Implementation of a new intelligent temperature me

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Implementation of a new intelligent temperature measuring instrument

1 Introduction temperature is an important parameter in industrial production. Before the 1980s, most of the measurement of temperature in China used moving coil instruments. Due to its low resolution, poor seismic resistance, parallax and reading error, this kind of instrument is often not competent for occasions requiring accurate temperature measurement. Since the 1980s, 3-1/2-bit a/d converter has been gradually used as the core component to manufacture measuring instruments, and has been active in various industrial production departments in China. However, due to the inherent defects such as complex alarm circuit and poor versatility of icl7107 temperature measuring instrument, In recent years, people use single-chip computers to form temperature measuring instruments, and the JSW intelligent temperature measuring instrument we developed is one of them. 2. Technical characteristics of JSW intelligent thermometer with the advent and commercialization of single chip microcomputer, using it to manufacture temperature measuring instruments with higher cost performance has the following obvious advantages:

● from the perspective of hardware, the instrument should be able to be used not only with thermal resistors but also with thermocouples, reducing the number of standby meters for users, thus bringing direct benefits

● when used with thermal resistor, it is not only applicable to three wire system, but also applicable to two-wire system

● when used with thermocouple, it is not only suitable for automatic temperature compensation of thermocouple cold end, but also for self zero calibration

● zero the amplifier to make the overall performance of the instrument more stable

● use the operation function of single chip microcomputer to make more accurate correction for the nonlinearity of thermal resistance and thermocouple

● it is very convenient to control the alarm by using the bit operation function of single chip microcomputer

the hardware circuit and logic block diagram of JSW intelligent thermometer designed according to the above concept are shown in Figure 1. It can be seen from the figure that when P1.1 of 8031 is at high level, the analog switch is connected to the thermal resistance measurement branch, and when P1.1 is at low level, the switch is connected to the thermocouple branch, so that the unity of thermal resistance/thermocouple on one meter can be achieved without adding any hardware

when P1.2 also sends out a high level, amplifier A2 is grounded, make 80C31 read the value and polarity of the a/d converter, and store it in a ram, then make P1.2 send out a low level, make the amplifier connect to the thermocouple potential, then make 8031 read the a/d value and polarity, and subtract this result from the last result, you can overcome the drift of the amplifier, so that you can also receive high-precision measurement results without expensive high-performance amplifier

when it is necessary to compensate the cold end temperature of the thermocouple, this instrument works in the alternating state of thermocouple and thermal resistance respectively

when the measurement result determines that there is an alarm, set or reset P1.6 and P1.7. The alarm circuit is quite simple

when users need sensors with different graduation numbers, they only need to replace the software. 3. Some problems in the design of JSW intelligent temperature measuring instrument 3.1 selection of R2 resistance

when measuring temperature with thermal resistance, the reference voltage of a/d converter ICL 7109 is taken from the voltage drop on r2=200 Ω, the measured voltage is obtained from RT, R1 is selected as 1K Ω, and 1403 is used to limit the current flowing through RT, so that it does not exceed 5mA, because excessive current will cause measurement error due to RT's own heating, On the other hand, there are two main development trends in the automotive industry at present. If R1 is not added, the high-end reference voltage of ICL 7109 will be equal to the 5V power supply, which will make ICL 7109 work abnormally. Therefore, the addition of R1 is necessary. The voltage on R2 is used as the reference voltage of a/d, and their size determination principle is based on the following considerations:

there are six new and old graduation numbers of thermal resistance sensors currently used in China: cu50, cu53, cu100, BA1 BA2、Pt100。 Their starting values at zero temperature are 50 Ω, 53 Ω, 100 Ω, 45 Ω, 100 Ω and 100 Ω respectively, while at the upper limit temperature, the maximum resistance of Pt100 and Ba2 has reached 310 Ω. According to the conversion characteristics of ICL 7109:

n=2048vx/vr (1)

the conversion formula of resistance can be deduced

n=2048 (irt/ir2) =2048rt/r2 (2)

the dynamic testing machine mainly includes fatigue testing machine. After r2=200 Ω is determined, when pT1 00 at 600 ℃, The a/d conversion value will not overflow, and for BA1 and cu50, the binary code of a/d conversion is larger than the binary code corresponding to the measured temperature, so nonlinear correction is required

since the voltage on R2 is used as the a/d reference, its accuracy and stability are directly related to the accuracy of the instrument. Therefore, R2 should choose a wire wound resistance with an accuracy of ± 0.1%

3.2 prevention of false alarm when blocking the thermoelectric line

when blocking the thermoelectric line, the potential of point a will rise to the power supply voltage, and the a/d converter will overflow. Using the overflow flag, 8031 not only adjusts the off character to display to 8255A, but also uses multiple controls to set P1.6 and P1.2, so that the relay does not engage, which eliminates the problem of false alarm. It is troublesome to do this in analog circuits, The bit function of the chip machine greatly simplifies the alarm circuit. Not only that, the alarm light on the panel makes every effort to build the aerospace new material industry base and the national military civilian integration demonstration base with program control. For example, the upper limit light is 8255A high 7 bits, and the lower limit light is controlled by B port high 7 bits. In this way, the single contact relay can be selected, and jzx-2f is selected in this example. Their price is usually 2 ~ 3 yuan/piece. In analog circuit, in order to synchronize the alarm light on the panel and the contact tag number output by the relay, double contact relays must be selected. Their price is 10 ~ 20 yuan/piece. Therefore, the cost of alarm components in this instrument is less. Not only that, its control mode is also very flexible. When used together with thermal resistors, The alarm methods for instruments are usually divided into high/low limit alarm and double upper limit alarm

in the alarm circuit composed of analog circuit, it is not easy to change the low limit alarm to double upper limit alarm, but in the instrument composed of single chip microcomputer, as long as the position control signal is taken out. 4. Thermocouple temperature measurement

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