Servo Information and Modification
for controllable continuous rotation (as a real small motor) for cannon rotation
by Donny Ciccimaro
What is an R/C Servo
The RC stands for Radio Control, which is what they are designed for. The requirements for R/C Big Gun Combat are quite demanding. The servo needs to be small, lightweight, strong, dependable, rugged, versatile, low cost, and easy to wire up. With the introduction of smaller inexpensive micro controllers, these servos work great for many demanding Big Gun functions as well.
Servo Theory of Operation
There are three wires, two for power, and one for the signal. Connect the red wire to 4.8 - 6 volts dc and the black wire to ground. Connect the yellow, or white signal wire to a positive going pulse, from 1ms to 2 ms to reference positioning. The output shaft will rotate to a position proportional to the input pulse width. The pulse off time can be from 10ms to 20 ms and is not critical. An on time of 1.5ms is the mid value, and will position the servo to its center of travel. The range of motion available is 90 degrees. The range can be extended to 180 degrees of travel by changing the pulse width by .5ms to 2.5 ms, but care must be taken to not exceed the physical limits of the servo. Although the servo and micro controller combination is looked at as an open loop system because the controller doesn’t know whether the servo has changed position. The servo feedback from a potentiometer connected to the output shaft, and is considered a closed loop system locally. The servo requires very little attention from the microcontroller yet continually monitors position and resists external forces to change its position.
Modification Procedure
Resistor Modification, wire colors are for Hitec, others may differ.
Modified Servo
General Information
This modification will give the servo the ability to turn continuously. You will be able to control the speed and direction, but you will no longer be able to control the absolute position of the output shaft. In general older servos that no longer position that well usually make great drive servos as the potentiometer, which is the part that usually wears out, is not used. This modification can’t be reversed, so make sure the servo is operating properly before you start. When done you will have created a durable, powerful, and easy to control drive motor. Just follow the step by step instructions in the included diagrams.
After the modification has been completed the servo can easily be controlled via the pulse train sent to it from the radio transmitter. The on time of the pulses are between 1.0mS and 2.0mS as before, but they are now used to control the speed and direction of rotation, as opposed to controlling the absolute position. A pulse of 1.0mS will try to position the output shaft full in one direction, but because the output shaft potentiometer is no longer connected, the servo will not be able to correct, so it keeps turning. A pulse of 2.0mS will try to position the servo in the other direction with similar results. A pulse of 1.5mS will try to position the servo in the center of travel, which is where the trimmer pot is adjusted, so no correction is needed, resulting in no motion of the output shaft. The speed is proportional to the amount the pulse varies from the stop value. The further the pulse is from 1.5mS the faster the rotation.
Checkout and Testing
Apply a 1.5mS pulse to the servo (you can use your transmitter and receiver if you don’t have a signal generator) and adjust the trimmer pot towards the center until the motor stops spinning. The servo stop value is now set. By changing the pulse length to 1.55mS the motor will slowly turn. The farther from 1.5mS you go, the faster the motor will turn. Changing the pulse width below 1.5mS will result in the motor turning in the opposite direction. When using fixed value resistor pairs, the only difference is you will find the point the motor stops turning by adjusting the pulse servo width with the trimmer on your radio.
What can you do with it?
You can use it as a small motor or whatever you can dream of that requires a small motor that can be easily controlled with your radio. It also makes a great cannon rotation servo. At least you learned how your servos work.