I am looking for a simple way to reduce the speed of my Ray Allen Trim servos.

 

Jeff,
I tested a variable power resistor, but reducing the voltage significantly reduces the power of the trim motor, and it tended to stall/not start well before I’d reached a 50% speed reduction. I consulted my nephew who’s well into high school robotics competition and he suggested a pulsed motor driver. This is a relatively simple electronic circuit that uses variable resistors to control the power output pulse width and duration. We made up a breadboard sample and were able to control the Ray Allen servo over an order of magnitude (factor of ten) speed range with less than a 20% variation in servo output power.

 

I installed a micro switch on the flap actuator to operate a pitch trim switching relay and wired it such that the trim motor is connected directly to ground when the flaps are moved out of the full –up position, and is grounded through the pulsing circuit when they are fully up. (Note that this circuit is designed to work on the ground side of the trim motor, rather than the power input side.) This gives me full speed authority in the landing configuration and reduced pitch trim authority in cruise configuration. The relay is really not needed; you could wire this circuit directly through the flap position sensor switch, but my logic was that the relay, locate with the other trim control components, provided a more “fail safe” system because the system reverts to “high speed” if any part of the sensing circuit wiring fails.

 

I could fax you a diagram of the circuit, but better if you get it directly from the source, as the explanation of how it works is useful. See Bill Davies’ “Electric Motors and Mechanical Devices for Hobbyists and Engineers,” page 105. Mr Davies also diagrams a more complex bi-directional motor controller, but the above simpler circuit works just fine to control the “ground.”

 

Contact me off-line if you need additional information.

Bob Pastusek