Gains for different position feedback encoders

[kmonroe023]

I have a question regarding how to update/change motor servo gains if the feedback device is changed.

 

The application is a rotary servo motor driving a vertical slide via a ball-screw.

 

There are two encoders in the system, a quadrature encoder that is built into the motor, and a BissC absolute encoder that is connected directly to the slide. The initial motor setup and tuning was performed using the quadrature encoder.

 

The existing gains work well with the slide & load attached and the quadrature encoder closing the position and velocity loops. I'd like to use the existing servo gains, but close the position loop using the absolute encoder; it's connected directly to the slide and has a much higher resolution than the quadrature encoder.

 

If I update pEnc and PosSf to use the absolute encoder, do I need to update the servo position gains to maintain the same bandwidth and performance I have with the quadrature encoder (See below for Motor.PosSf info)?

 

If I do need to update the position gains to use the absolute encoder, is there an easy way to scale the existing gains if I know the difference in resolution of the encoders?

 

// both encoder scaling factors setup to output millimeters
// absolute encoder, 1 nano-meter resolution (1 million counts per mm)
Motor[5].pEnc=EncTable[10].a
Motor[5].PosSf=1/1000000
// incremental encoder on ballscrew w/ 2mm pitch, 8192 counts per rev
Motor[5].pEnc=EncTable[5].a
Motor[5].PosSf=2/8192

[Clopedandle]

Feedback resolution acts as a servo loop gain, so yes, you have to compensate for it when changing feedback source. I wrote an example of this a while back here:

 

http://forums.deltatau.com/showthread.php?tid=1903&highlight=change+encoder

 

I believe changing the position loop scale factor was sufficient since it scales the servo loop gains.