Servos

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Combat robots use servomotors as actuators, positioning devices, and sometimes as drive motors. These servos are the same type used in remote-controlled devices such as model aircraft (to move flight control surfaces), boats and cars (for steering), as well as other hobbies. Most common radio-control receivers have built-in connections for several servos, allowing them to be remotely operated by moving the controls on the transmitter with very little setup required.

These devices take a pulsed electrical signal (usually PWM, or Pulse-width modulation), and respond by moving a lever, slide, or shaft to a specific position. This motion is usually over a range of rotation such as 90° or 180°. This motion can be used to operate flippers, lifters, self-righting mechanisms, and other features of the bot.

How to select a servo

In combat robotics, limiting factors for servos are weight, voltage, torque, and cost. For each servo you plan to use, obtain a specification sheet to find out whether it is suitable for the purpose.

  • Weight: A very durable servo with metal gears may be far too heavy for a small bot, but one with plastic gears may break quickly when the bot is struck or dropped.
  • Voltage: Many small hobby servos run on a limited range of voltages, often 4.8~6.0 volts. If your servo can't run directly from your bot's battery voltage, the receiver might need a battery eliminator circuit (BEC) module that generates enough power to run both the receiver and the servo(s).
  • Torque: Usually stated in kg-cm or ounce-inches. The servo needs to have enough strength to accomplish its task in the bot.
  • Cost: High quality servos cost more.

In addition, here are some other factors you should consider.

  • Dimensions and mounting method
  • Rotational speed and response time
  • Signal required (some servos may have more features with a digital signal)
  • Availability (you will want to have spares on hand in case of damage)
  • Continuous rotation ability (be careful not to order one of these if you need only a short range of motion)

ServoDatabase.com lists specifications for thousands of models.

How to connect and use

Nearly all servos have a common type of 3-wire connector, although the wire colors vary:

  • Ground wire – Black (or Brown)
  • DC power wire – Red (or Orange)
  • Signal wire – White (or Yellow)