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Standard Technologies of the Seattle Robotics Society

Hacking a Servo

Now that you understand how a servo works, the first thing that a eager engineering type will want to do is take it apart! Don't worry, this is completely acceptable behaviour in robotics. This section is going to describe how to take a R/C servo and make it into an excellent gearhead motor. The changes are quite easy to do, once you have seen the insides. This modification is known to work quite well on Futaba S-148 servos, which are commonly available.

The theory behind this hack is to make the servo think that the output shaft is always at the 90 degree mark. This is done by removing the feedback sensor, and replacing it with an equivalent circuit that creates the same readings as the sensor being at 90 degrees. Thus, giving it the signal for 0 degrees will cause the motor to turn on full speed in one direction. The signal for 180 degrees will cause the motor to go the other direction. Since the feedback from the output shaft is disconnected, the servo will continue in the appropriate direction as long as the signal remains.

The result of this is a really nice compact gearhead motor with built in electronics. The interface to this motor unit is a 1 wire control line, +5 volts for power, and a ground. All of this for around $15, which is an outstanding deal.

As for the details, there are actually only two modifications to make to the servo.

  1. Replace the position sensing potentiometer with an equivalent resistor network
  2. Remove the mechanical stop from the output shaft

Here are the steps. You will need a few supplies

The following steps will help you make the modifications.

Picture of servo gears

Picture of servo parts

(Click on picture for larger view)

Servo with top and gears removed

Disassembled servo motor

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Disassembled servo motor.

Picture of modified circuit boards

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An unmodified (left) and modified circuit board.

Picture of modified output shaft

(Click on picture for larger view)

An unmodified (left) and modified output shaft gear

The motor should now be able to turn all the way around. Connect a control horn, and carefully apply enough pressure to make the horn turn around. Feel for any mechanical problems, such as a gear catching on the cut off section of the tab. You should not feel any catching or resistance. It would be best not to play with turning the servo by hand too much. This device is not intended to be driven from the output shaft, and it may cause undo wear and tear on the servo motor.

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