tuning servo system

[Ya-hoo]

I am tuning my servo system which has PCI 7342 controller, and third party drive and motor. The drive is set to torque mode and controller in position mode. The motor jerked either in auto tune or manually tune mode, and drive got overspeed error message, got max overshoot 4000% in MAX.

Have some questions with tuning.
1. Can I set Kp less than 1?
2. How can I set desired position in step response?
3. except "understanding servo tune", is there any article helpful here?

Thanks.

[Jochen]

1. You can't set Kp less than 1
2. If you use auto tuning then you can set the desired relative target position for the step with the distance parameter. If you want to tune the system manually the parameter you should use is the "step length" setting on the Step Response tab.

To improve the controller behavior please try the following:

1. Make sure that you start with a low Kp value and with Kd and KI = 0.

2. Play around with the Control Loop Update Period. Depending on your velocity and encoder resolution you can increase the headroom for your control parameters. Please give me more details about these parameters so I can answer this more specificly.

3. Configure the drive for velocity mode if possible. From my experience in this mode drives are easie
r to control.

4. Are you using a brushless motor with hall sensors? Is your drive providing encoder feedback emulation? In this case you should be able to set the resolution for the encoder feedback emulation.
Don't use the highest resolution. The higher the resolution is the lower your PID control loop parameters need to be.

5. Use Kd for damping the system (Kd should be about 10 x Kp). Set the Derivative Sampling Period Td to a higher value like 6 or 8.

I hope this helps.

Best regards,

Jochen Klier
Applications Engineering Group Leader
National Instruments Germany GmbH

[Ya-hoo]

Hi Jochen,

Thank you for your reply.

The drive now is set to torque mode because I read the UMI 7742 manul and it recommands that drive set to torque mode and controller is easier to be tuned. Besides, the drive was auto tuned by itself. Is it possible that the auto tuned parameters are conflict with controller's parameter? If it is, should I decrease drive's gain? When Kd in MAX was set to 10, Kd=Ki=0, the motor got jerked and got overspeed error. Seems it only works with Kp=1.

I am using brushless motor which has a 1 speed resolver and drive has simulated incremental encoder output to controller. I don't think the encoder's resolution is adjustable. The simulated encoder resolution is 1024ppr and the controller should see 4096counts/rev. Am I right?


I will try velocity mode for drive and manually tune the drive.

Thanks.

[Ya-hoo]

Hi Jochen,

When I changed the gain parameters of drive, the step response is better in MAX, the settling time is around 350ms, with some steady state error. But trajectory response is not good, the velocity is oscillating around the desired velocity. Why?

The parameter in MAX are Kp=5, Kd=1500, Ki=0. Does it look ok?

Thanks.

[Ya-hoo]

If the motor works fine at lower speed, but always gets following error at higher speed. Is that because the proportional gain is not big enough?

Thanks