect from the robot.
"""
self.thisptr.disconnect()

the setup.py file:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

setup(
name = 'Demos',
ext_modules=[
Extension("test",
sources=["pyRobot.pyx"],
language="c++"),
],
cmdclass = {'build_ext': build_ext},

)

and then compiling!

run setup.py build_ext -i

With all of this working, a nice test.so file was created, and it was now possible to connect to the robot in Python with

import test
rob = test.pyRobot()
rob.connect()
target_angles = np.array([0, np.pi/2, 0, np.pi/4, 0, 0, 0, 0], dtype='float32')
rob.move(target_angles)
import time
time.sleep(1)
rob.disconnect()

In the above code we’re instantiating the pyRobot class, connecting to the robot, defining a set of target angles and telling the robot to move there, waiting for 1 second to give the robot time to actually move, and then disconnecting from the robot. Upon connection we have to pass in a set of joint angles for the servos, and so you see the robot arm jerk into position, and then move to the target set of joint angles, it looks something exactly like this:

6dof-connect-move

Neat, phase 1 complete.

At the end of phase 1 we are able to connect to the robot arm through Python and send commands in terms of joint angles. But we don’t want to send commands in terms of joint angles, we want to just specify the end-effector position and have the robot work out the angles! I’ve implemented an inverse kinematics solver using constrained optimization before for a 3-link planar arm, but we’re not going to go that route. Find out what we’ll do by joining us next time! or by remembering what I said we’d do at the beginning of this post.

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Dynamic movement primitives part 2: Controlling end-effector trajectories

The dynamic movement primitive (DMP) framework was designed for trajectory control. It so happens that in previous posts we’ve built up to having several arm simulations that are ripe for throwing a trajectory controller on top, and that’s what we’ll do in this post. The system that we will be controlling here is the 3 link arm model with an operational space controller (OSC) that translates end-effector forces into joint torques. The DMPs here will be controlling the (x,y) trajectory of the hand, and the OSC will take care of turning the desired hand forces into torques that can be applied to the arm. All of the code used to generate the animations throughout this post can of course be found up on my github.

Controlling a 3 link arm with DMPs
We have our 3 link arm and its OSC controller; this whole setup we’ll collectively refer to as ‘the plant’ throughout this post. We are going to pass in some (x,y) force signal to the plant, and the plant will carry it out. Additionally we’ll get a feedback signal with the (x,y) position of the hand. At the same time, we also have a DMP system that’s doing its own thing, tracing out a desired trajectory in (x,y) space. We have to tie these two systems together.

To do this is easy, we’ll generate the control signal for the plant from our DMP system simply by measuring the difference between the state of our DMP system and the plant state, use that to drive the plant to the state of the DMP system. Formally,

u = k_p(y_{\textrm{DMP}} - y)

where y_{\textrm{DMP}} is the state of the DMP system, y is the state of the plant, and k_p and is the position error gain term.

Once we have this, we just go ahead and step our DMP system forward and make sure the gain values on the control signal are high enough that the plant follows the DMP’s trajectory. And that’s pretty much it, just run the DMP system to the end of the trajectory and then stop your simulation.

To give a demonstration of DMP control I’ve set up the DMP system to follow the same number trajectories that the SPAUN arm followed. As you can see the combination of DMPs and operational space control is much more effective than my previous implementation.

Incorporating system feedback

One of the issues in implementing the control above is that we have to be careful about how quickly the DMP trajectory moves, because while the DMP system isn’t constrained by any physical dynamics, the plant is. Depending on the size of the movement the DMP trajectory may be moving a foot a second or an inch a second. You can see above that the arm doesn’t fully draw out the desired trajectories in places where the DMP system moved too quickly in and out and sharp corners. The only way to remedy this without feedback is to have the DMP system move more slowly throughout the entire trajectory. What would be nice, instead, would be to just say ‘go as fast as you can, as long as the plant state is within some threshold distance of you’, and this is where system feedback comes in.

It’s actually very straightforward to implement this using system feedback: If the plant state drifts away from the state of the DMPs, slow down the execution speed of the DMP to allow the plant time to catch up. The do this we just have to multiply the DMP timestep dt by a new term:

1 / (1 + \alpha_{\textrm{err}}(y_{\textrm{plant}} - y_{\textrm{DMP}})).

