There are several types of servomotor differing in size, control method and timing requirements. A typical servomotor has three terminals. The positive and 0 V terminals are connected to the supply. The third terminal is connected to a pulse generator, which delivers a stream of pulses of fixed length. Typically, the pulse length is between 1 ms and 2 ms and the pulse frequency is 50 Hz. The pulse length determines the angle through which the shaft turns and its direction. For example, a pulse length of 1 ms makes the shaft turn to the left (anti-clockwise) as far as it will go. A 2 ms pulse makes it turn as far right (clockwise) as it can turn. A pulse of average length, 1.5 ms, makes it turn to a central position.
Often a servo is limited to turning 45° on either side of its central position, but other types can turn 90° either way. Yet others are able to turn a complete circle. Consult the data sheets before buying, and when programming the motor.
Servomotors are connected to the moving parts of a robot by a variety of horns and discs which fit on to the output shaft. Kits containg an assortment of such devices are often supplied with the motor.
Servomotors are convenient to use for steering applications as they can be put into the 'straight ahead' position at the beginning of a program by sending a train of 1.5 ms pulses for about 200 ms. This gives the motor time to reach the central position from its previous (unknown position).
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