TABLE OF CONTENTS The amplitude of physical variables are converted into measurable parameters by sensors. The measurable parameters can be a visual indication, or an electrical signal which can be used as an actuator control signal or as a signal to a controller. Many sensors do not have a linear relationship between physical variable and output signal, and are temperature-sensitive. The output signals need to be corrected for the nonlinearity in their characteristics or conditioned for transmission, so that the necessary valves or actuators can accurately corrected for variations in the measured variable in a process control system. Signal conditioning refers to modifications or changes necessary to correct for variations in a sensor's input/output characteristics.
In the case of process control, the accuracy of transmission of the value of the variable is very important. Any errors introduced during transmission will be acted upon by the controller, and degrade the accuracy of the signal. Control signals can be transmitted pneumatically or electrically. Electrical or optical transmission are now the preferred methods, due to the following characteristics of pneumatic transmission: inflexible pluming, high cost, slow reaction time, limited range of transmission, lower reliability, lower accuracy, and increased requirements of the control systems. Electrical signals can be transmitted in the form of voltages or currents, as digital signals, and as wireless signals. Electrical signals can be converted to light signals and transmitted optically. Unfortunately, the terms transducer, converter, and transmitter are often confused and used interchangeably.
The choice of a...
