A servo is a device which controls the angle of a piece of machinery. A motor attached to it controls a rotating shaft which changes the angle of the shaft. This device can be used to control the angle of any number of things, such as the flight surfaces of a model airplane, the direction of the wheels on a remote controlled car, or how far a door opens or closes.
Whenever specific angular control of an object is required, a servo can do the job. Using a computer, remote control, or a simple tuning knob, the angle held by the device can easily be adjusted. This is achieved by using electrical signals to apply force to the shaft so that it turns to the desired angle.
The motor controlling the servo receives electrical signals from the control device at regular intervals in the range of milliseconds. The length of each signal pulse tells the motor the required angle. For example, a pulse of three milliseconds might set the servo at an angle of 120 degrees. A pulse of four milliseconds might increase the angle to 150 degrees. The manufacturer of each one determines the exact configurations, but the principal is the same.
Servos can be used in any number of electronic devices. A robotic arm would need one servo at each joint. Each one could be controlled individually, so that by controlling the angle of each servo, the arm could be moved in a great range of motion. The steering of a remote control car would only need one servo for the front tires if they moved at the same angle, or might need a separate one for each tire if the car were designed to spin in place.
Servos are relatively energy efficient, because the amount of force used by the motor is directly proportional to the amount of resistance it receives. If the control surfaces of a remote controlled plane are in a neutral position, there is relatively little force required in trying to move them, so the device will only use a small amount of electricity to keep them in place. The servo conserves electricity by working only as hard as it needs to. This is particularly important for battery operated machines. It means that the batteries can be smaller and lighter, and that they will have to be changed less frequently.