next, replace the 297 was Re: Was Chopper circuit - Now ver 3.2, G298

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> Hi,
>
> I now know the difference between the L297 and the L298. It will be
> called the G298. The PCB is layed out and goes to prototype next
week.
>
> In my opinion, a constant off time chopper does suck (not sure if I
> used that exact word, but it will do). Here's why:
>
> The constant off time algorithm works like this:
>
> (1) Apply power supply voltage to the coil. Current increases at a
> rate slightly less than V/L.
>
> (2) Sense the coil current. Is it less than the reference? If yes,
go
> to (1), if no, go to (3).
>
> (3) Reverse the voltage on the coil. Current will now decay at a
rate
> of slightly more than V/L.
>
> (4) Wait a fixed period of time (typically 20uS).
>
> (5) Go to (1).
>
>
> The problem is, though the "off" time is fixed, the "on" time will
> vary somewhat with current, typically from 20uS to 30uS. This means
> the frequency will range from 20 kHz to 25 kHz.
>
> The motor has 2 coils, so 2 choppers are needed. Each is a free-
> running oscillator.
>
> Anytime you have two free-running oscillators, they have a strong
> tendency to phase lock (both run in lock-step). While they do, the
> coil currents becomes divergent, so phase lock is broken after a
few
> cycles. The currents is restored to the correct values and phase
lock
> is re-established.
>
> This making and breaking phase lock generates the annoying
squealing,
> hissing and growling noises you hear. It also causes the motor to
> vibrate if the audible frequencies fall within the pass band of the
> motor.
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> > Alan,
> >
> > And while we're at it; can we get Mariss to describe in more
detail
> > an offhand comment he made some time ago, which has stuck with
me...
> >
> > That being something along the lines of: (Mariss "speaking")
> > Constant time off pwm sucks (he said something more like "has
very
> > poor performance"<G>). Doing it differently is harder to be sure,
> but
> > worth it, and I enjoy the challenge...
> >
> > (constant time off PWM is used in the L297)
> >
> > Ballendo
> >
> > --- In CAD_CAM_EDM_DRO@y..., Alan Marconett KM6VV <KM6VV@a...>
> wrote:
> > > Hi Mariss, Paul,
> > >
> > > Yes! It does sound like an interesting project. Now all we
have
> > to do
> > > is "publish" a simple plan (and code) for a PIC or PAL
> replacement
> > for
> > > the '97.
> > >
> > > Alan KM6VV
> > >
> > >