Re: [CAD_CAM_EDM_DRO] Re: breakout boards plus - hard choice

scyvt wrote:

> Jon
>
> Isn't the promise of closed-loop steppers much bigger than you just laid out? I don't know
> enough to understand what has to be implemented in software and what in hardware, but
> realization of the 'unstallable stepper" concept at the DIY level seems to be just about
> here, no? You must have had that in mind.

Because of all the delays involved (encoder readout rate =
computer PID loop rate = step pulse generator update rate, now
add the response time of the step driver, then probably not.
I'm not saying you could not do an unstallable stepper drive,
but that just using my stepper controller will not give you this
capability immediately.

To do the unstallable stepper, you need to monitor the phase lag
between the motor's winding current and the motor's position
with pretty good accuracy. So, you have to keep track of motor
position to one full step, including the step angle variation
within the motor and the errors in the encoder. Assuming the
motor is being driven pretty fast, say 1800 RPM, that is 30
rev/sec, and 6000 full steps a second. So, the USC and EMC
running at the default rate of 1 KHz servo updates a second
certainly won't cut it. You would probably need a servo update
rate of 12000 a second, minimum, to handle this speed. Well,
the USC can do that with a PCI parallel port and a fast
computer. I think Mariss' idea uses a step driver which has a
master current setting, so the controller can ramp the current
up as needed to drive the mechanical load. It might work with a
G201 or G203 drive without the pulse multiplier (which
complicates the phase relationship a lot). The motor would run
hotter as it runs at full current all the time. Then again, the
current modulation may be part of the key. I certainly have not
done this.

Jon