| Compared with the traditional dump truck with mechanical transmission, 60 tons articulated dump truck driven by six wheels electric wheels is more superior and has become the main conveyance in open coal mining industry. It has more control freedoms for being driven in this way. However, it also brings the problem of differential control for six wheels. Focusing on the problem that existing differential control strategies can only achieve differential control with poor dynamic performance for ignoring yaw control and drive anti slip control, this paper studies and designs a simple and feasible differential control strategy to make each wheel work in the best way under different conditions.Firstly, the structure composition, working principle and characteristics of electric drive system, steering system and suspension system are analyzed. The kinematic relationships between left and right steering hydraulic cylinder are derived and analyzed. The parameters like maximum turning angle, inlet pressure and oil return pressure are calculated. The parameters related to stiffness and damping of the hydro-pneumatic suspension are derived and calculated as well.Secondly, the relationships of velocity, acceleration and force between front and rear body are analyzed. The dynamics mathematical model of the whole truck which contains longitudinal, lateral, yaw and roll degree of freedom is derived based on the lagrange equation method. The Gim theoretical tire model and some auxiliary calculation models including wheel normal load model, tire velocity model, tire side slip angle model, wheel rotation dynamics model and road model are built. Finally, the simulink model of the whole truck is constructed and its correctness is verified.Thirdly, the steering kinematics relationships of six wheels are derived based on the Ackermann steering theory. The control principles of two common differential control strategies such as equal torque control and equal slip ratio control are summarized, and their Simulink control models are built. Simulation results show that both control strategies can achieve differential control, but still have some shortages. Thus, a differential control strategy based on driving force hierarchical control is put forward, which is composed of total driving power and yaw control power decision layer, differential drive distribution layer and driving anti-slip stability layer. In decision layer, the total driving power is acquired based on linear driving style, and the needed yaw control power is calculated by using auto disturbance rejection control algorithm to eliminate steering angle deviation. In distribution layer, the total driving power is distributed according to axle load while the needed yaw control power is distributed differentially. In stability layer, a wheel working condition and optimal slip ratio recognition algorithm is designed to modify the driving power of each wheel. Simulation results has showen the effectiveness of the recognition algorithm and the driving force hierarchical control strategy.Finally, virtual prototyping model of the whole truck based on Adams and co-simulation model by taking Matlab/Simulink as the main simulation environment are built. Co-simulation experiments for the three differential control strategies are carried on under the same condition respectively. The shortagies of equal torque control and equal slip ratio control and the effectiveness and superiority of the the driving force hierarchical control strategy are further confirmed. |
