Re: [CAD_CAM_EDM_DRO] access to Ametek motors- suitable for Servo application?

On Mon, 2003-12-22 at 21:34, Jon Elson wrote:

> Thanks, that helps define the problem better. I corresponded with Mariss
> about this, and he said the I term of the PID would drive it up to full
> current
> in a fraction of a second.

I don't think it will with only a step or 3 of error. It probably can't
or it wouldn't be stable. If it ever got to full current with just a
step or two of error, it would overshoot and start oscillating.

> Well, my experience did not show that happening.
> Your comment about the resolution of the encoder seems to confirm that.
> I wonder what the reason for this is, as I'm sure Mariss had this
> working fine
> at his lab. But, some combination of parameters seems to defeat the
> integral
> term of the PID.

I don't think there is any problem, I think they operate like they
should. The other thing to consider is how much torque you are putting
on the motor and what the current limit is set to. If the torque you
are applying exceeds the torque capacity of the motor or the torque
corresponding to the current limit, it won't get the error to zero.

I've done a lot of machining and testing and the repeatability of Amatek
motors with Gecko drivers is really, really good. I highly recommend
this system for small lathes (ie less than 14" swing) and mills.

> Does the G320/340 even sense the motor current, other than
> for fault trip? I wonder if motor inductance or encoder jitter could
> somehow
> keep resetting the integral term?

It <probably> senses the current it is sending to the motor. T is
proportional to I. It doesn't need to know much more about the motor.

> The motors in my local client's machine
> have 250 cycle/rev encoders (I think), so they are in the low part of
> that range.
> But, 250 Cycle/rev = 1000 counts/rev, and that gives 2.8 counts/degree.

Yep.

> If the shaft can move 15 degrees, that is 42 counts! Maybe the
> anti-diter circuit
> defeats the integral term when at standstill?

Turn up the gain and make sure you aren't overpowering the motors.

> >Turning up the gain until just below resonance helps too.
> >
> >
> Yes, clearly, and we have it as high as we can. Also, the raw gain of the
> PWM output stage is proportional the DC supply voltage. He was running
> his unit on 36 V, which is a bit marginal.

Agreed.

>
> >It makes a difference if you are pushing on the table or manually
> >turning the motors. I can't measure any deflection of the table with
> >the G320/Amatek setup that we use. And yet the motor will turn slightly
> >if you put enough torque on it.
> >
> >
> I'm talking about grabbing the motor pulleys. As my servo system was
> designed
> to deliver about 1000 Lbs linear force, pushing on the table will be
> insignificant.

I agree. Grabbing the pulley kind of defeats the purpose as it doesn't
take much to over power 500 oz in... 500 oz in = 30 in lb = 2.5 ft lb,
which isn't very much.

> I also can move the motors by hand, but the resistance is quite strong,
> even at
> very small displacement. On my client's machine, there is almost no
> resistance
> to turning the motors about 10 to 15 degrees from the center of the
> null. Then,
> it begins to resist strongly. If you leave it alone, the motors will
> twitch every
> 5 - 15 seconds, moving perhaps 5 degrees at a time, randomly, within
> that window
> of 'looseness'. This seemed totally wrong for what is supposed to be a
> precision
> positioning system.

Something is wrong with the system. Is this the first servo system
using Geckos you've been involved with ? I'd check what the power
supply voltage is at full current. It can't pull down or the Geckos
can't deliver enough power.

> Thanks for the comments! I'd still like to get back to my client's machine
> and make it more precise.
>
> Jon
>
>
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