Improving flow response of a variable-rate aerial application system by interactive refinement

Experiments were conducted to evaluate response of a variable-rate aerial application controller to changing flow rates and to improve its response at correspondingly varying system pressures. System improvements have been made by refinement of the control algorithms over time in collaboration with the system manufacturer, Houma Avionics, Houma, LA, USA. The variable-rate application system consists of Differential Global Positioning System (DGPS)-based guidance, AutoCal II automatic flow controller, and hydraulically controlled spray pump. The AutoCal II was evaluated for its ability to track desired flow rates set by the pilot. The system was then evaluated over several field trials to quantify its response to rapidly changing flow requirements and to determine the effect of the latest control algorithm improvements on response characteristics. System responses were analyzed while operating the AutoCal II in automatic mode over a pre-set field prescription containing four management zones (28, 47, 56, and 37 L ha−1 each 81 m long). To evaluate the effect of control algorithm improvements, areas under the flowrate-time curves were integrated and percentage differences in areas between those response curves and target flow rate curves were determined. Results for south–north runs indicated reduction of average error from 6.9% before control algorithm modification to 1.8% after algorithm modification. Benefits of a new flow monitor with capabilities for improved data acquisition resolution were illustrated by examining data from both 2005 and 2008 prescription runs. For the 2005 data, integration times per run matched expected values based on ground speed when using either the new flowmeter monitor or conventional monitoring via the AutoCal II with its irregular data integration intervals. The 2008 data showed inconsistencies in total integration time per run when reading flowmeter data via the AutoCal II; these intervals varied between 1.2 and 1.66 s. Integration timing intervals matched expected results when using the new flowmeter monitor instead of the AutoCal II to read and output data. Inconsistencies in AutoCal II timing were attributed to possible modifications in loop-timing portions of the AutoCal II control algorithm since the 2005 run was conducted. These results further support the value of the new flowmeter monitor in providing consistent results regardless of changes the manufacturer might make to the AutoCal II data acquisition and control program. The experiments served to illustrate an example of how iterative refinement of control algorithms in collaboration with the control system manufacturer could improve system response characteristics. System evaluation techniques described should also apply to aircraft that use propeller-driven spray pumps as well as hydraulically controlled spray pumps.