Re: Re: Low cost Servo Controller

----- 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