Research On Servo Control And Stable Walking For A Hydraulic Quadruped Robot

Robotics is a multi-discipline technique,including machinery,electronics,information science,cybernetics and artificial intelligence.Now,it has become the most prevalent research area in both academic and industrial organizations.Decades of developing have made these days a critical moment to gain huge breakthroughs for practical robot.Among legged robots,quadruped robots have high speed,flexibility and stability,which will help them be widely used in cargo transportation,disaster rescue,military surveillance and civil applications.Therefore,research on quadruped robots has gained theoretical and applicable significance.As an important research branch,hydraulic actuated quadrupeds have drawn extensive attentions of researchers.With hydraulic servo systems,hydraulic quadrupeds have fast moving speed and high load capacity,which make hydraulic quadrupeds the preferred robots for high load dynamic running.This thesis focus on some key points related to hydraulic quadrupeds,including precise hydraulic servo control,leg compliance control and robot static/dynamic stable walking.Electro-hydraulic servo cylinder is the basic actuating unit of hydraulic actuated robots,it's displacement control performance determines the trajectory tracking of the robot leg,then hugely affect the robot's walking stability.Due to its nonlinearity,unknown parameters and parameter variation,classical linear controller will simply fail in the precise control of electro-hydraulic servo cylinder.In addition,along with the switch of gaits,the load of cylinders will change drastically,which brings demand of high robustness to the servo controller.For a mobile robot,the resources of the central processing unit are limited,so the control algorithm needs to be simple,implementation convenient and computational economical.Due to these aspects,an adaptive robust controller with fast parameter estimation has been proposed.Moreover,improvements are also made to simplify the controller so that a high performance displacement controller suitable for the electro-hydraulic servo cylinder is derived.To reduce the foot-end contact force and improve the walking stability,leg compliance of the quadruped robot is analyzed.Under displacement servo control,robot legs have high stiffness,which will produce huge contact force when legs touch the ground.Spring damper can decrease the contact force,but the fixed stiffness and damping ratio make the legs load non-adaptive and also bring oscillations.To adapt to the load and regulate the stiffness and damping ratio,a compliant leg controller is proposed by combining the active compliance with the passive spring.With the proposed method,hydraulic cylinders and springs can reduce contact force and absorb contact energy so that low impact stable foot-ground interaction is obtained.Static walking is preferred for the robot to climb across obstacles and through uneven terrains.With the help of LIDARs and stereo vision,classical static walking method considers the robot's passing ability on rough terrains with known terrain information.In the absence of these information,we proposed a fuzzy algorithm to estimate the terrain,so that the pitch and roll angles of the terrain are obtained using only cylinder displacements and body posture.With body attitude and center of gravity adjustment,the robot shows an improved passing ability on unknown rough terrains.Trot gait is the most applicable gait for quadruped robots since it has a relatively high speed and energy efficiency.To enhance trot walking and running stability,swing leg retraction is involved and a Bezier curve base gait trajectory is designed.Retraction rate is analyzed for the influence on trot stability.Simulations and experiments show that the proposed method can improve both horizontal and vertical stability.Model based dynamic gait analysis is a popular method to achieve stable walking.Due to the complexity of the multi-degree-of-freedom leg,researchers proposed the spring loaded inverted pendulum(SLIP)equivalent model.Nevertheless,on account that the equivalent spring is not parallel to the real spring,the model accuracy is reduced.To increase the equivalent accuracy,an articulated SLIP model with coincided equivalent and real spring is proposed.In addition,the approximate analytic solution,return map and self-stability of the proposed model are analyzed,and a model based hopping controller is given.Theoretical analysis and simulation results show the effectiveness of the proposed model.Therefore,a novel idea for dynamic stable walking is provided and a theoretical and practical basis has been laid.