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.

[RoboFutbol]

Sometimes you can force arm orientation to avoid teaching more points.  Some examples are; FLIP, NOFLIP, RIGHTY, LEFTY, ABOVE and BELOW

I use FLIP and ABOVE in an application we have to keep the arm from going through the tray it places the item on.  We are at the limits for reach and can on occassion have the arm want to "unwind" into the tray.