Position and Motion Sensing
Position and Motion Sensing
Basic position definitions
Many industrial processes require both linear and angular position and motion measurements. These are required in robotics, rolling mills, machining operations, numerically controlled tool applications, and conveyers. In some applications it is also necessary to measure speed, acceleration, and vibration. Some transducers use position sensing devices to convert temperature and/or pressure into electrical units and controllers can use position sensing devices to monitor the position of an adjustable valve for feedback control.
Absoluteposition is the distance measured with respect to a fixed reference point and can be measured whenever power is applied.
Incrementalposition is a measure of the change in position and is not referenced to a fixed point. If power is interrupted, the incremental position change is lost. An additional position reference such as a limit switch is usually used with this type of sensor. This type of sensing can give very accurate positioning of one component with respect to another and is used when making master plates for tooling and the like.
Rectilinear motionis measured by the distance traversed in a given time, velocity when moving at a constant speed, or acceleration when the speed is changing in a straight line.
Angularposition is a measurement of the change in position of a point about a fixed axis measured in degrees or radians, where one complete rotation is 360° or 2π radians. The degrees of rotation of a shaft can be absolute or incremental. These types of sensors are also used in rotating equipment to measure rotation speed as well as shaft position and to measure torque displacement.
Arc-minuteis an angular displacement of 1/60 of a degree.
Angular motionis a measure of the rate of rotation. Angular velocity is a measure of the rate of rotation when rotating at a constant speed about a fixed point or angular acceleration when the rotational speed is changing.
Velocity or speedis the rate of change of position. This can be a linear measurement, i.e., feet per second (ft/s), meters per second (m/s), and so forth, or angular measurement, i.e., degrees per second, radians per second, rate per minute (r/m), and so forth.
Accelerationis the rate of change of speed, i.e., feet per second squared (ft/s2), meters per second squared (m/s2), and the like for linear motion, or degrees per second squared, radians per second squared, and the like, in the case of rotational motion.
Vibrationis a measure of the periodic motion about a fixed reference point or the shaking that can occur in a process due to sudden pressure changes, shock, or unbalanced loading in rotational equipment. Peak accelerations of 100 g can occur during vibrations which can lead to fracture or self destruction. Vibration sensors are used to monitor the bearings in heavy rollers such as those used in rolling mills; excessive vibration indicates failure in the bearings or damage to rotating parts that can then be replaced before serious damage occurs.