Stepper Motor Behaviour

Just come across a stepper motor behaving in a strange way and feel at a loss for an explanation of how it might be able to do this.
The stepper motor is rated at about 1kW is a bipolar device with two pairs of windings (four isolated coils) connected in series. All eight flying leads are connected as per the motor manufacturers guidance and is driven from a stepper driver package recommended by the motion controls company from which the stepper, controller and linear thrust unit were supplied. The installation seems to be connected in accordance with the installation instructions. The whole assembly is expected to provide a significant linear compressive force to the material that the end of the linear thrust unit contacts.
Our investigations so far have stripped the equipment down to just the stepper motor itself (removed from the linear thrust unit) driven from the stepper drive unit. The stepper drive unit takes a direction and step signal from a PLC and translates that into the appropriate motor coil power sequence. We still have a few things to measure on Monday when we return but here is what we have so far.
I will concentrate on the stepper motor part only as this exhibits the behaviour by itself without the lnear thrust unit attached.
When commanded to continuously step, in one direction only, the motor operates smoothly (for the 400 of steps per revolution it is expected to do) and continuously.
We have provided a mechanical stop to the motors ability to rotate at one position in the rotational arc. When this position is encountered the stepper motor slips back in the opposite direction before resuming to rotate up to the stop again. The expectation was that the motor would torque up and hold hard against the stop. The slip back is almost like a cog slipping (only there aren't any). The effort applied before slippage occurs is near maximum torque (as far as can be determined).
Any ideas or explanations for this behaviour?
TIA
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Paul E. Bennett ....................<email:// snipped-for-privacy@amleth.demon.co.uk>
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Paul Bennett Wrote

snipped some details on the system

If the amount of slip is small (a few degrees) then it is because there is effectively a magnetic cog slipping. The stepper works by moving the electromagnetic field generated by the coils and this pulls the fixed magnetic field of the rotor (generated by the permanent magnets) along with it.
The number of "teeth" or poles is determined by the mechanical construction. A 2 phase motor usually has 50 poles. The 2 phases of the windings can generate the same number of magnetic poles and moves the field by changing the phase of the winding currents. For a simple driver where each winding can have a fixed direction there are 4 "steps" per cycle which gives 200 steps per rev.
If the rotor is up agains the stop then it can't move forward with the field, but the driver continues to move the field. After about half a cycle, the electrical field is now effectively behind the rotor so it jumps back into alignment (slips a magnetic cog).
Stan
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Stan Katz wrote:

Thanks sent by email.
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Paul E. Bennett ....................<email:// snipped-for-privacy@amleth.demon.co.uk>
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Paul E. Bennett wrote:

This is perfectly normal behavior, per Stan's post.
The way a stepper works is to induce a preferred position for the rotor, then step that preferred position around the circle. The rotor will always fall toward the closest preferred position, as fast as it can. To get fine stepping, the rotor and windings are arranged so there are a number of identical cycles of this preferred position as you go around the circle.
When you block the rotor from moving in one direction and keep stepping, the rotor will press up against your stop as long as the closest preferred position is in front of it -- once you step through more than 1/2 cycle then the closest preferred position will be behind, and it'll go there.
This behavior of a stepper is both a blessing and a curse. Because it is so predictable you can use a stepper in open loop, trusting that it will go only the amount you commanded it, no more and no less. But because it does have a maximum torque you can run into problems, either because you need too much static torque or because you're stepping too fast. In either case your motor loses synchronization and life gets bad.
Worse, should your motor be stepping faster than it can keep up not only do you lose the dynamic torque from the coils, but your holding torque from the variable reluctance between rotor and armature can effectively go to zero. I had this happen to me with a small stepper driving a linear actuator -- the carrier that it was driving would actually fall back instead of driving forward. The root cause of this was a spec that called for moving the mechanism faster than the stepper could support.
Since every stepper motor that I've ever had to drive has been pushed past its reasonable limits I have developed a healthy dislike for them. If you need to drive a mechanism to the limits of its actuator then a stepper is simply the wrong thing to use -- a DC motor, brushed or brushless, with feedback, is a much more sensible choice.
DC motors will always give you what torque they can, and real position feedback won't leave you guessing about where your mechanism is. You don't have to pre-compute an acceptable acceleration profile and worry that after your system has had a few years for the grease to dry that it'll still work as designed. You don't have to worry that a bit of grit will fall into the mechanism and make it happily fail (it may still _unhappily_ fail, but at least your controller will know). You know that whatever the situation, a full torque command will get you full torque -- not a jittery mechanism that sags to whatever local minimum is closest.
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Tim Wescott
Wescott Design Services
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Tim Wescott wrote:

Paul,
Tim is leerier of steppers than I am, although he gives good advice. One approach is adding a digital absolute encoder (or even an incremental one if you know how to make them foolproof -- another discussion) to at least warn of trouble or perhaps allow recovery.
One rather standard approach is to ramp the speed. A stepper can reach and run at a higher speed than it can start at. Given its elastic operation, that's no surprise.
If a stepper has been disassembled, replace it. Its magnets were polarized with the entire magnetic structure in place. Separating the parts diminishes the flux, and it won't completely recover after reassembly.
I assume that your device worked once. That makes it likely that there is a new defect somewhere. Check the electronics. Check the currents in each winding that result from eight consecutive steps. If you discern no pattern, post it. If there is no pattern, you probably have a defective driver. Good luck.
Jerry
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Expected behaviour. The only way you are going to get a stepper to apply an adjustable torque or force is by putting a spring somewhere in the mechanics. --
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