Development and Test of On-Line Spectroscopic System Based on Thermostatic Control Using STM32 Single-Chip Microcomputer

Near-Infrared (NIR, wavelength range: 780 similar to 2 500 nm) on-line spectral analysis technique has the advantages of miniaturization, rapid detection, and stable and reliable results. Therefore, this technique is widely used in the field of industrial detection. Because the spectroscopic system is significantly affected by ambient temperature, the detector is always cooled in traditional on-line spectroscopic systems. However, measurement errors, such as wavelength drift, are still generated from the optical components when temperature changes. In addition, PCs are always used for system control and spectra acquisition, which significantly increase system instability. In order to solve these problems, this paper proposes an on-line thermostatic control spectroscopic system based on an STM32 single-chip microcomputer. Firstly, STM32 single-chip microcomputer is used to control the near-infrared spectroscopy for spectra data acquisition, configuration setting, preprocessing of the spectral data, and calculation of the physical and chemical parameters of the sample. Secondly, a constant temperature control system has been developed based onthe STM32 single-chip microcomputer, in which the proportional-integral-differential (PID) control algorithm is used. The closed-loop thermostatic control has been realized for the whole spectrometer, including the optical and the circuit part. At last, an industrial communication interface including Modbus protocol communication and 4 similar to 20 mA current signal communication has also been developed based on the STM32 single-chip microcomputer. The system test results show that the whole system can be operated stably without PC for a long time. The measure errors originating from the changing of ambient temperature also reduce obviously. During a test of 48-hour system operation, the temperature of the spectrometer is controlled stably at around (5 +/- 0.25) degrees C. A much smaller relative standard deviation of absorption spectra is obtained when the thermostatic control is applied. The system integrates spectra acquisition, pretreatment, calculation of physical and chemical parameters, industrial communication and thermostatic control, which can satisfy the requirements of industrial on-line detection.