The Study On Servo System For The Lower Extremity Exoskeleton Robot

Exoskeleton robot is a new technology on robot and has become one of the hotspots in the world. It’s a wearable robot which combine human intelligence with the power of robot.It can improve the user’s operation ability by strengthening the human body ability of defence,walking,loading and so on. Exoskeleton robot can be roughly divided into the whole body exoskeleton robot, upper limb exoskeleton robot, lower extremity exoskeleton robot,and exoskeleton joints used to joint correction or rehabilitation based on the mechanical structure. And according to the function of the exoskeleton system.it can be classified into two kinds.The first kind is the assisted exoskeleton robot which can help the elderly, the disabled people who can’t walk normally to move; the other kind is the enhanced exoskeleton robot which can strengthening the human body physiology. There is a wide range of applications prospect of exoskeleton robot.in the military and medical field, and so many countries begin to study the exoskeleton robot because of the great practical and study value of it.The servo system for exoskeleton robot can make the exoskeleton robot follow the human motion quickly and accurately. The central controller converts the human motion signal (raising hand, walking, running, jumping, squatting motion, etc.)from electrical, electrical, pressure, or displacement sensors into the control signal for exoskeleton robot driving system (hydraulic, electrical or pneumatic driving system, etc.),and as the same time, the sensor-system collects the motion signal of exoskeleton robot as the feedback signal which forms a closed-loop control so as to make the exoskeleton robot follow the human motion quickly and accurately. According to different types signal collected by the data acquisition system, the servo system can be divided into four types:the pre-programmed servo system, the servo system based on the human brain electrical signal (EEG), the servo system based on the human body muscle electrical signal (EMG) and the servo system based on human body motion signal. There is a limitation of motion scalability to the exoskeleton in the pre-programmed servo system. Some action of people and the external environment may interfere with the control of the servo system based on EEG. And the human body muscle electrical signal is difficult to collect, and the EMG sensor may fall off from the human body when people are moving. And there may be a seriously hysteresis for control of the exoskeleton system in the servo system based on body motion signal.The human body motion signal is e relatively easy to collect and the robot can move accurately based on it.We make deep study on the servo system based on human body motion signal for the lower extremity exoskeleton robot in this paper. And we design a sensor-system to acquire the human body motion signal based on the electronic sensors such as the foot-pressure sensor, the angle sensor, the joint encoder, etc., analyze the kinematics and dynamics analysis problems on the lower extremity exoskeleton robot for the servo system. Moreover we do some simulation experiment for the servo system on the basis of virtual instrument technology and robot control theory. The main research work in this paper is shown as follow.(1) There is a brief introduction of the development for the exoskeleton robot at home and abroad. And we analyze the mechanical structure, the overall performance and the control strategy for the domestic and foreign typical exoskeleton robot such as BLEEX, HULC, HAL, XOS and so on, expound the application value and research significance of the exoskeleton robot, and prospect development trend of exoskeleton robot.(2) We study the hip, knee and ankle and motion mechanism of lower extremity on the basis of human anatomy. We can design the mechanical structure of the exoskeleton robot based on the analysis of the movement and DOF of the joints. And then we analyze the gait cycle when people are walking and divide it into four parts: left-single supporting, left-double supporting, right-single supporting, and right-double supporting. This can help us to control the robot walking. In addition, we explain the theory of zero torque point (ZMP) and center of pressure (COP), which helps us, analyze the walking stability of the exoskeleton robot.(3) We design and analyze the mechanical structure of the lower extremity exoskeleton robot with the Solidwork software. In this paper we choose the DOF of hip, knee and ankle joints of each leg. The hip joint can finish abduction/adduction and flexion and extension motion; the knee joint can finish flexion and extension motion; the ankle joint can finish flexion and extension motion. And we analyze and choose the hydraulic drive system of the robot. In this paper, According to the output torque of different joints when people is walking, we configure driver module at hip joint and knee joint, and ankle joint is controlled by human body. And then we analyze the kinematics and dynamics analysis problems on the lower extremity exoskeleton, build the DH model of the robot and build the dynamic equation of the robot in different states when people are walking, provide theoretical reference for the robot servo system.(4) In this paper, we design a sensor-system to acquire the motion signal of human body and the robot based on the electronic sensors such as the foot-pressure sensor, the angle sensor, the joint encoder, etc. The controller makes these signals into the output torque of each joint driver. At the same time, the sensor-system collects the signal from the joint encoder as the feedback signal so as to adjust the posture of exoskeleton real-time, making the robot follow the human motion quickly and accurately.In this paper, we design a sensing boots based on pressure sensors, to detect the body foot pressure. According to the observation of the human body in walking, we find that the contact points between foot and ground are mainly concentrated at three parts:the foot heel, the first phalanges and the fifth toe bone. And in the test, the foot pressure signal detected at the three points is the most sensitive during people walking. So we place pressure sensors at the three points, observe the change of pressure signal, and summarize change rule. We can study control of the robot walking on the basis of that.As the same time, we place angle sensors at the lower thigh and the calf upper to collect the joints angle signal as the reference of output torque of each joint drive.(5) In this paper, we build the control model of the servo system for lower extremity exoskeleton robot. And we finish the simulation to verify the control efficiency of the servo system with the virtual instrument technology. Referring to robot dynamics theory, we build the dynamic equation of each joint of the exoskeleton robot (hip and knee joints in dynamic equations under different state of walking).And we finish some simulation experiment with Simulink toolbox in matlab, which can help us analyze the servo system. The virtual instrument technology can provides the experimental platform for the servo system design, reduce research cost, and shorten the research cycle. And then we put the mechanical structure of the robot in Solidwork software into Adams/view in which we can build the simulation model to carry out the simulation. In the simulation, we can observe the state of the robot, such as motion trajectory, angular velocity and angular acceleration of each link(thigh, calf and foot)and each joint(hip and knee).And these can help us assess the feasibility of the servo system, and provide experiment platform and theoretical reference for servo system study.In this paper, we study the servo system based on human body motion signal for lower extremity exoskeleton robot with the human lower extremity limb anatomy knowledge, the robot dynamics and kinematics theory the robot control theory. And we carry out carry out the simulation with virtual instrument technology. At last, we summarize the defects in the work, come up with emphasis in later work, and make the foundation for the study on exoskeleton robot.