How to finetune the force control of an AC Motor Linear Actuator?

Question

Hello,
I
am hoping to find some help here with the following problem, which I am trying
to solve for several days now....
I
am using the UMI-7774 and an AC servo-motor-driven linear actuator to do
force control. Basically, I need the actuator to apply a constant force and
react quickly to force changes. My system setup is as follows: I am using
a PCI-7344 Motion Control Card to provide the analogue command signal
(0-10V). This signal is supplied to the actuator controller (through a
UMI-7774), which transforms the command signal to an amperage command signal
(0-2.17Arms). This is the signal controlling the actuator’s torque. The actuator controller is working in torque mode. I use
a Kistler PZT force sensor (ring type) and a Kistler amplifier as my feedback
source. The signal from the sensor amplifier is directly connected to the ADC of the
UMI. I am using MAX to tune my system and define a PID controller that gives
me a good step response.
(1)
The results so far show that it works in principle, but I believe my system is
not yet tuned optimally. However, the best step response results I have
obtained and the PID parameters I used are shown in the attached files. As I am
relatively new to this area, could you please advise me on how I could further
improve the step response of the system? The current rise time and
overshoot values are very good (a 3% overshoot would also be acceptable),
but the observed undershoot is causing delayed settling. Also, the oscillation
observed during undershoot is something I would like to remove.
(2)
I have tried increasing Kp and Kd values. Increased Kd does not seem to correct
the oscillation problem. On the contrary, it destabilises the system. Increasing
Kp causes an unwanted overshoot.  I have also tried to reduce the
Derivative Sampling Period but that gave even worse results (System Inertia: J
= 1.8629*10^-5 Kgm^2).
At
Td=2 the actuator was retracting on its own to its home position when I set Ki>=8.
I got the most stable behaviour using Td=4. Is there any other way I could
improve the actuator’s response?
(3)
I have seen that in some cases people have used a “double PID loop” one
for the encoder feedback and one for the force feedback. Do you believe that
this would improve system behaviour? If so, how could
I implement two PID loops? I mean, MAX only allows me to enter
one set of PIDs....or is there a possibility to set up a second PID-loop
somewhere?....I have seen that the "Force Control Using Monitoring
Force" example essentially works with two Closed-loops, but one of them is
implemented in the application software and this would result in a
non-deterministic and comparatively slow response of the system, right??
Sorry for the long post and thanks
a lot in advance for help and advice.
Attachments:
Force Conrtol Loop Settings Actuator-BEST.JPG ‏41 KB
Step Responce Actuator-Force Feedback-BEST.JPG ‏58 KB

Answer 1

Hello Jochen and thank you for your very nice reply.
The past few days I have been trying to implement your advice, but unfortunately I did not have much success. Let me start by saying that explanations to my problem, the 2nd one looked more probable, as I always make sure that there is contact between my actuator and the workpiece I am pushing against. So, I unmapped my DAC by setting up the axis as a stepper axis with no feedback and I used the Load DAC to apply voltage and see when my actuator starts moving. That gave me the upper and lower limits for the Static Friction Compensation filter (please refer to attached image). I then implemented this filter and tried to retune the PID parameters, but the results I got were poor.
I can now adjust the parameters so that there is no initial spike and undershoot, but now the force ramps up fast to a specific value and then keeps rising slowly. Vibration is observed at the point of transition from fast to slow ramp-up (ref. Step Responce Actuator-Force Feedback-WITH FILTER_v2).
I could not eliminate the steady state error. When I tried to eliminate the steady state error by increasing Ki, or tried to create an initial overshoot by increasing Kp, I always ended up with an unstable system or a system with a response similar to the one shown in attached figure “Step Responce Actuator-Force Feedback-WITH FILTER_v1”.
  Do you perhaps have an explanation or comments for this behaviour? As for your other questions:
Sensor Bandwidth: N/A, Charge Amplifier Bandwidth: 200 kHz, Low pass filter: No (Amplifier has the capability though)
PID Update Period: 250 microseconds (fastest available)
NI Motion Version 2.3
Labview Version 8.5.1
  Thank you,
Thomas
Attachments:
Step Responce Actuator-Force Feedback-Filter Settings.JPG ‏28 KB
Step Responce Actuator-Force Feedback-WITH FILTER_v2.JPG ‏69 KB
Step Responce Actuator-Force Feedback-Filter Settings.JPG ‏28 KB