On Saturday 01 December 2007, Peter C. Wallace wrote:
>On Sat, 1 Dec 2007, Gene Heskett wrote:
>> Date: Sat, 01 Dec 2007 12:46:21 -0500
>> From: Gene Heskett <[EMAIL PROTECTED]>
>> Reply-To: "Enhanced Machine Controller (EMC)"
>> <[email protected]>
>> To: "Enhanced Machine Controller (EMC)" <[email protected]>
>> Subject: Re: [Emc-users] open loop galvanometer control
>>
>> On Saturday 01 December 2007, Klemen Dovrtel wrote:
>>> I have two open loop galvanometers (
>>> http://www.laserfx.com/Backstage.LaserFX.com/Systems/Pinouts/MFEdataLG.jp
>>>g ) which i would like to use emc controlled laser show
>>> (similar to this one:
>>> http://elm-chan.org/works/vlp/report_e.html ).
>>>
>>> I will use a hardware encoder for measuring the angle
>>> of galvanometer. I will configure hal using
>>> hal-encoder and hal-pid. I think i will manage to do
>>> this (i checked the etch-servo sample configuration).
>>>
>>> The galvanometer is current driven, so i was thinking
>>> of using a op-amp and R-2R resistor ladder for current
>>> source. So i need a 10 bit digital output for R-2R
>>> resistor ladder (instead of pwm used in etch-servo
>>> sample configuration). How can i archive this? Is this
>>> a good solution, or should i also use PWM (i didn't
>>> make the hardware driver yet).
>>
>> Right here, I'd change the thinking, rather than a fancy resistor ladder,
>> and suffering from the slow ballistics of the galvanometer, if you know
>> where it is by the feedback, use the servo gain to horse it to where its
>> supposed to be as you can sweep it from point to point many times faster
>> that way, and probably with much less overshoot than a free swing at x
>> current can ever do.
>>
>> The idea is that you drive it with 10x its rated current, but just for
>> about 1/2 the time it would take to move, and drive it in reverse just as
>> hard to stop it at the target point, then hold it in that position with
>> the servo feedback at its normal current until its time to make the next
>> move. One can also move it at slower speeds, with that speed totally
>> under control if the feedback is fine grained enough. One can mix the
>> back emf of its velocity with the feedback and get pretty smooth motions
>> down to the stop, but the stop would always be at an encoder count, not
>> completely analog. But then neither is the r2r idea.
>
>Thats pretty much just what Accelleration feed forward does. Dont know what
>its called in EMC, FF2?
>
I'll have to let someone else more fam with the terms comment on this aspect,
Peter. My setup with steppers doesn't use it.
My familiarity with that sort of thing comes from many years of tuning the
servo's in a 2" broadcast videotape machines, reading up on the theory as it
was practiced then in the 70's & 80's.
Ampex, in the AVR-1 & 2 models, at one time had some truly hellacious servos,
able to bring a 2" diameter headwheel into full servo-lock with maybe 5
microseconds (that's arc-second accuracy) worth of rotational error at 14,400
rpm in 400 milliseconds from a dead stop. I'd guess at 4 to 5 ounces of
rotational mass in that thing. The servo amps were built tough, but the
power supplies could have anchored the Forrestal if the wind wasn't too
strong.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Q: What's the difference between a car salesman and a computer
salesman?
A: The car salesman can probably drive!
-- Joan McGalliard ([EMAIL PROTECTED])
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