Re: Low cost Servo Controller

Hello Tom,
Good to see you are in here. We need your input and help.

You mentioned that you like to reduce your board to two channels only and then use multiple boards. I have not seen your
design, but I would it not be desirable to lock the 4 channel counters in one shot and then read the data. It will be much more
difficult if they are on separate boards.

Why not leave your 4-channel board as is and just have a separate 4-channel D-A output board with extra digital I-0?

I am on the way to your home page to see what you got...

Bertho
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> From: Tom Kulaga <tkulaga@...>
>
> OK, you guys have managed to drag me out of the woodwork on this one.
> Due to numerous Real Life issues, I've been very swamped, but I really
> want to chime in here, because this is the group project that I've been
> waiting for.
>
> I'm just going to hit the key points that I can help with or that I am
> curious about.
>
> Most of you have seen my work on the PC-based DRO, and those of you who
> haven't can see it at:
>
> http://www.mcs.net/~kulaga/dro.html
>
> This is the same card that Dan Mauch is selling.
>
> I think that it could serve as a good starting point for a servo
> controller card. I'd knock it back down to 2 axis, add 2 DAC outputs,
> a few digital I/O channels for limits, home, etc, and generally add alot
> more in the way of buffering and protection. I know that we sort of
> discussed this here on the list some time ago, but I haven't seen very
> great enthusiasm, and being pressed for time myself... you know the
> story. Anyway, there was some interest from Fred regarding setting up
> EMC to work with my card. Each card would support 2 axis, and users
> requiring more could just add cards in a modular fashion. It would be
> a little demanding on ISA slots for more complex machines, but any PC
> controlling something that complex would be dedicated and probably not
> running sound cards and such.
>
> I was also thinking that we could dovetail this project with all the
> talk about handwheels as controllers. Add one of my 4-axis cards for
> reading the handwheels, strip EMC down to just the PID loop stuff, add a
> little code to read the handwheels and feed commands to the servos, and
> you'd have a nice manual interface for your CNC machine. I don't know
> nearly enough about Linux, programming, or the Real Time patch to do any
> of this myself, but I'm sure someone could figure it out. It wouldn't
> be as clean a solution as running it all inside EMC, but it would be
> plenty useful.
>
> The debate about linear versus rotary encoders is one that I thought I
> could add something of interest on. Now, any backlash in the system is
> going to be annoying, and sometimes more than annoying. However, if you
> use linear encoders for feedback, that backlash will kill servos as a
> usable solution. For a PID loop to work right, the feedback to the loop
> *must* be exactly what the motor is doing. I have, however, seen this
> sort of thing done, and it resulted in a stable system. The trick was
> to use both rotary and linear encoders. The rotary, mounted directly
> to the motor shaft, would provide feedback for only the Derivative
> portion of the PID loop, with Integral and Proportional coming from the
> linear. It is not a perfect solution, but it does work. Still,
> backlash should be kept very very small, or the system is impossible to
> tune. Hands down, the best solution is zero-backlash ballscrews. It
> may be more expensive to retrofit, but the headaches and added cost of
> reading multiple encoders per axis is *far* more expensive and
> difficult, IMHO.
>
> When it comes to the amps, I'm not much of a help, but can anyone say
> whether it is possible to convert an analog signal to a PWM signal? It
> would be impossible for EMC to handle a software generated PWM signal,
> and the only way I know how to do it is by using a dedicated motion
> controller chip which is set up to output PWM. It's gotta be possible
> to convert one signal to the other, right? Especially given that we
> can decide here and now what level the signals will be (industry
> standards of +/-10VDC for analog, and 0-5VDC for PWM).
>
> OK, my brain is feeling pretty tapped right now, so I'll cut this off.
> I'll be monitoring the list daily, and I'll try to post if I've got any
> other thoughts. I know there was something else I wanted to say, but I
> can't seem to remember what it was.
>
> -Tom Kulaga