Why does the Ni-Motion Torque-Control Program give me such an oscillation???

Hello,
I really hope to find somee help here...I have a UMI-7744 and I would like to do force control for a servo motor. I have tried the NI-Motion example "Torque Cntrl Using Monitoring Force"...it seems to be quite easy - it constantly measures the current force and compares the ADC-values with the target force. The difference between the two is then new relativ position...
When I test it with an actuator that does not touch the workpiece (current force==>0N) and then tell it to apply 50N (ADC-value of ~1000) then of course it starts moving towards the workpiece, but then overshoots quite significantly when it touches it, then undershoots, then overshoots, then undershoots....etc....
 (1) Apart from the big overshoot, I would have assumed that the failure decreases over time and the force will be maintained at some point, but the actuator keeps on oscilating around the target force with quite high amplitudes.......does any body have an idea for the reason?????....How is it possible that the controller actually approaches the target force, ie. the over/undershoot should get smaller with every cycle...
 (2) I was assuming that the problem is because I load quite large relative positions (1000 - 0 = 1000) while the actuator is not yet in contact with the workpiece which then causes the overshoot when the actuator touches the workpiece. Hence, I tried limiting the relative positions to +/- 100, but it seems that it does not improve anything. The actuator still never maintains a steady force.Any idea, why limiting the relative positions does not have an effect on the oscillation??
(3) I have seen that the use a delay of 50ms. Hence every 50ms a new target position is loaded. Is there any experience of what the BEST time delay is for motion control???? Is it better to decrease the delay to adjust the relative position more often OR would this have negative effects because the moves don;t have enough time to complete??????
I am grateful for any reply...

Hi Jochen,
thanks for the clarification on the pros and cons of the two Torque control approaches...very enlightening......
To clarify my previous question: I am using the S200VTS-Servo Drive from Danaher Motion. The drive is able to run in Torque-mode, Velocity-mode and Position-mode. In the latter case it needs a "step and direction" input, instead of an analog input. My idea was to setup the S200-drive in Position-mode because in the "Torque Control using monitoring Force"-example the inner PID loop is for the position...so I thought, why shall I setup the S200-drive in torque mode, when actually the PID-loop tries to control the Position.....you know what I mean? It appeared to me, that with setting up the drive in torque mode and running the example code with the positional PID-loop I am mixing force and position....
Kind regards and thanks a lot for your efforts.
Well, let's take it like that: The second approach is ok, if the dynamics of the torque are very low. The worst case scenario for this approach is a solid actuator pressing against a solid workpiece. When these two pieces meet, a force occurrs almost instantaneously. That means, that very short move distances result in high force differences. With your current approach this means that it's very hard to control the force, as in fact the only way to generate force is to move the axis a bit. If you control the force directly instead (analog feedback), you will get a much better control behavior.
To clarify the idea behind that:
Just think of two metal plates lying on each other on the floor. Now you step on the upper metal plate. Does the upper metal plate move? No. Is there a change of the force applied to the lower plate? Yes.
To summarize: 'Torque Control using monitoring force' works best in systems with very low dynamics and a soft contact between actuator and workpiece. still all the limitations that I have mentioned in my first post are true, so I strongly recommend to use analog force feedback.
Your last statement about running the motor in position or torque mode is not clear to me. Are you referring to operation modes of your drive (by the way, which drive are you using?). In this case the clear recommendation is to configure your drive always in torque mode, regardless of the type of control provided by the 73xx device that you are using. From a drive's point of view 'torque mode' means, that the analog input voltage (command input) is converted into a proportional current signal and that's in both modes the best option.
Jochen

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