Servos

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.

ServoDatabase.com lists specifications for thousands of models.

Weight

In combat robotics servos with metal gears are common, because ones with plastic gears may break too easily when the bot is struck or thrown. Plastic-gear servos are most likely to be found on bots in weight classes lower than 150g.

Voltage

Traditional standard hobby servos run on a limited range of voltages, usually 4.8~6.0 volts, but you should choose servos that function on higher voltages (8V or 12V for 2S and 3S batteries are now common). This allows you to power the servo directly from battery voltage with needing a battery eliminator circuit (BEC).

Torque

Usually stated in kg-cm or ounce-inches. The servo needs to have enough strength to accomplish its tasks in the bot. For example, if you the servo will be self-righting your bot (flipping it right-side up), make sure it is strong enough to do that, even with the weight of another bot on top.

Cost

Stronger and higher-quality servos cost more. There's no shame in starting out with what you are willing to pay for.

Other factors

• 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)

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)