Re: Electronics gurus please help

--- In CAD_CAM_EDM_DRO@y..., <MIKEC@W...> wrote:
> dear Mariss,
> I have been developing a stepper motor controller for medical
use. I
> would appriciate you sending me the circuit you talked about if you
can.
> thanks, Mike Carpenter.
> ----- Original Message -----
> From: "mariss92705" <mariss92705@y...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Wednesday, September 11, 2002 9:45 PM
> Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
>
>
> > Scott,
> >
> > I took a quick look at your circuit and here are my comments
based on
> > some quick calculations of how I think your circuit behaves.
> >
> > When any MOSFET turns "on" current will build up in the coil at
the
> > rate of V/L where V is your power supply voltage and L is the coil
> > inductance. For a 2.4 mH inductance and a 24VDC supply, V/L would
be
> > 10,000 amps per second, so it would take 200 uS for the current to
> > reach 2A.
> >
> > Once this current reaches your reference threshold sensed by the
> > comparator, the MOSFET is turned "off". This is where trouble
begins.
> >
> > A property of inductance is it resists abrupt current changes. The
> > coil will generate a voltage that will avelanche (zener) the
MOSFET
> > drain to source in order to keep the current thru it flowing. This
> > will be about 100 to 120VDC if the parts are IRF530s. The V/L
would
> > now be 120V / 2.4 mH or 50,000 amps per second. It would take 40
uS
> > to have the current drop to zero.
> >
> > This presents two problems.
> >
> > The first is the decay current is not recirculated so it is
> > dissipated as heat. The amount of heat is 2A * 120V * 40uS /
(200uS
> > +40uS) or 40W, so I'll bet your MOSFETs get real hot.
> >
> > The second problem is your current is a saw-tooth that goes from
0 to
> > 2A. Step motors respond to the average current which would be 1A,
so
> > I'll bet your holding torque is going to be 50% of rated.
> >
> > I'm not saying this to tear apart your circuit, it is just to
point
> > out where the problem areas are. I can e-mail you an output
topology
> > that recirculates the current and thus would have a much lower
> > dissipation.
> >
> > Mariss
> >
> >
> >
> > --- In CAD_CAM_EDM_DRO@y..., "scottdbtnet" <scott@d...> wrote:
> > > I apologize for the quality, but I uploaded a JPEG of this
> > schematic
> > > to the files section as "Unipolar Stepper Driver.jpg" with
> > > description of "PIC Driven Unipolar Stepper Driver"
> > >
> > > Hope you can read it.
> > >
> > > It uses the PWM output of the PIC to set the comparator
threshold
> > for
> > > the current limit. When the voltage across the sense resistor
> > > exceeds this threshold the comparator signals an interrupt on
the
> > PIC
> > > at which point the interrupt code turns off the outputs for
about
> > > 25uS. It then turns the ouputs back on and the cycle repeats.
The
> > > proper output state is stored in a variable to make this easy.
> > >
> > > If anyone is interested I can provide more details. I am only
> > saying
> > > this circuit is simple and it does work. As I recall I can only
> > get
> > > about 300 RPM out of a Vexta PH264 using this circuit, but that
was
> > > adequate for my needs at the time.
> > >
> > > Scott
> > >
> > > --- In CAD_CAM_EDM_DRO@y..., "Ian W. Wright" <Ian@i...> wrote:
> > > > Hi Scott,
> > > >
> > > > This sounds like an interesting way of doing things - how
about
> > > putting your
> > > > circuit in the files section?
> > > >
> > > > Best wishes,
> > > > Ian
> > > > --
> > > >
> > > > Ian W. Wright
> > > > Sheffield, UK
> > > >
> > > >
> > > > The following added for automatic email harvesters!
> > > > abuse@w... abuse@p... abuse@b... abuse@y...
> > > > UCE@F... abuse@t... abuse@f...
> > > >
> > > > ----- Original Message -----
> > > > From: "Scott M. Thomas" <scott@d...>
> > > > To: <CAD_CAM_EDM_DRO@y...>
> > > > Sent: Wednesday, September 11, 2002 8:54 PM
> > > > Subject: RE: [CAD_CAM_EDM_DRO] Re: Electronics gurus please
help
> > > >
> > > >
> > > > > Hmmm. I suppose you could use 6 if you want to. I built
one
> > > using a PIC
> > > > > and chopped the input to the 4 "end" transistors, worked
very
> > > well. I
> > > > also
> > > > > used a common current sense resistor, so this simplified it
> > even
> > > further.
