A servo consists of a control board, a motor and a gearbox. You send a signal with a pulse every 20ms to the control board, the pulse varies from about 1ms to 2ms and the duration of this pulse determines what position the servo moves to. When a pulse is received the motor starts to move. The final gear in the gearbox is connected to a potentiometer so as the motor moves the resistance on this potentiometer changes and that in turn varies a voltage which is fed back to the control board to determine the servos position.
The gearbox will have a hard stop on the final gear which is supposed to mechanically stop the servo going past + or – 90 degrees so it will be the first thing you need to remove. After that you will locate a step inside the gear which matches a notch on the potentiometer, you need to remove the step in the gear so that the gear can spin freely on the potentiometer.
In this video the guy is able to cut a groove for a screw driver in the potentiometer and leave it adjustable from the outside, if you can do this definitely do but with smaller servos this is not possible. In the case of smaller servos you have two options
- Power the servo and give it a signal for the central position, the motor should start to spin. Adjust the potentiometer until the motor stops spinning. If you change the signal you should get speed control in either direction. When you are happy with the position glue the potentiometer in place so that it can no longer spin.
- An alternative method is to remove the potentiometer wires from the control board and solder two resistors in their place. The resistors should be equal, I usually use 1k Ohm resistors. After this you should have speed control of you motor by varying the servo signal. With this method you may have difficulty fitting the servo back together so try to plan for that, mount the resistors externally if you have to.
Once you rebuild your servo you should now have a continuous rotation servo.
If you have a motor e.g. a SIKU motor gearbox but no controller you could remove the control board from a servo (maybe one with broken gears), modify the board as above and then wire it to your motor to give you speed control. One thing to be careful of is if the motor in the servo is smaller than the one you are wiring to it may eventually damage the control board by drawing too much current.
I hope some of you find that useful and if you have any questions shoot over to the RC Tractors Forum and post them there. I am working on a Components Page for the site and it is taking me a long time to get round to everything but if you have suggestions for that let me know on the forum too.
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