All this new term does is slow down the canonical system when there’s an error, you can think of it as a scaling on the time step. Additionally, the sensitivity of this term can be modulated the scaling term \alpha_{\textrm{err}} on the difference between the plant and DMP states.

We can get an idea of how this affects the system by looking at the dynamics of the canonical system when an error term is introduced mid-run:

CSwitherror
When the error is introduced the dynamics of the system slow down, great! Lets look at an example comparing execution with and without this feedback term. Here’s the system drawing the number 3 without any feedback incorporation:

and here’s the system drawing the number 3 with the feedback term included:


These two examples are a pretty good case for including the feedback term into your DMP system. You can still see in the second case that the specified trajectory isn’t traced out exactly, but if that’s what you’re shooting for you can just crank up the \alpha_{\textrm{err}} to make the DMP timestep really slow down whenever the DMP gets ahead of the plant at all.

Interpolating trajectories for imitation

When imitating trajectories there can be some issues with having enough sample points and how to fit them to the canonical system’s timeframe, they’re not difficult to get around but I thought I would go over what I did here. The approach I took was to always run the canonical system for 1 second, and whenever a trajectory is passed in that should be imitated to scale the x-axis of the trajectory such that it’s between 0 and 1. Then I shove it into an interpolator and use the resulting function to generate an abundance of nicely spaced sample points for the DMP imitator to match. Here’s the code for that:

# generate function to interpolate the desired trajectory
import scipy.interpolate

path = np.zeros((self.dmps, self.timesteps))
x = np.linspace(0, self.cs.run_time, y_des.shape[1])

for d in range(self.dmps):

    # create interpolation function
    path_gen = scipy.interpolate.interp1d(x, y_des[d])

    # create evenly spaced sample points of desired trajectory
    for t in range(self.timesteps):
        path[d, t] = path_gen(t * self.dt)

y_des = path

Direct trajectory control vs DMP based control

Now, using the above described interpolation function we can just directly use its output to guide our system. And, in fact, when we do this we get very precise control of the end-effector, more precise than the DMP control, as it happens. The reason for this is because our DMP system is approximating the desired trajectory and with a set of basis functions, and some accuracy is being lost in this approximation.

So if we instead use the interpolation function to drive the plant we can get exactly the points that we specified. The times when this comes up especially are when the trajectories that you’re trying to imitate are especially complicated. There are ways to address this with DMPs by placing your basis functions more appropriately, but if you’re just looking for the exact replication of an input trajectory (as often people are) this is a simpler way to go. You can see the execution of this in the trace.py code up on my github. Here’s a comparison of a single word drawn using the interpolation function:

draw_word_traj

and here’s the same word drawn using a DMP system with 1,000 basis function per DOF:

draw_word_dmp
We can see that just using the interpolation function here gives us the exact path that we specified, where using DMPs we have some error, and this error increases with the size of the desired trajectory. But! That’s for exactly following a given trajectory, which is often not the case. The strength of the DMP framework is that the trajectory is a dynamical system. This lets us do simple things to get really neat performance, like scale the trajectory spatially on the fly simply by changing the goal, rather than rescaling the entire trajectory:

Conclusions

Some basic examples of using DMPs to control the end-effector trajectory of an arm with operational space control were gone over here, and you can see that they work really nicely together. I like when things build like this. We also saw that power of DMPs in this situation is in their generalizability, and not in exact reproduction of a given path. If I have a single complex trajectory that I only want the end-effector to follow once then I’m going to be better off just interpolating that trajectory and feeding the coordinates into the arm controller rather than go through the whole process of setting up the DMPs.

Drawing words, though, is just one basic example of using the DMP framework. It’s a very simple application and really doesn’t do justice to the flexibility and power of DMPs. Other example applications include things like playing ping pong. This is done by creating a desired trajectory showing the robot how to swing a ping pong paddle, and then using a vision system to track the current location of the incoming ping pong ball and changing the target of the movement to compensate dynamically. There’s also some really awesome stuff with object avoidance, that is implemented by adding another term with some simple dynamics to the DMP. Discussed here, basically you just have another system that moves you away from the object with a strength relative to your distance from the object. You can also use DMPs to control gain terms on your PD control signal, which is useful for things like object manipulation.

