12.1. Introduction

Since the end of the 1980s, many articles have appeared in scientific [BER 87], [GIA 86] and technical [BLA 87], [JOR 87] books using the term "intelligent", a word often associated with sensors, transmitters, actuators and instrumentation. In this chapter, we will define an intelligent sensor as the linkage of one or several measurement chains to a computing "machine". Its main function is providing reliable, useful information. However, sometimes we read that the sensor is the weakest link in the measurement chain.

As shown in Figure 12.1, we remember that the purpose of a sensor is essentially to provide information, that is, to measure a variable and to communicate representative information of the value of this variable. A sensor is always part of an action/reaction loop:

• This action/reaction loop may be "closed" in the technological sense through a certain number of components such as controllers, calculators, Programmable Logic Controllers (PLC) or actuators. The sensor is then an element in an automation loop that provides a representative signal of a physical variable: for example, it provides a signal to a regulator or to a PLC that will itself provide a command signal to be transmitted to an actuator. In the automotive field, a typical example is that of a rotation speed sensor placed on each of the wheels which transmits this rotation "speed" information to the central Anti-Breaking System (ABS), which makes the "right" decision by comparing the rotation speeds of the same axle.