Continuous rotation servos are popular with robot builders and in other applications where simple forward and reverse rotational control is needed. Standard radio-controlled (RC) aeroplane-style hobby servos are limited to a rotation angle of approximately 180 degrees. Modification of a standard servo to allow continuous 360 rotation is possible by making some changes to the internal gears. Using a modified servo motor in place of a DC motor adds some ability to control the speed and direction of the rotation without the need of a special circuit to control the analogue voltage to the motor. Simple pulse-width modulation (PWM) control is used instead, which is supported by a large number of available inexpensive control boards and wireless RC controllers.
- Skill level:
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Things you need
- Small Phillips-head screwdriver
- Micro-size Phillips-head screwdriver
- Table vice
- Wire cutter
- Small file
Locate the small screw on top of the servo gear holding the servo horn in place. Use the small Phillips-head screwdriver to turn the screw counterclockwise until the screw is removed. Put the screw and servo horn in a safe place for later use.
Locate the four micro-size screws on the bottom of the servo case. Use the micro-size Phillips-head screwdriver to turn each screw counterclockwise until each screw is removed. Put the screws in a safe place for later use.
Open the servo casing to expose the gears.
Remove the centre gear. The centre gear is in the way, so it needs to be removed for the next step. Put the gear in a plastic bag to avoid getting dirt on the grease that is stuck to the gear.
Remove the output gear. This is the tallest gear.
Cut the plastic stop tabs off the back of the output gear using the wire cutters. Use the small file to remove any excess plastic that might cause the servo gear to slow down or stop spinning.
Place the gear in the bench vice to hold it in place. Avoid damaging the gear by overtightening. Drill out the slot in the gear slightly larger than the pot that holds the gear in place.
Place the top gear back on the pot shaft. Take the centre gear out of the plastic bag and place it back in the middle next to the top gear. Align the teeth of the two gears.
Place the servo housing back on top of the gears and tighten the four micro-size Phillips-head screws by turning them clockwise with a micro-size screwdriver.
Replace the servo horn back on top of the servo and tighten the small Phillips-head screw to the top gear by turning it clockwise with a small screwdriver.
Tips and warnings
- Some servo manufacturers use servo gears with teeth that do not extend 360 degrees around the gear. These servos cannot be modified to continuous rotation. Mainstream servo manufacturers typically do not take this shortcut when manufacturing their servos. Check with the manufacturer to make sure the servo has gears with teeth all the way around each gear.
- Following this procedure of servo modification will void all manufacturer's warranties. In addition, this procedure can cause damage to servo motors. Use common sense when working with any electrical device. Only proceed with this procedure if you understand these risks.
- This procedure of servo modification removes the braking capability. The servo will come to a stop only after slowing down in a manner similar to that of an inexpensive remote control car. Additional modification is required to be able to apply braking through PWM control of the servo.
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