Servo motor control w/ electronic gearing

I'm looking for some opinions from those with experience controlling a servo drive with electronic gearing. (More specifically, step/direction signals from a stepper controller.)

I'm using an AMCI 3102 motion module, which I've used in the past to drive stepper motors and it has worked very well. I like this arrangement because I can do all the control directly from the PLC in the ladder logic. In my current application, due to higher speed requirements, I'd like to use this same setup to control a Kollmorgen AKD servo drive in electronic gearing mode.

The problem I'm having is that I can't get smooth motion across the entire speed range of the motor. If I adjust the AKD gearing ratios to match the max required motor speed (say 5000 rpm) to the max output frequency of the stepper card (100 kHz), it works fine at high speeds but the motion is extremely rough at slow speeds. I can adjust the ratios to provide smooth motion at slow speed, but this requires a high pulse output frequency and I then can't reach the high motor rpm required.

I guess my question is whether this problem is inherent to electronic gearing, or is it just the implementation of it in the Kollmorgen drive? I never had this issue with stepper motors (when using a microstepping drive).

100Khz is kind of weak for a max output frequency. Most higher controlers will give you resolutions up in the Mhz. The granularity at the low end is probably playing havoc with the control loops. Most decent amps can handle a 500k an up input signal as well.

Those AMCI modules are great when they work, but don't match up to the specs of a high control.

If I understand this right the Kollmorgen drive is geared to the stepper motor? I don't see how that will work because a drive wants to follow a smooth motion profile not a series of steps. Damian's comments are valid too. If the loop is being closed every millisecond then there are 0 to 500 steps per millisecond. If going slow, like 9500 steps per second then the drive will see 9 or 10 steps every millisecond. If the drive closes the loop faster the problem will be four times worse. The best way to do this is to gear both the stepper and the Kollmorgen to the same virtual master that is perfect and without steps.

Sorry let me clarify. There is no stepper motor, just a stepper controller (CW/CCW pulse outputs) connected to the Kollmorgen AKD servo drive (w/ AKM servo motor attached) set up for electronic gearing.

I don't see how that will work because a drive wants to follow a smooth motion profile not a series of steps.

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I guess I was assuming that in gearing/following mode, the servo drive would interpolate between the steps in the pulse signal to create a smooth motion profile (so a low pulse frequency wouldn't be a detriment). But my experience today has shown that the servo drive acts more like a non-microstepping stepper drive. I'm just trying to confirm whether this is normal behavior or unique to the Kollmorgen drive.

100Khz is kind of weak for a max output frequency.

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Good point, in fact the AMCI module will support 1MHz if I wire the outputs as differential. I'll try it.

kolyur said:

Good point, in fact the AMCI module will support 1MHz if I wire the outputs as differential. I'll try it.

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The good news is you have a full order of magnitude to explore on your resolution.

I think you will find that most servo drives in following mode will ty to follow that signal without "smoothing it" so to speak. At the low end it is getting stepwise jumps in the commanded position, and the only thing that is going to wash that out is de-tuning the gains.