Schmalzel. J.L. "instrume The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton CRC Press llc. 2000
Schmalzel, J.L.. “Instruments” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
108 struments 108.2 Physical Variables 108.3 Transducers 108.4 Instrument elements 108.5 Instrumentation System 108.6 Modeling Elements of an Instrumentation System 108.7 Summary of Noise Reduction Technique 108.8 Personal Computer-Based Instruments 108.9 Modeling PC-Based Instruments John L. Schmalzel e Effects of Sampling Rowan University 108.11 Other Factors 108.1 Introduction Instruments are the means for monitoring or measuring physical variables. The basic elements of an instru- mentation application are shown in Fig. 108. 1. A physical system produces a measurand, m(o), shown as time- varying, which is transformed by a transducer into an electrical signal, s(t), that is then processed by an instrument to yield the desired output information variable, i(t). Producing meaningful information from physical variables requires conversion and processing Electronic instruments require that physical variables be converted to electrical signals through a process of transduction, followed by signal conditioning and signal processing to obtain useful results 108.2 Physical variables The measurand can be one of many physical variables; the type depends on the application. For example, in process control, typical measurands can include pressure, temperature, and flow. Representative physical vari- ables with corresponding units are summarized in Table 108.1 108.3 Transducers Transducers convert one form of energy to another. To be useful for an electronic instrument, a transducer must produce an electrical output such as voltage or current to allow required signal conditioning and signal processing steps to be completed A variety of transducers are available to meet a measurement requirement; ome common examples are listed in Table 108.2. Transducers can be compared based on their operating principles, the measurand range, interface design, and reliability. Khazan [ 1994] gives a complete summary of transducer schemes c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 108 Instruments 108.1 Introduction 108.2 Physical Variables 108.3 Transducers 108.4 Instrument Elements 108.5 Instrumentation System 108.6 Modeling Elements of an Instrumentation System 108.7 Summary of Noise Reduction Techniques 108.8 Personal Computer-Based Instruments 108.9 Modeling PC-Based Instruments 108.10 The Effects of Sampling 108.11 Other Factors 108.1 Introduction Instruments are the means for monitoring or measuring physical variables. The basic elements of an instrumentation application are shown in Fig. 108.1. A physical system produces a measurand, m(t), shown as timevarying, which is transformed by a transducer into an electrical signal, s(t), that is then processed by an instrument to yield the desired output information variable, i(t). Producing meaningful information from physical variables requires conversion and processing. Electronic instruments require that physical variables be converted to electrical signals through a process of transduction, followed by signal conditioning and signal processing to obtain useful results. 108.2 Physical Variables The measurand can be one of many physical variables; the type depends on the application. For example, in process control, typical measurands can include pressure, temperature, and flow. Representative physical variables with corresponding units are summarized in Table 108.1. 108.3 Transducers Transducers convert one form of energy to another. To be useful for an electronic instrument, a transducer must produce an electrical output such as voltage or current to allow required signal conditioning and signal processing steps to be completed. A variety of transducers are available to meet a measurement requirement; some common examples are listed in Table 108.2. Transducers can be compared based on their operating principles, the measurand range, interface design, and reliability. Khazan [1994] gives a complete summary of transducer schemes. John L. Schmalzel Rowan University
PHYSICAL Lm( TRANSDUCER Ls(D_ INSTRUMENT Li(t) SYSTEM FIGURE 108.1 Generalized block diagram of an instrument applied to a physical measurement. TABLE 108 1 Representative Physical Variables, Physical Variable Symbol SI Units, Abbreviations N EFQfLmPPRrtvv he flow rate, m /s hertz, Hz m kilogram, kg N/m2 Watt. w ohm. Q Temperature Velocity TABLE 108.2 Representative Transducers Measurand Transducer Operating Principles Displacement Resistive Change in resistance, capacitance, Capacitive inductance caused by linear or angular Inductive displacement of transducer element Force Strain gage Resistance, piezoresistivity Temperature Thermistor Resistance Thermocouple Peltier, seebeck effect Pressure Diaphragm motion sensed by a displacement technique. Differential pressure Pressure drop across restriction Turbine Angular velocity proportional to flow rate 108.4 Instrument elements ignal conditioning consists of amplification, filtering, limiting, and other operations that prepare the raw nstrument input signal for further operations. The signal may be the output of a transducer or it may be an electrical signal obtained directly from an electronic device or circuit Signal processing applies some algorithm to the basic signal in order to obtain meaningful information. Signal conditioning and processing operatic may be performed using analog or digital circuit techniques, or using a combination of methods. There are a variety of trade-offs between them. For example, analog methods offer bandwidth advantages, whereas digital techniques offer advanced algorithm support and long-term stability. The use of microprocessors within an instrument makes it possible to perform many useful functions including calibration, linearization, conversion, storage,display, and transmission. A block diagram of a representative microprocessor-based instrument is e 2000 by CRC Press LLC
