A Velocity Tracking Approach Based on Neural Networks for a Multimodal Wheel-Legged Composite Gait on a Parallel Structure Robot

Parallel-legged robots are favored by many researchers due to their potent environmental adaptability and load capacity. However, the work efficiency of the parallel leg structure is always confined due to its uptight workspace and extremely uneven force distributed among actuators. Therefore, this article proposes a new type of wheel-legged composite gait for the parallel structure, inspired by the characteristic of variation in the pitch angle of the foot-end when a human walks. First, the kinematic inverse solution of the wheel-legged composite gait is derived, where the reverse rotation of the wheel alleviates the slipping phenomenon of the foot-end on stance stage in humanoid gait from the kinematic level. Meanwhile, numerical simulation and experiments have verified that the proposed gait can enlarge the workspace of a single leg for 25.4% and alleviate the maximum force on the actuators for 16.4% minimizing actuator wear. Besides, a legged motion speed tracking approach is implemented for dynamics of the wheel-legged composite gait. Among them, a whole body dynamics model is constructed to adjust the neural network weights enabling real-time adjustment of incremental proportion–integration–differentiation control parameters based on the foot longitudinal force, to scale up its control efficiency. Finally, to ameliorate the issue of limited contact friction between the robot and the ground, an encoding approach to identify the sliding state of the robot's foot-end in the X-Y plane has been proposed. This approach adaptively modifies both the longitudinal force threshold of the foot-end and the desired linear velocity of the robot body. These adjustments are guided by specific control rules, which aim to enhance the accuracy of speed tracking during legged motion across varying terrains. The resulting linear and yaw velocity steady errors are constrained to 0.05 m/s and 0.012 rad/s, respectively.