RE: Low cost Servo Controller

Bertho-

Take a look at the LM629 Motion Controller chip from National Instruments.
It's an all-in-one motion controller that handles the encoders, current
sensing, programmable PID loop, and either 8/12 bit DAC output or 8 bit
sign-magnitude PWM output to the servo amplifier. They show a simple
circuit with the LM629 using PWM to talk to their own LMD18201 3A 55V
H-bridge. You need to talk to the LM629 with a micro of course, but the
language is very straightforward. It seems like a good starting point for
figuring out the basic functions and architecture for an all-digital
low-cost system we're talking about. These chips are expensive, as far as
chips go ($25 ea.), but they do just about everything. I don't have much
time now, but I've got the data book in front of me if you need more info.
(National Power ICs Databook).

Carlos Guillermo

> -----Original Message-----
> From: Bertho Boman [mailto:boman@...]
> Sent: Friday, September 17, 1999 7:43 AM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO]Low cost Servo Controller
>
>
> From: Bertho Boman <boman@...>
>
> 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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> ------------------------------------------------------------------------
> Welcome to CAD_CAM_EDM_DRO@...,an unmoderated list for
> the discussion of shop built systems in the above catagories.
> To Unsubscribe, read archives, change to or from digest.
> Go to: http://www.onelist.com/isregistered.cgi
> Log on, and you will go to Member Center, and you can make changes there.
> For the FAQ, go to http://www.ktmarketing.com/faq.html
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>