On the design and application of thermocouple sensor temperature measurement system

As a typical temperature measuring element, the thermocouple sensor has been widely used.
The following is a typical temperature measurement system controlled by a single chip microcomputer, which consists of three parts: (1) measurement amplifier circuit; (2) A/D conversion circuit; (3) display circuit. It is widely used in temperature measurement and temperature control systems in power plants and chemical plants.
1. Hardware design
(1) Thermocouple temperature sensor
This system uses nickel-chromium-nickel-silicon thermocouples. The measured temperature range is 0-655℃. The cold end compensation adopts the compensation bridge method. The potential generated by the unbalanced bridge is used to compensate for the change in thermoelectric potential caused by the change in the cold end temperature of the thermocouple. The unbalanced bridge consists of four bridge arms of resistors R1, R2, R3 (manganese copper wire winding), Rcu (copper wire winding) and a bridge voltage regulator, which are connected in series in the thermocouple circuit. Rcu and the cold end of the thermocouple are at ±0℃, and R1=R2=R3=1Ω. The bridge power supply voltage is 4V, powered by a voltage-stabilized power supply. Rs is a current-limiting resistor, and its resistance value varies with different thermocouples. The bridge is usually balanced at 20℃. At this time, the four bridge arm resistances of the bridge are R1=R2=R3=Rcu, and there is no output at ends a and b. When the cold end temperature deviates from 20℃, for example, when it rises, Rcu increases, while the thermoelectric potential of the thermocouple decreases as the cold end temperature increases. Uab is equal to the decrease in the thermoelectric potential, and the output potential remains unchanged after Uab is superimposed on the thermoelectric potential, thereby achieving automatic completion of cold end compensation.
(2) Measurement amplifier circuit
In actual circuits, the signal output from the thermocouple is no more than a few tens of millivolts (<30mV), and contains common-mode interference such as power frequency, static electricity and magnetic coupling. To amplify this kind of circuit, the amplifier circuit needs to have a high common-mode rejection ratio, high gain, low noise and high input impedance. Therefore, a measurement amplifier circuit is suitable. The measurement amplifier is also called a data amplifier, instrument amplifier and bridge amplifier. It has a high input impedance and is easy to match with various signal sources. Its input offset voltage, input offset current and input bias current are small, and the temperature drift is small. Due to the small time temperature drift, the measurement amplifier has good stability. The measurement amplifier is composed of three op amps, and the differential input terminals R1 and R2 are connected to the in-phase terminals of A1 and A2 respectively. The input impedance is very high, a symmetrical circuit structure is used, and the measured signal is directly added to the input terminal, thereby ensuring a strong ability to suppress common-mode signals. A3 is actually a differential follower, and its gain is approximately 1. The gain of the measuring amplifier is: AV=V0/(V2-V1), AV=Rf/R(1+(Rf1+Rf2)/RW). In this circuit, as long as the performance of op amps A1 and A2 is symmetrical (mainly referring to input impedance and voltage gain), their drift will be greatly reduced. They have high input impedance and common mode rejection ratio, are very sensitive to tiny differential mode voltage, and are suitable for measuring signals transmitted over long distances. Therefore, they are very easy to use with sensors with tiny outputs. RW is an external resistor used to adjust the gain, and a multi-turn potentiometer is used here.
In the actual circuit, A1 and A2 use low-drift, high-precision op amp OP-07 chips, whose input offset voltage temperature drift αVIOS and input offset current temperature drift αIIOS are very small. OP-07 uses ultra-high technology and "Zener fine-tuning" technology to make its VIOS, IIOS, αVIOS and αIIOS very small, and is widely used in stable integration, precision addition, comparison detection and precision amplification of weak signals. OP-07 requires dual power supply, and the operating temperature range is 0～70℃. Generally, zero adjustment is not required. If zero adjustment is required, RW can be used for adjustment. A3 uses 741 chip, which requires dual power supply, and the power supply range is ±(3～18)V. The typical power supply is ±15V, which should generally be greater than or equal to ±5V. It contains compensation capacitors inside, and no external compensation capacitors are required.
(3) A/D (analog-to-digital) conversion circuit
The voltage signal amplified by the measurement amplifier has a voltage range of 0～5V. This signal is an analog signal and cannot be accepted by the computer, so A/D conversion must be performed. In the actual circuit, the ICL7109 chip is selected. ICL7109 is a high-precision, low-noise, low-drift, low-cost dual-integral 12-bit A/D converter. Since the current 12-bit successive approximation A/D converter is relatively expensive, the cheap dual-integral 12-bit A/D converter ICL7109 can be used in situations where the speed is not too high, such as in high-precision measurement systems for various sensor signals such as weighing, measuring pressure, and measuring temperature.
Through the above analysis of the thermocouple sensor temperature measurement system, I hope it will be helpful to everyone's work and study.