| Hydraulically driven large-scale heavy-duty hexapod robots,an essential subset of legged mechanical systems,boast robust load-bearing capacities,versatile terrain navigation abilities,and elevated stability.These attributes collectively enable their operation within intricate and severe environmental conditions.Nevertheless,the deployment of such robots in pragmatic engineering applications remains limited.This is attributed primarily to the high energy demands and suboptimal energy efficiency characterizing hydraulic systems,culminating in diminished operational endurance.Compounding these challenges are the unpredictable perturbations emanating from external environments,coupled with intrinsic hydraulic shocks that exacerbate the complexity of system-wide control.Furthermore,the significant mass of these robots induces flexible deformation within the support limbs,compromising the integrity of the robot’s hybrid series-parallel mechanism.Such deformations lead to a deterioration in the kinematic performance of the entire system.Given these concerns,investigating motion control strategies that address energy dissipation and holistic stability for large-scale heavy-duty hexapod robots,specifically those exhibiting rigidflexible coupling properties,becomes crucial.Such inquiry aims to amplify energy utilization and refine the overall motion capabilities of the robotic systems in question.To this end,the present study embarks on an in-depth exploration of a multiactuator load adaptive drive system.Additionally,it delves into a comprehensive robot control approach predicated on the aforementioned drive system and the hierarchical control strategies that accommodate the robot’s rigid-flexible coupling traits.The main research work of the paper is as follows:(1)A multi-actuator load adaptive drive system based on digital hydraulic technology is proposed to address the problem of low energy utilization of the hydraulic system.This paper takes a large-scale electro-hydraulic heavy-duty six-legged robot as the research object,and carries out research on the energy-saving method of the hydraulic drive unit in view of the problems of throttling loss in its swing leg and low energy utilization of the hydraulic drive unit.A multi-actuator load adaptive drive system based on digital hydraulic technology is proposed to adjust the input pressure and flow rate of the power source of each leg according to the robot’s motion trajectory planning,so as to reduce the system’s throttling loss and overflow loss.Meanwhile,a drive system control strategy including power control,pressure/flow control,and a multi-objective function displacement matching method considering energy consumption and pressure impact are proposed.Aiming at the pressure and flow impact problem during the displacement switching process of the discrete variable pump group,a timing control method based on iterative learning is proposed to improve the smoothness of the system during the switching process by adjusting the switching time.After simulation analysis and experimental verification,the drive system and its overall control strategy proposed in this paper can effectively reduce the throttling loss of the swing leg of the foot robot,and the proposed timing control strategy can eliminate the downward impact problem during the state switching of the drive system and ensure the smoothness of the system.(2)Robot whole-body control strategy based on multi-actuator load adaptive drive is proposedIn order to improve the stability of the robot,a whole-body control strategy including attitude control and outrigger force/position flexibility control is proposed.Based on the robot center-of-mass dynamics model,the mapping relationship between the center-of-mass trajectory and the foot-ground interaction force and joint acceleration is established,and the desired foot-ground interaction force and joint acceleration to maintain the stability of the torso are solved by the quadratic programming(QP)algorithm with the minimum deviation of the center-of-mass moment as the objective function.The adjusted amount of the foot-end motion trajectory is generated by the robot’s attitude control and adaptive impedance control.On the basis of the desired motion trajectory,an electro-hydraulic actuator control strategy combining the sliding mode control based on the expanded state observer and the feed-forward compensation of the pressure shock error is proposed for the hexapod robot with large external loads and pressure shocks in the internal hydraulic drive unit.Based on the developed large-scale heavy-duty robot,it is verified through experiments and simulations that the above control strategy can effectively improve the accuracy,flexibility and environmental adaptability of the robot motion.(3)A hierarchical control strategy for rigid-flexible coupled robots is proposed,based on the analysis of the rigid-flexible coupling characteristics of a heavily loaded hexapod robot.In order to solve the problem of degradation of motion performance of heavy-duty robots with rigid-flexible coupling caused by leg deformation,a hierarchical control strategy based on centre-of-mass dynamics is proposed.Firstly,according to the robot trajectory and foot-ground interaction force,the flexible deformation feed-forward compensation control is used to reduce the flexibility error of the foot trajectory,so as to make its kinematic characteristics approximate to that of a rigid robot.Then,in order to eliminate the disturbance of the body by the residual deformation of the feedforward compensation,the vibration isolation control strategy of the hydraulic drive unit is proposed,and the optimal driving force of the hydraulic actuator to maintain the stability of the body is solved by using the quadratic programming algorithm based on the rigid-flexible coupled dynamics model of the whole machine with the dynamics of the centre of mass of the rigid body and the dynamics of the flexible leg.Finally,the synergistic control method combining the impedance control of the legs and the force/position control of the hydraulic actuator unit is used to complete the force tracking and reduce the disturbance of the flexible deformation of the leg mechanism on the carcass.Through experimental verification and simulation analysis,it is proved that the hierarchical control strategy can effectively reduce the deviation of the centre of mass trajectory caused by the deformation of the mechanism of the rigid-flexible coupled robot,and improve the stability of the robot’s motion. |
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