Problems with PLC and Stepper Motor Results

Assuming that the PLC supplies the final drive for the stepper. that is 4 or more wires and not 2 wires (step and direction).

The phase of the controller must match the current phase of the motor when switching over between the multiplexed steppers. if not erratic steps and speeds will be experienced.

I will advise two memory allocations to keep the last status of an axis that must be reloaded for switching over.

Note that for 3 axis movement the maximum speed obtainable on the shared axes will be much lower than for the normal one.

A separate stepper driver for each axis is very inexpensive and steppers are also available with the driver built in.

If the PLC is supplying only step and direction then speed might be your problem.

It is not clear whether there are driver boards between the PLC and the stepper motors. Unless these are tiny motors like in an inkjet printer, there should be drivers. How big are these steppers and their drivers, if any? Especially if there are no drivers, it is quite possible that one of the phases has blown, or there could be a loose or broken wire or connector.

"programmed PLC with one of the stepper motor output redirected to common of PLC outputs".

Does this statement tell us typical PLC outputs are operating the step motor? I've not tried this before, but if I were to, I would check rated frequency responce of the outputs (slew rate).

If there are no drivers with the motors running direct from the outputs, you should know that drivers allow many adjustments which may help with speed problems.

When all else fails, break out you oscope and look at the pulse shape. This should give an indication of what is going on.

I think Hendrik hit the nail on the head with the stepper motor driver that is being used to drive two motors getting out of phase with one or both of the motors it is trying to drive. Stepper motors have multiple coils, commonly four, that must be energized one at a time in sequence. As you step from one coil to the next the motor rotates through a fixed angle. There are a couple of things that can be going on:

1. Firstly the motor must be in the same position as the driver. If the motor is in the position for coil A to be energized and the driver thinks that coil C is energized then you will have problems.
2. Stepper motors when being operated are designed to have one coil energized at all time. If the power is removed from all the coils then they can easily freewheel and be rotated by something pushing on the device they are driving. While one of the coils is energized the rotor is essentially locked and the motor can not easily be rotated by external forces. If you were using a common driver to drive something like a 3 axis milling machine platform while the PLC is connected to one motor the force being applied to the object being machined may cause the motor that dose not have power applied to it to slip and move to a different position.

When I see thing like this being done it makes me wonder what people are thinking of. If the people that designed and manufactured the PLC intended it to drive three stepper motors then it would either have or be capable of having three stepper motor drivers connected to it. The question then becomes why are you trying to do something with the PLC that it was not designed to do in the first place?

My suggestion would be to either get an upgrade for your PLC so that it can drive all three motors directly or get a PLC that was capable of driving all three stepper motors simultaneously.

Oh yes, one thing else, get a new software consultant, this guy likes to cut corners and that is never a good thing.