Stepper motor control

R

Thread Starter

R_T

Hi there,

I would be grateful if someone could shed some light on this for me. I have read various tutorials on stepper motors but have not been able to clear up a question I have.

I understand that if a force greater than the holding torque of a motor is applied to it then the motor position could become out of synch with the position held by the controlling program.

A way around this could be to advance until a limit switch is triggered and then to reset it's position from this known position. But this causes a lot of disruption in the application.

An better solution would be to interrogate the motor controller on the motor's position and then correct the deviation. My question is whether the motor controller device holds the actual physical position of the motor or the position where it thinks it is, i.e. (before it was forced).

Once again I would be thankful for any help.
 
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Robert Scott

Stepper motor controller are open-loop devices. They have no inherent feedback from the motor telling them where it is or if it has been forced out of sync. The controller only counts steps, up and down, to deduce the position.

Some closed-loop controllers make use of an independent encoder feedback to gain this information, but that is an entirely different type of controller.

Open-loop stepper applications are built around the assumption that the motor does not get pushed out of sync. Home references are often established as you described, with a limit switch, or by running into a hard stop and stalling.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
 
C

Curt Wuollet

The controller can only know the position the motor is supposed to be in unless it has an encoder for feedback. Of course, you can also
add an encoder and keep track of the real position with a PLC or PC with a quadrature encoder or absolute index counters.

Regards
cww
 
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William Sturm

Unless the stepper motor has a position encoder that feeds back to the drive, the stepper drive only knows how many pulses it put out. It will not have any way of knowing if the motor actually moved. Conversely, if the motor moves due to an outside force, the drive will not know about the movement.

Open loop stepper systems (without encoder feedback) must be carefully designed to prevent unintended motion. This is why servo systems or closed loop stepper systems are so popular for custom applications. It is often more economical to add feedback than to do the required engineering to minimize the chance of position loss.

Bill
 
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