secondary encoder feedback

[wayne g]

I would like to be able to supply an analog feedback into the primary feedback axis and thus drive to this given analog feedback position. My problem is controlling the velocity at this time. The system runs off at a velocity controlled by (initially) the P of the pid loop. This can be any velocity and is usually quite large.

Ideally I would like to be able to drive the velocity at a mm/sec rate and thus this would lean to having the encoder connected to secondary feedback and using the Pv control. This though is not distinct in its velocity as it is function of driving parameters etc and the pid loop parameters. The driving velocity I would like to be able to set accurately say 2mm/sec variable) etc.

I have had a bit of a playaround wi
th altering the maximum output voltage from the board via the load torque limit setting vi, although this is not an elegant solution to the problem and may in fact create more headaches as then I am losing torque from my motor and not achieving any high torque requirement driving.

[Amaury]

Wayne,

If the motor is properly tuned it should not run faster than the velocity which you loaded unless you run into a following error at which point the PID gains will kick in and may take you above your target velocity.

The degree of precision with which you can reach a target velocity depends on how well the motor PID is tuned and the resolution of the encoder. You may want to look at Understanding Servo Tune

You mention that the system starts by running off at a velocity controlled by P. This implies that you are introducing some discontinuity which makes the error term increase. I recomend looking for the source of the discontinuity. It may be a res
ult of static friction you may play around with the Static Friction parameters in the servo tune section.

If none of the above sugestions work, there is a velocity threshold parameter. You can set this to the max velocity, and then throtle back via the load torque limit setting that way you can get your full torque the rest of the time.

Hope this helps!
Amaruy
NI Applications Engineer