Monitoring feedback?

clavius

Member
Join Date
Jul 2008
Location
Massachusetts
Posts
8
Say you have an application where you used some manner of feedback do closed loop control of the speed for a motor. For example, picture a motor the drives the axis of a machine like a gantry router or maybe an overhead crane.

You want feedback so that you can maintain a set feed rate on the axis, so you have some sort of encoder on that axis that tells the drive how fast it's moving and tweaks the speed to maintain. Speed would be user settable and will vary with the job at hand. Feedback could be an encoder, a DC tachometer signal, or whatever. So should the feedback device fail, the drive decides it's stalled and it floors the gas. In some instances, very bad.

This question is somewhat hypothetical for the moment though it may become part of an actual application at some point. In my case, such a failure would not create a danger of anyone getting injured or killed (this would not be crane, etc) however such a failure could potentially cause significantly costly damage to equipment.

Any thoughts on some good methods to detect and deal with the loss of feedback and guarding against the drive going "open loop"?

The motor would likely be 2-3HP with a VFD drive. The rest is pretty much a blank slate so I have lots of leeway design-wise.

Ideas?

Appreciate any insights you guys could offer.

Thanks.
 
Depends on the hardware you choose.

In a closed loop system, you normally have the option to specify a delta, based on expected position and current positions feedback, if outside the range, the drive will fault.
 
Wouldn't you implement two alarm conditions such as "unintended movement" which would be feedback without control, and "no feedback" which would be the opposite and stop the system to counter this?
 

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