Electrical – PPM/PWM controlled, closed-loop stepper motor?

motorservostepper motor

(It is easy to misunderstand what I'm describing if you read it too fast)

I've google this for a few days now. I'm looking to make or buy something that bolts onto the back of a stepper motor that makes the stepper motor function exactly like an RC servo. That means it receives PPM/PWM input, HAS A POTENTIOMETER to measure shaft position, and automatically moves the stepper CW or CCW until it is in the correct position according to pulse width. Think of it like RC servo electronics with a big stepper motor in place of the little DC motor that normally is found in the servo casing.

If for some crazy reason this doesn't already exist, one way that it could be made is by buying a cheap servo, desoldering the small DC motor from the control board, making some kind of interface that translates the -5v to 5v DC voltage to stepper pulses, and outputting that to a stepper motor.

A stepper with an optical encoder is not what I'm looking for. A stepper that receives a direction and speed as input, regardless of what is attached to it, is not what I'm looking for. I'm basically looking for something with the physical characteristics of a stepper (The ability to direct-mount other hardware to the output shaft, metal frame), the high torque of a stepper, but with the control characteristics of a RC servo. It needs to be able to be overloaded, externally knocked out of position, interrupted while moving yet still know where it is. Guessing where it is is not acceptable like it is with open-loop control.

Doesn't this already exist? If not, what would be the simplest design, with the fewest separately-bought pieces, for something like this?

Best Answer

The whole reason for a stepper motor is to get accurate positioning without a closed loop feedback system. It could be done, but will have a larger deadband and be much slower than a normal servo. However there might be applications where it could be useful to operate a stepper motor this way.

one way that it could be made is by buying a cheap servo, desoldering the small DC motor from the control board, making some kind of interface that translates the -5v to 5v DC voltage to stepper pulses, and outputting that to a stepper motor.

This won't work properly because the servo control board is tuned to the characteristics of the motor and gear train.

The simplest design with fewest parts would probably be a small MCU and stepper motor driver IC. The MCU would receive the PWM signal and measure its width with a timer, operate the stepper motor phases via the driver, and read the pot with an A/D converter. Modules and source code to do all these things are readily available (would just take a bit of programming to get it all working together).