Heat source automatic tester based on microcontroller control

　　In the process of drug quality monitoring, temperature measurement of heat source reaction is an important content. Drug testing has its own particularities and has high requirements on the accuracy, stability, consistency, linearity and other indicators of the testing system. Most of the traditional old-fashioned instruments are various types of thermometers, and their testing efficiency and accuracy are unsatisfactory. According to the requirements of on-site monitoring, an automatic heat source tester based on microcontroller control was developed, which realized the automatic circuit test of 30 heat sources in the laboratory, met the high standard requirements of accurate and stable on-site temperature measurement, and successfully completed the drug testing laboratory testing instrument. of upgrading.

　　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 the multi-point test is sent to the instrument amplifier through the multi-channel switch to achieve difference amplification, and then the V/F converter is used to convert the voltage signal into a certain frequency. The pulse signal is 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 implements high-precision A/D conversion through the V/F converter. Finally, the data is sent to the 8051 Perform analysis and processing, and cooperate with the input and display module circuits to complete functions such as multi-point temperature display, monitoring, early warning, and printing.

　　In order to ensure the accuracy, stability and linearity of the instrument's temperature measurement, two main measures have been taken on the hardware. On the one hand, it is to select the appropriate 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 suitable 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. The integrated temperature sensor AD590KH is selected here. Its temperature measurement accuracy is 0.1°C, its temperature measurement resolution is 0.01°C, and its nonlinearity is less than 0.5% in the range of 0 to 150°C. Its parameter performance ensures on-site collection The data accuracy, stability and linearity after converting the heat source data into voltage signals are higher than the measurement standards. In addition, the integrated temperature sensor AD590 has good consistency, and users can easily replace the probe by themselves after using the instrument for a certain period of time. On the other hand, in the process of signal selection, transmission, amplification and A/D conversion, some interference will inevitably be introduced, causing certain errors in the data. In order to ensure that the performance indicators of the entire instrument meet the requirements of measurement standards, higher requirements are put forward for the structure of the forward channel of the instrument and the selection of device performance parameter indicators. In terms of software design, digital filtering and linear software correction are used. .

　　As shown in Figure 1, the 30-channel temperature sensor AD590KH uses two analog switches 4067 to form a 30-select 1 heat source data selector. The 30-channel signals are controlled on and off through P1.0~P1.4 of the P1 port of the 8051 microcontroller. The 30 channels of heat source data are sent to the instrument amplifier AD524 in sequence to realize difference amplification one by one.

　　The instrument design requires a temperature measurement resolution of 0.01°C, a temperature measurement range of 30 to 40°C, and an amplifier output voltage range of 0 to 10 V. Since the system has very high performance requirements for temperature measurement parameters, 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 AD524 The gain is set to 100 times. The temperature measurement voltage value at 30°C 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/°C), when the instrument amplifier is connected to the non-phase terminal, the input voltage of the actual field detector changes by 10 mV, and the output voltage changes by 1 V; for every 0.1°C 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°C 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 on parameters such as amplifier gain stability, offset voltage, zero drift, and nonlinear distortion. It is not suitable to use a general-precision operational amplifier as an amplifier, otherwise the operational amplifier may affect the signal amplification. The error caused by one link reduces the parameter performance of the instrument and fails to meet the measurement standards. This instrument uses the AD524 with high performance parameters as the instrument amplifier. The zero drift, offset voltage, nonlinear distortion and other parameters of AD524 are very small, so the error generated by AD524 in the process of amplifying the signal is within the performance parameters required by this instrument. It is negligible in magnitude.

　　2 V/F conversion

　　The system uses LM331 as a V/F converter to convert the 0~10 V voltage output by the AD524 into a pulse signal with a frequency of 0~100 kHz, and sends 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°C, 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°C, the voltage output by the instrument amplifier is 10 V, and the frequency value after V/F conversion is 100 kHz. Using a V/F converter to implement A/D conversion eliminates the interference in the process of sending the converted data to the microcontroller. By adjusting the timing time of the T1 port and changing the number of pulses received by the T0 port counter, the A/D can be changed. Number of digits to convert. 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 the T1 port, the number of A/D conversion digits is increased, thereby improving the accuracy and resolution of the system temperature measurement data. In order to better improve the performance of the instrument, increase the timing time T1 by n times (n is a positive integer), and divide the pulse data received by the T0 port by n to implement software filtering.

　　In addition, this tester is equipped with keyboard input and LCD display modules, making the instrument fully configured and user-friendly and reliable.

　　3 system software

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

　　

　　4 Conclusion

　　Practice has shown that the test system built based on the above ideas is correct and feasible. The instrument was developed in the past six months and has met the design requirements in its stable operation in the future. When the system allows one probe to measure temperature data for a few seconds, using a V/F converter can both reduce instrument costs and improve system performance.

　　References

[1] He Limin. Microcontroller application system design - system configuration and interface technology [M]. Beijing: Beihang University Press, 1990.
[2] Li Chaoqing. Microcontroller principles and interface technology［M］．Beijing: Beihang University Press, 2001.