7344 PID proportional gain = 1 but actuator still unstable.

Question

Hi LabView Gurus,
Problem: With the 7344 PID P gain set to 1 (I & D to zero), I cant get correct performance from my Dynaserv servo drive using encoder feedback.
I'm driving a Dynaserv (ActuatorM1075B60 ControllerD1075B02) in torque mode with an NI PCI7344.  All is working Ok, except that with a proportional gain of 1 the system is close to oscillation and overshoots when doing step velocity changes or position moves.  It appears that the system requires a gain of less than 1 (with a gain of 3 or greater the actuator becomes unstable)
The controller is working in absolute position mode with encoder feedback.  The Dynaserv encoder provides 425984 counts per revolution - which is much lager than I've seen before.  I normally work with encoders that provide 2000 counts/rev and these work well with a wide range of PID params usable depending on the mechanical set-up.
I require the Dynaserv to operate at anywhere from zero to greater than one rotation per second with accurate step changes in velocity and position with less than 0.1 degrees or 100 counts of overshoot.  I can't achieve this even with a gain of one currently.  It would appear that the encoder is simply providing too many counts.
I've tried limiting the 7344's output both in software and using a voltage divider on the output, but this doesn't solve the problem - I can't reach full torque as the output is reduced.  I've also looked into gearing and other settings on the 7344 but can't see anything helpful.  Note also i have the Dynaserve controllers gain at the minimum setting also.
Any ideas?  Your thoughts would be much appreciated!
Cheers,
Shane.

Answer 1

Shane,
I agree with your assumption, that the issue is caused by the high resolution of the encoder. If you increase the encoder resolution, this has exactly the same effect as increasing Kp.
So compared to a standard 2000 counts/rev encoder, your encoder scales Kp with a factor of about 213. As the 73xx boards run the PID loop on a DSP without floating point unit, this encoder eats up all the headroom for your Kp value.
From a functional point of view, such a high resolution encoder rarely makes sense, as all the mechanical positioning errors of your system are typically much higher than the resolution of the encoder, so it doesn't add real accuracy to your system. In fact it may even add more issues if you want to run the motor at higher velocities, as thie means very high frequencies that need to pass the encoder cable. The 7344 supports up to 20 MHz encoder input signals, but at these frequencies cable damping and noise can become serious problems.
So if possible I recommend to exchange the encoder with a lower resolution version. By the way, is this a real quadrature encoder or are you using an emulated encoder signal, that is generated by your drive (typical setup for BLDC motors). In this case you may be able to reduce the resolution in the drive's configuration.
Kind regards,
Jochen Klier
National Instruments