Compliant Joint Based On Magnetorheological Fluid Transmission And Its Control

While robotic technology exploring its application rapidly and deeply in all kind of fields,the traditional stiff and isolated industrial robots gradually find their unsufficiency in these new scenarios.With this background,the research and development of collaborative robot emerges as the times require,especially for assembly,polishing,grasp and operation for fragile or soft objects where force-position coordination or force/torque holding are required by collaborate control methodologies.However,most of the current research on collaborate robots were based on the conventional industrial robot arm and realized by advanced compliant control algorithms with help of diversified high-precised sensors.All these work raised a big challenge for system modeling precision,accuracy and adaptability for control algorithms.Although many researchers and technicists made great efforts in this field,the collaborative robots fails to satisfy the compliance and safe requirement in application.In order to realize the compliance and safety of robot in cooperation process,the research and development of flexible robot with inherent flexibility,that is,flexible robot with active adjustable stiffness,is a more direct way,which can fundamentally solve the problem of robot safety.Aiming at this goal,a new type of MRF(magnetorheological fluids)compliant joint is proposed in this dissertation,which takes MRF as an important part of the transmission component and realizes the hybrid control of the position and torque of the robot joint by using its continuous and controllable transmission properties,so that the robot joint has intrinsic safety and flexibility.One of the basic problems in applying magnetorheological fluids to robot joints is to establish a rheological model of its shear transmission.In this research,Bingham plastic model,Herschel-Bulkley model,Casson model and their combined models are introduced and analyzed in detail.According to the compliance requirements of robot joints,combined with the pre-motion/fretting characteristics of magnetorheological fluid shear,shear thinning characteristics and constant torque transmission characteristics,a segmented full-domain shear stress model for magnetorheological fluid is proposed,which is the basis of motion analysis,dynamic characteristics analysis and control research of the proposed MRF compliant joint.The compliant joint structure based on magnetorheological fluid is designed.The structure design and magnetic circuit design of MRF compliant coupler are analyzed in detail.The analytical model between current and magnetic induction intensity,i.e.I-B model,is established.The concept of coupling stiffness is proposed and a coupled stiffness model is established based on the I-B model.Based on the concept of coupling stiffness,the transmission stiffness of MRF compliant joint is defined,which provides the structure and model foundation for the variable stiffness control and stiffness constraint of the compliant joint and the flexible manipulator based on this joint.Based on the I-B model,the torque coupling relationship and position coupling relationship of MRF compliant joint coupler are analyzed.The torque coupling model and position coupling model are established.The compliant operation process of robot is analyzed and divided into two stages: free space and constraint space.The constraints and requirements of robot joint control in the two stages are further defined.It is proposed that when the MRF compliant joint works in free space,its internal MRF is in synchronous transmission stage,and when the MRF compliant joint works in constraint space,its internal MRF is in yield stage.The transmission stiffness is regulated by actively adjusting the coupling stiffness.Aiming at the motion characteristics and control requirements of MRF compliant joint,an active compliant dual-loop control strategy is proposed,which includes position control loop and torque control loop.The position control loop is used to track the target position,and the torque control loop is used to control the output torque based on the coupling stiffness constraint.The impedance model is used to transform the position change into the torque constraint correction between the position control loop and the torque control loop.Fuzzy adaptive tuning PID control algorithm is used for coupling position control,and a coupling torque control algorithm of MRF joint based on constrained model predictive control is proposed.The global shear stress model,torque coupling model and coupling torque control based on MPC control algorithm are simulated and analyzed respectively.A prototype system based on MRF compliant joint coupler is built,and the coupling performance experiment and closed-loop torque control experiment of MRF compliant joint coupler are carried out respectively.Aiming at the urgent requirement of flexibility and safety in robot application,this dissertation fully explores the rheological characteristics and multi-mode traditional characteristics of magnetorheological fluid materials from the perspective of robot ontology security,proposes and studies the compliant joint of robot based on MRF,establishes the basic model of the joint,and puts forward the effective strategy and algorithm for variable stiffness and torque output control.The research work provides a new idea and realization method for the research of novel active flexible manipulator with variable stiffness.