Will the robot move differently when it stops and resumes on its path to a target position?

[arisigor 0]

I'm new to Fanuc robots.

 

I have a simple program that does  J PR[R[162]] R[20:SPEED]% CNT R[23:BLEND] ;

I also have a digital output which whenever I set it to 1 kills the program and the robot stops. It does not fault the robot though. This is the desired behavior. So, whenever I rerun this program, with the same speed and blend values, the robot reaches the target position PR[R[162]]

 

My question is, is there any chance the robot will move differently when it stops on its way to the target position? I mean, let's assume that I don't stop the robot. The robot will move the joints in a certain way to reach the target position. Then if I move the robot to its original position and start the robot again, but now I stop it and then resume it, as described above, should it move the joints the same way?

 

My tests show that it moves the same path, but I just wanted to check with more experienced people here.

[BobLfoot 301]

It has been a couple of years since I worked with a Fanuc Robot, but let me see if I can answer your question.

First a joint move doesn't move the tool along a path it moves each joint individually.

The joints are different size servo motors and when your program stops them they decelerate to a stop.

They then resume when you ask, so the relationship of each axis to the others will be slightly different than when you don't stop mid flight.

This could result in a different flight path for the tool.

[pturmel 157]

In a joint move, the axis that will take the most time to reach its target "controls" the move, and the other axes' speeds and accel/decel are scaled to arrive at the endpoint at the same time.  As long as the robot continues using these scaled values during your stopping function, the path will be identical when you resume.  For all of the robots that I've worked with (quite a few, this would be true for program stops, but not safety stops.  But safety stops are so fast that the deviation from the path would be small.  You should experiment with high-speed position data logging.  Some brands are smart enough to return to the intended path before resuming a move that was interrupted by a safety stop.  (ABB, IIRC.)

[arisigor 0]

How do I check those scaled values on Fanuc controller? I'm new to robots, so if you could give as much information it would be great.

[pturmel 157]

11 hours ago, arisigor said:

check those scaled values

You mean for high-speed data logging?  I'm not current on Fanuc, but I would use an EtherNet/IP adapter with a fast RPI from a suitable PLC to collect the axis values continuously.  The Fanuc would have to be configured to place its axis values in the I/O table.  Once the data is collected, make XY plots using the slowest axis as the plot's X and the other axes as multiple plot Y's.  Overlay runs with stops on top of uninterrupted runs.  They should overlap near-perfectly.  Deviations will show poor overlap.

[arisigor 0]

I meant how do I check/make sure the robot uses same scaled values?

[pturmel 157]

Yes, use high speed datalogging to confirm the same path is followed.  Or ask technical support from Fanuc.  Or both, if you have reason to think Fanuc might be mistaken.

[panic mode 246]

did not use Fanuc in ages but there can be some differences:

as i recall Fanuc robots only use two stop types (0 and 2). 

stop 2 is a programmed (controlled) stop which is when robot reaches target point while controlled by program.

stop 0 is used in case of an error or fault or safety stop (estop, fence etc).

 

in case of stop 2, robot should always be on path - regardless if stopped or not.

in case of stop 0, brakes are applied and motors are deadlocked (short circuit breaking). so from this moment on and until robot comes to a rest, robot is actually out of control and possibly no longer on path. think of one axis as a car without ABS that was moving fast and then wheels are locked... as expected, car will slide in the general direction in which it was traveling. some cars may remain on the road, some may end up in the ditch, its hard to give precise prediction...

now back to the robot. each axis is another car. and each is different. so each will decelerate on its own depending on torques acting on it and obviously this depends on many factors: robot type, assigned payload, actual payload, pose, speed and trajectory at the moment of interruption, number of axes in motion, coefficients of friction, state (wear) of the individual brakes, gear ratio at each axis etc. so the place where robot actually ends up will be more complicated because EOAT depends on position of all joints. but it will still be in the general direction in which was moving initially.

the question here is how the recovery works and that is something that may be product or brand specific so check the documentation, ask Fanuc or test.

 

Nowadays I work with KUKA which also supports stop 1, and that means more complex behavior. Naturally there are more possibilities where the robot may end up in the case of stop. One nice feature here is that KUKA will always do a BCO if not on path. This function is moving robot at reduced velocity until on path, then stops and waits for another start. The exception is the EXT mode which does not do a BCO. This is type of auto mode in which robot is controlled by something external (PLC), equivalent to using PNS on Fanuc. So in this mode, blindly clearing faults and issuing start will make robot move like before - without BCO and without speed reduction (even if not on path). Obviously when commissioning/programming is not done correctly this could be a problem. For example PLC does not see if robot is on path or not on its own, so blindly issuing another start in EXT is a great way to create a lot of carnage. Which is why KUKA recommends to stick with manual recovery in T1. This makes sense since auto recovery needs proper solution and here - attention to detail is pretty important.