Re: [CAD_CAM_EDM_DRO] Re: 20 Amp 80 V $99 DC Servodrive

Mariss Freimanis wrote:

> Jon, thank you for the kind words. Actually there is no proccesor
> involved, just a pair of 4-bit adders and 4 4-bit U/D counters for
> that section. The sum output is indeed a 2s-compliment weighed 8-bit
> value.

Ah, fascinating. Did you use an FPGA for this? I just designed a quad
24-bit
encoder counter to be packed into a single Xilinx FPGA - a really neat
capability!

> Regards to how many bits resolution is enough for following error,
> the snap answer would be "no amount is enough". My view is following
> error is a dynamic phenomena in response to inertial loads and is
> meant to be taken out within a few mechanical time-constants of the
> motor.

Depends on the servo system. The best I could do with my ancient
Allen-Bradley
late 70's CNC control was about .001" per IPM, so at 60 IPM, you were
saddled
with .060" following error! Ugh!

With EMC and the same servo amps (mine) and using a little bit of
feedforward
compensation, I have gotten that down to about .005" following error at
60 IPM!

> Your point intrigued me so I set up a test to measure following error
> on the test motor I am presently using. The motor is a NEMA 23
> rated at 25 Amps stall (300 in-oz) and 28VDC. I commanded it to turn
> 1,000 RPM with no ramping. For a max following error of 100 counts,
> the motor accelerated from zero to 1,000 RPM in 0.0032 sec. and
> caught up to the now moving command position in an additional 0.0035
> sec. This I think is considerably better than what a step motor could
> do.

No doubt! 3 mS from standing start to 1000 RPM!

> I can see your point though if very large load inertias are present
> and the mechanical time constant is very long; then 12 or 16 bits of
> resolution may be needed. However this drive is supposed to be an
> inexpensive alternative to step motors while offering some
> improvement in performance. Consequently some comprimises are
> necessary while still meeting the requirements of the majority of
> step motor applications.

Sure, but I hate compromising performance. I guess that is why you can
think of
selling your unit for $99!

Jon