Re: [CAD_CAM_EDM_DRO] Linear encoder and rotary motor servo

elliot@... wrote:

>This has been discussed before: a linear encoder combined with a rotary
>motor and some sort of rotary motion/linear motion conversion. The
>consensus has been that this is a bad idea because play in the rotary/linear
>motion conversion will cause the servo to go wild, hunting and vibrating.
>
>

That's llikely to be true in a machining environment. The cutter causes
an oscillation in force, since there's more force when a tooth is
engaged than when there isn't. The PID loop for the motor will have
some oscillation - even if this is just the "singing" between encoder
counts. If the gain is too high (gives you a nice stiff machine) and
there's too much backlash or you hit the natural frequency of the belt
or something, then boom - you get oscillation.

>I was convinced that this was so until I took apart a inkjet printer
>yesterday and found that it had a rotary motor and a linear encoder. The
>rotary to linear motion bit is a cogged belt.
>
>

The load on an inkjet print drive mechanism is nearly constant, and they
don't hold position - they need to maintain velocity. They need
position feedback so that the ink droops can be clocked out to the jets,
but that's easily done. you can tune the latency as well - the older
inkjets had a test mode where they would print some dashes with
alternate jets when moving in one direction, then would do the same in
reverse. You would then change some parameter until the print looked
nice and even. The position encoder only needs to know that you're in
the same place going one direction or the other - it doesn't need to
know exactly where that is, since that's takes care of with the software
offsets (and there's usually an index mark on the encoder strip as well).

>Since millions are of these were made, it clearly works. Is the common
>problem with servos of this sort caused by backlash between a screw and nut?
>
>

See above - that also takes care of backlash.

>The encoder grating pitch is 0.0055 inch, so by detecting all edges they
>could get the precision they need.
>
>The motor cogs less than some DC motors its size, but still is not exactly
>smooth when turned by hand.
>
>Does anyone know if this 0.0055"/4 is about the limit of what can be done
>with a cogged belt? It would be a really neat way to make a hexapod.
>
>regards-
>Elliot
>
>

There's also dual-loop control, where you have an encoder on the motor
and a scale on the table. The encoder is used as the feedback to the
motor control PID, and the linear scale is used as the final position
feedback for the higher level position loop. I think the motor encoder
is used for the most part, but the scale allows minor corrections.
Also, since the motor velocity/position loop uses the rotary encoder,
there's no issue with oscillation.

Hopefully, someone who understands dual-loop better can fill in the gaps
(or replace what I've written, as the case may be :) )
- Steve