Detect stepper motor rotation without access to shaft / rotating parts

Question

The problem is the following:

For an existing application I need to develop an addon-board, and one of the functions it needs, is to detect if a stepper motor inside the machine is running. The limitations are as following:

• The stepper motor is not easily possible to disassemble
• The shaft and everything that rotates cannot be accessed
• The driver is quite complicated and there is some safety relevant factor, so I don't want to tinker with it.
• I have basically only access to 2 sides of the motor, the cable and the backside of the stepper motor

Is there a reliable way(sensor) for this situation to detect, if the motor is running?

(P.s. I was thought about to attach a hall sensor to the surface of the stepper motor Detect stepper motor movement using a Hall Effect sensor - which also has no answer)

Answer 1

If you're able to watch the current through one of the motor leads, that'll tell you whether the motor is being driven or not. If the current is approximately zero, the motor is not being driven; if the current only goes in one direction, then the motor is being actively held still; and if the current alternates directions, the motor is being driven to turn.

Another option is to monitor the voltage between two wires of the same phase. If the voltage alternates between fully positive and fully negative, the motor is being driven to turn. Otherwise, it is not. (If the voltage alternates between fully positive and slightly negative, then the driver is probably actively holding the motor still.)

Answer 2

A Hall effect sensor won't tell you if the shaft is actually turning - only that the magnetic fields are changing. And I'm pretty sure there is very little external magnetic field for the sensor to detect, anyways.

You can always use a clamp-on sensor on the wires to verify that the phases are being driven correctly. You could make a fairly small printed circuit board which would be plugged into the connection point of the power connector to measure current and format it into some standard interface such as USB.

I'd suggest an acoustic or vibration sensor. At least at low step rates, the acceleration and deceleration of the shaft should provide information. Even at higher step rates, I'd suspect that the vibrations associated with phase drives would be different if the shaft is turning vs when it's not.

Answer 3

There are very sophisticated stepper drivers for cheap today. Some of them analyze the back-EMF coming from the motor. This way they can detect all kinds of things like motor stall, load, missing steps, etc..

They also usually have a digital interface, which means you could get the desired feedback from the driver.

In general, you could analyze back-EMF yourself, though I believe that wouldn't be an easy task.

Answer 4

You say you have access to the back side of the motor - if it has a through shaft (even a small bit), you can color half of the round shaft black and use an optical sensor to see shaft reflection vs black absorption.