Combining shaft+linear encoders (was: Gecko Servo Control)

> -----Original Message-----
> From: astrehler [mailto:deruhrmacher@...]

> Is it possible to connect a linear encoder on the slides
> instead of an angular encoder on the motor.
> I would like to increase the accuracy of the machine this way,
> because this would eliminate pitch errors of the screws.
> I know that some "big" machines works like this.

Larry Ragan wrote:

> I have done this on my knee mill. I had an anilam cnc control that died and
> I used the scales that were on the machine. I am using Gecko 340 drives with
> the original servo motors. You need to make sure that there is very little
> backlash. My system has ballsrews on all 3 axis and the y axis would
> oscialte rapidly. There was about .001 play in the screw mounts and until
> this was removed it would shake the machine pretty violently. It can be
> done and does work quite well but any slop between the motor and the encoder
> will result in this osciallation.

Hi,

I am currently designing my own servo control chip (step/dir and encoder
inputs, +-10V or PWM output). I consider adding additional input pins to
connect a second (linear) encoder. The idea is to mix the two position
inputs for the PID control in such a way that the shaft encoder on the
motor provides the velocity (D) value and the linear encoder provides
the P and I signal.

That way, I think, the control loop should be more stable and a higher
gain (better stiffness) should be possible as if using a linear encoder
alone. Of course, backlash is still to be avoided but a little "elastic"
delay between motor and slide movement is not that harmful if the
damping is calculated from the faster input from the encoder mounted
directly to the motor shaft.

Will that work? Does someone have experience with combined shaft and
linear encoders? How do the "big" machines handle this?

Regards

Bene

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