Re: [CAD_CAM_EDM_DRO] Servo amplifier boards.

johndroc@... wrote:

> I'm working on an EMC retrofit of a Milltronics Partner VII. I've
> actually
> gotten movement out of the Y axis! Right now I'm having difficulty
> adjusting
> the amp boards, because I have no documentation. They are made by IIC
>
> (Industrial something Controls - I forget what it stands for). There
> are 5
> or 6 tuning resistors labled:
>
> Bal
> CL
> CMP
> Tach
> Sig
>
> Does anyone know how they might work? I have no idea how to adjust
> the amp
> boards to the signals from the STG2 card. When it is not in "Machine
> On"
> mode, it seems to "drift". Any suggestions would be great. Thanks!

This is normal. There will always be some offset voltage between the
DAC and the amp, which will cause drift. I have differential inputs on
my amps, so the drift can be set very close to zero. It still drifts
.0001"
every couple of seconds. Because of this, you don't want to leave the
machine in this state for a long time. It could cause a very
slow-motion
crash.

The Bal pot is the offset adjustment, and should approximately null
out the drift. CL is the current limit, and should be set to a low
value
during testing, and turned up later. CMP is loop compensation,
but it is not too clear what loop (there are 3, position, velocity and
armature current). You would generally turn this up when there is
vibration, humming or fluctuation in the speed. The tach and Sig
pots set the gain on the tach feedback and the velocity command
signal from the STG board. Depending on the tach you have and
the maximum expected RPM, you can set the tach pot to get more
or less feedback from the physical system. If you set it too high,
it will overload the input of the servo amp, and the system will
go haywire. Also, if the feedback gain is too high, it will cause a
condition of instability, where the gain is greater than unity at a
frequency where the phase shift exceeds 180 degrees. This can
cause anything from mild humming to violent shaking that can
cause the machine to tip over. (I know, I have had to e-stop
mine a couple of times during tune-up.) The Sig pot needs to
be adjusted so that (at the input to the error amp) the velocity
command voltage from the CNC control is equal to the tach
fedback for that commanded velocity, plus a little bit to compensate
for friction. The way to adjust this is to use a jog button to
move the machine, and have the logging fucntion turned on
during the jog. When everything is optimum, you get a perfect
trapezoid - a ramp up to speed, steady speed, and a ramp
down to stop. When the Sig gain is not enough, you will
see the ramp up, then a slightly rising line, where position
error is increasing the velocity command to make up for
insufficient gain. If this is too high, velocity will overshoot.
If the straight portion has wiggles, you may need to turn down
the tach pot.

It gets vastly more complicated than this, with all sorts of
control theory, Bode plots, partial differential equations, etc.
And, it is hard for the control experts to do if you don't know
the weight of every part, the spring rate of the leadscrew,
friction of many forms, electromagnetic damping in the motor,
ad infinitum. The logging and graph functions of EMC make
this MUCH easier to do, without digital storage scopes, etc.

Jon