And of course I haven’t touched on rhythmic DMPs or learning with DMPs at all, and those are both also really interesting topics! But this serves as a decent introduction to the whole area, which has been developed in the Schaal lab over the last decade or so. I recommend further reading with some of these papers if you’re interested, there are a ton of neat ways to apply the DMP framework! And, again, the code for everything here is up on my github in the control and pydmps repositories.

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Robot control part 7: OSC of a 3-link arm

So we’ve done control for the 2-link arm, and control of the one link arm is trivial (where we control joint angle, or x or y coordinate of the pendulum), so here I’ll just show an implementation of operation space control for a more interesting arm model, the 3-link arm model. The code can all be found up on my Github.

In theory there’s nothing different or more difficult about controlling a 3-link arm versus a 2-link arm. For the inertia matrix, what I ended up doing here is just jacking up all the values of the matrix to about 100, which causes the controller to way over control the arm, and you can see the torques are much larger than they would need to be if we had an accurate inertia matrix. But the result is the same super straight trajectories that we’ve come to expect from operational space control:

3link

It’s a little choppy because I cut out a bunch of frames to keep the gif size down. But you get the point, it works. And quite well!

Because this is also a 3-link arm now and our task space force signal is 2D, we have redundant space of solutions, meaning that the task space control signal can be carried out in a number of ways. In other words, a null space exists for this controller. This means that we can throw another controller in our system to operate inside the null space of the first controller. We’ve already worked through all the math for this, so it’s straightforward to implement.

What kind of null space controller should we put in? Well, you may have noticed in the above animation the arm goes through itself, here’s another example:

3linknonull

Often it’s desirable to avoid this (because of physics or whatever), so what we can do is add a secondary controller that works to keep the arm’s elbow and wrist near some comfortable default angles. When we do this we get the following:

3linknull

And there you have it! Operational space control of a three link arm with a secondary controller in the null space to try to keep the angles near their default / resting positions.

I also added mouse based control to the arm so it will try to follow your mouse when you move over the figure, which makes it pretty fun to explore the power of the controller. It’s interesting to see where the singularities become an issue, and how having a null space controller that’s operating in joint space can actually come to help the system move through those problem points more smoothly. Check it out! It’s all up on my Github.

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Robot control part 6: Handling singularities

We’re back! Another exciting post about robotic control theory, but don’t worry, it’s short and ends with simulation code. The subject of today’s post is handling singularities.

What is a singularity

This came up recently when I had build this beautiful controller for a simple two link arm that would occasionally go nuts. After looking at it for a while it became obvious this was happening whenever the elbow angle reached or got close to 0 or \pi. Here’s an animation:

singularity

What’s going on here? Here’s what. The Jacobian has dropped rank and become singular (i.e. non-invertible), and when we try to calculate our mass matrix for operational space

\textbf{M}_\textbf{x}(\textbf{q}) = (\textbf{J} (\textbf{q}) \; \textbf{M}^{-1} (\textbf{q}) \; \textbf{J}^T(\textbf{q}))^{-1},

the values explode in the inverse calculation. Dropping rank means that the rows of the Jacobian are no longer linearly independent, which means that the matrix can be rotated such that it gives a matrix with a row of zeros. This row of zeros is the degenerate direction, and the problems come from trying to send forces in that direction.

To determine when the Jacobian becomes singular its determinant can be examined; if the determinant of the matrix is zero, then it is singular. Looking the Jacobian for the end-effector:

\textbf{J}(\textbf{q}) = \left[ \begin{array}{cc} -L_0 sin(q_0) - L_1 sin(q_0 + q_1) & -L_1 sin(q_0 + q_1) \\ L_0 cos(q_0) + L_1 cos(q_0 + q_1) & L_1 cos(q_0 + q_1) \end{array} \right].

When q_1 = 0 it can be that sin(q_0 + 0) = sin(q_0), so the Jacobian becomes

\textbf{J}(\textbf{q}) = \left[ \begin{array}{cc} -(L_0 - L_1) sin(q_0) & -L_1 sin(q_0) \\ (L_0 + L_1) cos(q_0) & L_1 cos(q_0) \end{array} \right],

which gives a determinant of

(L_0 - L_1)(-sin(q_0))(L_1)(cos(q_0)) - (L_1)(-sin(q_0))(L_0 + L_1)(cos(q_0)) = 0.

