Re: Need to convert small stepper to larger motor

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "vrsculptor" <vrsculptor@...>
wrote:

>
> I have a machine that uses a small 4 wire stepper motor for a feed
> mechanism. The controller is a surface mount electronic black box that
> houses a lot of other electronics and has no apparent stepper driver
> and no standard step/direction signals. The motor only spins in one
> direction. I need to replace the motor with a larger one and don't
> want to stress the existing board/driver.
>
> Any suggestions on how to drive a bigger motor from the existing 4
> wires? Power an external H-bridge? Convert to an electronic step
> signal that I could use, after digital gearing, to drive a gecko?
> Thought about mounting an encoder on existing motor to generate step
> signals for a gecko/servo with identical encoder but I don't think
> that would work. I don't know if encoder a/b pulses equate directly to
> gecko step pulses. Direction is not an issue.
>
> Roger
>

Hmmm an interesting problem ;)

I don't think the stepper outputs can be sent directly to a bridge
because as a microstepping drive they may not always be digital as it
tries to emulate a sinusoidal drive. Even if that is not the case
then you will need to drive a bridge with a current limit of some sort
as the original driver will be driving a pretty much open circuit load
and won't take care of that. Perhaps you could feed a current sense
voltage from the new bridge circuit back into the system after first
removing the original sense resistor. I'm not sure.

You can probably just decode the open circuit stepper drive using an
ADC and a microcontroller and then generate normal S/D from them,
would be worth checking out the waveforms. Or depending if you REALLY
need the microstepping use comparitors or something to get a half
stepping output waveform to decode.

You could use an encoder on the shaft definitely, a couple of options.
You could choose the CPR and the gear ratio between the bigger motor
and your drive carefully so that you can drive the new motor driver
directly from one of the phases of the encoder, you won't have
quadrature so you may need a high resolution encoder.

You could use a quadrature decoder chip to provide a pulse train (4 X
CPR) and as it only turns in one direction you will only get pulses
from one of the outputs, that could again be used to drive a motor.

Simple counters can also be used to divide the pulse train down if
needs be.

Or just use a microcontroller to decode the encoder and produce a
divided pulse train to the new stepper drive, its easy stuff
especially if you keep the ratio a power of 2 (2 4 8 ... etc).

Graham