Grain loss in the harvesting process of combine harvesters not only causes economic losses to farmers but also affects the soil environment because of the lost grain covering the soil, influencing crop growth in the next season. Grain sieve loss-monitoring sensors represent an important accessory in combine harvesters, as they can not only provide current grain loss levels for the operator to adopt a rational action in time but also serve as an important performance signal for the control system. To reflect the rice grain sieve loss level of combine harvesters in real time, an indirect grain sieve loss-monitoring system is proposed in this paper. First, the grain collision rise time was obtained by the finite element method (FEM), and the parameters of the grain loss sensor signal processing circuit were determined accordingly to upgrade the monitoring accuracy. Then, grain loss distribution behind the cleaning shoe was analyzed in detail under different working parameters. Grain loss distribution functions at the end of the sieve and a monitoring mathematical model with relevant variables were established based on the laboratory experiment results. Finally, calibration experiments were carried out to verify the measurement accuracy of the sensor on a cleaning test bench, with an obtained relative monitoring error ≤6.41% under different working conditions.

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Grain loss rate is an important indicator to evaluate the performance of combine harvesters. It is indicted that signal voltage amplitude and signal frequency are the key factors for designing a grain loss sensor. In this work, the high-speed photography and signal high-speed acquisition technique were utilized to capture grain collision attitude and the corresponding collision signal characteristics and the effect of grain moisture content and collision angle on signal voltage amplitude and signal frequency was studied in detail, which lays a good foundation for optimizing grain loss signal processing circuit parameters. Then, monitoring resolution of the grain loss sensor was improved by adding constrained damping layer to the sensitive plate under the instruction of experimental modal analysis. At last, a field experiment was carried out; the field experiment results indicate that the monitoring performance improved.