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Physical Principles of Sensing


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 where the final step must 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 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 (excitation) light is generated by a light emitting diode (LED), transmitted via an optical fiber to the object and reflected from its surface. The reflected photon flux enters the receiving optical fiber and propagates toward a photodiode where it produces an electric current representing the 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, reflection, and conversion back into electric current. Therefore, such a sensing process includes two energy conversion steps and a manipulation of the optical signal as well.


  • Light Emit Diode
  • Electric Charge
  • Thermal Radiation
  • Seebeck Coefficient
  • PVDF Film

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  1. 1.

    The prefix tribo- means “pertinent to friction”.

  2. 2.

    A Russian physicist of German extraction Georg Wilhelm Richmann (1711–1753) was killed in St. Petersburg during a thunderstorm experiment when a ball lightning having a size of a fist jumped from the electrometer and struck him in a forehead.

  3. 3.

    Now, the U.S. pennies are just copper-plated (2.5%), but till 1982 they contained 95% of copper.

  4. 4.

    Excluding superconductors, which are beyond the scope of this book.

  5. 5.

    Since resistance of a metal increases with temperature, a tungsten filament in a light bulb acts as a self-regulator of temperature, so the filament does not burn out. When temperature increases, the resistance goes up and the current drops, causing the temperature to come down. If α for metals were negative, the filaments would instantly burn out and we would not have electric lights.

  6. 6.

    See Section 16.3.1.

  7. 7.

    The term is based on a similarity between the electrode shape and fingers (digits) of two human hands grasping one another.

  8. 8.

    The complete set of coefficients also includes shear stress and the corresponding d-coefficients.

  9. 9.

    The electrode, not the crystal area! Piezo induced charge can be collected only over the area covered by the electrode.

  10. 10.

    Remember, a piezoelectric sensor is an AC device, so it will not respond to a constant or slowly changing force.

  11. 11.

    This is a misnomer as the prefix ferro, meaning iron, is used despite the fact that most ferroelectric materials do not have iron in their lattice. It is used by analogy with ferromagnetics.

  12. 12.

    A Thompson effect was discovered by William Thompson around 1850. It consists of absorption or liberation of heat by passing current through a homogeneous conductor which has a temperature gradient across its length. Unlike in the Joule effect, the heat is linearly proportional to current. Heat is absorbed when the current and heat flow in opposite directions, and heat is produced when they flow in the same direction.

  13. 13.

    Or perhaps the same material in two different states, for example, one under strain, the other is not.

  14. 14.

    Joule heat is produced when electric current passes in any direction through a conductor having finite resistance. Released thermal power of Joule heat is proportional to squared current: P = i 2/R, where R is resistance of a conductor.

  15. 15.

    There is an anecdote about the American physicist R. W. Wood (1868–1955). A theatrical director from New York asked Wood to invent a mysterious sound effect for a play about travel through time. Wood built a huge organ pipe (sort of a whistle) for the infrasonic frequency of about 8 Hz. When during a dress-rehearsal Wood activated the pipe, the entire building and everything in it started vibrating. The terrified audience ran out to the street, feeling uncontrollable fear and panic. Needless to say, the pipe was never used during performances.

  16. 16.

    More precisely, not “by him” but rather “for him”.

  17. 17.

    A calorie that measures energy in food is actually equal to 1,000 physical calories, which is called a kilocalorie.

  18. 18.

    After all, Fahrenheit was a toolmaker and for him 96 was a convenient number because to engrave the graduation marks, he could easily do so by dividing a distance between the marks by two: 96, 48, 24, etc. With respect to nationality of the blood, he did not care if it was blood of an Englishman or not. Now, it is known that blood temperature of a healthy person is not really constant. It varies between approximately 97°F and 99.5°F (36°C and 37.5°C) but during his times, Fahrenheit could not find a better thermostat than a human body.

  19. 19.

    There is a difference of 0.01° between the Kelvin and Celsius scales, as Celsius’ zero point is defined not at a triple point of water as for the Kelvin, but at temperature where ice and air-saturated water are at equilibrium at atmospheric pressure.

  20. 20.

    This assumes that there is no phase change during warming up, like from solid to liquid.

  21. 21.

    More precisely, thermal expansion can be modeled by higher order polynomials, however, for the majority of practical purposes, a linear approximation is usually sufficient.

  22. 22.

    Likely, this is because of a better compatibility between the animal protein molecules and structures of the water crystals at that temperature.

  23. 23.

    In 1918, Max K. E. L. Planck (Germany, Berlin University) was awarded Nobel Prize “in recognition of his services he rendered to the advancement of Physics by his discovery of energy quanta”.

  24. 24.

    In 1911, Wilhelm Wien (Germany, Würtzburg University) was awarded Nobel Prize “for his discoveries regarding the laws governing the radiation of heat”.

  25. 25.

    Here we discuss the so-called thermal sensors as opposed to quantum sensors that are described in Chapter 13.

  26. 26.

    This simplified analysis assumes that there are no other objects in the sensor’s field of view.

  27. 27.

    Dual band detectors use two narrow spectral ranges to detect the IR flux. Then, by using a ratiometric technique of signal processing, temperature of an object is calculated. During the calculation, emissivity and other multiplicative constants are cancelled out.

  28. 28.

    In a thermally balanced IR sensor, the sensor’s temperature is constantly controlled (warmed up or cooled down) to bring the net thermal flux close to zero. Then, according to (3.138), the emissivities are multiplied by zero and thus their values no longer make any difference.


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Fraden, J. (2010). Physical Principles of Sensing. In: Handbook of Modern Sensors. Springer, New York, NY.

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