Geckodrive

Sorry for the cross posting but it's been a stellar day here. This
has to do with the "Unstallable Step Motor Project" that may be of
interest to some:
-----------------------------
Hi,

The very crux of the G100 project is to have PID servoed step motors
that cannot be stalled under load. The G100 hardware was designed with
this goal in mind; that is why it has 6 encoder input channels, one
for
each axis. I have recently made/found the time to work towards this
goal.

Before anyone asks "When will it be done?", don't. It will be done
when
it's done. A lot of steps, work and time stands between what has been
recently accomplished and a finished product.

The goal: Imagine you are doing 3-D contouring (X,Y,Z) when someone
decides to sit on your gantry. An ordinary open-loop stepper or servo
motor would stall/fault-out and that would be that.

Now imagine the overloaded X-axis servo-stepper would just slow down
under this load. The other axis (Y,Z) would slow down as well, keeping
their speed ratios with the overburdened X-axis. The 3-D contouring
path would be strictly maintained albeit at a much reduced speed. Once
your friend got off of the gantry, speed would pick up to where it was
before. Your work would not be ruined.

How would this be done? Your step motors would be mounted with
encoders. The G100 would sense 1 or more motors are about to stall
(develop a phase lag of 1.8 degrees). The G100 would slow them down
and
back up the speed-torque curve to where more "grunt" (torque) was
available.

Steppers have the unique characteristic of having a speed-torque curve
where the slower you go, the more torque that is available. Can't do
that with a regular servomotor.

OK. That's the goal; that's the picture in my mind. Here's what has
been done by today:

I have a G100 driving a G201 running a step motor at 3,000 RPM (NEMA
23, 3A/phase, 24VDC). Open-loop, you so much as look at it and it will
stall. A flea landing on the shaft would stall it open-loop. I take a
pair of pliers and clamp down on a wet paper towel folded over the
motor shaft and all I get is steam. The motor will not stall. It slows
down under load but it delivers 50W to the paper towel/pliers combo to
make said steam. It goes all the way down to zero RPM without losing
position.

I ease up on the pliers and the motor is back up to 3,000 RPM
lickedly-
split, gives a chance for the boiling-hot paper towel to cool off. Not
a step has been lost; you just can't kill the motor (stall it).

This is just the first step (though a very important one) to integrate
the unstallable feature into a working CNC firmware. There are many
problems this feature presents.

Just right now, having it do what it does, has me totally stoked. It,s
so cool, jerk down on the pliers, motor slows down. Instruments say
it's delivering 95%+ of all it has to give at all speeds. Let up and
bing-boom-bam, 3,000 RPM again. Just can't kill it no matter what I
do;
very fine Friday nite indeed.

Before you ask #2: Yes, it will traslate into all step motors of all
sizes at all supply voltages at all speeds.

Mariss