# COMP Robotics: An Introduction

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COMP790-072 Robotics: An Introduction
Kinematics & Inverse Kinematics UNC Chapel Hill M. C. Lin

Forward Kinematics UNC Chapel Hill M. C. Lin

What is f ? UNC Chapel Hill M. C. Lin

What is f ? UNC Chapel Hill M. C. Lin

Other Representations
Separate Rotation + Translation: T(x) = R(x) + d Rotation as a 3x3 matrix Rotation as quaternion Rotation as Euler Angles Homogeneous TXF: T=H(R,d) UNC Chapel Hill M. C. Lin

Forward Kinematics As DoF increases, there are more transformation to control and thus become more complicated to control the motion. Motion capture can simplify the process for well-defined motions and pre-determined tasks. UNC Chapel Hill M. C. Lin

Forward vs. Inverse Kinematics
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Inverse Kinematics (IK)
As DoF increases, the solution to the problem may become undefined and the system is said to be redundant. By adding more constraints reduces the dimensions of the solution. It’s simple to use, when it works. But, it gives less control. Some common problems: Existence of solutions Multiple solutions Methods used UNC Chapel Hill M. C. Lin

Numerical Methods for IK
Analytical solutions not usually possible Large solution space (redundancy) Empty solution space (unreachable goal) f is nonlinear due to sin’s and cos’s in the rotations. Find linear approximation to f -1 Numerical solutions necessary Fast Reasonably accurate Yet Robust UNC Chapel Hill M. C. Lin

The Jacobian UNC Chapel Hill M. C. Lin

The Jacobian UNC Chapel Hill M. C. Lin

The Jacobian UNC Chapel Hill M. C. Lin

Computing the Jacobian
To compute the Jacobian, we must compute the derivatives of the forward kinematics equation The forward kinematics is composed of some matrices or quaternions UNC Chapel Hill M. C. Lin

Matrix Derivatives UNC Chapel Hill M. C. Lin

Rotation Matrix Derivatives
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Angular Velocity Matrix
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Computing J+ Fairly slow to compute Instability around singularities
Breville’s method: J+(JJT)-1 Complexity: O(m2n) ~ 57 multiply per DOF with m = 6 Instability around singularities Jacobian loses rank in certain configur. UNC Chapel Hill M. C. Lin

Jacobian Transpose Use JT rather than J+ Avoid excessive inversion
Avoid singularity problem UNC Chapel Hill M. C. Lin

Principles of Virtual Work
Work = force x distance Work = torque x angle UNC Chapel Hill M. C. Lin

Jacobian Transpose Essentially we’re taking the distance to the goal to be a force pulling the end-effector. With J-1, the solution was exact to the linearized problem, but this is no longer so. UNC Chapel Hill M. C. Lin

Jacobian Transpose UNC Chapel Hill M. C. Lin

Jacobian Transpose In effect this JT method solves the IK problem by setting up a dynamical system that obeys the Aristotilean laws of physics: F = m v ;  = I and the steepest descent method. The J+ method is equivalent to solving by Newtonian method UNC Chapel Hill M. C. Lin

Pros & Cons of Using JT + Cheaper evaluation + No singularities
- Scaling Problems J+ has minimal norm at every step and JT doesn’t have this property. Thus joint far from end-effector experience larger torque, thereby taking disproportionately large time steps Use a constant matrix to counteract - Slower Convergence than J+ Roughly 2x slower [Das, Slotine & Sheridan] UNC Chapel Hill M. C. Lin

Cyclic Coordinate Descend (CCD)
Just solve 1-DOF IK-problem repeatedly up the chain 1-DOF problems are simple & have analytical solutions UNC Chapel Hill M. C. Lin

CCD Math - Prismatic UNC Chapel Hill M. C. Lin

CCD Math - Revolute UNC Chapel Hill M. C. Lin

CCD Math - Revolute You can optimize orientation too, but need to derive orientation error and minimize the combination of two You can derive expression to minimize other goals too. Shown here is for point goals, but you can define the goal to be a line or plane. UNC Chapel Hill M. C. Lin

Pros and Cons of CCD + Simple to implement + Often effective
+ Stable around singular configuration + Computationally cheap + Can combine with other more accurate optimizations - Can lead to odd solutions if per step not limited, making method slower - Doesn’t necessarily lead to smooth motion UNC Chapel Hill M. C. Lin

References UNC Chapel Hill M. C. Lin