Re: [CAD_CAM_EDM_DRO] Closed-loop steppers, was: breakout boards plus - hard choice

On 7/31/07, Jon Elson <elson@...> wrote:

>
> Michael Fagan wrote:
> > The Newall system scales are available in a 5 um resolution and would
> > probably work for this application (they're not optical scales, they're
> > based on capacitance I think). Also, I believe that the Acu Rite
> MillPWR,
> > their turnkey cnc system, uses servos with rotary encoders for gross
> > positioning but relies on feedback from their glass scales for
> closed-loop
> > feedback, as well as the 3-axis DRO component of the system.
> Newall scales are based on steel bearing balls in a stainless
> tube, and use a magnetic sensing and interpolation scheme.
>

Right. I knew about the balls (I've got a set on the BP) but I didn't know
how they gathered data from them. I know they are advertised to have a
faster "refresh rate" than optical ones but other than that I don't know the
exact specs.
The newall dro unit does have a serial port output on the back that gives
real inch measurements for each axis regularly (don't know the exact timing)
so I think it can be interfaced for some things.



I

> have never seen Newall scales used in a CNC application, and
> therefore I worry that they may be too slow to readout for
> motion control. They say "sample period = 100 us", but I really
> don't know what that means. If it means that the max pulse rate
> is 10 KHz (1/100 us) then that may be a problem at 5 um.
> 5 um * 10 KHz = 50 mm/second, which is roughly 120 IPM. If the
> spec means that the pulses will not come out faster than 10 KHz,
> then the servo loop will get unstable as you near that speed.
> (I assume it would catch up later, no problem for a DRO,
> disastrous for a servo.)
>

Right, which is why the MillPWR has rotary encoders for doing the rapid
movements (like the 120 ipm) and uses the DRO optical scales only for fine
positioning.



With the dual-encoder approach, what software supports that?

>

It's Acurite's proprietary MillPWR unit, it's pretty unhackable AFAIK. It's
not software, it's got an entire hardware unit as well. See
http://www.acu-rite.com/index.cfm?PageID=56ABA3B5-F620-11D4-BC6400A0CC271CB6
I don't know exactly how they do it, but I do know that it's pretty
accurate.


Jon

>
>

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