DC motor control

Started by Tom Becker May 22, 2004
I've been using H-bridges conventionally by applying direction and
braking to the driver inputs and PWM speed to the enable input. I see
another method in an appnote that intrigues me.

If opposing PWM is applied to the H-bridge driver inputs (i.e. one input
is inverted), then zero speed is midrange, with full speed forward and
full speed reverse at the PWM range extremes. The enable input remains
free to use as another control input.

Is this a common method that I've missed somehow? Anyone with
experience? Tom
Tom Becker
--... ...--
www.RighTime.com
The RighTime Clock Company, Inc., Cape Coral, Florida USA
+1239 540 5700


Tom,

>I've been using H-bridges conventionally by applying direction and
>braking to the driver inputs and PWM speed to the enable input.

This method is called Sign-Magnitude

>If opposing PWM is applied to the H-bridge driver inputs (i.e. one input
>is inverted), then zero speed is midrange, with full speed forward and
>full speed reverse at the PWM range extremes. The enable input remains
>free to use as another control input.

Do you mean locked anti-phase? The PWM is applied to the direction
pin and the enable pin is kept high. When PWM is 50% the motor is trying
to go CW and CCW for the same amount of time and yields zero speed.
This is a common method and the advantage is said to be more torque at
low speed. The application notes for the LMD182000 power mosfet
discusses both methods.

I've used both methods; they both work; I haven't tried to make a
quantitative
comparison of torque at low speed and can't recommend one method over the
other.

John-




> ... the advantage is said to be more torque at low speed. The
> application notes for the LMD182000 power mosfet discusses
> both methods.

Thanks, John. Low-end torque is inviting. I'll see if it delivers. Tom
Tom Becker
--... ...--
www.RighTime.com
The RighTime Clock Company, Inc., Cape Coral, Florida USA
+1239 540 5700


> ... [as opposed to Sign-Magnitude...] Do you mean locked anti-phase?
The PWM is applied to the direction pin and the enable pin is kept high.
When PWM is 50% the motor is trying to go CW and CCW for the same amount
of time and yields zero speed. This is a common method and the advantage
is said to be more torque at low speed.

I can report that these two methods of PWM motor speed control are very
different and yield very different results.

"Sign-Magnitude" mode (i.e. direction/braking selection to the H-bridge
phase control inputs, PWM speed control to the H-bridge enable input)
produces an exponential PWM/speed curve. Perhaps 60% speed change comes
from only 15% duty cycle input change. Lower PWM frequencies produce
better torque at the expense of noise chatter and pulsing motion, and
very slow motion is possible due to armature hammering by pulses as
narrow as 3% before stall. Except for those effects, low PWM
frequencies require no particular concern.

"Locked Anti-Phase" mode (i.e. PWM applied in push-pull to the H-bridge
phase control inputs, on/off to the enable input) produces an
almost-straight-line bipolar linear PWM/speed curve, with zero-speed at
50% PWM duty cycle. My device yields 30 degrees/second at 100% PWM and
15 degrees/second at 75% PWM, -30 degrees/second at 0%, and -15
degrees/second at 25% PWM, pragmatically perfect PWM/speed tracking. My
motors stalled at about 18% PWM, drawing 150mA average at 449Hz. Higher
PWM frequencies provide higher stall speeds at less current; lower PWM
frequencies produce lower stall speeds, but at greatly-increased current
demand and motor heating. In my testing, for instance, 113Hz PWM
overheated a motor at 36% PWM, yielding 8 degrees/second motion. On the
other hand, a 904Hz PWM frequency produced almost the same control as
449Hz, drawing 80mA instead of 150mA, and stalling a little earlier.

Dynamic braking is possible with Sign-Magnitude, as is coasting. Locked
Anti-Phase, though, is essentially dynamically braked continually and
offers no coasting except by disabling the H-bridge.

FYI. Tom
Tom Becker
--... ...--
www.RighTime.com
The RighTime Clock Company, Inc., Cape Coral, Florida USA
+1239 540 5700