On Dec 31 2008, 1:44*am, "Bill Noble" wrote:
top posted to annoy those who hate top posting

*if the servo is vibrating, it is unstable. *You can do a root locus plot,
or you can fake it
the most likely cause is lag in the feedback system - you can accommodate
additional lag by reducing the forward path gain (that's what your gain pot
is for) - you might try playing with the TAC input, you may not have enough
rate feedback - it sounds like your servo has an inner rate loop and an
outer position loop, so the rate is primarily responsible for damping and
responsiveness, and the position loop gets you where you want to go.

Just a little reading on feedback systems will go a long way towards giving
you some understanding of what is going on.

Given that this happened when you changed the position sensor, I am guessing
that it is a phase lag type phenomena because the new sensor is slower
(maybe only by a few milliseconds) than the old one. *Digital sensors
introduce pure lag which is the worst thing for a standard feedback control
system.

"oldjag" wrote in message

...
On Dec 29, 4:11 pm, Dave B wrote:

On Mon, 29 Dec 2008 12:50:25 -0800, Dave B wrote:
Are the amps manufactured by Westamp?

Dave B

I have never worked on a Hurco, however it would seem there would be
parameters that allow you to alter the position loop gain.

Do you have parameters for backlash etc ?

Dave B

Dave;

The amps are Servomate Randtronics. They have the following Pot
adjustments: GAN = Gain, TAC = tachometer input, AUX=auxillary, (not
used), BAL=Balance, SIG=Signal and CLM=current limit. *Standard
preliminary setup is to MAX out, (20 turns CW) the CLM and the TAC,
set the BAL in the middle, adjust SIG to get the A/D out from the
servo control board to the Servo amps around 0.65V during a 20 in/min
feed, and use the BAL to get the voltage the sam in both directions.
Also the gain gets adjusted up to instability and backed off a few
turns. *The problem is with the base settings the servos are vibrating
like crazy. *Backing off the Sig kills the vibration, but with the sig
backed off to where the vibration stops, the servos are very slow to
respond, and the A/D voltage gets to only .45 volts. *Things checked
so far:
1.)Tach inputs to servo amp are okay and correct polarity.
2.)Encoder pulses look good on A, B and Z channels on scope at the
servo control board.
3.)Backlash on the ballscrews is less than 0.0002"
4.)Main cap on servo amp 80Vdc supply checks out okay.
5.) +12V and -12V Servo amp supply looks good.
6.) Position display is reading in the correct direction and is
accurate.
7.) A/D signal from servo amp looks noisy, but I don't know what
normal looks like.

At this point I'm not sure what to check next. *Vendor thinks maybe
problem is the new encoders, but I'm not convinced.

** Posted fromhttp://www.teranews.com**

Bill;

TAC does not have much effect. With a dual trace scope I measured the
delay time from an encoder pulse input state change to the Servo
control board output changing state, and it's 9.6 millisec, which
seems a bit long. The new encoders are good for 100Khz, or 6000 rpm,
much faster than this machine can go. I measured the encoder rise
time at less than 0.4 micro seconds. By the way this "oscillation"
problem I am having begins as a "ticking" noise that occurs several
seconds apart, and increases in intensity and frequency off occurance
as the SIG pot is increased.

Each "Tick" is preceded by a gradual motor rotation of about 1/8 turn
and then a "snap" back to starting position, which makes the ticking
noise. All axis are doing the same thing, but for testing I've
disconnected all except the Z axis for now. I thinking about trying
an open loop run with a signal generator feeding the servo amp a slow
sine wave, say to start maybe 0.2 Hz. and increasing the frequency
from there. just to verify correct amp operation on the velocity
loop. The tach signal looks okay at the amp input.

Now I have noticed another issue, every so many bootups of the MPU,
instead of getting +0.45/-0.45 Vdc D/A output from the servo board it
gives +1.75/-1.75vdc on encoder state change. On some boots I get
nothing...nada. Maybe this is related, maybe not but it's very
weird. I double checked my connections when this happened, checked
the encoder inputs, and everything else was normal. Even the 0.45vdc
output is suspect, as the D/A is supposed to put out at least 0.65 -
0.8vdc at 20 IPM feed. This aspect seems like a board issue, but the
boards were sent back last week to the vendor and they checked out
okay on their test rig....