Abstract
Existing agricultural tractor path tracking methods rarely consider tracking accuracy and driving stability simultaneously. To solve this problem, a novel path tracking method of tractor is proposed in this paper. The method contains two layers of controllers. The first layer is the desired front wheel steering angle controller based on path prediction, and the desired yaw rate of tractor can be obtained by reference model expression. The second layer is the front wheel steering angle compensation controller based on improved second-order sliding mode (SOSMC) to improve tractor driving stability. The innovation of SOSMC is a combination of nonlinear integral sliding mode (NISM) surface and improved Super-Twisting algorithm, which can not only improve the rate of the control system to the sliding mode surface, but also suppress chattering. To verify the performance of SOSMC, the simulations are carried out on three paths at the constant speed of 10 km/h through Matlab/Simulink and Carsim. Three working paths including arched, pear-shape and lemniscate are constructed in Carsim. The results show that the control effect under three controllers is better than no control. Compared with no control, the maximum value and root mean square (RMS) of lateral error under SOSMC are reduced by 13.09% and 25.07% on average in three paths, and that of yaw rate are decreased 53.29% and 54.64%. The tracking accuracy of SOSMC is the best of three controllers, and that of NISMC is better than SMC. Compared with NISMC, the maximum value and RMS of yaw rate under SOSMC are reduced by 37.74% and 43.62% on average. The yaw rate, front wheel steering angle compensation value and chattering amplitude of front wheel steering angle under SOSMC are significantly smaller than those of SMC and NISMC. In summary, the novel path tracking method can simultaneously satisfy the accuracy and stability of the tractor when it tracks the field working path.
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