Similarly, when q_1 = \pi, where sin(q_0 + \pi) = -sin(q_0) and cos(q_0 + \pi) = -cos(q_0), the determinant of the Jacobian is

\textbf{J}(\textbf{q}) = \left[ \begin{array}{cc} -(L_0 - L_1) sin(q_0) & L_1 sin(q_0) \\ (L_0 + L_1) cos(q_0) & - L_1 cos(q_0) \end{array} \right].

Calculating the determinant of this we get

(L_0 + L_1)(-sin(q_0))(L_1)(-cos(q_0)) - (L_1)(sin(q_0))(L_0 + L_1)(-cos(q_0)) = 0.

Note that while in these cases the Jacobian is a square matrix in the event that it is not a square matrix, the determinant of \textbf{J}(\textbf{q})\;\textbf{J}^T(\textbf{q}) can be found instead.

Fixing the problem

When a singularity is occurring it can be detected, but now it must be handled such that the controller behaves appropriately. This can be done by identifying the degenerate dimensions and setting the force in those directions to zero.

First the SVD decomposition of \textbf{M}_\textbf{x}^{-1}(\textbf{q}) = \textbf{V}\textbf{S}\textbf{U}^T is found. To get the inverse of this matrix (i.e. to find \textbf{M}_\textbf{x}(\textbf{q})) from the returned \textbf{V}, \textbf{S} and \textbf{U} matrices is a matter of inverting the matrix \textbf{S}:

\textbf{M}_\textbf{x}(\textbf{q}) = \textbf{V} \textbf{S}^{-1} \textbf{U}^T,

where \textbf{S} is a diagonal matrix of singular values.

Because \textbf{S} is diagonal it is very easy to find its inverse, which is calculated by taking the reciprocal of each of the diagonal elements.

Whenever the system approaches a singularity some of the values of \textbf{S} will start to get very small, and when we take the reciprocal of them we start getting huge numbers, which is where the value explosion comes from. Instead of allowing this to happen, a check for approaching the singularity can be implemented, which then sets the singular values entries smaller than the threshold equal to zero, canceling out any forces that would be sent in that direction.

Here’s the code:

Mx_inv = np.dot(JEE, np.dot(np.linalg.inv(Mq), JEE.T))
if abs(np.linalg.det(np.dot(JEE,JEE.T))) > .005**2:
    # if we're not near a singularity
    Mx = np.linalg.inv(Mx_inv)
else:
    # in the case that the robot is entering near singularity
    u,s,v = np.linalg.svd(Mx_inv)
    for i in range(len(s)):
        if s[i] < .005: s[i] = 0
        else: s[i] = 1.0/float(s[i])
    Mx = np.dot(v, np.dot(np.diag(s), u.T))

And here’s an animation of the controlled arm now that we’ve accounted for movement when near singular configurations:

fixed

As always, the code for this can be found up on my Github. The default is to run using a two link arm simulator written in Python. To run, simply download everything and run the run_this.py file.

Everything is also included required to run the MapleSim arm simulator. To do this, go into the TwoLinkArm folder, and run python setup.py build_ext -i. This should compile the arm simulation to a shared object library that Python can now access on your system. To use it, edit the run_this.py file to import from TwoLinkArm/arm_python to TwoLinkArm/arm and you should be good to go!
More details on getting the MapleSim arm to run can be found in this post.

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Robot control part 5: Controlling in the null space

In the last post, I went through how to build an operational space controller. It was surprisingly easy after we’ve worked through all the other posts. But maybe that was a little too easy for you. Maybe you want to do something more interesting like implement more than one controller at the same time. In this post we’ll go through how to work inside the null space of a controller to implement several seperate controllers simultaneously without interference.
Buckle up.

Null space forces

The last example comprises the basics of operational space control; describe the system, calculate the system dynamics, transform desired forces from an operational space to the generalized coordinates, and build the control signal to cancel out the undesired system dynamics. Basic operational space control works quite well, but it is not uncommon to have several control goals at once; such as `move the end-effector to this position’ (primary goal), and `keep the elbow raised high’ (secondary goal) in the control of a robot arm.

If the operational space can also serve as generalized coordinates, i.e. if the system state specified in operational space constrains all of the degrees of freedom of the robot, then there is nothing that can be done without affecting the performance of the primary controller. In the case of controlling a two-link robot arm this is the case. The end-effector Cartesian space (chosen as the operational space) could also be a generalized coordinates system, because a specific (x,y) position fully constrains the position of the arm.

