RE: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations

I agree Jon.

I said/meant in an earlier post you are about one of the only guy's that has
got most of this cracked, or compensated enough to work, and probably one of
the few into really big machines.

I think your FF1 and FF2 cycles will offer a good compensation method to
increase the dynamic range, hadn't thought of it that way before, but quite
effective. It compensates nicely for the machines ability to accelerate.



Rutex do have what they call a fold back system, which does seem to start
clipping the pwm short when it starts to get into problems. Makes a horrible
grunchy sound. It just doesn't do anything when real problems occur bar blow
the fets, and when into foldback the noise this creates causes massive
encoder / position errors.

The Tek's do not have any circuitry to achieve this, they seem to have
conduction commutation problems when they are starting to get into trouble
just before they trip, they just trip way too quick.

Shuting off within the pwm cycle but allowing it to continue is a good
method as you say, it doesn't compensate for dynamic range problems, but it
stops them becoming a problem, and provides a lot of protection just as it
is required with good recovery. With yout FF1 and FF2 it should make quite
an aggressive acceleration viable and also allows much wilder inductance
figures, though your filters will mask this issue as you say.



Seems you have much of the problems in this area licked. I'll have to go
back and look again at the data you sent me a few months back.





Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Jon Elson
Sent: 27 May 2007 19:43
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Big servo motors drive recommendations



Mark Vaughan wrote:

>
> Consider for the minute you have just started the motor and the error has
> gone quite big. Reducing acceleration would reduce this. But the servo has
> sent a command of lets say 50% on time to the Fets in the bridge stage.

Big

> motors have very low resistance, mine are about 1 ohm so we are looking at

a

> target current of 80 amps here. Ooops, the fets go bang or the overload
> trips.

My servo amps have "live" current limit, so that any time the
instantaneous current exceeds the limit setting, the transistors
are switched off until the next PWM cycle. This is totally
separate from the fault trip limit, which is set higher, and
requires a manual reset. Also, the output filters slow down the
rate of the rising current, so the PID as well as the current
limiting has enough time to react to it.

A proper PID contol loop should never go from zero to 50% duty
cycle in one servo cycle. The D term should slow down the rate
at which the duty cycle command rises to a level which allows
stable response. Within a couple one millisecond servo loops
the motor/encoder should begin to respond (move) and so the duty
cycle should hold at a percentage that maintains the acceleration.

Something is wrong with the servo loop tuning in your case. I
know I could make this work well with EMC and my servo amps.
EMC has the advantage of not only very flexible P, I and D
terms, but FF1 and FF2 (Feed-Forwards for velocity and
acceleration) as well.

Jon





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