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Servo Motor 
A Servo Motor is a cheap rotary actuator used for radio control and small-scale robotics.
A servo motor is normally connected using just 3 wires.
The angle of the servo is determined by the width of the PWM signal that is applied to the control wire. Most servos expect to see a pulse every 20 ms. The angle is calculated as the fraction of the pulse width
with the pulsewidth , the current angle .
For most RC servos the exact refresh rate is irrelevant as long as it is in a range of 40 Hz to 200 Hz.
A pulsewidth of will turn a servo with a maximum angle to ° (neutral position).