Re: looking for THE dc drive motor speed controller solution

Hi JCD,

pogo wrote:
Well I am frustrated. I thought had "THE" solution for driving my
DC drive gear motors, but it appears that PWM from one device is
not the same as PWM from another. crap! ( or --- I am not asking
the right questions / lack of understanding, etc ... ) I thought I
had the perfect H-bridge device that I could control from either a
Acroname Moto 1.0 module or Servo Controller module until I
discovered (if I am interpreting the technical advice correctly)
that I can't get the speed control granularity I want that way ---
it sounds like I can only get near full speed that way.

When you use an H-Bridge to drive a permanent DC motor, you use PWM.
There are a couple of different types of drives (signed magnitude, or
locked anti-phase).

With signed magnitude, you have two control lines, one is normally used
as direction and one is normally used as the PWM control signal. The
percentage of on time (or off-time depending on the direction) in the
PWM signal controls the speed.

With locked-antiphase, a single control line is used. 50% duty cycle
means stopped. Greater than 50% means move in one way, and less than
50% means move in the other.

RC servos also use PWM, but quite differently. With a servo, the width
of the pulse typically vaires from 1 millsecond to 2 milliseconds. The
spacing between the pulses is less crictical and typically about 50
pulses per second are sent out. A 1 millisecond pulse tells the servo
to move to one extreme, and a 1.5 millsecond pulse tells it to go to
the middle, and a 2 millisecond pulse tells it to go to the other
extreme.

RC servos often contain a permannent DC motor (at least on the
inexpensive servos). You can remove all of the servo electronics,
connect the motors up to an H-Bridge and drive the motors just like
regular motors.

Here are my specs:

The brains of the robot will be a mini-PC board (ITA, etc.)
It will need to control a servo controller (like Parallax,
Lynxmotion, etc. for arms/grippers/camera pan/tilt)
It will need to control 2 reversable DC gear motors for
differential drive and steering

I want as few external boards as possible, for power consumption,
weight, etc. In other words, if I could get the servo controller to
also send signals to the motor speed controller that would great!

You'll probably need one a servo controller to drive the servos and an
H-Bridge to drive the DC gear motors.

Dave Hylands

.

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