Skip to content

Servo Basics

  • A servo is a device that takes in a commanded position and drives an actuator to that desired position.
  • The scope of this page is mainly limited to PWM input servos, but there are servos that accept other types of signals such as CAN bus.

PWM input

  • Many servos have a Pulse Width Modulation (PWM) input. These are signals that alternate between a "high" and "low" state and can encode information. Important terms to describe PWM signals are:
    • Frequency - number of pulses per second. Many servos expect 50 Hz.
    • Pulse Width - length of time that signal is "high". Servos usually encode the minimum rotation with a 1000 μs pulse width, and a maximum rotation with a 2000 μs pulse width. 1500 μs encodes the center (neutral) position.
    • Duty Cycle - % of time that signal is "high". Usually not used to describe PWM input for servos, in favor of pulse width. Duty cycle = pulse width * frequency
  • Consult the datasheet to see what voltage the device expects as the "high" logic level.
  • Some servos can accept a wider input range, for instance 500 μs to 2500 μs.

Wiring

  • PWM servos have 3 wires - power, ground, and signal.
  • Large servos can pull a substantial amount of current depending on their load. Always consult the datasheet to see what the peak current is, and make sure your wires, connectors, and power supply can handle it.
  • Many servos accept 5V power input. Some accept higher voltages, like 6V or 7.2V, and this can increase max torque and power consumption.
  • Long runs of signal wire can pick up noise from the environment. This can cause servos to twitch or make unexpected movements.

Mechanical Considerations

  • Make sure to install servos with the arm in the desired neutral position. Power the servo and "center" it by giving it a 1500 μs signal while installing to ensure that the neutral position is where you want it to be. Depending on your design, it may be difficult to adjust the position of the arm after the servo is installed.
  • Not all servos have the same range of movement. +/- 90 degrees is common, but you should always consult the datasheet to see what the minimum and maximum positions of your specific servo are.
  • Control linkages and hinges should be carefully considered for flight control surfaces. Poorly made linkages can have "slop" that can lead to unexpected behavior or flutter. POORLY MADE LINKAGES WILL RESULT IN CRASHES
  • This is a good resource for designing control linkage geometry: RCModelReviews Video