Research On The Quasi Passive Dynamics Of Bio-quadruped Robot With Elastic Body

The quadruped robot is an important area in the research of robots, because of itsstability and the capability of realizing dynamic gait. Since the1960s, the quadrupedrobots have developed towards high-mobility, high-speed as well as heavy-loading.To enable the quadruped robots to run at a high speed, some researchers attempted toadopt biomimetic design using cheetah as the original version. During high speed run-ning, the cheetah will flex and then extend its elastic body. These actions are believedto be used to increase the running speed. Obviously, the flexion and extension of thebody may affect the running performance since the limbs are connected to the bodydirectly. Therefore, it is necessary to study the mechanism of the bending of the bodyduring continuous running. This is critical before using body joint in quadruped robot.However, the most of the present quadruped robots have a stiff body, which make itimpossible to use them in the research of body joint mechanism. Some quadrupedrobots are designed with body joints, while the researches are mainly focused on therealization of continuous movement under active control. The mechanism of bodyjoint has not been study specifically.In this paper, a quadruped model with elastic body joint is introduced based on thequadruped mammals with flexible body. Then, a specific running pattern of this model,named quasi passive running, is proposed to analysis the basic characteristic of bodyjoint motion; To obtain the quasi passive running automatically, an efficient algorithmis derived to search the initial state parameters. Based on the searching process, theproperties of body joint stiffness is further studied to match the requirements of bionicmotion. At last, the motions of the system with elastic body joint are analyzed.A appropriate model is the basis of analysis of the body joint movements. Basedon the analysis of the trunk skeleton system and joint connection, two models with elastic body are introduced. Then, the model with multi body joints is simplified,and a planar model with one elastic body joint is obtained according to the evolutionsof spine joints. Despite the simplification, the sagittal bending of trunk can still beobtained avoiding affections of unnecessary structures. With these reductions, the re-search can be focused on the basic qualitative properties of spine flexion and extensionduring quadrupedal running in the sagittal plane.To obtain the periodical motion of the model with elastic body joint, the symmet-rical gait is proposed based on the analysis of the system dynamics. Then, a controlmethod is developed to control the motion of body joint according to the analysis oftrunk motion and the electromyogram of quadruped mammals. In this method, thebody joint is locked when the body reaches its maximum bend. It is held until thesystem reaches the symmetry point of the locking instant. These actions results ina specific type of bounding, which is defined as quasi passive bounding. Using thiscontrol strategy, the system’s energy is maintained being beneficial to the analysis ofbasic property of spine motion.Comparing with the traditional models, the model with elastic body joints hasmore state parameters which are very difficult to set exact values manually. Therefore,to obtain the quasi passive running efficiently, modified map functions are developedbased on the Poincare′Return Map. In these functions, the touch-down angles of limbsare mapped to the symmetrical parameters. Numerical iteration is adopted to obtainthe required touch-down angles. Then, a automatical search process is developed toimprove efficiency based on the analysis of data range of the map functions.To obtain more realistic body bending, the characteristic of body joint stiffnessis further studied in this paper. Modifying the original dynamics, a new equation isobtained which can reflect the relations between limbs and body joints directly. Then,based on this function and the motion of real quadruped mammals, the design strategyis introduced for the design of body joint.The stance duration and limb peak force are analyzed using the proposed modeland gait. The results show that, running speed exerts effect on stance duration bymeans of centrifugal force. No matter which model it is, this force increase with theincrease of running speed reducing the stance duration indirectly. In each stance, the motion of body joint could reduce the leg peak force suggesting the use of body jointfor quadruped robot at high speed.