Study On The Compliance Control In Robot-Environment Interaction Using The Electromagnetic Variable Stiffness Principle

In recent years,variable stiffness has been one of the research hotspots in the field of robotic compliance control.Using the compliance devices with variable stiffness to tackle the compliance control between the robot and the environment can not only improve the safety of robot-environment interaction,but also decouple the motion control and force control of the robot system.Ultimately,the processing quality of the robotic tasks can be enhanced.However,the current variable stiffness devices are designed mainly in terms of mechanical or pneumatic principles.The variable stiffness in the mechanical designs is achieved based on the adjustment of the built-in elastic components using the dedicated mechanical transmission system.For the pneumatic designs,the adaptable compliance is realized by controlling the air pressure.Both designs suffer problems such as complicated mechanical structure,nonlinear mechanical characteristics,and slow response of stiffness regulation.In consequence,the accuracy and stability of compliance control using the current variable stiffness devices is unsatisfactory.In view of the above problems,this paper proposes the principle of variable stiffness based on the non-contact electromagnetic interaction effects.The electromagnetic compliance device and the matching compliance control system are also developed.Compared with the normal variable stiffness devices,the electromagnetic compliance system can significantly improve the response speed,accuracy and stability of force control.In order to enhance the compliance ability of robot in unknown interaction environment,this paper further researches the realization of spatial variable compliance characteristics.In the end,some experiments focusing on physical robot-environment interaction are carried out with the aid of electromagnetic compliance.The experiments have fully demonstrated the potential of electromagnetic compliance in ensuring the safety of robot-environment interaction and improving the machining quality of robotic tasks.The main research contents of this paper include the following parts:Principle of Variable Stiffness based on the Electromagnetic Interaction Effect: In terms of the experimental analyses on the mechanical properties of different combinations of magnetic sources,the electromagnetic principle of variable stiffness is proposed based on the combination of coil and coaxial magnet.According to the theories of electromagnetism,an analytical mapping model from the input current to the output force and stiffness of the electromagnetic interaction system composed of coil and coaxial magnet is established.Using the derived model,the structural parameters of electromagnetic interaction system are optimized to realize stronger load capacity and more linear force-deflection characteristics.Then a physical prototype of the electromagnetic compliance device is manufactured.The prototype has fast stiffness adjustment response and can achieve approximately linear mechanical performance.Force Control System of the Electromagnetic Compliance Device: The control system model of electromagnetic compliance is established.The influence of electromagnetic induction on the mechanical properties is also studied.On the basis of PID controller,PID controller of single neuron and PID controller of BP neural network,the simulation analyses of control system are implemented in order to derive the best force control strategy for the electromagnetic compliance device.Inspired by the pulse width modulation(PWM)technology,the force control system for the electromagnetic compliance device is built using an arduino microcontroller and an H-bridge driver board.Both the tests on the control system and the comparisons with the pneumatic compliance devices show that the electromagnetic compliance has the characteristics of higher control precision,better stability and faster response.Design and Realization of Spatial Compliance Characteristics with Variable Stiffness: After replacing the actuators of parallel mechanism with the electromagnetic variable stiffness springs,the design model of electromagnetic parallel compliance device with configuration of 3UPU-4SPS is proposed.The parallel compliance device can realize spatial translational stiffness performance at the center of stiffness,and the eigenvectors and eigenvalues of the translational stiffness matrix respectively represent the directions of force compliant axes and the stiffness values in the corresponding compliant axes.Furthermore,in order to realize the desired spatial compliance characteristics,a synthesis algorithm for designing the spring stiffness value in each limb of the parallel compliance device is studied.The algorithm can help the electromagnetic parallel compliance device to achieve any desired spatial compliance characteristics by adjusting the coil current in each spring limb.Experimental Research on Compliance Control in Physical Interaction with the aid of Electromagnetic Compliance System: With the aid of electromagnetic compliance,three experiments including impacting the raw egg,squeezing the raw egg and robotic automatic polishing are carried out.Owing to the abilities of fast stiffness regulation and robust force control of the electromagnetic compliance device,the fragile eggs remain unscathed under the acting force of mechanical system.On the other hand,the electromagnetic compliance device is used to control the positive polishing pressure in the automatic polishing process in real time,so that the motion control of industrial robot and the force control of compliance device are decoupled,which makes the material removal effect in the polishing process be smooth and fluent.Through the above experiments,the potential application value of electromagnetic compliance in industrial automation is visually revealed.In summary,on the basis of the electromagnetic variable stiffness principle and compliance control technology proposed by this paper,the response time of stiffness regulation is shortened from hundreds of milliseconds required by many traditional variable stiffness designs to less than 30 ms,and the accuracy of force control is significantly improved compared with the common pneumatic compliance devices.Generally speaking,this paper provides a new technical path for achieving safe and controllable physical robot-environment interactions.