Simultaneous Measurement of Tempearture and Strain Using Four Connecting Wires

This paper describes a new signal-conditioning technique for measuring strain and temperature which uses fewer connecting wires than conventional techniques. Simultaneous measurement of temperature and strain has been achieved by using thermocouple wire to connect strain gages to signal conditioning. This signal conditioning uses a new method for demultiplexing sampled analog signals and the Anderson current loop circuit. Theory is presented along with data to confirm that strain gage resistance change is sensed without appreciable error because of thermoelectric effects. Furthermore, temperature is sensed without appreciable error because of voltage drops caused by strain gage excitation current flowing through the gage resistance. INTRODUCTION Strain measurement during hot structural testing requires simultaneously measuring strain and temperature to correct for apparent strain output at the strain gage [1]. The traditional approach is to attach two independent sensors and signal-conditioning equipment for a strain gage and a thermocouple at the desired location. This paper shows that both sensors can be combined to form one sensor called the thermostrain gage. This combining process mixes the signals of both sensors. In theory, appropriate signal processing separates and measures the voltages representing temperature and strain [2]. Such a system can use thermocouple wire to connect a strain gage to the data readout equipment. A new demultiplexer design accomplishes this goal through a combination of transducer wiring, alternating excitation, and signal processing to separate thermoelectric (thermocouple) and resistance change (strain gage) signals in such a manner that each signal contributes negligible contamination to the other. This design for signal separation is comprised of three main parts: the transducer wiring scheme, the Anderson current loop technique [2], and the analog demultiplexer design. The key to system performance rests in the analog demultiplexer design. The demultiplexer separates the two signals which represent temperature and strain and which exist concurrently on the same pair of sense wires. The driving force behind the development of the new demultiplexer design was the need to acquire the temperature data at a strain gage location without using additional connecting wires. The demultiplexer design separates temperature and strain signals which originate at the strain gage location on a test specimen while using the same set of connecting wires. This paper describes a fully developed and laboratory-demonstrated signal demultiplexer design which implements Anderson’s theory [2] with performance normally encountered in systems employing separate, independent strain and temperature sensors. NOMENCLATURE A1, A2, A3, A4 difference amplifiers Cu copper eAB Seebeck voltage, V emf electromagnetic force, V Iac alternating current level, mA Idc direct current level, mA INV inverter amplifier J thermocouple junction L distance between two transducers metal A, metal B metal types for thermocouples metal C metal type for strain gage PLD programmable logic device Rgage total gage resistance, Ω RI initial gage resistance, Ω Rref reference resistance, Ω Rw resistances of connecting wire, Ω TC thermocouple Vgage strain gage voltage, V Vout demultiplexer-sensed voltage, V VoutA demultiplexer A half-cycle voltage, V VoutB demultiplexer B half-cycle voltage, V Vref reference resistor voltage, V VT thermocouple voltage, V ∆R change in gage resistance, Ω μe microstrain Ω ohm THERMOCOUPLE THEORY The thermocouple (TC), a commonly used device for measuring temperatures [3], is used in many hightemperature applications because of its wide temperature range and ability to be optimized for use in various atmospheres. Each of the several types of TC’s has its own set of properties. The process of selecting a specific type of TC for a particular application is beyond the scope of this paper. A thermocouple is formed when two metal wires composed of different alloys are joined to form an electrical circuit. Figure 1 shows a thermocouple circuit. The joining of the two dissimilar metal wires completes an electrical circuit which sums the voltages produced when a temperature gradient exists along the different conductors. This 2 Figure 1. Thermocouple circuit. +