56.2 Physical Sensors Physical measurands include temperature, strain, force, pressure, displacement, position, velocity, acceleration optical radiation, sound, flow rate, viscosity, and electromagnetic fields. Referring to Table 56.1, all but those transduction mechanisms listed in the chemical column are used in the design of physical sensors. Clearly, they comprise a very large proportion of all sensors. It is impossible to illustrate all of them, but three measurands stand out in terms of their widespread application: temperature, displacement(or associated force), and optical radiation Temperature Sensors Temperature is an important parameter in many control systems, most familiarly in environmental control stems. Several distinctly different transduction mechanisms have been employed. The mercury thermometer was mentioned in the Introduction as a nonelectrical sensor. The most commonly used electrical temperature sensors are thermocouples, thermistors, and resistance thermometers. Thermocouples employ the Seebeck effect, which occurs at the junction of two dissimilar metal wires. A voltage difference is generated at the hot junction due to the difference in the energy distribution of thermally energized electrons in each metal. This voltage is measured across the cool ends of the two wires and changes linearly with temperature over a given range, depending on the choice of metals. To minimize measurement error the cool end of the couple must be kept at a constant temperature, and the voltmeter must have a high input impedance The resistance thermometer relies on the increase in resistance of a metal wire with increasing temperature As the electrons in the metal gain thermal energy, they move about more rapidly and undergo more frequent collisions with each other and the atomic nuclei. These scattering events reduce the mobility of the electrons, and since resistance is inversely proportional to mobility, the resistance increases. Resistance thermometers onsist of a coil of fine metal wire. Platinum wire gives the largest linear range of operation. To determine the resistance indirectly, a constant current is supplied and the voltage is measured. a direct measurement can be made by placing the resistor in the sensing arm of a Wheatstone bridge and adjusting the opposing resistor to"balance"the bridge, which produces a null output. A measure of the sensitivity of a resistance thermometer is its temperature coefficient of resistance: TCR=(AR/R)(1/AT) in units of resistance per degree of temperature Thermistors are resistive elements made of semiconductor materials and have a negative coefficient of resistance. The mechanism governing the resistance change of a thermistor is the increase in the number of conducting electrons with an increase in temperature due to thermal generation, i.e., the electrons which are the least tightly bound to the nucleus (valence electrons) gain sufficient thermal energy to break away and become influenced by external fields. Thermistors can be measured in the same manner as resistance thermon eters, but thermistors have up to 100 times higher TCR value Displacement and Force Many types of forces are sensed by the displacements they create. For example, the force due to acceleration of a mass at the end of a spring will cause the spring to stretch and the mass to move. Its displacement from the zero acceleration position is governed by the force generated by the acceleration(F= m a)and the restoring force of the spring. Another example is the displacement of the center of a deformable membrane due to a difference in pressure across it. Both of these examples use multiple transduction mechanisms to produce an electronic output: a primary mechanism which converts force to displacement(mechanical to mechanical)and then an intermediate mechanism to convert displacement to an electrical signal (mechanical to electrical). Displacement can be measured by an associated capacitance. For example, the capacitance associated with gap which is changing in length is given by C=area x dielectric constant/gap length. The gap must be very small compared to the surface area of the capacitor, since most dielectric constants are of the order of 1 x 10-1 farads/cm and with present methods, capacitance is readily resolvable to only about 10-l farads. This is because measurement leads and contacts create parasitic capacitances the same order of magnitude. If the capacites 8 is measured at the generated site by an integrated circuit(see Section III), capacitances as small as 10- far c 2000 by CRC Press LLC© 2000 by CRC Press LLC 56.2 Physical Sensors Physical measurands include temperature, strain, force, pressure, displacement, position, velocity, acceleration, optical radiation, sound, flow rate, viscosity, and electromagnetic fields. Referring to Table 56.1, all but those transduction mechanisms listed in the chemical column are used in the design of physical sensors. Clearly, they comprise a very large proportion of all sensors. It is impossible to illustrate all of them, but three measurands stand out in terms of their widespread application: temperature, displacement (or associated force), and optical radiation. Temperature Sensors Temperature is an important parameter in many control systems, most familiarly in environmental control systems. Several distinctly different transduction mechanisms have been employed. The mercury thermometer was mentioned in the Introduction as a nonelectrical sensor. The most commonly used electrical temperature sensors are thermocouples, thermistors, and resistance thermometers. Thermocouples employ the Seebeck effect, which occurs at the junction of two dissimilar metal wires. A voltage difference is generated at the hot junction due to the difference in the energy distribution of thermally energized electrons in each metal. This voltage is measured across the cool ends of the two wires and changes linearly with temperature over a given range, depending on the choice of metals. To minimize measurement error the cool end of the couple must be kept at a constant temperature, and the voltmeter must have a high input impedance. The resistance thermometer relies on the increase in resistance of a metal wire with increasing temperature. As the electrons in the metal gain thermal energy, they move about more rapidly and undergo more frequent collisions with each other and the atomic nuclei. These scattering events reduce the mobility of the electrons, and since resistance is inversely proportional to mobility, the resistance increases. Resistance thermometers consist of a coil of fine metal wire. Platinum wire gives the largest linear range of operation. To determine the resistance indirectly, a constant current is supplied and the voltage is measured. A direct measurement can be made by placing the resistor in the sensing arm of a Wheatstone bridge and adjusting the opposing resistor to “balance” the bridge, which produces a null output. A measure of the sensitivity of a resistance thermometer is its temperature coefficient of resistance: TCR = (DR/R)(1/DT) in units of % resistance per degree of temperature. Thermistors are resistive elements made of semiconductor materials and have a negative coefficient of resistance. The mechanism governing the resistance change of a thermistor is the increase in the number of conducting electrons with an increase in temperature due to thermal generation, i.e., the electrons which are the least tightly bound to the nucleus (valence electrons) gain sufficient thermal energy to break away and become influenced by external fields. Thermistors can be measured in the same manner as resistance thermom￾eters, but thermistors have up to 100 times higher TCR values. Displacement and Force Many types of forces are sensed by the displacements they create. For example, the force due to acceleration of a mass at the end of a spring will cause the spring to stretch and the mass to move. Its displacement from the zero acceleration position is governed by the force generated by the acceleration (F = m · a) and the restoring force of the spring. Another example is the displacement of the center of a deformable membrane due to a difference in pressure across it. Both of these examples use multiple transduction mechanisms to produce an electronic output: a primary mechanism which converts force to displacement (mechanical to mechanical) and then an intermediate mechanism to convert displacement to an electrical signal (mechanical to electrical). Displacement can be measured by an associated capacitance. For example, the capacitance associated with a gap which is changing in length is given by C = area ¥ dielectric constant/gap length. The gap must be very small compared to the surface area of the capacitor, since most dielectric constants are of the order of 1 ¥ 10–13 farads/cm and with present methods, capacitance is readily resolvable to only about 10–12 farads. This is because measurement leads and contacts create parasitic capacitances the same order of magnitude. If the capacitance is measured at the generated site by an integrated circuit (see Section III), capacitances as small as 10–15 farads