Electrically preloaded Dual motor drive for R&P

andypugh wrote:

It is possibly worth mentioning that I think I have heard of systems that use opposed motors to achieve backlash compensation.

I believe I saw in passing hardware to implement such a system on a Components suppliers web site. The name escapes me but they are an importer of motion control products from Europe if I remember correctly.

I think it would be done by asymmetric PID controllers, with the asymmetry switching depending on direction of motion.
The problem is likely to be that the direction that the cutting force acts is not necessarily the direction in which the axis is moving. This is the problem with software backlash comensation, and the dual-motor arrangement is likely to suffer the same problem.

I sitting here wondering if there isn't a simple solution here. For example a adac motor set up to apply a controlled amount of torque that varies with the direction of travel. You could use the direction signal to reverse the applied torque. Seems like it might lead to bouncing around though. Plus you end up wasting stepper power to over drive the applied torque.

With servo control you might be able to have a small bias current to take up the slack, then a slow I-term in a PID controller to adapt to the encoder-counts offset between the pinions. Any change in counts-offset could be seen as a "backlash rattle" and counteracted.

I can't see a workable way to do it open-loop with stepper motors, though.

The traditional way to deal with backlash in rack and opinions is to split the pinion or in some cases the rack. I've seen this in machines built pre WW2. Even in the days of beefy arms winding the cranks backlash was something to minimize.

I do believe though that the modern solution would be to not use a rack.

wizard69 wrote:

andypugh wrote:

I do believe though that the modern solution would be to not use a rack.

What do you suggest instead of rack and pinion?