Using hobby servos

Hobby servos are used as the main actuators in Opsoro robots. These motors originate from the RC world, where they are used to control model cars or airplanes. These actuators have the advantage of being inexpensive and relatively easy to control. However, when using these actuators, some things need to be kept in mind:

  • They have 3 wires: ground, 5V and signal. The servo will only work if these wires are connected correctly. The colors of these wires can vary. Common color codes include:

    • ground (brown), 5V (red), signal (orange)
    • ground (black), 5V (red), signal (white)
  • Hobby servos have a limited range of motion. Usually, they are restricted to 180° of rotation. Beyond that, they hit a plastic tab inside the servo. These actuators cannot be used for continuous rotation, but they can be told to rotate to a certain position and stay in that position.

  • To control them, a pulse-width modulation signal needs to be sent. The length of the pulse determines the angle of the servo:

    • 500μs corresponds to minimum position (usually -90°)
    • 1500μs corresponds to neutral position (0°)
    • 2500μs corresponds to maximum position (usually +90°).
  • The gearbox of the servo is internally connected to a potentiometer, which is used to sense the position of the servo. As a result, it is important to first put the servo in neutral position before connecting a mechanism to the output shaft. Not doing so can result in unexpected behavior, such as the servo destroying itself because it cannot rotate further.

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