Re: [CAD_CAM_EDM_DRO] Tachs and encoders

antson20022 wrote:

> I am a little confused about using tachs and encoders. I thought I
>had everything straight in my head about a servo system untill today.
>I saw an advertisement by a company selling servo systems and they
>used encoders ONLY, with no tachs. I thought you had to have both. Is
>a system with the tach and encoder a better way to go? Are there any
>disadvantages to using an encoder only? Any input would be greatly
>appriciated.
>
>

If you have a very high encoder resolution, the tach becomes much less
important. Velocity information can always be derived from the encoder,
but in between encoder counts, the velocity generator has to "guess".
At extremely slow speeds, the servo essentially becomes open-loop until
the next encoder pulse comes along. This can cause some amount of
jumpiness at low speeds. This can be seen when milling large holes or
cutting angles that are nearly parallel to one axis.

I did some experiments when I set up my own servo amps, and was able to
see stick-slip friction take over at about .01 IPM (That is one inch every
100 minutes!) There was no way to detect it was actually moving, but I
could see about 3 encoder counts per second going by, and also see the
jumps in the amplified tachometer signal. The DC tach was producing
a calculated output of about 7 uV. I was quite satisfied with this (amazed,
really, at how well it worked). On these amps, you can stop the computer
from updating the DACs, and still can't turn the pulleys on the motor.
So, the tach is the only thing keeping the machine from being moved.

So, the coarser your encoder resolution, the more a tach helps keep the
motion smooth. My X and Y encoders give 20,000 counts/inch.

Jon