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The potentiometer is perhaps the most common position sensor. This type of sensor is basically a fixed resistor with a movable tap that allows the amount of resistance between the tap and either end of the resistor to provide a portion of the total resistance (0-100%). ill. 1 shows an example of the linear potentiometer and one can see the main parts are the two terminals of the fixed resistor and the center tap. The center tap is also called the wiper, slider, or tap. Potentiometers are sized by the amount of resistance that is available between the two fixed terminals.
Above: ill. 1: A linear potentiometer with a diagram of fixed ends and wiper.
The reason the potentiometer is so usable is that it's fairly linear along its entire length. The resistor is usually made of fine resistance-type wire that is tightly wound around an insulating core. The slider makes contact with the wire and provides a means to alter the amount of the total resistor that is used. e.g., if the total resistance is 1000 Ω, and the slider is positioned at a point that is the center of the resistor, the amount of resistance between either end and the slider will be 500 Ω. If the slider is moved 75% of the way toward one end, the amount of resistance between the slider and one fixed end will be 750 Ω and the amount of resistance between the slider and the other end will be 250 Ω.
The linear potentiometer is usable in applications where the amount of linear movement must be measured accurately. In many of the previous applications of sensors and transducers, such as the low-pressure sensor whose diaphragm moves a small amount, a sensor like the linear potentiometer is used to measure the movement. The linear potentiometer is useful with the linear part of the rack and pinion or ball screw mechanism. Another application for the linear potentiometer is on plastic injection molding machines where the travel of the movable section of the mold must be accurately measured and converted to a position. The linear potentiometer in this application may be over 48 in. long, since the amount of travel will be up to 48 in. In this application, the linear potentiometer is fixed and the movable center tap is connected to the movable part of the mold so that it will move with the mold as it opens and closes and continually indicates the position.
