Servo - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 242

Introduction to Servo Motors

Most motors are simple devices that turn on their axle when current is supplied.
When a motor turns, the user generally has no idea about the angle or speed;
to get this information, sensors are required. Servo motors differ by knowing
exactly the angle that they are at and adjusting their position as required. Most
servo motors cannot turn 360 degrees; instead, they are often limited to a range.
Most servo motors have 180 degrees of rotation, as shown in Figure 14-1.

90 °

0 ° 180 °

Figure 14-1: Servo motor movement

To know the exact position, servo motors can use a wide variety of techniques.
Most use a potentiometer, using electrical resistance to understand how far the
arm has turned, while more advanced systems use a coded optical wheel to get
precise information.
Servo motors were originally designed in the dark times of war. They were
used in radar and anti-aircraft artillery during World War II. Radar requires
the angle of the emitter and receiver to be known because the position of the
aircraft needs to be calculated and displayed on a screen. Anti-aircraft artillery
needs to be placed at a precise angle depending on the results of the calculation,
and servo motors could place heavy loads at the right angle much faster than
humans and with more reliability.
Although it might seem strange to have a motor that does not make complete
turns, servo motors have a wide range of uses. They are used in industrial systems
to open and close valves; they are still used on radar or tracking equipment to
point a device in the right direction with a high level of precision; and robots
use servo motors to keep arms at a precise angle, while providing enough force
to keep the arm in place with a high load. Hobbyists making remote controlled
vehicles are familiar with servo motors because they are used to control steering.
When the front wheels of a car turn left or right, this is a servo-motor acting,
keeping the direction in place despite resistive force.