Heat source automatic tester based on 8051 single chip microcomputer control

        In the process of drug quality monitoring, the temperature measurement of heat source reaction is an important part. Drug testing has its own particularity, and has high requirements for the accuracy, stability, consistency and linearity of the test system. Traditional old instruments are mostly various types of thermometers, and the test efficiency and accuracy are unsatisfactory. According to the requirements of on-site monitoring, a heat source automatic tester based on single-chip microcomputer control was developed, which realized the automatic circuit testing of 30 heat sources in the laboratory, met the high standard requirements of accurate and stable on-site temperature measurement, and successfully completed the renewal of test instruments in the drug testing laboratory.

      1 Test instrument system composition and working principle

　　The working principle diagram of this test instrument is shown in Figure 1. The heat source data of multi-point test is sent to the instrument amplifier for difference amplification through a multiplexer switch, and the voltage signal is converted into a pulse signal of a certain frequency by a V/F converter and sent to the T0 port of the 8051 microcontroller. T0 is a counter and T1 is a timer. It receives the pulse signal within the timing time and realizes high-precision A/D conversion through the V/F converter. Finally, the data is sent to 8051 for analysis and processing, and the input and display module circuit is used to complete the display, monitoring, early warning and printing of multi-point temperature.

　　In order to ensure the accuracy, stability and linearity of the instrument's temperature measurement, two measures are mainly taken in hardware. On the one hand, it is necessary to select a suitable temperature sensor. The parameter performance of the temperature sensor is the key to whether the performance of the whole machine can meet the design requirements. As a measuring instrument, it is not appropriate to use a general PN junction temperature sensor because its temperature measurement accuracy, stability, linearity and consistency are relatively poor and cannot meet the design requirements. Here, the integrated temperature sensor AD590KH is selected. Its temperature measurement accuracy is 0.1℃, the temperature measurement resolution is 0.01℃, and the nonlinearity is less than 0.5% in the range of 0-150℃. Its parameter performance ensures that the data accuracy, stability and linearity of the heat source data collected on site after being converted into a voltage signal are higher than the measurement standard. In addition, the integrated temperature sensor AD590 has good consistency. After the instrument has been used for a certain period of time, the user can easily replace the probe by himself. On the other hand, in the process of signal selection, transmission, amplification and A/D conversion, some interference is inevitably introduced, causing certain errors in the data. In order to ensure that the performance indicators of the instrument meet the requirements of the measurement standards, higher requirements are placed on the structure of the instrument's forward channel and the selection of device performance parameter indicators. In software design, digital filtering and linear software correction are used.

　　As shown in Figure 1, 30-channel temperature sensor AD590KH is connected to two analog switches 4067 to form a 30-to-1 heat source data selector. The on and off of the 30-channel signal is controlled by programming P1.0~P1.4 of the P1 port of the 8051 microcontroller, so that the 30-channel heat source data are sent to the instrument amplifier AD524 in turn to realize the difference amplification one by one.
                     

　　The instrument design requires a temperature measurement resolution of 0.01°C, a temperature measurement range of 30-40°C, and an amplifier output voltage range of 0-10 V. Since the system has very high requirements for temperature measurement parameter performance, in order to meet the design standards, the amplifier adopts differential amplification of the signal. The higher the gain of the differential amplification, the higher the resolution and sensitivity of the instrument's temperature measurement output results. Here, the gain of AD524 is set to 100 times. The temperature measurement voltage value of 30℃ is used as the reference voltage value Vr of the high-performance reference voltage source LM399 and connected to the inverting input terminal of the instrument amplifier AD524 as the temperature measurement zero-scale reference point (the zero drift and offset voltage of LM399 are both less than 5 PPM/℃). The instrument amplifier is connected to the actual field detector in the same phase. The input voltage changes by 10 mV and the output voltage changes by 1 V; for every 0.1℃ change in temperature, the input voltage of the instrument amplifier changes by 1 mV and the output voltage changes by 100 mV; for every 0.01℃ change in temperature, the input voltage of the instrument amplifier changes by 0.1 mV and the output voltage changes by 10 mV.

　　As a high-performance measuring instrument, it has extremely high requirements for the parameters such as amplifier gain stability, offset voltage, zero drift and nonlinear distortion. It is not suitable to use general precision op amps as amplifiers. Otherwise, the instrument's parameter index performance may be reduced and fail to meet the measurement standards due to the error caused by the op amp in the signal amplification link. This instrument uses AD524 with high performance parameters as the instrument amplifier. The parameter index values ​​of AD524 such as zero drift, offset voltage and nonlinear distortion are extremely small, so that the error generated by AD524 in the process of amplifying the signal can be ignored in the order of magnitude of the performance parameters required by this instrument.

       2 V/F conversion

　　The system uses LM331 as a V/F converter to convert the 0-10 V voltage output by AD524 into a pulse signal with a frequency of 0-100 kHz and send it to the T0 port of the 8051 microcontroller. The measured temperature data is linearly proportional to the frequency of the pulse signal after V/F conversion. The higher the temperature, the higher the voltage output by the instrument amplifier and the higher the frequency value output by the V/F converter. When the temperature is 0℃, the voltage output by the instrument amplifier is 0 V, and the frequency value after V/F conversion is 0 kHz; when the temperature is 40℃, the voltage output by the instrument amplifier is 10 V, and the frequency value after V/F conversion is 100 kHz. Using the V/F converter to realize A/D conversion eliminates the interference of the converted data in the process of sending it to the microcontroller, and by adjusting the timing time of the T1 port and changing the number of pulses received by the T0 port counter, the number of bits of A/D conversion can be changed. This system sets the timing time of the T1 port to 100 ms, making the V/F converter equivalent to a 14 b A/D converter. By increasing the timing time of port T1, the number of bits of A/D conversion is increased, thereby improving the accuracy and resolution of the system temperature measurement data. In order to better improve the performance of the instrument, the timing time T1 is increased by n times (n is a positive integer), and the pulse data received by port T0 is divided by n to achieve software filtering.

        In addition, this tester is equipped with a keyboard input and LCD display module, making the instrument configuration perfect and the user's operation convenient and reliable.

       3 System Software

　　The temperature measurement system software is compiled in MCS-51 series assembly language. Figure 2 is the program flow chart of the system software.
                                      

       4 Conclusion

　　Practice shows that the test system built based on the above ideas is correct and feasible. The instrument was developed in nearly half a year and met the design requirements in subsequent stable operation. When the system allows a probe to measure temperature data for a few seconds, the use of a V/F converter can reduce instrument costs and improve system performance.

         References

[1] He Limin. Single-chip microcomputer application system design - system configuration and interface technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1990.
[2] Li Chaoqing. Single-chip microcomputer principle and interface technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2001.