RE: [CAD_CAM_EDM_DRO] Re: Heck of a time tuning servo motors.

The problem is most likely in the tune. When you get high current, but
nothing is going anywhere, I suspect you are getting a high oscillating
current and the servo is doing a high frequency hunting.

The problem is difficult to describe without quickly getting into second
order differential equations, root locus plots, and other issues related to
dynamics. Without getting into long textbook discussions, I might give you
a few suggestions:

PID works fine with linear systems. Leadscrews, and their associated
backlash, create a very non-linear system. Conventional PID cannot handle
backlash without serious detuning.

If you have access to the gains, the first one to lose is the integrator,
that's the I term. I would always start out with I as zero. Proportional
will act like a spring, with higher numbers being a stiffer spring.
Derivative will act like an oil filled damper and calm it down a bit. Using
only P and D terms is like tuning a car suspension with springs and shocks.

Also, friction before the backlash will help, like the derivative term in
the controller. Friction after the backlash will hurt and will emphasize
the backlash non-linearity. Tightening the gibs is friction after the
backlash.

G. Jackson

-----Original Message-----
From: Erik Reikes [mailto:ereikes@...]
Sent: Thursday, October 10, 2002 2:31 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Heck of a time tuning servo motors.

Ok, So I tried a couple of things. I hooked up a 1.5VDC D cell
battery and the motor turned smoothly albeit slow. Not sure if that
is the be-all end-all test for cogginess in industrial versus servo
motors.

It is still sort of "stally" when turning the screws. Sometimes
depending on my gain and damping settings it will loudly "sing" and
draw a ton of current.

The lead screws are not "easy" to turn, but I can turn them with one
hand. To give an example of how tight I have them adjusted I
have .0025" backlash in the Y direction and about .004" in the X
direction (along the lathe axis). There is no measurable flex (best
I can measure is about 0.0002") in the gibs when I push on the table.

I am going to loosen things up tomorrow and see if I really just
don't have enough torque. This concerns me because when actually
cutting I often have a significant back pressure on the wheels.

I suppose there is still possibly a problem with cogging in the
motors themselves, but I'm not sure how to test for that condition.

Thanks in advance.

-Erik Reikes

--- In CAD_CAM_EDM_DRO@y..., "Erik Reikes" <ereikes@r...> wrote:
> I'll just reply to all the replies at once to save nouse on the
list :
>
> MikeC :
> I figured as much power wise. It seemed like a lot of power. I
have
> run them with the pulley disconnected and it moves smoothly and
with
> a reasonable amount of torque (seems like). I'm not sure if I'd
say
> the handwheel is "free and easy" its not tough, but its not easy.
I
> have the gibs adjusted fairly tighly to eliminate slop. Also the
> anti-backlush nuts are a bit on the tight side. Next experiment
for
> tonigt will be to loosen the machine up and see how it performs.
>
> deanc500 :
> Not sure about whether they are industrial or servo. They had
double
> ended shafts and mounting position for encoders which I put on my
> self. I'll try the low voltage test with a D cell battery in a
> little bit. It does have symptoms consistent with sticjing at slow
> speed. Seems to buzz when slow, but low current and fine when
moving
> quickly.
>
> bill :
> Doesn't feel especially tight, but I'll try messing around a bit in
a
> sec. Figured I'd take a break and eat some food and veg out before
> attacking it again. The coupling is really just a set screw to a
> timing belt pulley. I'm not sure how that could be misaligned
enough
> to cuase a problem.
>
> Thanks for all the advice. I'll let you all know how it works out
as
> I know more.
>
> -Erik Reikes


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