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Extreme noise when using Analog input position mode

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I'm currently working on a linear air bearing setup, the setup consists of an air bearing, a tecnotion UL6n motor and a renishaw linear encoder for feedback.

The system is all set up however we're experiencing issues with controlling the system.

The set up works fine in service - position mode (despite a high pitch noise which is rather annoying), it is tunined using the performance servo tuner.


However when we switch to analog input mode things start to get weird, first of all the result; the stage starts to make a lot of noise and uses a lot of current, see the video below:


The analog input value is adjusted to be exactly zero and also stays at exactly zero, thus I would expect the position command to be 0.000mm and thus having the same behaviour as in the service mode.

When looking at the current loop the current command keeps on switching between the min and max current which is allowed, I can adjust the PI controller however the behaviour stays the same.

The setup is going to be used for testing of nonlinear systems and needs to be snappy, thus decreasing the P-gain isn't really a solution.

But again, I would expect the system to react in a same manner as in the service mode.

Does anyone what is going wrong and what direction to look for?


Thanks in advance!



Does the analog input maybe have something to do with it?

My input voltage is zero however my position command is equal to the position of the linear stage:


I would expect my position command to be 0,000V x 20,000mm/V = 0,000mmm or maybe the inverse of the position of the stage.




1 Answer

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Since the position command resolution is so low in Analog Position mode, the Acceleration Feedforward needs to be set to zero.  You can find this in the Scope screen and Servo Gains tab.  The parameter name is IL.KACCFF.  Set it to zero.  If you are using the autotuner, you need to uncheck "Tune Acceleration FF".

Analog Position mode may still be jerky, due to the low command resolution. The larger the scale of the analog input, the lower the resolution. As the analog input voltage fluctuates, the motor will move a larger distance, causing a jerky motion.


Dear Jcoleman02,

Thank you for the reply, I'll try do to this first thing next week and let you know.

Stefan - Fri, 07/17/2020 - 06:00
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