Extending Tekkotsu to New Platforms

Creating a Kinematics File and Forward Kinematics Support

The first step towards performing cognitive robotics with the arm was to create a kinematics descriptor file. With this, we could accomplish two major goals. First, we could use forward kinematics and solve for where the gripper is in terms of the base's coordinate frame. Second, and more importantly, we could use inverse kinematics and position the joints in order for the gripper to reach a certain point.

To construct the kinematics file, we used the following procedure from Robert Schilling's Fundamentals of Robotics: Analysis and Control:

0. Number the joints from 1 to n starting with the base and ending with the tool yaw, pitch, and roll, in that order.
1. Assign a right-handed orthonormal coordinate frame L0 to the robot base, making sure that z0 aligns to the axis of joint 1. Set k = 1.
2. Align zk  with the axis of joint k + 1.
3. Locate the origin of Lk at the intersection of the zk  and zk+1  axes. If they do not intersect, use the intersection of zk  witha common normal between zk  and zk-1.
4. Select xk  to be orthogonal to both zk  and zk-1. If  zk  and zk-1 are parallel, point  xk  away from zk-1.
5. Select  yk  to form a right-handed orthonormal coordinate frame Lk.
6. Set k = k+1. If k < n, go to step 2; else, continue.
7. Set the origin of Ln at the tool tip. Align zn  with the approach vector, yn  with the sliding vector, and xn  with the normal vector of the tool. Set k = 1.
8. Locate point bk  at the intersection of the xk  and zk-1  axes. If they do not intersect, use the intersection of xk  with a common normal between xk  and zk-1 .
9. Compute Θas the angle of rotation from xk-1  to xk  measured about zk-1 .
10. Compute das the distance from the origin of frame Lk-1 to point bk  measured along zk-1 .
11. Compute aas the distance from point bk to the origin of frame Lk  measured along xk .
12. Compute αas the angle of rotation from zk-1  to zk measured about xk .
13. Set k = k+1. If k ≤ n, go to step 8; else, stop.

(Schilling, page 54)

We followed this procedure and ended up with a kinematics file where each entry resembles the following:
joint_type: 0
immobile: 0
theta: -1.570796326794895
d: 0.0
a: 154.0
alpha: 0.0
theta_min: -1.570796326794895
theta_max: 1.570796326794895
m: 0.0
cx: 0.0
cy: 0.0
cz: 0.0
Ixx: 0.0
Ixy: 0.0
Ixz: 0.0
Iyy: 0.0
Iyz: 0.0
Izz: 0.0
tekkotsu_output: 2
The basic parameters are obvious from the Denavit-Hartenburg procedure from Schilling's book. However, Tekkotsu adds a few other notable fields.
Once we did this, we could run our forward kinematics solver and see that it worked. One problem that we ran into was that the servos on the arm were not strong enough to fully support the arm's weight, so our results were always slightly off. If we held the arm at the point of zero resistance from any of the servos, everything would work fine. A related problem was the fact that we could not use PID control over the servos. Since there was only one-way communication, we could only control the P value. As a result, the arm would often start to oscillate and would continue until we stopped it manually.

Regardless, forward kinematics were functional if we were to assume that our arm existed in a perfect world where the servos were always where we told them to be. It was the best that we could possibly do.

Next: Inverse Kinematics