TREXLHPKLWS3S Design and application of thermocouple sensor temperature measuring system
First, thermocouple sensor temperature measurement system design and application
The following introduces a typical temperature measurement system controlled by 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) TREXLHPKLWS3S Thermocouple temperature sensor
The system uses a nickel-chromium-nickel-silicon thermocouple, the measured temperature range is 0 ~ 655℃, the cold end compensation adopts the compensation bridge method, the potential generated by the unbalanced bridge to compensate the thermocouple due to the temperature change of the cold end caused by the change in the value of the thermocouple. The unbalanced bridge consists of resistors R1, R2, R3(manganese copper wire wound), Rcu(copper wire wound) four bridge arms and bridge voltage regulator, which are connected in series in the thermocouple circuit. Rcu and the cold end of the thermocouple are at the same ±0℃, and R1=R2=R3=1Ω, the bridge supply voltage is 4V, powered by the regulated power supply, Rs is the current limiting resistor, and its resistance value is different because of the thermocouple, the bridge is usually balanced at 20℃, then the resistance of the four bridge arms R1=R2=R3=Rcu, no output at the a and b ends. When the cold end temperature deviates from 20℃, for example, when it rises, the Rcu increases, while the thermoelectric potential of the thermocouple decreases with the increase of the cold end temperature. The reduction of Uab and thermoelectric potential is equal, and the output potential remains unchanged after Uab and thermoelectric potential are superimposed, thus achieving the automatic completion of cold end compensation.
(2) Measurement amplifier circuit
In the actual circuit, the signal output from the thermocouple is no more than a few tens of millivolts (<30mV), and it contains common mode interference such as power frequency, electrostatic and magnetic coupling, and the amplification of this circuit requires the amplifier circuit to have a high common mode rejection ratio and high gain, low noise and high input impedance, so it is appropriate to use the measurement amplifier circuit. Measurement amplifier is also known as data amplifier, instrument amplifier and bridge amplifier, its input impedance is high, easy to match with a variety of signal sources, and its input offset voltage and input offset current and input bias current is small, and temperature drift is small. The stability of the measuring amplifier is good because the time temperature drift is small. The measuring 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, the symmetrical circuit structure is adopted, and the measured signal is directly added to the input end, thus ensuring a strong ability to suppress the common mode signal. The A3 is actually a differential follower with a gain of approximately 1. The magnification of the measurement amplifier is: AV=V0/ (V2-V1), AV=Rf/R(1+(Rf1+Rf2)/RW). In this circuit, as long as the operation amplifier A1 and A2 performance symmetry (mainly refers to the input impedance and voltage gain), its drift will be greatly reduced, with high input impedance and common mode rejection ratio, very sensitive to small differential mode voltage, suitable for measuring signals transmitted over long distances, so it is very easy to use with small output sensors. RW is an external resistor used to adjust the magnification, where a multi-turn potentiometer is used.
In the actual circuit, A1 and A2 adopt low-drift and high-precision OP amp OP-07 chips, whose input offset voltage temperature drift αVIOS and input offset current temperature drift αIIOS are very small.TREXLHPKLWS3S Op-07 adopts ultra-high technology and “Zener fine-tuning” technology, making its VIOS, IIOS, αVIOS and αIIOS very small. It is widely used in stable integration, precision addition, ratio correction detection and precision amplification of weak signals. OP-07 requires dual power supply and the operating temperature ranges from 0 to 70 ° C. Generally, zero adjustment is not required. If zero adjustment is required, RW can be used to adjust it. A3 uses 741 chip, it requires dual power supply, power supply range is ±(3 ~ 18)V, typical power supply is ±15V, generally should be greater than or equal to ±5V, its internal compensation capacitor, no need to external compensation capacitor.
(3) A/D(analog-to-digital) conversion circuit
After measuring the voltage signal amplified by the amplifier, its voltage range is 0 ~ 5V, this signal is an analog signal, the computer can not accept, so it must be A/D conversion. In the actual circuit, ICL7109 chip is selected. The ICL7109 is A dual-integral 12-bit A/D converter with high accuracy, low noise, low drift and low cost. Due to the high price of the current 12-bit successive approximation A/D converter, the cheap double-integral 12-bit A/D converter ICL7109 can be used in situations where the speed is not too high, such as high-precision measurement systems for weighing pressure, temperature and other sensor signals. ICL7109 mainly has the following characteristics: (1) high precision (accurate to 1/212=1/4096); (2) Low noise (typical value is 15μVP-P); (3)TREXLHPKLWS3S Low drift (<1μV/℃); (4) High input impedance (typical value 1012Ω); (5) Low power consumption (<20mW); (6) The conversion speed is up to 30 times/s, and when the 3.58MHz crystal excitation source is used, the speed is 7.5 times/s