But often when using operational space control for more complex robots this is not the case. In these situations, the forces controlled in operational space have fewer dimensions than the robot has degrees of freedom, and so it is possible to accomplish the primary goal in a number of ways. The null space of this primary controller is the region of state space where there is a redundancy of solutions; the system can move in a number of ways and still not affect the completion of the goals of the primary controller. An example of this is all the different configurations the elbow can be in while a person moves their hand in a straight line. In these situations, a secondary controller can be created to operate in the null space of the primary controller, and the full control signal sent to the system is a sum of the primary control signal and a filtered version of the secondary control signal. In this section the derivation of the null-space filter will be worked through for a system with only a primary and secondary controller, but note that the process can be applied iteratively for systems with further controllers.

The filtering of the secondary control signal means that the secondary controller’s goals will only be accomplished if it is possible to do so without affecting the performance of the first controller. In other words, the secondary controller must operate in the null space of the first controller. Denote the primary operational space control signal, e.g. the control signal defined in the previous post, as \textbf{u}_{\textbf{x}} and the control signal from the secondary controller \textbf{u}_{\textrm{null}}. Define the force to apply to the system

\textbf{u} = \textbf{u}_{\textbf{x}} + (\textbf{I} - \textbf{J}_{ee}^T(\textbf{q}) \; \textbf{J}_{ee}^{T+}(\textbf{q})) \textbf{u}_{\textrm{null}},

where \textbf{J}_{ee}^{T+}(\textbf{q}) is the pseudo-inverse of \textbf{J}_{ee}^T(\textbf{q}).

Examining the filtering term that was added,

(\textbf{I} - \textbf{J}_{ee}^T(\textbf{q}) \textbf{J}_{ee}^{T+}(\textbf{q})) \textbf{u}_{\textrm{null}},

it can be seen that the Jacobian transpose multiplied by its pseudo-inverse will be 1’s all along the diagonal, except in the null space. This means that \textbf{u}_{\textrm{null}} is subtracted from itself everywhere that affects the operational space movement and is left to apply any arbitrary control signal in the null space of the primary controller.

Unfortunately, this initial set up does not adequately filter out the effects of forces that might be generated by the secondary controller. The Jacobian is defined as a relationship between the velocities of two spaces, and so operating in the null space defined by the Jacobian ensures that no velocities are applied in operational space, but the required filter must also prevent any accelerations from affecting movement in operational space. The standard Jacobian pseudo-inverse null space is a velocity null space, and so a filter built using it will allow forces affecting the system’s acceleration to still get through. What is required is a pseudo-inverse Jacobian defined to filter signals through an acceleration null space.

To acquire this acceleration filter, our control signal will be substituted into the equation for acceleration in the operational space, which, after cancelling out gravity effects with the control signal and removing the unmodeled dynamics, gives

\ddot{\textbf{x}} = \textbf{J}_{ee}(\textbf{q}) \textbf{M}^{-1}(\textbf{q}) [\textbf{J}_{ee}^T(\textbf{q}) \; \textbf{M}_{\textbf{x}_{ee}}(\textbf{q}) \; \ddot{\textbf{x}}_\textrm{des} - (\textbf{I} - \textbf{J}_{ee}^T(\textbf{q})\;\textbf{J}_{ee}^{T+}(\textbf{q}))\;\textbf{u}_{\textrm{null}}].

Rewriting this to separate the secondary controller into its own term

\ddot{\textbf{x}} = \textbf{J}_{ee}(\textbf{q}) \textbf{M}^{-1}(\textbf{q}) \textbf{J}_{ee}^T(\textbf{q}) \; \textbf{M}_{\textbf{x}_{ee}}(\textbf{q}) \; \ddot{\textbf{x}}_\textrm{des} - \textbf{J}_{ee}(\textbf{q}) \textbf{M}^{-1}(\textbf{q})[\textbf{I} - \textbf{J}_{ee}^T(\textbf{q})\;\textbf{J}_{ee}^{T+}(\textbf{q})]\;\textbf{u}_{\textrm{null}},

it becomes clear that to not cause any unwanted movement in operational space the second term must be zero.

