On 2009-06-15, Bob Engelhardt wrote:
RogerN wrote:
I have an older Anilam CNC Bridgeport and Anilam CNC lathe. They both use
motors with tach feedback, the mill uses DRO scales for position feedback to
control and the lathe uses encoders coupled to the ballscrews. I replaced
the original amplifiers with AMC amplifiers similar to those in your link,
they work great.

How does you system use both tach feedback & encoders? It seems like
the encoders would give all the feedback that's needed.

The controller puts out a voltage proportional to the desired
speed, and the tach generator produces a voltage which is compared to
that voltage and is constantly adjusted by the amplifier to maintain the
speed -- all without the CPU needing to pay attention once it has put
out the voltage.

The computer checks the counter which keeps track of the encoder
pulses and compares that to the predicted position every so often, and
if necessary adjusts the output speed-command voltage. The counter is
part of the controller card, and also does not need constant CPU
attention, so the computer system can be doing look-ahead calculations
-- and visiting the internet if you so desire.

In contrast -- with steppers, the computer has to keep putting
out pulses -- and if the system is fitted with an encoder, to compare
what the encoder says has happened with what the computer *thinks*
should have happened.

Mostly -- every pulse requires an interrupt and a task switch
which is a rather busy thing for the computer to do. In contrast,
telling a D/A converter to output a voltage, and every so often reading
and resetting the encoder counter is a much lower load on the computer.

Using the encoder as a speed feedback requires a lot of work on
the CPU's part -- checking *every* pulse from the encoder. Using the
tach feedback simply involved telling it what to do and then relaxing.

And without the tach feedback, the computer will also need to
constantly be changing the voltage output to the amplifier -- or to
constantly be putting out pulses to an amplifier which makes the servo
motor pretend to be a stepper motor.

Depending on the precision needed, it is possible to cut a
shallow angle by outputting two voltages -- one for the X axis and one of
the Y axis (and perhaps one for the Z axis as well, if you are being
fancy) and then forget about it until it thinks that one of the axes is
getting close to position.

Enjoy,
DoN.

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