Re: [CAD_CAM_EDM_DRO] Re: Looking for a servo amplifier.....

dougrasmussen@... wrote:

> Matt,
>
> Yes, I have the tuning info, etc. The problem is the BAL pot does
> not seem to be active, the drive won't balance. When the control is
> put in Servo adjust mode to tune the axis lag, it's impossible to get
> the value adjusted properly. The axis lag is much higher in one
> direction than the other, and impossible to adjust that difference
> out. This IM-balance causes in-position errors and occasionally a
> servo error during rapid, a while back I reset the max rapid down to
> 250 IPM to help eliminate errors.

My understanding of the BAL pot is it adjusts the DC offset of the
velocity loop. In other words, with a zero Volt velocity command
at the input, you should get no movement from the motor. Quoting from
the Servo Dynamics book for the SDFPO 1525-17, The balance pot is
used to adjust internal offsets in the amplifier. The function of this pot
is such that for zero input volts the output current should be at zero
amps.

> I tuned per my book, results were not good. Tech suggested using the
> test points off a good axis (X) card and tuning bad axis (Y) to the
> same reading. That improved axis performance a bit.
>
> This is the Y axis, which is notorious for allowing coolant to leak
> onto the motor. That was my problem 5 years ago, the motor got
> coolant into the encoder area, new motor cured problem. Performance
> symtoms were similair to now, but the Servo Dynamics tech tells me
> the balance problem is strictly related to the amplifier card and not
> the motor.

What is the velocity feedback from your motor to the servo amp? Does
it read the encoder or a tachometer of some sort? When in E-stop,
does the CNC control display position correctly when you manually
move the machine? IF not, you may have a bad encoder feeding bad
position data to the CNC control. IF that is happening, no work on
the servo amp will be any help.

> I do intend to switch motors between X & Y to make sure the problem
> doesn't follow the motor. Meanwhile due to work load I need to keep
> this machine running since it has the longest travel of my mills.

If coolant gets into the encoder, I wouldn't be surprised if you have
trouble there. Can the encoders be opened up, or are they sealed?
If you can get them open, you may be able to clean the glass disc
(Carefully!) and fix the problem. (There will also be analyzer gratings
very close to the patterned side of the disc, you may be able to
clean them with a lint-free cleaning paper like Kim-Wipes wetted
with alcohol, by sliding the Kim-Wipe between the disc and the
analyzer. This is a tricky operation, and shouldn't be tried without
good access and good light.

If a DC tach is used for velocity feedback to the amp, and the amp
shows problems when not even connected to the CNC control,
this could also be the problem. It could just be worn out. Also, check
for loose couplings between motor, tach and encoders.

Jon