Research On Levitation Control And Stability Of Permanent Magnetic Levitation Transportation System

Magnetic levitation takes the magnetic field as the medium,where the force is generated,and there is no physical contact between the magnet and the levitated target,hence achieving stable levitation in space.Magnetic levitation technology has the unique advantage of contactless magnetic force as support.It shows a booming trend in industrial applications such as transportation,precision machining,aerospace,and turbomachinery.With the increasing requirements for clean transport technology in the semiconductor processing industry,the application of magnetic levitation technology in the dust-free workshop to achieve clean transport solutions has aroused extensive concern from researchers.The electromagnetic levitation transport is easy to achieve and has good stability.However,there is a large loss of energy consumption under long-term operating conditions.The coil heating problem significantly affects the stable temperature condition of the dust-free workshop.Although the PML(permanent magnetic levitation)transport system shows low energy consumption,its levitation control under the multi-point support structure is difficult to realize.Currently,there is no device or prototype to realize clean transport by using PML technology in China.The control of PML systems is still a difficult problem to be solved urgently,and the research is of great significance.This paper presents a magnetic levitation platform supported by four-point permanent magnet units.The magnetic force is adjusted by changing the rotation angle of the permanent magnet.Hence the levitation air gap can be kept constant when the platform is loaded.Besides,the stable levitation state only needs to input a small current to overcome the load torque of the permanent magnet.Therefore,the PML platform has the characteristics of low steady-state energy consumption,high controlling stiffness,and anti-eccentric load.The research content of this paper is divided into the following six parts:(1)Based on an analysis of the existing shortcomings of the electromagnetic levitation platform and the traditional PML platform,a variable magnetic circuit permanent magnetic levitation platform was proposed.The structure and working principle of the system were introduced in detail.The nonlinear model of magnetic force and torque of the PML unit was established.Besides,it was verified by finite element analysis and experimental measurement.The three-degree-of-freedom motion control model was obtained by using the coordinate transformation method,and the stable levitation conditions of the system were analyzed.(2)The double PD controller for the variable magnetic circuit PML system was designed based on the decentralized control strategy.The decentralized controller takes four PML units as independently controlled targets,where the air gap length and the rotation angle are selected as the controlled variables according to the requirement of levitation.The double PID controller and LQR controller of the PML unit was proposed respectively.The parameters of the controller were quickly tuned by the particle swarm optimization algorithm.Then the performance of the controller was compared by levitation experiments.In order to eliminate the adverse influence of the different characteristics of each PML unit,an improved PD controller with integral compensation was proposed,and its control effect was verified by a suspension experiment.The results show that the system can float quickly without bias under the decentralized strategy PD.However,there is a certain static error in air gap length.(3)With the centralized control strategy,the levitation platform is regarded as a controlled system,where the outer loop of the controller was designed for three degrees of freedom.The inner loop was designed for the rotation angle of each permanent magnet.Based on the PID control law,the separate integral controller,nonlinear feedforward compensation controller,and fraction-order controller were designed.Finally,the experimental system of the permanent magnetic levitation platform was built.On the premise of stable levitation of the system,the performance of the controller was tested by floating experiment,step input experiment,and sine tracking experiment.The control accuracy,stability,and response speed were compared,and the actual effects of different improvement strategies were analyzed.The experimental results show that the platform can achieve a 1mm floating height with the above three controllers,which solves the problem that the traditional PID controller can not float stably.The platform can realize translation along Z-axis,rotation around X-axis or Y-axis,and joint motion with high control precision.The feedforward compensated fractional PID controller has good sinusoidal tracking performance.When input sinusoidal signal with 0.05 mm amplitude,the maximum traceable frequency of the closed-loop system could be 11 Hz.(4)When the permanent maglev platform works,the influence of frequent loading or unloading is equivalent to a variable disturbing force.Based on the stable levitation requirement of the platform,the bearing characteristics and levitation power consumption of the maglev platform were further analyzed.Firstly,the bearing stiffness model of the levitation platform with three degrees of freedom was established,and the controlling stiffness characteristics of the PML platform with PID control and PD control were analyzed.The eccentric load experiments were carried out to demonstrate the characteristic of infinite control stiffness.The results show that the response speed of the permanent magnetic levitation system under the PD controller is faster,but the tilt problem of the platform under disturbance input is also obvious.Under the action of integer-order PID controller and fractional-order PID controller,the stable suspension air gap of the platform does not change after partial load,which indicates that the permanent magnetic levitation platform under PID control has the characteristics of the constant air gap and anti-bias load.(5)In order to investigate the power consumption of the PML system,the steady current model is established.The steady-state current of the PML unit was compared with that of the electromagnetic levitation system and the hybrid levitation system.The fixed air gap controller and fixed angle controller were designed,and the quasi-zero power characteristics of the permanent magnetic levitation unit were analyzed and verified by continuous loading experiments.The results show that the value and variation range of the steady current of the PML unit is small when the suspension mass increases continuously.With a fixed air gap controller,the PML unit can maintain low suspension power consumption and constant suspension air gap.(6)Finally,the research work and related results of this paper are summarized,and the innovation is clarified.Future research is prospected according to the development of permanent magnet transportation technology and the characteristics of the PML system.