2D Robot Arm Inverse Kinematics using Mixed Integer Programming in Cvxpy

Mixed Integer programming is crazy powerful. You can with ingenuity encode many problems into it. The following is a simplification of the ideas appearing in http://groups.csail.mit.edu/robotics-center/public_papers/Dai19.pdf . They do 3d robot arms, I do 2d. I also stick to completely linear approximations.

The surface of a circle is not a convex shape. If you include the interior of a circle it is. You can build a good approximation to the circle as polygons. A polygon is the union of it’s sides, each of which is a line segment. Line sgements are convex set. Unions of convex sets are encodable using mixed integer programming. What I do is sample N regular positions on the surface of a circle. These are the vertices of my polygon. Then I build boolean indicator variables for which segment we are on. Only one of them is be nonzero \sum s_i == 1. If we are on a segment, we are allowed to make positions x that interpolate between the endpoints x_i of that segment x = \lambda_1 x_1 + \lambda_2 x_2, where \lambda_i >= 0 and \sum \lambda=1. These \lambda are only allowed to be nonzero if we are on the segment, so we suppress them with the indicator variables \lambda_i <= s_i + s_{i+1}. That’s the gist of it.

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Given a point on the circle (basically sines and cosines of an angle) we can build a 2d rotation matrix R from it. Then we can write down the equations connecting subsequent links on the arm. p_{i+1}=p_{i} +Rl. By using global rotations with respect to the world frame, these equations stay linear. That is a subtle point. p and R are variables, whereas l is a constant describing the geometry of the robot arm. If we instead used rotation matrices connecting frame i to i+1 these R matrices would compound nonlinearly.

All in all, pretty cool!

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