© 2000 by CRC Press LLC 108.4 Instrument Elements Signal conditioning consists of amplification, filtering, limiting, and other operations that prepare the raw instrument input signal for further operations. The signal may be the output of a transducer or it may be an electrical signal obtained directly from an electronic device or circuit. Signal processing applies some algorithm to the basic signal in order to obtain meaningful information. Signal conditioning and processing operations may be performed using analog or digital circuit techniques, or using a combination of methods. There are a variety of trade-offs between them. For example, analog methods offer bandwidth advantages, whereas digital techniques offer advanced algorithm support and long-term stability. The use of microprocessors within an instrument makes it possible to perform many useful functions including calibration, linearization, conversion, storage, display, and transmission. A block diagram of a representative microprocessor-based instrument is shown in Fig. 108.2. FIGURE 108.1 Generalized block diagram of an instrument applied to a physical measurement. TABLE 108.1 Representative Physical Variables, Symbols, and Units Physical Variable Symbol SI Units, Abbreviations Current I ampere, A Energy E joule, J Force F newton, N Flow Q volume flow rate, m3 /s Frequency f hertz, Hz Length L meter, m Mass m kilogram, kg Pressure P N/m2 Power P Watt, W Resistance R ohm, W Temperature T Kelvin, K Time t second, s Velocity V m/s Voltage V volt, V TABLE 108.2 Representative Transducers Measurand Transducer Operating Principles Displacement Resistive Change in resistance, capacitance, or (Length) Capacitive inductance caused by linear or angular Inductive displacement of transducer element Force Strain gage Resistance, piezoresistivity Temperature Thermistor Resistance Thermocouple Peltier, seebeck effect Pressure Diaphragm Diaphragm motion sensed by a displacement technique. Flow Differential pressure Pressure drop across restriction Turbine Angular velocity proportional to flow rate
DISPLAY Physical TRANSDUCER TIONENO ESSING INPUTHOUTP MICROPROCESSOR WER SUPPL STORAGE FIGURE 108.2 Block diagram of generalized, microprocessor-based instrument. 108.5 Instrumentation System An instrument is never used in isolation. The instrumentation components contribute to an overall system response in a number of ways that are based on the measurement system elements present. These elements include:(1)sources,(2)interconnect, (3)device or system under test, (4)response measuring equipment, and (5)environmental variables. Figure 108.3 shows the elements of a typical instrumentation system 108.6 Modeling Elements of an Instrumentation System est results are achieved when the instrumentation system is clearly understood, and its effects compensated for when practical. Lumped parameter modeling of the elements shown in Fig. 108.3 provides a means for determining the contribution each element makes to the overall system behavior. Of particular importance are the input and output impedances of each element. In addition, the effects of interconnect and environmental variables can also be modeled to determine their influence on the system. The relative dimensions of the measurement system with respect to the highest frequencies encountered-whether signal or noise--determine whether simplified circuit theory models, or generalized solutions to Maxwell's equations must be used. theory models can be used. Operation in this regime also allows impedance matching to be largely ignored e.g., not requiring mandatory use of 50 Q2 sources, 50 Q2 transmission lines, and 50 Q2 terminations which is commonly encountered in high-frequency systems. Table 108.3 summarizes several common instruments and input or output impedance models corresponding to Fig 108.4. At low frequencies, interconnect can be modeled r ignoring the ver ce(Zsl, Zs2)terms, and considering only the shi TAL INTERCONNECT DEVICE UNDER INTERCONNECT I NSTRUMENTS MEASUREMENT SYSTEM FIGURE 108.3 Fundamental elements of an instrumentation system. e 2000 by CRC Press LLC
© 2000 by CRC Press LLC 108.5 Instrumentation System An instrument is never used in isolation. The instrumentation components contribute to an overall system response in a number of ways that are based on the measurement system elements present. These elements include: (1) sources, (2) interconnect, (3) device or system under test, (4) response measuring equipment, and (5) environmental variables. Figure 108.3 shows the elements of a typical instrumentation system. 