Unexpected -310040 error (could not converge) in LabVIEW for Robotics: Inverse Kinematics VI

Question

Hi,
I downloaded the robot kinematics example Vis off of this page from devzone, and they work fine.
Basics of Robot Kinematics
http://zone.ni.com/devzone/cda/tut/p/id/10687
“This is great!” I thought to myself.  I really wanted to play with the IK Vis.  So I went about defining my own serial arm, using my own lengths and configurations for each joint.
First, I ripped out all the dynamic parameters in Create Puma 560.vi.  Didn’t think I needed those just for the IK calculations.  I went about defining an arm with only one link with a length of 1 and a twist angle of 0.  I set my desired position to (1,0,0) and lo and behold, the VI worked with no error.  I then set position to (0,1,0), but this time the -310040 error occurred (could not converge).  Why Is this happening?  Am I missing something in the arm definition?
I then went on to doing some more trial and error, I added a second link, again with a length of 1 and a twist angle of 0. I tried (2,0,0), and it worked.  Even (1,1,0) worked, to my surprise, the two links would form a 90 deg angle.  But still, (-2,0,0) didn’t work.
Another question I have is, can the Robotic Arm Vis account for singular points that may occur (positions with no solutions or infinite solutions?)  An initial sweep of the arm workspace would be nice.  Also, for positions that have more than one solution, is it possible to input constraints on the joint angles so that only one solution is possible?    
Thanks for your response.
John Wu
Project Manager
Riobotics Technology Consulting
Attachments:
IK Question.zip ‏36 KB

Answer 1

Hi,
Thanks for your reply.  Fortunately I'm not dealing with prismatic joints in my application, so the visualization bug that you mentioned won't be a big issue to me.  I'm only dealing with revolute joints for now.  Your explanation for the prismatic joint was right on the spot, however I found a couple more quirks with revolute joints.
Consider the case of only one revolute joint, length=1, twist angle=0.  I would expect that this single joint allows the arm to revolve around the base in the XY plane.  So I plug in (1,0,0), I get a solution.  I plug in (0,1,0), no solution (see pic).  Same with (-1,0,0) and (0,-1,0), no solution.  So I fiddled around and added one more revolute joint of length=0 to the series.  Now the VI gives solution to all of the 4 positions I tried earlier.
"Ok" I thought to my myself, maybe I just need to tag a joint of length=0 to the end of my series, everything should work out.  Not so.  I tried having 3 revolute joints in total, 2 of them with length=1, the last one length=0, tried some different positions but most positions failed to converge.
My goal is simple ... to define a serial arm with 3 revolute joints, all rotating in the same plane, and try out the LabVIEW IK algorithms.  For simplicity's sake, all 3 links lengths=1.  Do you think you can provide an example?
-John
Attachments:
IK failed to converge.jpg ‏84 KB