Re: New Servo ?

Hi Arne,

I do not imply the digital servo amp will be, or is, "better" than an analog input section one.

All I am saying is that we can get rid if the analog tach, the D-A and associated amps in the PC, op-amps, filters and pots in
the servo amp. A full blown analog servo amp is complex with all its compensation and adjustments.

Regardless how we start, we need the power output stage which basically would have a TTL level input for control.

Now would it not be nice if we just could plug in ONE micro controller and be done with it? All the analog stuff would be
gone! The controller would be programmed to talk to the PC, watch the encoder signals and the motor curret via its built in A-D
and continuously calculate and generate the PWM.

All tuning would be via software from the PC. If you ever have crawled behind equipment in an industrial noisy, dirty
environment and tried to hook up scopes and DVMs and try to find some pot to tweak, you know what advantage it would be to do it
from the PC.

I am sure most of the software is more or less available in the public domain. It would be a lot simpler "Kit" for amateurs to
build and it can be upgraded in software.
Bertho
==========================

> From: "Arne Chr. Jorgensen" <instel@...>
>
> Hi again,
>
> I made a fool of my self too many times, - so I guess nobody would
> be surprised if I did it again. So here goes:
>
> Ok Bertho, I read some of what you wrote. I had not seen all of
> it, because of the line wrapping. The thing is that this list
> should have been sponsored by a toner and paper company. I am
> amazed with the activity. I receive things faster than I can think
> ;-)
>
> Still, I don't understand the concept. What I mean is, why this
> should be any nicer or better than a classical design, something
> like Jon's.
>
> So, let try something else: We start backwards.
>
> You need a power output. I would say a MOSFET bridge. If this is
> the case, I think I would add another ADC. Put one in each leg of
> the bridge. This way I think it would be easier to measure the
> currents. You will have freewheeling currents that would be
> difficult to measure otherwise.
> In order to measure the currents, then you would like as small
> resistors as possible and amplify them. Let's add some op amps, and
> you may have to adjust the gain, so add some variable resistors.
> >From the processor, you will need to translate the voltage suitable
> for the high end transistors, and also the low end. Okay - add some
> more discrete components. Then add some input buffers and output
> buffers for the encoder. This may be differential drives or just
> 5V. Then we need some input latches, or buffers. You may want to
> have fifo ram. If we say a cmd byte + 16 bit, you would like to
> download 3 bytes fast, and independent of the processor. And before
> adding the processor, maybe ram/eeprom/flash, reset circuits,
> oscillator, .... I think we have something.
>
> I think we have passed an analog design in the amount of
> components. Now we need a program, and we need to initialize some
> parameters for gains, etc. How to we do that ? Well, we could use
> the cmd-byte to do several things, but already we will have some
> problem with the current feedback. The reason is that we don't know
> the size of the motor, so we have to adjust the current gain. If
> you want it all electronic, then we need to replace the gain
> resistors with some electronic variable resistors, ( maybe from
> Xicor ) or we could use some DACs.
>
> We can approximate the velocity by reading the encoder count and
> compare it to the previous sample. We can plug in some digital
> filters, and we have our servo.
>
> More or less the same as the analog version, - so what have we
> accomplished. It is more expensive, harder to make. We still need
> encoder input in the PC, so the digital interface don't make much
> difference. Sorry, we need more wires, maybe buffers and shielding
> too.
>
> If this circuit doesn't do anything else than replace an analog
> servo amp, then I don't see any use in it. A digital tacho on the
> other hand, is nice enough, but in this way - I just don't see any
> point in it.
>
> I may have missed some of the ideas you all had, - but I can not see
> the point in this, - if it doesn't do anything more.
> --------- > Snip