Optimal Control of Wheel Loader Operation in the Short Loading Cycle Using Two Braking Alternatives

The optimal control of wheel loader operation is used in order to investigate the potentials for fuel cost and cycle time minimization during the short loading cycle. The wheel loader is modeled as a nonlinear system with three control inputs and four state variables where a diesel engine generates the power utilized for lifting and traction. The lifting system is modeled considering the limitations in the hydraulics and also the structural constraints. A torque converter is included in the driveline model which introduces nonlinearities into the system and operates in different modes affecting the fuel consumption. The gear shifts during the loading cycle impose a discrete variable into the system and this is taken care of by representing the loading cycle as a multi-phase optimal control problem with constant gearbox gear ratio in each phase. Minimum fuel and minimum time system transients are calculated and analyzed for two alternative cases one where the torque converter is used to stop the vehicle before reaching the reversing point and another where the service brakes are utilized. The optimal control problem is iteratively solved in order to obtain the tradeoff between fuel consumption and cycle time for both braking alternatives. It is shown that although the engine operates at lower speeds when the torque converter is used for braking, the fuel consumption increases as higher torques are demanded from the engine during braking. The increase in fuel consumption is higher in faster cycle operations as the vehicle travels at higher speeds and larger torques are required to stop the vehicle. Wheel loader operators tend to use torque converter braking alternative as it is more convenient; however, it accompanies higher fuel consumption which highlights the importance of developing intelligent and easy to use braking systems.