3 - Sensors and interfacing

Abstract

Sensors are the primary input element involved in reading physical quantities into a data acquisition and control (DA&C) system. They are generally used to measure analogue signals although the term `sensor' does in fact encompass some digital devices, such as proximity switches. This chapter discusses the attributes of sensors, actuators, signal conditioning, and digitization circuitry that have a direct bearing on software design. Most types of sensor operate in a purely analogue manner, converting the measurand to an equivalent analogue signal. The sensor output generally takes the form of a change in some electrical parameter such as voltage, current, capacitance or resistance. Sensor characteristics cannot be considered in isolation. Sensors are often closely coupled to their signal-conditioning circuits and one must, therefore, also take into account the performance of this component when designing a DA&C system. Signal-conditioning and digitization circuitry can play an important role in determining the characteristics of the measuring system as a whole. Although signal-conditioning circuits can introduce undesirable properties of their own, such as noise or drift, they are usually designed to compensate for inadequacies in the sensor's response.