Re: Low cost Servo Controller

I am very interested in the design of an all digital servo amp. I have mentioned the pieces in several previous post but here
is a summary:

I like to stay away form the analog circuitry if possible and have a well defined simple digital unit. I know how to design
analog circuits, that is not the issue. Simply we will have a more repeatable and easier to buil system if it is all digital.
Also, do not be scared of the "DSP". It is basically a microprocessor that has been optimized for certain math calculations but
a regular microprocessor can also be used.

The interface to the PC would be all digital and the PC would send a two-byte speed command to the servo amp. As it is now, the
software sends the 2-bytes to a D-A (Digital to Analog) on an internal I-O board so there would not be any significant software
changes. The actual interface could be through a digital I-O card, the printer port, USB or even high speed serial I-O.

A "real" servo amp needs a tach input but we would use the encoder signals to calculate the speed and avoid the analog issues
and extra mounting headaches of a tach.

The last input needed is the motor current. We would sense the current and feed it to an A-D converter. Some
microprocessors/DSP even have built in A-D.

The central component of course would be a microprocessor or DSP chip, programmed to do the servo loop calculations and the gain
& filtering would be software adjustable.

If this approach is followed we basically would end up with the controller chip with all the smarts and it would do all the
calculations and produce the PWM output drive to the high power output switching sections.

Envision the best case: One DSP chip with inputs from the PC, the encoder and the motor current and it produces a PWM that is
feed to the power output switching stage.

Questions?

What digital I-O is used today for servo amps that would be similar to what I am proposing. I am sure I am reinventing the
wheel and do not claim any originality. It is simply the obvious answer to a problem. Is there any type of standard? What
digital output options do high end commercial software have?

Bertho Boman



Dean Franks wrote:

> From: "Dean Franks" <dean@...>
>
> ----- Original Message -----
> From: Tom Kulaga <tkulaga@...>
> To: <CAD_CAM_EDM_DRO@onelist.com>
> Sent: Thursday, September 16, 1999 5:00 PM
> Subject: [CAD_CAM_EDM_DRO] Re: Low cost Servo Controller
>
> <snip>
>
> > 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).
> >
>
> Yes, use a linear ramp oscillator and a comparator. Configure the circuit
> so the oscillator runs at the desired PWM frequency, the oscillator and
> input ref signal are connected to the comparator. Set things up so the
> comparator generates a "true" output when the input ref signal > linear ramp
> signal. If you want to prevent 100% duty cycle, put a divider (and maybe a
> zener) on the input ref signal to scale/clip it to the desired max duty
> cycle value.
>
> You can generate a close linear ramp using analog methods, or you can use a
> counter connected to a D/A converter. Assuming that you use an 8 bit
> counter and D/A, run the counter at 256x the pwm frequency. You could also
> use a cheap PIC with a software counter driving a single D/A source,
> although using the same ramp for all PWM channels will generate a bucket of
> EMI. If you do this in software, you might take the opportunity to stagger
> the master PWM clock, or use spread spectrum PWM generation to reduce noise
> [drop me a note if you want details on this].
>
> Hope this is useful.
>
> Dean
>
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