[TowerTalk] Prop Pitch Power Supply and Control Circuit

[Kipton Moravec]

On Sun, 2008-03-16 at 13:50 -0400, Bob Maser wrote:
> I am in need of a schematic for providing an adjustable 24VDC for a small 
> prop pitch.  Also need a circuit for controlling the direction other than 
> selsyn motors.  I understand that some people have used a Hall effect device 
> and a magnet mounted on the PP motor shaft.
> 
> Bob W6TR 

I am rather new, but I do not understand what you are asking for.

First I do not know what a "small prop pitch" is. I am guessing you are
adjusting the pitch of a propeller, but that does not make sense on a
tower talk board.

DC motor control in robotics is usually done with an H-Bridge and a
motor with a built in quadrature encoder. You count the pulses in
hardware to tell where you are. You use the H-Bridge and Pulse Width
Modulation (PWM) to move the motor in the direction and speed you want.
Using a PID Algorithm you can optimally (or near optimally) move the
motor where you want.

An encoder gives higher resolution on the motor shaft than a hall effect
sensor. Usually a hall effect sensor can give you 1 or 2 counts per
motor revolution. An encoder can give anything from 8 (home made) to
1024 counts (some Pittman motors) per revolution.

There is a lot on motor control in the robotics clubs like www.dprg.org
or www.seattlerobotics.org . We use small DC motors for controlling
robot movement and speed. Change the polarity of the motor with the
H-bridge and it changes direction, change the on/off ratio of PWM and
you change the speed.

The main issues you have to worry about with a controller, is what is
the maximum current of the motor? You can measure it by finding out what
is the current at 24V when you hold the shaft to keep it from turning?
How accurately do you need to position the prop?

I prefer to use a small microcontroller with built-in PWM and up/down
timers. I can program it to do exactly what I want.

Kip

-- 
Kipton Moravec KE5NGX
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain