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.
68 SERVO 07.2015
Analog Servos
for
Robotics
Figure 1.
By Eric Ostendorff