Benchmarking the dynamic performance of two commercial variable-rate controllers and components

Several commercial variable-rate (VR) control systems are available for implementation of single tree-zone fertilization for Florida citrus. There has been little performance evaluation of these control systems and their sub-components (including hydraulic flow-control valves and encoders) and trigger modes. This study developed test procedures and two evaluation criteria, total application error (TAE) and total single tree-zone application error (TSAE), to determine the effect of each component on performance. The time delays inherent in the controller modules for application rate changes in response to GPS triggering were higher compared to triggering by real-time sensors. It was found that the performance of a proportional solenoid valve was 36% and 43% better than that of a DC motor-operated valve for the TAE and TSAE criteria, respectively. The resolution of the encoder had a direct effect on system performance. Performance of a higher-resolution encoder was 54% and 58% better than that of a lower-resolution encoder for the TAE and TSAE criteria, respectively.