Environmental Adaptive Control Method Of Trotting Quadruped Robot Based On Contact Force Control

With the development of scientific research and engineering,the robot plays a more and more important role in human society.However,because the opetation environment is very complex,the exercise ability and environmental adaptability of the robot faces great challenge.The researchers hope to initate the geometric configuration of the bios in the nature for improving performance of the robot.So the bionic robot has become a hot research direction in recent years.As a successful example,the quadruped robot has great potential for adapting the complex environment by initating the configuration and locomotion mechanism of the quadruped mammals.However,current technical level,especially in China,is much lower than expectation.The performances include low velocity and energy efficiency,inadequate compliance and adaptability for environment and so on.The main reasons are complex system,various gaits,high dimension dynamical model.So there is difficult to analyse the trot gait mechanism based on dynamical model directly.Furthermore,how to improve the compliance of the robot,how to keep the posture of the body stable in unknown environment is the key technology which has not been solved completely.In this paper,the gait dynamical mechanism,the dynamical stability analysis and the environmental adaptive control algorithm is studied for the trotting quadruped robot,mainly including the following contents and creative points:The dynamical model of the quadruped robot is built based on lagrangian method to analyse the controllability and static stability of the body posture,study the trot gait dynamical mechanism in a single stance phase.Based on gait mechanism,the dynamical model is decoupled and simplified by approximating as a 3D seven-link mechanism and a linear inverted pendulum model.So 6-DOF posture of the body is approximately decoupled as “5+1” form.Simultaneously,by approximately decoupling the diagonal swing legs,the body and the diagonal stance legs,the high-demension nonlinear dynamical model is transformed to multiple low-demension models.And the trot gait dynamical mechanism is described more clearly.As a creative point of this paper,considering the switch process between the adjacent stance phases,the approximate dynamical model in a single stance phase is extended to the whole trotting process.And the switch dynamical model is built to analyse the controllability of the body posture,the stability of the quadruped robot and the amount of the input variable under static and dynamic conditions.Furthermore,the influence on the stability of the body posture caused by the unknown rough terrains is analysed from the perspective of model,as the theoretical basis of designing controller.Aiming at the compliant control problem in the process of quadruped trotting,a dual-loop control strategy based on contact force at the feet is proposed in this paper.The inner-loop is single leg control loop.A landing phase is added between the swing phase and the stance phase to decreasing the impact force caused by the unknown rough terrains.And the optimization algorithms for the joint torque of the stance legs and the joint angular velocity of the swing legs are designed to decrease the desired force and linear velocity of the hydraulic actuators at joints.Based on the optimization results,aiming at the patameter uncertainty in the dynamical model,a hybrid motion/force robust controller and a motion robust controller are designed to realize the tracking of the contact force of the stance legs and the foot position of the swing legs,while the constraints of the feet landing angles are fulfilled simultaneously.The outer-loop is the body posture control loop.Aiming at the bounded disturbances owing to the model patameter uncertainty and landing impact force,a contact force distribution algorithm based on adaptive sliding mode is proposed to adjust the desired contact force of the stance legs.And a optimization distribution algorithm based on SCI coefficient is designed to decrease the unexpect slippage between the stance feet and the ground.Based on above dual-loop control strategy,the body posture of 3D seven-link mechanism keeps asymptotically stable in a single stance phase.Considering the switching process between the adjacent stance phases,the quadruped robot system transforms a periodically switch system from an autonomous system.In this paper,aiming to the locomotion control problem about the quadruped robot in the switching process,based on the switching dynamical model,the following two contents have been studied.Simultaneously,some and theoretical and experimental conclusions have been obtained: 1.Uniform stability criterion and Uniform asymptotically stability criterion about body posture of the trotting quadruped robot are proposed;2.The switching controllers based on 3D seven-link mechanism and linear inverted pendulum are proposed to realize uniform asymptotically stability of the body's orientation and height,simultaneously the horizontal position error being kept in a bounded region which is relevant to gait cycle.Aiming at different rough terrain,for example: step,lateral external force lashing,slope environment and so on,the strategies including virtual contact force ecaluation method,one order elastic damping model,adjusting the transformational relation between the coordinate systems are added to the proposed switching controller to realize the quadruped robot balanced trotting locomotion.