Re: That encoder stuff

> From: "Arne Chr. Jorgensen" <instel@...>
>Subject: That encoder stuff.
>

<snip>

>Bertho, - you suggest a micro controller servo, and you would like
>to get rid of the tacho generator. Well, when I just wrote this,
>then another solution appeared. If this chip was read with ADC and
>you used lookup tables, then you might be able to correct it for an
>"off the shelf" encoder. But that doesn't look like any good
>solution - when I think of it.
>
>What would be the fastest way to read the velocity ?
>
>tmp = read_encoder_cnt;
>current = tmp - previous;
>previous = tmp;
>velocity = look_up_table( current );
>?
>I just don't like it. The dynamic range you need is very large,
>the response has to be rapid, and you also need direction info.
>
>I made a digital servo like this, with a 68HC11. I generated the
>PWM and everything, and had a command interface similar to LM629.
>The stupid thing I did, was to use some encoder inputs with very low
>resolution, and this may be some of the reason that the whole thing
>did not behave as I had hoped for. I also had PID filtering, and
>the thing was quite okay when it was running in mid range - say you
>were to run it at 50% speed, then change it to 75%. The trouble
>came when you came down to a crawl or changed direction. It was
>difficult to get a linear transfer function, and good stability at
>the same time.

Sometime back I read a report on an pulse coder to tach converter that used
the frequency of pulses for the high speed D/A conversion and the time
between pulses for low speed D/A conversion. This system gave a resonably
accurate velocity signal for both rapid moves and slow feedrates.

Something like this could be developed even if you wanted to stay in
digital for the entire velocity loop.

Ray