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Lateral motion of a tractor in rotary tilling is disturbed by reaction force in the tilling operation, spatially variability of tilled soil and bumped surface of till sole. Therefore, it is difficult for the operator to steer so that the tractor runs along his desirable course.

it is prevented driving straight by the disturbances on the above though an operator keeps its running course straight. Furthermore, in his handling for putting it back on his desirable course, static and dynamic relationships between the wheel angle and the lateral motion of the body are not stable and the control response of the tractor is undesirable for his operating.

First, yaw angle controller (PI controller) of an agricultural tractor was designed. The experimental results with the controller showed that braking force distribution is effective for lateral motion control of the tractor. In the early part of the paper, the control system configuration and the experimental results are presented.

In the later part of the paper, direct yaw moment control designed through the gain-scheduled H control is presented. The performance of this controller was evaluated in a numerical simulation. The numerical value of the direct yaw moment, output of this controller is calculated so that the lateral motion of the tractor body follows the reference responses stably without steady errors. In general, lateral motions of vehicles are changed due to the traveling speed. Therefore, the gain-scheduled controller whose parameters are scheduled utilizing the measured traveling speed is designed for involving the property. The simulation results showed adequate performance of the controller in attenuating several disturbances.