Development and evaluation of multi-mode four-wheel electrohydraulic steering system on a sprayer vehicle
The trend in North American agricultural equipment is to produce larger, faster machinery in order to increase productivity. Agricultural sprayer vehicles are being designed with boom lengths exceeding 100 ft and field speeds up to 20 mph. These two parameters, along with the movement towards automatic guidance of sprayers, make steering control of the utmost importance. The availability of four-wheel multi-mode electrohydraulic steering provides steering options to potentially increase the spray accuracy and efficiency, while reducing crop damage and increasing vehicle maneuverability. In this research, a self-propelled agricultural sprayer was modified to allow the rear wheels to be steered using electrohydraulic control valves. This enabled the vehicle to be steered in multiple modes. The research focused on evaluating the effect of multi-mode four-wheel steering on the vehicle handling characteristics and vehicle performance of the sprayer. This thesis presents the research conducted in modeling the vehicle, developing the control and evaluation methods, and interpreting the results of the testing. Four-wheel multi-mode steering enables the vehicle to have the option of steering with two wheels (conventional), all four wheels in the same direction (crab), or steering the front two wheels in one direction and the rear two wheels in the opposite direction (coordinated). The multi-mode steering system was evaluated by driving the sprayer through specified paths in the different steering modes. The position and heading of the vehicle was measured for each mode using two dual frequency DGPS receivers. From the GPS data, sprayer performance measures such as over/underspray, skip and overlap, and crop damage were assessed for each steering mode. This research showed that multi-mode four-wheel electrohydraulic steering is a practical option for sprayer vehicles. The results of the research show that there are performance advantages and disadvantages for each mode of steering. Coordinated steering increased the maneuverability of the vehicle, while crab steering reduced the misapplication during a lateral path adjustment. The research also showed that there are different levels of driver variability between modes of steering. This indicates that for multi-mode steering to be an efficient option, the driver needs to be trained to operate the vehicle in each modes of steering.