PulseWidthModulation comments

[Terry_Sare at dell.com]

>From another list for robots: PWM comments. Funny that it shows up on two
different list at the same time.

>>>>>>>>>>>>>>>>>>Inserted from srs list <<<<<<<<<<<<<<<
Hi,

        The speed of the PWM for speed control of a DC motor is *highly*
dependent on the motor you are controlling.  For example, the little 3/8
inch
diameter motor that I use require a PWM frequency of at least 20 KHz (which
isn't doable with a standard HC11 setup).   Luckily, most of us aren't
pushing the performance envelop, so most of the typical motors used in
mobile
robots can be run with a wide range of frequencies.
        As far as the inductance, if you run the frequency up, the inductive
field doesn't collapse as much, so the impedance isn't such a problem.
Depending on the motor, there are bad ranges, however.

                Cheers,        Bill Harrison
                                    http://www.sinerobotics.com

Peter McCollum wrote:

> 1000 ticks at 8 MHz would be 8 KHz, right? I'd say that that's much too
> high. The inductance of a DC motor is probably high enough that the 8 Khz
> signal is seeing a very high impedance. Thus, very poor power transfer.
>
> I would try 20,000 or 30,000 ticks instead - giving you 300-400 Hz.
> That's the range I'm using on two projects so far, and it seems to work
> OK.
>
> Pete M.
>
> On Sun, 31 Jan 1999 22:27:08 -0500 (EST) Derek Konigsberg
> <octo at evcom.net> writes:
>
> >       Anyways, does anyone know what period works best for maximum
> >torque in PWM motor control?  Right now, low speeds yield almost no
> >torque
> >and I can effortlessly stop the wheels with my hands.  I'm using a
> >period
> >of 1000 clock ticks (8MHz clock) on a 68HC11E1.  My high/low (duty
> >cycle)
>
> ___________________________________________________________________
>>>>>>>>>>>>>>.end insert
-----Original Message-----
From: rauscher at icst.com [mailto:rauscher at icst.com]
Sent: Thursday, February 04, 1999 1:21 PM
To: DIY_EFI at efi332.eng.ohio-state.edu
Subject: Re: PulseWidthModulation comments



Ludis wrote:
>Years ago as a summer intern I worked on a motor controller which used
>PWM.  A brushless DC motor ran on up to 120 to 170 volts at up to 10 to
>15 amps.  A switching (PWM) power supply controlled the voltage applied
>to the motor.
<snip>
>An improved version used a microcontroller controlled digital PWM.  I
>think the PWM frequency was about 20 KHz and the PWM had 5 bits of
>range.  The same microcontroller monitored the motor position/speed to
>form a closed loop system.

Was this motor designed specifically for this freq? When I first developed
this PWM motor controller, I was trying to run it at 20KHz also. But the
motor was down on power. Motor was an AstroFlight cobalt, 9.6V at 35Amps,
such as those used in R/C model aircraft. Going to the lower freq brought
the power back up. I looked at possibly skin effect, but the wire used in
the 
winds wasn't that large.

>Some comments applicable to fuel pump / coolant fan PWM control:
>
>The pulse width and frequency needs to be very stable, otherwise all the
>inductors in the circuit will squeal like stuck pigs.  The original
>analog system screamed constantly.  The digital system (running the same
>switcher!) was totally silent as long as the pulse width wasn't
>changing.  Software in the microcontroller updated the pulse width every
>millisecond.  When the pulse width needed to be changed often, this
>produced a 1 KHz tone from the electronics.  The amount of noise was
>directly proportional to the load on the motor.  You could actually hear
>the computer "straining" against a high load.

Sounds as thou the noise is from the P/S, and not the motor. I've only
heard a light whine from the motors, and only at lower speeds. So hopefully
the noise won't be enough to drive folks out of the car <g>.

>The main switching transistor was a TO-220 package mounted on a dinky
>little heat sink.  You don't need a massive TO-3 transistor.  In the
>above system, cooling the rectifier diodes was actually more of a problem.

Oh so true, in the writeup I showed that even at 15 Amps, the MOSFET only
needed to dissipate a little more than 5 watts. Only a small heat sink
required.
The key to the proper MOSFET, is low voltage drop through it. This keeps
losses down and heat also.

>BTW, in one mode, this motor was reversed every several tenths of a
>second.  This applies to a thread from several weeks ago.

My brain is fading here, possibly relative to a motor driven FPR?

BobR.

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