MoveAbsJ/MoveJ joint interpolation

Robot Controller
1

Hello everyone,

Let, for example,

var jointtarget j1:=[[-90, 0,-60, 0,-30, 0],[9E9,9E9,9E9,9E9,9E9,9E9]];
var jointtarget j2:=[[-80,10,-50,10,-20,10],[9E9,9E9,9E9,9E9,9E9,9E9]];
var robtarget r1;
var robtarget r2;
…
r1:=CalcRobT(j1,tool0);
r2:=CalcRobT(j2,tool0);
…

Let the robot be at j1 (i.e., at r1).

What is the profile of the six joint values during the execution of
MoveAbsJ j2,v100,fine,tool0;
or
MoveJ r2,v100,fine,tool0;

I know that in both cases, all joints start and finish at the same time, so I always thought that after acceleration and prior to deceleration, the profile for each joint is linear, but I was surprised to see that it is not. In fact, in some cases, a joint that has to go from say 10 to 30 degrees, will first go to say -5 degrees and then to 30 degrees.

Does anyone know details on the actual trajectory planning? I want to be able to foresee the path of a robot in a MoveJ or MoveAbsJ motion. Is this some kind of an approximation of the minimum-time trajectory?

Thanks,

Ilian

2

Greetings,

Is it by chance that the robot wrist is at some point directly above the center of axis one? This is an additional robot singularity, I have seen axis one swing around quite a bit with a MoveJ instruction in these cases. A MoveAbsJ cures that.

3

Hi,

I did some further tests and I realized that I was wrong. Besides, it is clearly explained in the ABB manuals that the central portion of the joint-space trajectory of any joint is linear, during a MoveJ (or MoveAbsJ) motion. And you are right – there are exceptions when near singularities.

By the way, you are referring to what is called a “shoulder singularity,” which in the popular inline wrist six-axis robot occurs when the distance between the center of the robot and the axis of joint 1 is minimum. See slide 27 of my lecture notes: http://www.gpa.etsmtl.ca/cours/gpa546/Notes/Cours11.pdf

Cheers,

Ilian