On Tue, 30 Dec 2008 22:44:17 -0800, "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 from http://www.teranews.com **
I tend to agree with the vendor, I looked all over the net for some
information on the MPU unit you replaced and didn't find much.
The symptons are related to a resoultion problem, however the fact the
move amount is correct confuses the issues.
Would be nice to get in an open loop condition and run only the
velocity loop from a battery box. If the D/A puts out 9-10 volts at
max then you could use a battery box and put lets say a 1 volt speed
command and monitor the tach to ensure the scaling is correct for
proper speed.
If the rapid rate is 200 ipm and the tach is 3 volts per 1000 rpm
(screw) or whatever you could at least determine how close you are.
Once this is correct in your mind then you know the problem is in the
position loop.
If you disable the servo and command a rapid move what is the output
voltage to the drive?
Regards
Dave B