> > > > >
> > > > > I'll have to check out the voltage doubling effect, hadn't
> > > thought of
> > > > that.
> > > > > But this shouldn't have a huge effect on things depending
upon
> > > the power
> > > > > supply voltage.
> > > > >
> > > > > Thanks
> > > > >
> > > > > Scott
> > > > >
> > > > >
> > > > > -----Original Message-----
> > > > > From: mariss92705 [mailto:mariss92705@y...]
> > > > > Sent: Wednesday, September 11, 2002 3:38 PM
> > > > > To: CAD_CAM_EDM_DRO@y...
> > > > > Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help
> > > > >
> > > > >
> > > > > Scott,
> > > > >
> > > > > If it were an R/L drive you would be correct. The center-
tap of
> > > the
> > > > > motor coil would go to the + supply thru a large current
limit
> > > > > resistor and the winding ends would have the transistors.
> > > > >
> > > > > Since a unipolar chopping drive was mentioned, 6
transistors are
> > > > > required; the current limit resistor is replaced with a
> > > transistor to
> > > > > which pulse width modulation (chopping) is applied. The end
> > > > > transistors still would behave the same.
> > > > >
> > > > > A center-tapped (6-wire motor) winding is a 2:1 step-up
> > > transformer.
> > > > > because the center tap goes to the + supply, pulling one end
> > wire
> > > to
> > > > > ground causes 2 times the supply voltage to appear on the
other
> > > end.
> > > > >
> > > > > Mariss
> > > > >
> > > > > --- In CAD_CAM_EDM_DRO@y..., "Scott M. Thomas" <scott@d...>
> > wrote:
> > > > > > I count 4 mosfets not 6 for a unipolar drive, did I miss
> > > > > something? And,
> > > > > > because there are half the parts and no high side drivers
> > > involved,
> > > > > I would
> > > > > > say that a unipolar drive is much simpler. I also don't
> > > understand
> > > > > why the
> > > > > > voltages would be different.
> > > > > >
> > > > > > Scott
> > > > > >
> > > > > > -----Original Message-----
> > > > > > From: mariss92705 [mailto:mariss92705@y...]
> > > > > > Sent: Wednesday, September 11, 2002 12:59 PM
> > > > > > To: CAD_CAM_EDM_DRO@y...
> > > > > > Subject: [CAD_CAM_EDM_DRO] Re: Electronics gurus please
help
> > > > > >
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > It takes 6 power transistors and 6 high speed rectifiers
to
> > > build a
> > > > > > unipolar "chopper", it takes 8 power transistors and 8
high
> > > speed
> > > > > > rectifiers to make a bipolar (full bridge) one.
> > > > > >
> > > > > > The power rectifiers are not across the power transistors
in
> > the
> > > > > > unipolar drive. If MOSFETs are used in a bipolar drive,
the
> > > > > > rectifiers are included in the transistors.
> > > > > >
> > > > > > The power transistors "see" twice the power supply
voltage in
> > a
> > > > > > unipolar drive. In a bipolar drive, the maximum
> > voltage "seen"
> > > by
> > > > > the
> > > > > > power transistors is equal to the supply voltage.
> > > > > >
> > > > > > The performance of a unipolar drive is about equal to a
> > bipolar
> > > > > drive
> > > > > > running a motor in a half-winding connection.
> > > > > >
> > > > > > A bipolar drive is prefered because it is simpler (when
> > MOSFETs
> > > are
> > > > > > used) and the circuit voltages are lower.
> > > > > >
> > > > > > Mariss
> > > > > >
> > > > > > --- In CAD_CAM_EDM_DRO@y..., Vajk Fekete
<vajk.fekete@o...>
> > > wrote:
> > > > > > >
> > > > > > > question1: is a chopper unipolar driver much worse than
a
> > > chopper
> > > > > > bipolar drive?
> > > > > > >
> > > > > >
> > > > > >
> > > > > >
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reach it if
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> > http://groups.yahoo.com/group/jobshophomeshop I consider this
as a
> sister site to the CCED group, as many of the same members are
there, for OT
> subjects, that are not allowed on the CCED list.
> >
> > NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY
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