SOLID Learning Robot – Servos

“ANALYSIS: Servos provide a more flexible option that is very easy to control in large numbers using digital electronics, best suited to mobile lightweight robotic designs where the servo’s rigid outer shell can be incorporated as a structural element of the robot itself.

A sample RC hobby servo with a six-horn servo arm attached

The most flexible type of actuator for our SOLID Learning robots is the Servomechanism (Servo), which can be found in many radio-control toys and is available through RC hobby centers and online electronics suppliers.

Exploded servo depiction revealing internal gears, electronics and motor

Available in a variety of sizes and formats, servos integrate control electronics together in the same package as a motor and gearbox mechanisms to simplify control and use.

Kondo KHR-1 robot body using multiple servos for many degrees of freedom in movement

The rigid outer case of the servo allows it to be used as a structural element in robot designs, which can use many servos to achieve multiple degrees of freedom in movement. Servos used in RC hobbies are small, powerful, inexpensive and readily available.

Pulse-Width Modulation (PWM) servo control

Most servos are configured for precise positioning within a 180 degree arc (half a circle) based on digital control using pulse-width modulation (PWM). Digital signals direct the servo to move to a specific angle based on the width of an electronic pulse transmitted over the control channel. The built-in electronics translate a single data channel to bidirectional control of the servo.

Servos can be converted for continuous operation, removing exact position sensing within the 180 degree arc and translating the same PWM signal to achieve precise control of speed rotating backwards and forwards bidirectionally. When acquiring servos from an electronics supplier, it is important to specify whether a standard or continuous rotation model is desired. Standard servos can be permanently modified to continuous operation if necessary.

Like stepper motors, servos have no continuous ability to operate from power alone. Servos use their onboard control circuits to translate a single PWM data stream into forward or backward movement to a specific position (standard 180 degree mode) or at a specific speed (continuous mode). Servos draw less current the slower they are operated, by require a constant voltage over their full range of deflection (or speed) since all control comes only from manipulation of the PWM signal.

A mixed servo assortment controlled by a two-wire I2C servo controller

Each servo consumes only a single control channel from the electronics, but servo controllers are made to handle many servos using an even more limited number of control channels (32, 64 or more servos using two or more connections) for more complex robots. Some controllers are even chainable to achieve control for over 990 servos using only two channels from the I2C control electronics.

Summary for Servos:

  • Price: Moderate ($11 and up) with no additional components required
  • Availability: Sourced from some RC toys, RC hobby shops and electronic suppliers
  • Control: Can be directly controlled by digital electronics
  • Operation: Specific position within a 180 degree arc or specific speed in continuous rotation
  • Outputs: One (1) for each servo, Two (2) for a multi-servo I2C driver
  • Caveats: Cannot operate without logical PWM control

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