Satellite TV and internet uses geostationary satellites--satellites that are always in the same position in the sky. A signal is bounced off the satellite from a broadcasting station down to the satellite dish at the consumer's house. This satellite dish doesn't need to move since the satellite doesn't. Because different services use different satellites, some enthusiasts use satellite dishes controlled by servo motors. These dishes can be pointed at any satellite in the sky.
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Stepper Motor Basics
Most movable satellite servos use stepper motors. A stepper motor is a motor which rotates in small steps. Often, it takes more than 100 steps for a stepper motor to make one rotation. Because of the small size of the steps, a satellite dish can be precisely adjusted to face in any direction.
How Stepper Motors Work
Stepper motors have a metal gear in the middle attached to the rotor. Around this gear are several equally spaced electromagnets. Each electromagnet also has teeth like a gear, but these teeth don't actually touch the gear in the middle. When one of the electromagnets is turned on, it pulls on the gear. The teeth of the gear line up with the teeth of the magnet, holding the motor in position.
Turning the Motor
When the teeth of the gear and the first magnet line up, the teeth of the second gear are slightly out of sync with the magnet. When the first magnet is turned off and the second one is turned on, the rotor moves slightly so that it lines up with the second magnet. When the third magnet is turned on and the second one is turned off, the motor takes another small step forward. The process continues until the motor is in the desired position. As the motor turns, the satellite dish turns.