Servo - July 2015

Figure 1 shows three different servo sizes, from a large “quarter scale” servo (left) to a small nine gram servo (right). A standard sized servo (center) measures about 40 mm L x 20 mm W x 36 mm H, and weighs 40-50 grams. The cheapest (~$5-$10) servos use plastic gears and bushings. Better ($30+) ones use metal or “karbonite” gears and ball bearings for bigger loads and longer life.

Servos generally require 4. 5-6 volts to operate and consume most of the electric power in a robot. While most microcontrollers require very little electrical power and could run on tiny batteries, servos require larger batteries — either alkaline or rechargeable. The minimum practical power supply to drive one or two servos is three AA alkaline cells.

Servos are motorized gearboxes which move to a certain location or speed specified by a control signal. The focus of this article is on analog servos which are the ones traditionally used in R/C cars and airplanes, and, more recently, are used in hobby robotics. Digital servos will also work, but are often more expensive and consume more power than analog servos. They do offer extra features, but we won't get into those here.