Re: [CAD_CAM_EDM_DRO] Re: Electronics gurus please help

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@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
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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subjects, that are not allowed on the CCED list.
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