K-type thermocouple temperature measurement compensation program
Fix the probe of the filament-shaped K-type thermocouple on the monitoring point of the printed board with solder or high-temperature adhesive. , The recorder automatically samples the temperature signal of the thermocouple at predetermined time intervals, and saves the temperature data that changes with time in the recorder's non-volatile memory. In this process, the probe of the filamentary K-type thermocouple is fixed on the monitoring point of the printed board with solder or high-temperature adhesive, and the temperature recorder and the printed board are passed through the furnace with the conveyor network or conveyor chain of the furnace. At the same time, the recorder automatically samples the temperature signal of the thermocouple at predetermined time intervals, and saves the temperature data that changes with time in the recorder's non-volatile memory. During this process, the external temperature of the temperature recorder may reach over 270 ℃, and its internal temperature is also around 60 ℃ after adopting the necessary heat insulation technology. The theoretical cold junction temperature of the thermocouple is the freezing point temperature of pure water (0 ℃), so it must be compensated. 1 Introduction In order to meet the needs of automated mass production in the SMT industry, the vast majority of enterprises use tunnel-type continuous conveying structure reflow ovens. This kind of reflow oven generally has at least 3 temperature zones. Because the temperature change on the printed board is much more complicated than the display temperature of the instrument, it is difficult for the operator of the reflow oven to adjust the temperature and transmission speed of the reflow oven in a short time. Best state. 2 Scheme selection 2.1 Hardware system scheme Existing products mostly use 3 methods to measure the ambient temperature of the cold junction. (1) Directly borrow the internal temperature sensor of the CPU, such as Cygnal's CF020. However, firstly, the internal temperature field of the recorder is not uniform, and there is a difference between the temperature of the hot-spot couple compensation line and the surface temperature of the CPU; secondly, the sensitivity of the integrated temperature sensor is generally 0.1 ℃, and the accuracy is ±2 ℃, which is difficult to meet the measurement requirements. Require. (2) Use a new type of intelligent temperature sensor, such as Maxim DS1626, 12bit sampling accuracy, 3-wire serial data communication, 0 ℃ to + 70 ℃, 2.7V (3) High-precision A/D sampling chip + remote temperature sensor. After theoretical analysis and practice, we adopted the improved third scheme. , The hardware system is mainly composed of a reference voltage source (ADR420), a high-precision sampling chip (MAXIM1403), a temperature-sensitive transistor (3DG6) and a CPU (CF320). ADR420 provides a 2.048V reference voltage with an accuracy of 0.05% and a temperature drift of 3PPM/℃. MAX1403 is an 18-bit, over-sampling AD chip. It uses a sigma-delta modulator and digital filter to achieve true 16-bit conversion accuracy. MAX1403 can provide three true differential input channels with independent programming (gain ranges from 1V/V to +128V/V), and can compensate for the DC offset of the input parameter voltage. And these three true differential input channels can also form five pseudo differential input channels. In addition, the chip also has two additional differential correction channels to correct gain and offset errors. The on-chip digital filter can process the line frequency and related harmonic frequencies, and make the amplitude of these frequencies zero, so that a better filtering effect can be obtained without an external filter, and the output terminal can be improved. The quality of the digital signal. Taking the reference voltage of this system 2.048V as an example, MAX1403 can perceive the minimum voltage (1 times PGA), that is, 1LSB corresponds to 2.048/216 u003d 0.03125mV, which is much smaller than the temperature-sensitive transistor 2mV/℃, and the perceivable temperature change is less than 0.02 ℃. After taking anti-fluctuation measures, the error requirements of PN junction 0.2 ℃ and system 0.5 ℃ can be guaranteed. 2.2 Software calculation method The overall program flow chart is shown in Figure 2. Before application, first measure the voltage value of the temperature-sensitive transistor at the freezing point (ice-water mixture) and boiling point (in these two states, the water temperature is relatively constant and can be measured with an industrial high-precision mercury thermometer) as the end point of the difference calculation; The temperature-sensitive transistor is used to measure the internal ambient temperature of the instrument; finally, the temperature at the measuring point is obtained by the temperature compensation formula (Equation 1). T u003d TC+k·T0 (1) where T is the temperature of the measuring point, TC is the temperature before compensation obtained by the thermocouple, T0 is the ambient temperature of the cold junction of the thermocouple measured by the transistor, and k is the proportional coefficient ( Even the medium and temperature compensation interval vary). 3 Theoretical basis 3.1 Thermocouple principle The third of the basic laws that need to be applied to the use of thermocouple temperature measuring instruments-the 'intermediate temperature law' is explained as follows: In Figure 3, the thermocouple AB is at the junction temperature T and the cold junction temperature. The thermoelectric potential EAB(T,0) at 0 ℃ is equal to the thermoelectric potential EAB(T,To) of thermocouple AB at junction temperature T and cold junction temperature To, and EAB at junction temperature To and cold junction temperature 0 ℃ The algebraic sum of (To,0). That is, EAB(T,0) u003d EAB(T,To) + EAB(To,0). The proof is as follows: where: e —— unit charge; k —— Boltzmann's constant; NA, NB —— the electron density of conductors A and B, they are all functions of temperature. EAB ——The total thermoelectromotive force in the closed circuit of the thermocouple. 3.2 Principle of temperature measurement of PN junction semiconductor theory and experiments have proved that in the range of -50 ℃～+150 ℃, when the emitter junction is forward biased, regardless of the collector junction reverse bias or zero bias, under a certain collector current form, The base-emitter forward voltage UBE of the NPN silicon transistor decreases as the temperature T increases. And there is a good linear relationship, and its voltage temperature coefficient is about -2.1mv/ ℃
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