A Hybrid Approach to Ground Speed Control of Large Wheel Loaders with Engine at Full Throttle

Ground speed control of large wheel type loaders using the impeller clutch and brakes while the engine is at full throttle is investigated in this paper. The control strategy requires the engagement and disengagement of the impeller clutch or the brakes, making the closed loop system discontinuous. Use of a single control strategy such as a Proportional plus Integral (PI) or Sliding Mode (SM) controller may not result in a satisfactory response, particularly with respect to response time, vehicle jerk & acceleration, and chatter due to brake/impeller clutch switching. The sliding mode controller has a fast response time without causing instability, but may cause controller chatter in situations where the boundary layer thickness (an envelope around the sliding surface) is not sufficient. On the other hand, a PI controller can be made free of such controller chatter with appropriate gain selection. However, it tends to be slow responding when high overshoot is to be avoided. As a result, a hybrid approach, combining the quick responsiveness of the SM controller and the chatter free, damped response characteristics of a PI controller, is taken to control the impeller clutch or the brakes. A switching criterion is used to turn on either the impeller clutch controller or the brake controller, preventing the simultaneous engagement of the brakes and the impeller clutch. The hybrid control strategy consists of utilizing a sliding mode control law to compensate for relatively small speed error while using a PI control law for large speed errors to ensure smooth engagement of the actuators. Simulation results show that the ground speed of the wheel loader can be adequately controlled using the hybrid strategy, with the engine at full throttle.