Re: 20 Amp 80 V $99 DC Servodrive

> 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!

No, just 4000 series CMOS. It pokey but that's just what you need for
noise immunity in these applications. It's just fast enough to do
what's need without being so fast it responds to noise.

> 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!

My guess would be it did not have an integrator in the loop. An
integrator would not permit a constant following error.

> 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!

Compromises are inevitable in any endeavor. The art is in finding the
right amount to accomplish what you must do and to leave what you
just fancy. The former costs 10%, the latter 90%, IMHO.

Mariss