POSITION SENSORS

Linear and rotary motion is used in many of the previous sensors to detect pressure, flow, level, and temperature. For this reason it's important to be able to measure linear and rotary position or distance moved. It's also important to be able to measure linear and rotary motion and distance for many positioning and robotic applications. This section will discuss the operation of linear and rotary potentiometers, linear variable differential transformers (LVDTs), magnetostrictive sensors, encoders, and resolvers that are all used to measure distance or position. It's also important to realize that the two main types of motion, linear and rotary, are virtually interchangeable with the use of ball screw mechanisms and rack and pinion devices. This means that if the output motion of a sensor is linear, it can be changed to rotary motion with a rack and pinion, and if the output motion is rotary, it can be changed to linear motion with a rack and pinion or a ball screw mechanism. This simple conversion allows a larger variety of transducers to be used to measure distance or position.

An example of a rack-and-pinion and a ball screw mechanism is shown in Figs. 1a and 1b. The ball-screw mechanism is shown in ill. 1a. From this diagram it's somewhat visible that the screw part of the ball screw looks like a threaded rod. The number of threads per inch will determine the amount of resolution the ball screw will have. When the ball screw is rotated, the ball bearings will rotate with the screw and cause the traveling portion of the mechanism to move in a linear motion. This is similar to having a threaded rod with a nut on it. If the nut is kept from turning and the rod is rotated, the nut will move back and forth along the threads of the rod.

ill. 1b shows the rack and pinion mechanism. From the diagram note that if rotary motion is used to turn the pinion gear, the rack will change this motion into linear motion. If linear motion is input into the rack, the pinion gear would transfer this motion into rotary motion.

Above: ill. 1 (a) A ball-screw mechanism that converts rotary motion into linear motion. (b) A rack and pinion mechanism used to convert rotary motion into linear motion, or to convert linear motion into rotary motion.