Re: Open loop control

[Fred Proctor <proctor-at-cme.nist.gov>]

Arne Chr.Jorgensen wrote:

> A stepper motor don't use any feedback.  Is it possible to do a
> crude hack, and have a STG board do the same ?
> 
> Say I wanted to machine a "circle".   Is there an easy way to pass
> this out on the DACs,  without feedback from encoders.

I would do this:

1. Set all the gains to 0 except for FF1, the velocity feedforward gain.
This will cause the STG to output a voltage proportional to the
commanded velocity. The units of this value are in volts per inch per
second. Measuring this can be tricky. I record the starting position,
put 1 volt on the DAC, and tick off the subsequent positions each
second. The slope is the velocity feedforward gain. Do this for 2 volts,
etc. and if the value varies you may have a problem. If you have a tach
then they should be the same. If you are in current mode, it will be
different. I deduce you have some other type of system, like a
voltage-to-frequency converter attached to steppers? If so, boldy go
where no man has gone before.

2. Set the FERROR (and MIN_FERROR if you have one) to something larger
than the range of motion for each axis. This will prevent a fatal
following error from stopping motion.

--Fred