Study On Variable Stiffness Rotatianal Hip Joint Of A Robotic Hopping Monopod And Its Feature

During the million-year course of evolution, the terrestrial animals gradually optimized the body structure and functionality to suit their living environment. The leg stiffness adjustment mechanism is one of the many adaption features. Research on legged robots regularly refers to the running animals as study model, and tried to design robotic legs purposely mimicking their counterparts in land animals. Altho ugh the research product is abundant, the study on the hip joints’ rotational elastic feature and its influence on hopping dynamic is currently not often reported. This thesis is aim at studying this specific area, and devoted to explore and take advantage of the elasticity in hip joint to success a good dynamic feature. In legged robots, such functional hip joint should be achieved by applying variable stiffness joint. Because this particular application in legged locomotion, the task brings upon a big challenge to the design and motion control.Aimed on the application of variable stiffness rotational hip joint on robotic monopod, we employed the Poincare mapping fixed point analysis method. Targete d at the analysis of hopping monopod model, stability of passive gait is studied. By comparing the dynamic feature of hopping monopod with and without the compliant rotational hip joint, the influence of hip joint’s passivity on hopping dynamic is revealed. Further, based on the flight phase active control strategy the stable fixed point could be found in an enlarged parameter space, based on this, with the analys is on the energy cost evaluation function, the effect of hip joint stiffness on hopping energy recycling is studied. By comparing the energy feature with varied hip joint stiffness, their relation is established and obtained the relation of hip joint stiffness with the movement parameters. Simulation study is carried out to prove the effectiveness and could be used as a guide for developing hopping robotic monopod.On summarizing the currently developed variable stiffness joints, the design principle based on stepless transmission ratio adjusting concept is adopted. To fit the joint design, lever mechanism is utilized and adjusted and embedded in the joint structure. For legged robot, weight and size constrains are satisfied through compact design while maintaining sufficient driving capacity. In the joint, the stiffness adjusting and motion control are realized via independent driving chains, this eliminated the motion coupling for the control, makes it easier for joint control tasks, it is an advantageous precondition to the monopod hopping control. The stiffness output feature is thus studied and the effectiveness is tested by both simulation and joint prototype test.As this thesis aimed at, the hopping dynamic is adjusted by the joint stiffness control activities, this task cannot be fulfilled with the absence of reliable joint stiffness online identification. To achieve this, the Tailor expansion is employed in the first step to linearize the internal compliant torque with the joint for a good estimation, and in order to remove the influence of stiffness adjusting parameter on joint stiffness, we here decoupled two separate joint parameters, and Kalman filter is further used for estimating expression function coefficients. Combined with the identified inertia and damping, joint stiffness is identified and this method is tested by simulations. Base on the stiffness identification result we then developed the feedforward base stiffness close loop control strategy and achieve good result.Finally, experimental system of the variable stiffness rotational joint is built, and carried out experiment on joint stiffness output feature, it is proved that the lever mechanism with stepless transmission ratio adjusting feature could be as core component. Joint control experiments are also launched and achieved close loop control result. The effectiveness is verified. Further the hopping monopod with variable stiffness hip joint is developed, the test bed hardware compartment is demonstrated and the hopping test is carried out to validate the availability for variable stiffness joint in hopping robotic monopod experiment system.