There is only one free term left in the second term, and that is the pseudo-inverse. There are numerous different pseudo-inverses that can be chosen for a given situation, and here what is required is to engineer a pseudo-inverse such that the term multiplying \textbf{u}_{\textrm{null}} in the above operational space acceleration equation is guaranteed to go to zero.

\textbf{J}_{ee}(\textbf{q})\textbf{M}^{-1}(\textbf{q}) [\textbf{I} - \textbf{J}_{ee}^T (\textbf{q})\textbf{J}_{ee}^{T+}(\textbf{q})] \textbf{u}_{\textrm{null}} = \textbf{0},

this needs to be true for all \textbf{u}_{\textrm{null}}, so it can be removed,

\textbf{J}_{ee} (\textbf{q}) \; \textbf{M}^{-1}(\textbf{q}) [\textbf{1} - \textbf{J}_{ee}^T (\textbf{q}) \; \textbf{J}_{ee}^{T+} (\textbf{q})] = \textbf{0},

\textbf{J}_{ee}(\textbf{q}) \; \textbf{M}^{-1}(\textbf{q}) = \textbf{J}_{ee}(\textbf{q}) \; \textbf{M}^{-1}(\textbf{q}) \; \textbf{J}_{ee}^T(\textbf{q})\; \textbf{J}_{ee}^{T+}(\textbf{q}),

substituting in our inertia matrix for operational space, which defines

\textbf{J}_{ee} (\textbf{q}) \textbf{M}^{-1} (\textbf{q}) = \textbf{M}_{\textbf{x}_{ee}}^{-1} (\textbf{q}) \textbf{J}_{ee}^{T+} (\textbf{q}),

\textbf{J}_{ee}^{T+}(\textbf{q}) = \textbf{M}_{\textbf{x}_{ee}} (\textbf{q}) \; \textbf{J}_{ee}(\textbf{q}) \; \textbf{M}^{-1}(\textbf{q}).

This specific Jacobian inverse was presented in this 1987 paper by Dr. Oussama Khatib and is called the `dynamically consistent generalized inverse’. Using this psuedo-inverse guarantees that any signal coming from the secondary controller will not affect movement in the primary controller’s operational space. Just as a side-note, the name ‘pseudo-inverse’ is a bit of misnomer here, since it doesn’t try to produce the identity when multiplied by the original Jacobian transpose, but hey. That’s what they’re calling it.

The null space filter cancels out the acceleration effects of forces in operational space from a signal that is being applied as part of the control system. But it can also be used to cancel out the effects of any unwanted signal that can be modeled. Given some undesirable force signal interfering with the system that can be effectively modeled, a null space filtering term can be implemented to cancel it out. The control signal in this case, with one primary operational space controller and a null space filter for the undesired force, looks like:

\textbf{u} = \textbf{J}^T_{ee}(\textbf{q}) \; \textbf{M}_\textbf{x}(\textbf{q}) \; \ddot{\textbf{x}}_\textrm{des} - \textbf{g}(\textbf{q}) - \textbf{J}^T_{ee}(\textbf{q}) \;\textbf{J}^{T+}_{ee}(\textbf{q}) \; \textbf{u}_{\textrm{undesired force}}.

We did it! This will now allow a high-priority operational space controller to execute without interference from a secondary controller operating in its null space to complete it’s own set of goals (when possible).

Example:

Given a three link arm (revolute-revolute-revolute) operating in the (x,z) plane, shown below:

rotation and distance2

this example will construct the control system for a primary controller controlling the end-effector and a secondary controller working to keep the arm near its joint angles’ default resting positions.

Let the system state be \textbf{q} = [q_0, q_1, q_2]^T with default positions \textbf{q}^0 = \left[\frac{\pi}{3}, \frac{\pi}{4}, \frac{\pi}{4} \right]^T. The control signal of the secondary controller is the difference between the target state and the current system state

\textbf{u}_{\textrm{null}} = k_{p_{\textrm{null}}}(\textbf{q}^0 - \textbf{q}),

where k_{p_\textrm{null}} is a gain term.

