[jim_stoll]

I have a cable-driven mechanism that consists of a spindle on a stepper motor shaft. A flexible metal cable is wrapped around the spindle, and runs through a sheath (bicycle cable style) to the actuator on the other end. Cable is taken in/out by stepping the motor forward/backward, and the actuator is kept under tension by a spring, thus ensuring that the cable stays taught when it is payed out by the spindle.

When the system starts up, I have the stepper attempting to take in a full rotation of cable (the cable travel is just under a full revolution of the spindle), resulting in the stepper motor stalling (ie, with the cable fully pulled in), and I then take this as my 'home' position for the stepper.

I'm running the stepper motor in 1/16 micro stepping mode, using a Pololu DRV8826 driver.

When I reach the stall state, instead of the stepper making noises of protest over the stall (the axes on my CNC mill give a little bit of a shriek if they are stalled - granted, using an entirely different class of driver - Gecko), this stepper is taking a bit of a jump 'backward' (ie, letting the cable loosen). Sometimes it is a very small jump, and the cable remains taught, and other times it is a larger jump, with some slack being allowed back into the cable. The amount of slack varies from instance to instance.

So, I'm trying to understand this behavior, and see if there is a way that I can manage it.

Thoughts that have occurred to me to try to mitigate the behavior, but which I haven't yet tried are:

- increase or decrease the current going to the stepper - I don't need brute strength on this stepper, so reducing the current a bit wouldn't hurt things. (At present, I've got the current limited to about 1.8A)
- try moving to full-step mode for the 'force home' operation - just in case the jump is a matter of the motor reverting back to the last full step when the stall occurs (though if this were the case, I'd think it would at least jump back to a consistent position)
- experiment with other micro-step increments, and motor speeds, to see what effects this would have on the stall behavior

If anyone can offer any insight or advice on dealing with this, I'd very much appreciate it.

I realize that a limit switch would solve this problem, but its not entirely practical with this particular application, so I'm trying to find a way to make this 'forced home' approach work (or some other logical/mechanical approach, vs adding an encoder or switch or such), if possible

Thanks!

[James Newton]

I believe that is normal operation for a stepper at stall. It will lock to the nearest step which does not pass by the mechanical limit. If that step position doesn't happen to be right at the point of the mechanical limit, there would normally be a bit of a space between the position and the limit. Each step is a specific, discrete position... like integers on a number line. If you are moving from 1 to 2, and you are stopped at 1.4, you don't come to rest at 1.4, but at 1. There is then 0.4 distance between your position and the stop.

- - - Updated - - -

The other way to solve this problem is to introduce a friction clutch. The clutch will slip at the exact mechanical stop and take up the difference in step position and the stop.

[sswitaj]

This is pretty typical behavior for a small stepper. Imagine you're slowly advancing into your stop, one microstep at a time. As you load up against the stop the motor finds it harder and harder to take each step, until you get to the holding torque of the motor, and the motor lets go.

In an ideal world, the rotor would just fall back one step, the magnetic fields would align again and the rotor would be captured and stop.

But the problem is that in the real world you've been storing energy in the load, especially something like a cable it's a bit springy. Also, when the rotor lost lock, it probably popped back quite abruptly, so for a brief instant it has significant reverse velocity. Step motor rotors are big magnets, so they have pretty good mass for their size and therefore rotary inertia, which adds to the load you now have to "catch" to halt the backwards motion.

Also, the act of having the magnetic rotor pop backwards swirls the magnetic fields in the stator a bit, and an effect called "back EMF" comes into play, reducing the available torque in the magnetic field.

For these reasons, a rotor in stall always has less torque available than a captured rotor. The "pull-in" torque, available to recapture the rotor will always be significantly less than the "breakout torque", which is the torque that will cause a rotor to loose lock.

This all happens in a fraction of a second, but for that fraction, the energy stored up in the system conspires against the motor, which has to overpower these effects to capture the rotor again, and the rotor can bounce quite a few steps backward before things settle down again.