Servo motors are a type of electromechanical actuators that do not rotate continuously like DC/AC or stepper motors, rather they used to position and hold some object. They are used where continuous rotation is not required so they are not used to drive wheels (unless a servo is modified). In contrast they are used where something is needed to move to particular position and then stopped and hold there. Most common use is to position the rudder of aircrafts and boats etc. Servos can be used effectively here because the rudder do not need to move full 360 degrees nor they require continuous rotation like a wheel. The servo can be commanded to rotate to a particular angle (say 30) and then hold the rudder there. Servos also employs a feedback mechanism, so it can sense an error in its positioning and correct it. This is called servomechanism. So if the air flow exerts pressure on rudder and deflects it the servo will apply force in opposite direction and try to correct the error. Say if you ask servo to go and lock itself to 30 degrees and then try to rotate it with your hand, the servo will try hard and its best to overcome the force and keep servo locked in its specified angle.
Servos are also used to control the steering of RC cars, robotics arms and legs.
Their are many types of servos but here we will concentrate on small hobby servos. Hobby servo has motor and its control mechanism built into one unit. They have 3 wire connector. One is for positive supply other for ground and the last one for control signal. The image below shows a common hobby servo from Futaba, its S3003.
Futaba S3003 wiring.
- RED -> Positive supply 4.8v to 6v
- BLACK -> GND
- WHITE -> Control Signal.
Controlling a Servo Motor.
Controlling a servo is easy by using a microcontroller, no external driver like h-bridge etc are required. Just a control signal is needed to be feed to the servo to position it in any specified angle. The frequency of the control signal is 50hz (i.e. the period is 20ms) and the width of positive pulse controls the angle.
Rread More: Servo Motor Control by Using AVR ATmega32 Microcontroller