A self-propelled agricultural sprayer with four-wheel steering (4WS) was developed. A digital controller was designed and built to control the rear steering angle based on that of the front wheels through electrohydraulic control valves. Three modes of steering were enabled and investigated. Experimental methods were developed to determine what potential 4WS has in improving machine performance. In particular, machine performance of the sprayer was evaluated by measuring turning radius and performance metrics in headland turning and lateral path shift procedures. Coordinated 4WS resulted in smaller turning radii than conventional two-wheel steering (2WS). In the headland turning tests, significant mean increases in aligning distance of 5.58 m and significant mean decreases in rear wheel off-tracking area of 9.3 m2 were observed in 4WS over 2WS. In lateral path correction tests, crab 4WS substantially decreased the area and magnitude of estimated application errors over conventional 2WS, while coordinated 4WS resulted in increased application errors. These results provide evidence that 4WS could enable improvement in sprayer machine performance.
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