Research on Real-Time Joint Stiffness Configuration of a Series Parallel Hybrid 7-DOF Humanoid Manipulator in Continuous Motion  
[PERSON]; [PERSON]; [PERSON]; [PERSON] Research on Real-Time Joint Stiffness Configuration of a Series Parallel Hybrid 7-DOF Humanoid Manipulator in Continuous Motion  
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[PERSON]  
[PERSON] Zhang  
School of Automation, Beijing University of Posts and Telecommunications, No 10, Xitucheng Road,  
Haidian District, Beijing 100876, China; [EMAIL_ADDRESS] (H.S. ); [EMAIL_ADDRESS] (Y.Z.) [PERSON]  
typical variable stiffness actuators are VSA (variable stiffness joint actuator) [12], AMASC (actuator with mechanically adjustable series compliance) [13], pVSJ (passive variable stiffness joint) [14], etc. [15; 16; 17; 18]. Although research of variable stiffness joint actuators is increasing, research on robots with flexible joints is not yet at a mature stage. Many researchers have laid the foundation for the research of humanoid manipulator based on flexible joints. It is known that the human arm has good joint flexibility, redundant mechanism, and surface contact flexibility, so that it can flexibly and safely complete many complex tasks. Assuming that the humanoid manipulator is a serial parallel hybrid 7-DOF manipulator, and the stiffness of each joint can be changed continuously, compared with the serial redundant manipulator and the manipulator of the rigid joint, the humanoid manipulator and the human arm have more in common. The joint flexibility of the human arm is adjusted passively according to its' needs, while the joint flexibility of the humanoid manipulator in continuous motion can be configured actively according to the working environment. The working environment of a robot can be roughly divided into the environment of no interference and the environment of human robot interaction. In the environment of no interference, there is no need to worry about the harm of robot movement to human bodies; in the environment of human robot interaction, it is very important to ensure the safety of the human body. In 2016, the international organization for standardization (ISO) issued the ISO/TS 15,066 technical standard, which provides technical guidance for operators to ensure their safety when working with robots [19; 20]. In the human-robot interaction environment, it is very likely that there will be accidental collisions between robots and humans. Many researchers have studied how to avoid these human-robot collisions [21; 22], which is important, but it is equally important to reduce injuries sustained to the human body in the event of accidental collision. The flexible joint manipulator can adjust the stiffness of each joint in real-time to meet environmental requirements. Therefore, it is necessary to study how we might reduce the collision force by adjusting the joint stiffness of the humanoid manipulator. [PERSON] et al. conducted special research on collisions between cooperative robots and human bodies, and obtained the threshold of pain in various parts of the human body when subjected to force and pressure [23; 24]. According to this research, when the human and robot collide accidentally, the force on the human body should be lower than the pain threshold to ensure that the human body is safe. In addition, [PERSON] et al. studied the collision problem between a single joint flexible robot and human head, and adopted HIC (Head Injury Criterion) as the injury index of human-robot collision [25; 26; 27]. The smaller the HIC is, the safer the human body is. On this basis, [PERSON] and others proposed MSI to predict the injury of human-robot collision [28]. The above research can provide theoretical support for the evaluation of human-robot collision injury. It was found that the velocity of the robot and the stiffness of the human robot contact surface both play an important role in the safety of human-robot collisions.