What is a Servo Following Error?

[travis]

What is a Servo Following Error?

Hello,
I am looking for a better understanding of servo following errors.

[3rdshiftguy]

What is a servo following error?

A servo following error is one of the more common faults or alarms to rear their ugly head in a motion control system. I will try to explain a servo following error in two ways. First the technical definition, and then an explanation that would be more relevant to a maintenance technician or engineer. I'm not perfect so if anyone can add to this, please chime in.

For the engineers in the room...

Definition of Servo Following Error: Every control loop in a servo motion control system relies on position and velocity feedback to adjust its command signal. The feedback is read from either analog voltage fed back to the controller, or an encoder signal. This indicates the actual physical position of the motor.

The controller then compares the desired signal to the feedback signal. It instantly verifies that the motor is precisely following the controller's command. The difference between these values is called the "servo following error" (or simply following error).

For the maintenance techs in the room...

This can be caused by a few different things. If you are getting a servo following error on startup of a new system it can be a number of things.

• Encoder resolution is not correct, improper encoder, encoder wiring, or improper parameters related to pulses or counts.


• Command wires can be reverse polarity causing the motor to attempt a move in the wrong direction.


• PID loop is not tuned properly in the servo motor and drive combo. You would need to consult your manuals since each manufacturer has its own method of tuning.


• Following error setting is too precise. You would need to consult your controller manual as these procedures also vary among manufacturers.

OK, now for a system that has already been tested and successfully used for any period of time. We look at the causes of servo following errors due to component failure of a tested and previously operable system.

Remember, this error appears as a result of the motor not reaching its required position in the allowed time period.

• Check encoder connections, sometime wiggling the cable can cuase these errors to appear and disappear. Connection problems almost always appear in the connector or connector pins. The likely hood of broken wires within the cable are far less than the connection points, that is, unless you see a pinched or crushed cable.


• Bad encoder coupling is next in line. Encoder couplings can break or come loose causing slippage. This is easy to check.


• Bad encoder is next in line. Swap encoders with a known good one if you've got a spare, or interchange it with another available encoder.


• If the feedback doesn't seem to be the case. You would need to do a velocity loop test to verify the the motor and drive are functioning properly.

Velocity loop tests isolate the motor and drive from everything else. You would need to research the velocity loop test for your particular motor and drive combo. Hope that helps you. And if anyone can add to this, please feel free.

[treqilio]

This is a great explanation for servo following errors. Thanks for the detailed info.