108.6 Modeling Elements of an Instrumentation System Best results are achieved when the instrumentation system is clearly understood, and its effects compensated for when practical. Lumped parameter modeling of the elements shown in Fig. 108.3 provides a means for determining the contribution each element makes to the overall system behavior. Of particular importance are the input and output impedances of each element. In addition, the effects of interconnect and environmental variables can also be modeled to determine their influence on the system. The relative dimensions of the measurement system with respect to the highest frequencies encountered—whether signal or noise—determine whether simplified circuit theory models, or generalized solutions to Maxwell's equations must be used. Generally, if measurement system dimensions are on the order of 1/20 of the shortest wavelength, simple circuit theory models can be used. Operation in this regime also allows impedance matching to be largely ignored; e.g., not requiring mandatory use of 50 W sources, 50 W transmission lines, and 50 W terminations which is commonly encountered in high-frequency systems. Table 108.3 summarizes several common instruments and input or output impedance models corresponding to Fig. 108.4.At low frequencies, interconnect can be modeled by ignoring the very low series resistance and inductance (Zs1, Zs2) terms, and considering only the shunt FIGURE 108.2 Block diagram of generalized, microprocessor-based instrument. FIGURE 108.3 Fundamental elements of an instrumentation system
CASSINI SPACECRAFT O ne of NASAs latest planetary systems research segments is called the Discovery Program. This program is an effort to develop frequent, small planetary missions that perform high quality scientific investigations. Discovery missions planned for 1997 include the sending of a Mars lander to the planet and launching the Lunar Prospector to map the moon's surface composition. The principle planetary mission of NASAs Discovery Program is the 1997 launch of Cassini. Cassini is a joint project of NASA, the European Space Agency(ESA), and the Italian Space Agency, and managed by the Jet Propulsion Laboratory (PL) The flight vehicle consists of the main Cassini spacecraft and the ESA-built Huygens Probe, a 750-pound, six instrument package that will descend into the atmosphere of Saturns moon Titan, which is believed to be chemically similar to the atmosphere of early Earth Launched towards the end of 1997, Cassini will make flybys of Venus and Jupiter en route to a rendezvous with Saturn in July 2004. Cassini will release the Huygens Probe during its first orbit, then make approximately 40 revolutions over a span of four years, while the spacecrafts 12 instruments onduct a detailed exploration of the whole Saturnian system, including Titan and the planets other icy moons. (Courtesy of National Aeronautics and Space Administration. This artist,'s concept shows the Cassini spacecraft orbiting around Saturn just after deploying a probe that will descend into the atmosphere of Saturns moon Titan. Launched October 1997, Cassini will reach Saturn in July 2004 and orbit the planet for four years thereafter.(Photo courtesy of National Aeronautics and Space Administration. capacitance(Zp) which is in the range of 50 to 150 pF/m for different types of cable. At high frequencies, the haracteristic impedance of the interconnect is used;e.g, 50 Q2 or 75 @2 for commonly used coaxial cables; 120 Q2 for twisted pair The response of an entire instrumentation system can be modeled by interconnecting the individual me elements. Figure 108.5 shows an le that was obtained by substituting models for an operational amplifier e 2000 by CRC Press LLC
© 2000 by CRC Press LLC capacitance (Zp) which is in the range of 50 to 150 pF/m for different types of cable. At high frequencies, the characteristic impedance of the interconnect is used; e.g., 50 W or 75 W for commonly used coaxial cables; 120 W for twisted pair. The response of an entire instrumentation system can be modeled by interconnecting the individual model elements. Figure 108.5 shows an example that was obtained by substituting models for an operational amplifier CASSINI SPACECRAFT ne of NASA’s latest planetary systems research segments is called the Discovery Program. This program is an effort to develop frequent, small planetary missions that perform high quality scientific investigations. Discovery missions planned for 1997 include the sending of a Mars lander to the planet and launching the Lunar Prospector to map the moon’s surface composition. The principle planetary mission of NASA’s Discovery Program is the 1997 launch of Cassini. Cassini is a joint project of NASA, the European Space Agency (ESA), and the Italian Space Agency, and is managed by the Jet Propulsion Laboratory (JPL). The flight vehicle consists of the main Cassini spacecraft and the ESA-built Huygens Probe, a 750-pound, six instrument package that will descend into the atmosphere of Saturn’s moon Titan, which is believed to be chemically similar to the atmosphere of early Earth. Launched towards the end of 1997, Cassini will make flybys of Venus and Jupiter en route to a rendezvous with Saturn in July 2004. Cassini will release the Huygens Probe during its first orbit, then make approximately 40 revolutions over a span of four years, while the spacecraft’s 12 instruments conduct a detailed exploration of the whole Saturnian system, including Titan and the planet’s other icy moons. (Courtesy of National Aeronautics and Space Administration.) This artist’s concept shows the Cassini spacecraft orbiting around Saturn, just after deploying a probe that will descend into the atmosphere of Saturn’s moon Titan. Launched October 1997, Cassini will reach Saturn in July 2004 and orbit the planet for four years thereafter. (Photo courtesy of National Aeronautics and Space Administration.) O