Let the centres of mass be

\textrm{com}_0 = \left[ \begin{array}{c} \frac{1}{2}cos(q_0) \\ 0 \\ \frac{1}{2}sin(q_0) \end{array} \right], \;\;\;\; \textrm{com}_1 = \left[ \begin{array}{c} \frac{1}{4}cos(q_1) \\ 0 \\ \frac{1}{4}sin(q_1) \end{array} \right] \;\;\;\; \textrm{com}_2 = \left[ \begin{array}{c} \frac{1}{2}cos(q_2) \\ 0 \\ \frac{1}{4} sin (q_2) \end{array} \right],

the Jacobians for the COMs are

\textbf{J}_0(\textbf{q}) = \left[ \begin{array}{ccc} -\frac{1}{2} sin(q_0) & 0 & 0 \\ 0 & 0 & 0 \\ \frac{1}{2} cos(q_0) & 0 & 0 \\ 0 & 0 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 0 \end{array} \right],

\textbf{J}_1(\textbf{q}) = \left[ \begin{array}{ccc} -L_0sin(q_0) - \frac{1}{4}sin(q_{01}) & -\frac{1}{4}sin(q_{01}) & 0 \\ 0 & 0 & 0 \\ L_0 cos(q_0) + \frac{1}{4} cos(q_{01})& \frac{1}{4} cos(q_{01}) & 0 \\ 0 & 0 & 0 \\ 1 & 1 & 0 \\ 0 & 0 & 0 \end{array} \right]

\textbf{J}_2(\textbf{q}) = \left[ \begin{array}{ccc} -L_0sin(q_0) - L_1sin(q_{01}) - \frac{1}{2}sin(q_{012}) & -L_1sin(q_{01}) - \frac{1}{2}sin(q_{012}) & - \frac{1}{2}sin(q_{012}) \\ 0 & 0 & 0 \\ L_0 cos(q_0) + L_1 cos(q_{01}) + \frac{1}{4}cos(q_{012}) & L_1 cos(q_{01}) + \frac{1}{4} cos(q_{012}) & \frac{1}{4}cos(q_{012}) \\ 0 & 0 & 0 \\ 1 & 1 & 1 \\ 0 & 0 & 0 \end{array} \right].

The Jacobian for the end-effector of this three link arm is

\textbf{J}_{ee} = \left[ \begin{array}{ccc} -L_0 sin(q_0) - L_1 sin(q_{01}) - L_2 sin(q_{012}) & - L_1 sin(q_{01}) - L_2 sin(q_{012}) & - L_2 sin(q_{012}) \\ L_0 cos(q_0) + L_1 cos(q_{01}) + L_2 cos(q_{012}) & L_1 cos(q_{01}) + L_2 cos(q_{012}) & L_2 cos(q_{012}), \end{array} \right]

where q_{01} = q_0 + q_1 and q_{012} = q_0 + q_1 + q_2.

Taking the control signal developed in Section~\ref{sec:exampleOS}

\textbf{u} = \textbf{J}^T_{ee}(\textbf{q}) \; \textbf{M}_{\textbf{x}_{ee}}(\textbf{q}) [k_p (\textbf{x}_{\textrm{des}} - \textbf{x}) + k_v (\dot{\textbf{x}}_{\textrm{des}} - \dot{\textbf{x}})] - \textbf{g}(\textbf{q}),

where \textbf{M}_{\textbf{x}_{ee}}(\textbf{q}) was defined in the previous post, and \textbf{g}(\textbf{q}) is defined two posts ago, and k_p and k_v are gain terms, usually set such that k_v = \sqrt{k_p}, and adding in the null space control signal and filter gives

\textbf{u} = \textbf{J}^T_{ee}(\textbf{q}) \; \textbf{M}_{\textbf{x}_{ee}}(\textbf{q}) [k_p (\textbf{x}_{\textrm{des}} - \textbf{x}) + k_v (\dot{\textbf{x}}_{\textrm{des}} - \dot{\textbf{x}})] - (\textbf{I} - \textbf{J}^T_{ee}(\textbf{q}) \textbf{J}^{T+}_{ee}(\textbf{q})) \textbf{u}_{\textrm{null}} - \textbf{g}(\textbf{q}),

where \textbf{J}^{T+}(\textbf{q}) is the dynamically consistent generalized inverse defined above, and \textbf{u}_{\textrm{null}} is our null space signal!

Conclusions

It’s a lot of math, but when you start to get a feel for it what’s really awesome is that this is it. We’re describing the whole system, and so by working with these equations we can get a super effective controller. Which is pretty cool. Especially in relation to other possible controllers.

Alright! We’ve now worked through all the basic theory for operational space control, it is time to get some implementations going.

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