Research On Near-infrared Moisture Detection Based On Digital Lock-in Amplifier

Accurate measurement of material moisture content is important for process control in many industrial fields.In many water detection methods,near-infrared moisture detection by virtue of continuous online,non-contact measurement,fast response and other advantages,has been widely used in many areas.In the sintering production process,sintering mixture mois-ture is one of the important parameters.On-line measurement of moisture content of sinter mixture is very difficult due to the interference of water content in the sintering process.At present,the application of a wide range of sintering mixture moisture content on-line detection equipment is mainly based on near-infrared absorption spectrum of near-infrared moisture me-ter.The moisture meter modulate high-power halogen light source by cutting disc,with power consumption,Components,modulation frequency by the mechanical speed limits and other deficiencies,affecting the instrument's measurement accuracy,stability and service life.Aiming at the shortcomings of the traditional near-infrared moisture meter,a new near-infrared moisture detection scheme is adopted in our research,that is,infrared LED light source instead of the traditional light source,electrical modulation optical signal instead of mechanical modulation.However,due to the low luminous power of the LED light source,the signal received by the Photoelectric detector is weak and there is a problem that it is difficult to detect accurately.Otherwise,based on full understanding of previous research work,the use of digital phase-locked amplifier technology to detect the Photoelectric detector received signal processing.Digital lock-in amplifier not only greatly simplifies the circuit design,but also effectively improve the signal to noise ratio.The research contents are as follows:Firstly,the theory of moisture detection and weak signal detection is studied.Based on the design and simulation of the digital lock-in amplification technology,the overall scheme of near-infrared moisture detection system is designed.Secondly,the optical system of the new near-infrared moisture detection system is analyzed and calibrated,so that the Photoelectric detector can receive more optical signals.Then,the analog signal processing circuit,such as preamplifier circuit and filter circuit,is optimized,and a signal driver circuit based on adjustable frequency and stable current and DSP-based signal detection circuit are designed.At the same time,the digital signal processing program based on DSP processor and the human-computer interaction program of serial-port touch-screen were written at the same time of building the near-infrared moisture measurement system hardware platform.Finally,a series of validation experiments were carried out using the near-infrared moisture measurement system.In the laboratory conditions,we use the drying method to calibrate the moisture measure-ment system.Then the moisture content of 3%to 8%of the sintering mixture for moisture measurement experiments.The experimental results show that the maximum absolute mea-surement error is 0.32%water content and the relative standard deviation is 1.91%when the measuring distance is 0?150mm.