After using RUN_HOLD button, robot changes PATH ??

[Davros]

Hi
I have a 6 axis RX60B with CS7MB controller. The machine i work on has been working OK for years. Yesterday i was testing some changes at slow speed & using the RUN_HOLD pause button on the MCP control panel. After pausing the robot and then letting it continue, i noticed that the robot was taking a very strange path. It was like axis 4 was trying to do a 360 ! It seems that because I pause the robot, the robot re-calculated a new path to the final position. This new path may lead to a crash or motor overload as robot tries to move to a position it cannot get too i.e "soft envelope error".

The code looks like:

   SPEED speed.movepo*0.5
   MOVE #helppoint

   WAIT DISTANCE(HERE,DEST) < 50
   SPEED speed.movepo*0.5
   MOVE pos2

   WAIT DISTANCE(HERE,DEST) < 50

I pause robot when it was moving to #helppoint.
Strange robot movement is seen when robot was moving to within 50mm of pos2. Pos2 is calculated from a teachposition using the SHIFT command.

Can anyone explain this or how a robot calculates its path?
Thanks alot.

[TygerDawg]

A couple of possibilities:

You're moving from a precision point (joint angle) position to a cartesian [X,Y,Z,y,p,r] position.  Certainly not a problem normally, but you must make sure your joint angles during the transition from one to the other are not near a limit. 

The operating system calculates the easiest way to move the arm from one point to another and if it is near a limit it may decide to do some strange unexpected motions in this case.  I describe it as "unwinding" one or more of the axes.  Monitor the joint angles from the teach pendant and see what's going on.  I had a similar situation once and it required a couple of extra precision point locations to "force" the arm into a desired configuration through a transition, otherwise the robot swung around strangely and crashed.  The arm configuration & joint angles were good at the beginning of the path and at the end of the path.  But during the middle of the path a limit was approached.  When I stopped and jogged there, I got strange motion.  Fixed it, then ran it in AUTO and slowly increased speed to verify the motion.

I have also seen completely unexpected motion with no explanation, but it was EXTREMELY RARE...once in five years, and only under a very specific and repeatable set of circumstances.  I would not expect that to be the problem.