كتيب الهندسة الكهربائية – الطبعة الثالثة / The Electrical Engineering Handbook – Third Edition

The Electrical Engineering Handbook – Third Edition:

Electrical Engineering: Sensors are critical components in all measurement and control systems. The need for sensors that generate an electronic signal closely followed the advent of the microprocessor and computers.

Together with the everpresent need for sensors in science and medicine, the demand for sensors in automated manufacturing and environmental monitoring is rapidly growing.

In addition, small, inexpensive sensors are finding their way into all sorts of consumer products, from children’s toys to dishwashers to automobiles.

Because of the vast variety of useful things to be sensed and sensor applications, sensor engineering is a multidisciplinary and interdisciplinary field of endeavor.

This chapter introduces some basic definitions, concepts, and features of sensors, and illustrates them with several examples.

The reader is directed to the references and the sources listed under Further Information for more details and examples.

There are many terms that are often used synonymously for sensors, including transducer, meter, detector, and gauge. Defining the term sensor is not an easy task; however, the most widely used definition is that which has been applied to electrical transducers by the Instrument Society of America (ANSI MC6.1, 1975): ‘‘Transducer—

A device which provides a usable output in response to a specified measurand.’’ 

A transducer is more generally defined as a device that converts energy from one form to another. Usable output can be an optical, electrical, chemical, or mechanical signal.

In the context of electrical engineering, however, an Ausable output is usually an electrical signal. The measurand is a physical, chemical, or biological property or condition to be measured.

Most but not all sensors are transducers, employing one or more transduction mechanisms to produce an electrical output signal.

Sometimes sensors are classified as direct or indirect sensors, according to how many transduction mechanisms are used.

For example, a mercury thermometer produces a change in the volume of mercury in response to a temperature change via thermal expansion. The output signal is the change in height of the mercury column. Here, thermal energy is converted into mechanical movement and we read the change in mercury height using our eyes as a second transducing element.

However, in order to use the thermometer output in a control circuit or to log temperature data in a computer, the height of the mercury column must first be converted into an electrical signal. This can be accomplished by several means, but there are more direct temperature sensing methods, i.e., where an electrical output is produced in response to a change in temperature.