An Image-Based Application Rate Measurement System for a Granular Fertilizer Applicator

Abstract

<abstract> <bold><sc>Abstract.</sc></bold> With the tremendous advances and further adoption of precision agriculture in the last decade, variable-rate fertilizer applicators have been introduced to the market. However, the sensors used for measuring application rate on variable-rate fertilizer applicators are inaccurate and incapable of generating high-resolution maps of the application rate. Therefore, the objectives of this study were to develop an image-based application rate measurement system (IBARMS) that can monitor the application rate with a high accuracy and high resolution and to test the performance of the IBARMS with an open-loop controller and a proportional integral derivative (PID) controller. The variable-rate fertilizer application system consisted of a fertilizer applicator, the IBARMS, and an application rate control system. A charge-coupled device digital color camera was used to acquire images of the falling fertilizer granules. The fertilizer granules in an image were separated from the background, and the diameter of each granule was measured. The application rate was estimated from the diameter of each granule, the number of granules counted by the IBARMS, and the granule density of the fertilizer determined from preliminary tests. The application rate estimated by the IBARMS was compared with that obtained by weighing the total discharged fertilizer for a specific time. The application rate of the granular fertilizer was controlled with an open-loop controller and a PID controller. Three step changes of the rotational speed of the roller (6.2, 9.6, and 13.1 rpm) were set for both controllers. The application rate estimated by the IBARMS was close to that obtained by the weighing method. The coefficient of determination (R²) between the application rates measured by the two methods at five roller speeds (2.2, 5.3, 8.7, 12.4, and 16.9 rpm) was 0.995. The root mean square error (RMSE) of the controlled roller speed for the PID controller was 0.49 rpm, which was smaller than the RMSE (0.75 rpm) for the open-loop controller. The accuracies of the IBARMS for measuring the application rate during all roller speed steps with the open-loop controller and the PID controller were 93.0% and 93.7%, respectively. From the results, we believe that the IBARMS can measure the application rate of granular fertilizers with a high degree of accuracy. Future work includes installing the variable-rate fertilizer application system on an agricultural tractor and testing its on-the-go performance in the field.