Active Variable-Impedance Based Compliant Landing Control For Hydraulically-Actuated Legged Robots

Legged robots can adapt to complex terrain because of their excellent motion capability and diverse locomotion gaits,which gives them wide application prospect.Stable locomotion in complex terrain is the fundamental problem to be solved among the legged robots motion control.The key but difficult point of stable locomotion is the controlling of compliant contact between robots and environment,which is the compliant landing control for legged robots.This paper mainly focuses on the actively compliant landing problem of hydraulically-actuated legged robots based on variable-impedance control.It includes the output-force controlling of joints cylinder,the variable-impedance parameter designing of the bionic leg during compliant landing,the stability analysis of the variable-impedance based compliant landing controller and the performance evaluation of the controller.The experimental results of the hydraulically-actuated testing single leg platform showed the superior performance and application value of the designed controller.Some achievements are listed as bellow:1.Through the derivation and analysis of the dynamics model of the hydraulic actuator output force,it is found that the output force model has a natural feedback of the load velocity which limits the bandwidth of the force close-loop control system and the force tracking performance.In order to eliminate the influence of the load velocity natural feedback,a load velocity positive feedback term and a compensation term are introduced in the output force control loop with PI controller.With this controller,accurate tracking of the 2Hz sinusoidal signal is obtained with load free motion.2.An actively variable-impedance based compliant landing controller is introduced with the load velocity positive feedback based joints force controller to reduce the shock force during landing.The compliant landing controller consists of a Finite State Machine,a foot-end Planning Trajectory and actively Variable Impedance.The Finite State Machine is based on foot-end contact force and system time.It divides the landing motion into Swing phase,Landing phase and Support phase and a specific control law is introduced in different phase.The planning trajectory is based on polynomial interpolation which meets the need that foot-end velocity equals 0 at the TouchDown,LiftOff moments to enhance compliance during landing.The parameters of variable impedance are inspired from human leg impedance characteristic during walking to emulate the human compliance.Finally,a Lyapunov function is introduced to prove the stability of the compliant landing controller.3.Finally,a hydraulically-actuated leg testing platform is constructed to test the performance of the compliant landing controller.With the variable-impedance based compliant landing controller,DropDown experiments,consecutive compliant Landing experiments and consecutive compliant Hopping experiments are introduced.Compared with the shock force of dogs during galloping gait,the experimental results showed the superior performance and application value of the designed controller in decrease shock force during landing.