Physical Principles of Sensing

Abstract

Since a sensor is a converter of generally nonelectrical effects into electrical signals, one and often several transformation steps are required before the electric output signal can be generated. These steps involve changes of types of energy or physical properties of materials, wherein the final step shall produce electrical signal of a desirable format. As it was mentioned in Chap. 1, generally there are two types of sensors: direct and complex. A direct sensor is the one that can directly convert a nonelectrical stimulus into electric output signal. Many stimuli cannot be directly converted into electricity, thus multiple conversion steps would be required. If, for instance, one wants to detect displacement of an opaque object, a fiber optic sensor can be employed. A pilot light beam (excitation signal) is generated by the light emitting diode (LED). Then the light flux enters the optical fiber and propagates through it, then exits toward the object and is reflected from its surface. The reflected photon flux enters the receiving optical fiber and propagates toward a photodiode, where it is detected to produce electric current representing a distance from the fiber optic end to the object. We see that such a sensor involves transformation of electrical current into photons, propagation of photons through some refractive media (the fiber), reflection from the object, propagation again through the fiber, and conversion back into electric current. Therefore, such a sensing process includes two energy conversion steps and also manipulation of light.