Sliding Mode Control of Large Wheel Loader Powertrain for Full Throttle Directional Shifts

Ground speed control of a large wheel loader (LWL) is a very important part of a truck loading cycle. Since the engine is at full throttle for most part of a loading cycle, the ground speed is controlled by an impeller clutch/brake pedal. Essentially, this mechanical pedal, when engaged, disconnects the engine from the driveline and applies the service brakes. However, in order to properly control the ground speed of a large wheel loader, an appropriate powertrain control strategy is needed for the directional shifts (1R-1F, 2R-2F, etc.). These shifts are usually associated with unacceptable levels of jerk and acceleration. A reference trajectory for the vehicle speed based on the desired jerk and acceleration traces can be generated which, when properly tracked by appropriate control of the impeller clutch and the brakes, results in the desired levels of jerk and acceleration. A tracking controller is therefore appropriate. As indicated by simulation results, a conventional proportional plus integral (PI) tracking controller results in high jerk levels due to integrator windup effects, particularly when finite valve dead times are accounted for. A proportional controller is not suitable for such case, as it is subject to finite tracking error. Sliding Mode control algorithm is, on the other hand, a nonlinear controller with better stability characteristics and can overcome the problems stated above. Simulation results show that the sliding mode controller for the LWL powertrain results in smoother directional shifts as compared to a PI controller for the same desired shift time.