This arrangement is known as a shaft encoder. To implement the feedback loop you will need to fix the shaft of a rotary potentiometer to the foundation of the motor while allowing the potentiometer’s body to rotate freely with the motor shaft. This sort of a ‘feedback’ system is also used in commercially available servo motors. Voltage applied to the motor terminals will be so as to cause the shaft to turn to reduce ‘positional error’ to zero. This ‘current position’ will be compared against an ‘desired position’ and a ‘positional error’ will be generated. In order to control the shaft position of a DC motor (and thereby convert it into a servo motor), you need to be able to ‘encode’ the position of the shaft. This can be very useful when we want to move a control surface such as a rudder or a thruster to a particular position. Servo motors on the other hand, allow us to control the position (or angle) of the motor output shaft. In a DC Motor, speed control can be achieved by varying the terminal voltage but position control of the shaft is very difficult to implement. The problem with DC motors is that when they have a voltage applied to their terminals, they tend to rotate forever in a particular direction, stopping or reversing the motor can only be achieved by cutting off electric supply or reversing polarity. In most motors, like the one shown below, the gear train scales up the torque of the motor by using a reduction gearing that outputs a much higher torque (albeit at the cost of a much reduced output RPM). The torque that is generated at the output shaft can be scaled up or scaled down by using a gear train. DC Motors can be made to turn either clockwise or counter-clockwise by changing the polarity of the voltage applied to their terminals.
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