ARS | Publication request: Field Assessment of A Variable-rate Aerial Application System

Submitted to: Asian Conference on Precision Agriculture
Publication Type: Proceedings
Publication Acceptance Date: August 15, 2009
Publication Date: October 14, 2009
Citation: Thomson, S.J., Huang, Y., Hanks, J.E., Martin, D.E. 2009. Field Assessment of A Variable-rate Aerial Application System. Book of Abstracts, Joint International Conference (JIAC) (formerly Asian Conference on Precision Agriculture, Beijing, China, pp. 185-186.

Interpretive Summary: Agricultural aircraft are used to apply chemical and biological pest-control agents, harvesting aids, and nutrients to assure a high-yielding and economically viable crop. Only recently have agricultural aircraft been equipped to implement variable-rate application to match site-specific needs of the crop. Variable-rate aerial application systems have seen only limited use since about 2004, and very little information has been presented on the accuracy of these systems for placement of chemical and response of these systems to changing rate requirements. Aerial flow control systems must also adjust flow properly to accommodate changes in ground speed. Previous studies conducted in our fields using an Air tractor 402B aircraft have quantified application error with respect to field boundaries whose precise locations were known. Spraying system response for a single group of spray runs was also illustrated, and responses tended to vary from overdamped to slightly underdamped depending on rate change levels. Potential improvements to the control algorithms were proposed. This study was designed to quantify system response to changing rates before and after the system manufacturer changed the control algorithm to improve response. Results for a set of south-north runs indicated reduction of average error from 6.9% before control algorithm modification to 1.8 % after algorithm modification. North-south runs indicated response lags after control algorithm modification and the system’s inability to meet flow demands at the highest flow rates. This was due to the system calling for more spray than the booms were able to deliver at high ground speeds. These factors biased post-modification results unfavorably when compared with results obtained before the control algorithms were modified. The experiments served to illustrate an example of how recursive refinement of the control algorithms in collaboration with the control system manufacturer could improve system response characteristics. System evaluation techniques described herein should also be applicable to aircraft that use propeller-driven spray pumps as well as hydraulically controlled spray pumps.

Technical Abstract: Several experiments were conducted to evaluate the system response of a variable-rate aerial application controller to changing flow rates. The research is collaboration between the USDA, ARS, APTRU and Houma Avionics, USA, manufacturer of a widely used flow controller designed for agricultural aircraft. The variable-rate application system consists of Differential Global Positioning System (DGPS)-based guidance, AutoCal II automatic flow controller, and hydraulically controlled 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 improvements made in the control algorithms 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 each 81 m long). To evaluate the effect of control algorithm improvements, areas under the flow rate-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. North-south runs indicated response lags after control algorithm modification and the system’s inability to meet flow demands at the highest flow rates. This was due to the system calling for more spray than the booms were able to deliver at high ground speeds. These factors biased post-modification results unfavorably when compared with results obtained before the control algorithms were modified. The experiments served to illustrate an example of how recursive refinement of the control algorithms in collaboration with the control system manufacturer could improve system response characteristics. System evaluation techniques described herein should also be applicable to aircraft that use propeller-driven spray pumps as well as hydraulically controlled spray pumps.