AC Servo Motor | Stepper Motor

AC Servo Motor | Stepper Motor

Servos are controlled by sending an electrical pulse of variable width, or pulse width modulation (PWM), through the control wire. There is a minimum pulse, a maximum pulse and a repetition rate. A servo motor can usually only turn 90° in either direction for a total of 180° movement. The motor's neutral position is defined as the position where the servo has the same amount of potential rotation in the both the clockwise or counter-clockwise direction. The PWM sent to the  motor determines position of the shaft, and based on the duration of   the   pulse   sent   via   the   control   wire the  rotor will turn to the desired position. The servo motor expects to see a pulse every 20   milliseconds (ms) and the length of the pulse   will  determine  how  far  the  motor  turns.  For   example, a 1.5ms pulse will make the motor   turn  to  the  90°  position.  Shorter  than  1.5ms 

moves it to 0° and any longer than 1.5ms will turn the servo to 180°, as diagramed below.

Variable Pulse Width Control Servo Position

of a servo motor (L) and an assembled servo (R)

When these servos are commanded to move, they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is called the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.