RE: Low cost Servo Controller

Mike -

I don't have direct experience with the LM629, but I suspect, as with any
servo system, you'd have a following error during motion, and the "out of
sync-ness" would be in there somewhere. I've heard the trick is to have
small enough "moves", the smaller the better, and the microcontroller or PC
would have to command these moves in the right proportions to the axes
controllers to result in linear, circular, or combined motions
(interpolation). You have to rely heavily on a good PID filter with the
right parameter settings. At this level of motion, in my understanding, the
difference between stepper and servo control schemes start to blur, since
most motion controllers of either type end up commanding tiny point-to-point
moves anyway.

Carlos Guillermo

> -----Original Message-----
> From: Mike Gann [mailto:mikegann@...]
> Sent: Saturday, September 18, 1999 12:47 PM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO]Low cost Servo Controller
>
>
> From: Mike Gann <mikegann@...>
>
> I could be way off base here, as I haven't reviewed the LM629
> in quite a while. But if I recall, don't you set up the parameters
> in the LM629 telling it where to go and it handles the rest?
>
> If thats the case, again its been awhile since I looked at this chip,
> the problem would be keeping the axis' in sync.
>
> Mike
>
> CG wrote:
> >
> > From: "CG" <cnk@...>
> >
> > 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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> the discussion of shop built systems in the above catagories.
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