Grain Loss Detection Utilizing Array Structure PVDF Films

Harvesting is one of the most important processes in agricultural production.Online grain loss detection is one of the key indicator for evaluating the quality of harvesting operations.Piezoelectric grain loss sensors have become the mainstream in such application for both scientific research and industry.The array structure sensor has been proven to effectively increase the frequency of grain loss detection.The grain loss sensor with double-crossed-layer PVDF films,developed based on the soft and thin characteristics of PVDF films,has also been proven to be able to identify and count multiple grains colliding at the same time,which can further increase the detection frequency.However,there are some problems in the grain loss sensor with array of PVDF films,and also difficulties in extracting signals from strong noise background when the harvester is running,making it is difficult to improve detection accuracy in grain loss detection.Aiming at the above problems and difficulties,this paper has carried out research on the grain loss detection based on the sensor with the array of PVDF films.The main research content and results of this article are as follows:(1)Starting from the basic structure of the PVDF film sensor,the structure was optimized with sensitivity,uniformity and detection frequency as indicators.The vibration model of the sensor was investigated through finite element simulation and particle impact test.The three methods of fixing PVDF films,the four material hardness and the five thickness of the elastic layer were compared respectively.Eventually,the optimized installation method is to fix PVDF film by one-side,and the elastic layer is 4 mm thick foamed silicone rubber.And the signal-to-clutter ratio of the sensor is improved by dividing the elastic layer.Then,the optimized was designed to be waterproof,dustproof and shockproof.And its performance was evaluated through particle collision tests.It was found that the sensitivity of the sensor was poor at the gap of the PVDF film array.When the value at the gap was eliminated,the grain loss sensors with single-layer and double-crossed-layer PVDF films can basically meet detection requirements.(2)An on-board detection device for grain loss in combine harvesters was developed based on the performance characteristics of grain loss sensors.It contains a sieve,a multiple outlet funnel,multiple sensor attachments and grain loss sensors.The sensor attachments have internal baffles that match the gaps between the PVDF films,adjusting the falling positions of grains to avoid collision with the gaps.Grain collision experiments were carried out.The characteristics of the grain collision signals and unit vibration noise were analysed under different accelerators.A three-dimensional vibration isolation platform was developed and its vibration isolation effect was evaluated.The results show that the three-dimensional vibration isolation platform has a significant effect on high-frequency noise reduction,and the vibration isolation efficiency under50%,75%and 100%accelerators are 66.39%,63.15%and 70.24%,respectively.(3)Digital signal processing methods were used to correct for the baseline drift and low-frequency noise in the grain loss signals.By comparing the effects of the two classical filters,the FIR high-pass filter based on the Kaiser window function is selected to correct the baseline drift.The upper cut-off frequency of the stop-band is 2 Hz,the lower cut-off frequency of the pass-band is 6 Hz,and the attenuation amplitude is 40 d B.An active active noise reduction system was constructed by changing the internal structure of the sensor attachment,and the active noise reduction algorithm based on LMS was investigated.It is shown that after baseline drift correction and active noise reduction,the average peak voltage at 50%,75%,and 100%accelerators drops by 46.0%,47.5%,and 57.8%,respectively,and the root mean square value drops by 57.1%.56.0%and 60.6%,respectively.(4)The recognition and counting algorithm of one sensor unit based on threshold is studied,and the influence of the trigger threshold(±V_T)and trigger interval time(T_S)on the algorithm was explored.The recognition and counting algorithm of the grain impact sensors with single-layer and double-crossed-layer PVDF films based on multi-sensor fusion are studied.After testing,the average relative errors of the algorithms of the grain impact sensors with single-layer and double-crossed-layer PVDF films are 2.88%and 3.22%,respectively,and the theoretical detection frequencies are 2778 and 4630 per second,respectively.