Re: PWM generation details
- From: John Nagle <nagle@xxxxxxxxxxx>
- Date: Fri, 27 Jul 2007 01:34:35 GMT
Joe Pfeiffer wrote:
e c kern <eckern@xxxxxxxxx> writes:
On Jul 25, 5:36 pm, D Herring <dherr...@xxxxxxxxxxxxxxxxxxx> wrote:
PWM is a technique that allows smooth changes in motor output, using
simple on/off switches. This is achieved by switching faster than the
mechanical system can respond, such that the mechanics smooth out the
switching to average the switch state.
The point of PWM is not that you can use on/off switches. The most
common "switches" used in motor drives are transistors and are quite
capable of operating in linear mode to achieve speed control.
However, doing that is inefficient because it's essentially the same
as using a big variable resistor.
Also, you can use one bit per motor rather than enough bits to get the
desired resolution.
Further, low frequency PWM is not a great idea. Driving motors with
pulsed current makes them whine annoyingly and may cause damage. It
is also less efficient because the pulsed nature of the current
results in greater resistive losses in the windings. Ideally you
operate PWM drives at a sufficiently high frequency that the current
is smoothed by inductance. In this mode, the motor driver is
essentially a variable dc-dc down-coverter. The "inductance" here can
actually be the windings of the motor but in that case you cannot use
noise suppression capacitors.
I think you misread his post as suggesting that you sit there toggling
a physical switch back and forth with your index finger. He pretty
clearly meant something that could be switched "fast enough" to work
well, though he focussed on the mechanical system's response speed
rather than induction.
Generally PWM rates are in the audio range, from hundreds of hertz to
hundreds of kilohertz. The trend is towards higher frequencies, outside
the human hearing range. See the Atmel ATA6831, which is a dual PWM
driver for automotive applications designed to run at up to 25KHz.
This is for brush-type DC motors. There's also polyphase PWM, where
several phases are driven separately and in sync with motor rotation.
Brushless variable-speed motors are driven that way. This requires
more elaborate drive electronics but is more efficient.
These things are actually quite difficult to design well. They
have to deal with brush noise, inductive kickback, short circuit and
overcurrent protection, and thermal protection. The ones designed
for RC motor speed control are usually quite good at current capacity,
efficiency, and protection, but dumb - no feedback or computer interface.
The ones designed for hobbyist robotics tend to be smarter, but bigger and
not as good on protection circuitry. The ones for industrial motor control
are all over the place; some are very robust, and some are underdesigned,
and they're all much bigger and more expensive than the R/C units of
comparable current carrying capacity.
It's still harder than it should be to get small, fully-protected
motor controllers with encoder feedback at low cost.
John Nagle
.
- References:
- PWM generation details
- From: dbbarua
- Re: PWM generation details
- From: D Herring
- Re: PWM generation details
- From: e c kern
- Re: PWM generation details
- From: Joe Pfeiffer
- PWM generation details
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