Evaluating the Response Time of a Rate Controller Used with a Sensor-Based, Variable Rate Application System

Abstract

Technology for real-time sensor-based variable rate application equipment can involve intricate systems. A sensor system typically provides the rate-controller a set-point every second creating a challenge in terms of the response time to achieve the desired rate using existing rate-controllers, valves, and nozzles. Response time of a commercially available rate-controller was evaluated with two applicator configurations: a pulse width modulated (PWM) system with fixed orifice nozzles (PWM-applicator) and a standard system with a fast close (FC) valve and variable orifice nozzles (FC-applicator). These applicator configurations were tested with two input signals: a step input rate change common in a map based system using variable rate technology (VRT) and a simulated real-time sensor-based VRT input that updated every second. Pressure, flow rate, and controller input from the simulated sensor system were measured and recorded with a data acquisition system. The data were analyzed to determine if the applied rate correctly followed the desired set-point rate while evaluating the associated response time of the rate-controller for different valve speed and brake point settings. Results suggest that corresponding minimum response time had to be determined for each configuration for optimum performance of VRT equipment. The PWM-applicator response time with a simulated sensor input was 0.5 s and with a simulated map input was 1.5 s. The FC-applicator response time with a simulated sensor input was 0.6 s but 2.1 s for a simulated map input. Although the PWM-applicator had a slightly lower response time than the FC-applicator, flow rate and pressure were more stable with the FC-applicator and the application error was less. However, if a variable rate controller is optimized for map-based application, it will likely function adequately in a sensor-based system. Results from our study can assist in determining the spatial resolution for variable rate application using commercially available VRT equipment.