> I need to control a small valve. It will need to be constantly adjusted
> back and forth to maintain a precise pressure within the tube it will
> be mounted.
>
> I figure I have 3 basic options: Servo motor, stepper motor, and
> gear-drive motor (forward & reverse movement)
>
> Range of motion would be <= 90 degrees.
>
> I know a little about servos. Have done some programming on them. The
> others, no experience or knowledge.
>
> What would be the easiest and least expensive, programming and hardware
> wise?

My first knee jerk reaction is to use an ordinary DC motor geared down.
This is driven via PWM thru an H bridge to provide continuously variable
torque in both directions.  A software PID control loop then regulates
pressure with the motor drive PWM value as its output.

I've done something similar, except my motor was controlling the throttle
of an engine and the PID controller was regulating the engine speed.
Floating point definitely helps.  I had 100Hz update of the motor PWM
value being computed in a 16F876.  Worked very nicely.


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