Re: [CAD_CAM_EDM_DRO] access to Ametek motors- suitable for Servo application?

On Mon, 2003-12-22 at 10:08, Jon Elson wrote:

> The standard Ametek motor that lots of people use (36 V @ 900 RPM) have
> pretty high armature resistance. I'd suspect that any other motor they
> make in
> the same frame would be wound the same way. it seems that these motors
> don't allow a Gecko drive to reach its proper performance in terms of DC
> gain. In other words, you can take the shaft of the motor and move it maybe
> 10 - 15 degrees with VERY little resistance, then the Gecko starts pumping
> current through it and it resists the deflection.

That is not the case with the Amatek motors we use, whatsoever. If one
has the loop gain properly adjusted, the motors hold position very well,
and as good as any other motor will.

It should be noted that in a typical PID loop, such as what might be
used in a Gecko driver, the voltage/current applied to the armature is
proportional to the error, ie the difference between the count stored in
the Gecko and the position sent by the encoder. Thus, NO motor is going
to stay exactly on position when manually rotated when standing still.

How much the motor will rotate depends upon the error count and the
higher the encoder resolution, the more steadfast the motor holding will
be in terms of allowable deviation.

We've got several types of servo motors setup up with Gecko 320s, some
of which are Amateks as being discussed. WE've also run various encoder
resolutions, from 100 counts per rev to 1000 counts per rev. The motors
with the low resolution encoders feel really loose at standstill. Put a
high resolution encoder on the same motor and it feels tight. This
happens with all motors including Amateks.

Turning up the gain until just below resonance helps too.

> I haven't gotten any
> explanation
> from Mariss as to why it behaves like that, it isn't supposed to!

I don't think there is anything wrong with the motors or the drives.

> My own
> servo drives get very stiff, and reach full current with a deflection of
> .001"
> or so, which is a small fraction of a degree.

It makes a difference if you are pushing on the table or manually
turning the motors. I can't measure any deflection of the table with
the G320/Amatek setup that we use. And yet the motor will turn slightly
if you put enough torque on it.

You don't state what your resolution is, but no system can go from 0
current to a high current with an error signal of 1 or 2 steps. That
would be really high gain on the I term and pretty unstable.

> And, it appears, although I haven't tested it,
> that this
> may only apply to the Ametek motors. I've had several people report similar
> behavior with Ameteks, and no other motors.

We've got other, bigger motors that feel loose as well with a low
resolution encoder.

--
Kim Lux <lux@...>