Study On Rack Gear Type Eddy Current Damping Device And Its Control Performance

Eddy current damping is a new type of non-contact damping that uses the electromagnetic induction.Compared with traditional viscous damping and friction damping,the eddy current damping has advantages of no fluid,less maintenance,long service life,high sensitivity,strong reliability,etc.,which makes it have broad prospects in the field of vibration control.However,the vibration characteristics of civil engineering structures are low velocity and large mass,so the energy dissipation efficiency of the eddy current damping needs to be improved.The gear and rack are mature industrial products with low cost and strong reliability,and can be used to amplify the energy dissipation efficiency of the eddy current damping.Furthermore,the required inertia force can be achieved by the combination of the gear and rack,which provides a novel way for structural vibration control.In this study,two novel devices combining eddy current damping with rack and gear were firstly developed,and their mechanical properties were systematically investigated by using theoretical analysis,prototype tests and numerical simulations.In addition,the effect of apparent mass in structural vibration control was studied.The works included in this paper are summarized as follows:（1）A novel eddy current damping device（ECD-RGD）with a directional decomposition characteristic for isolated structures was developed by adopting rack,gear,sliding rail and eddy current damping.The ECD-RGD can simultaneously export independent damping force and independent inertial force,in which the damping force and the inertial force are connected in parallel.The directional decomposition characteristic overcomes the shortcoming that the axial direction of a damper is inconsistent with the seismic direction.A series of prototype tests were carried out to study the mechanical properties of ECD-RGD,which are compared with the results of theoretical analysis and electromagnetic finite element numerical simulation.In addition,a nonlinear theoretical model for calculating eddy current damping force at high velocity was proposed based on the results of numerical simulation.The results show that the hysteretic curves of the ECD-RGD obtained by the prototype tests are considerable plump,indicating that the ECD-RGD has the excellent mechanical properties.The combination of rack and sliding rail can be adopted to realize the directional decomposition of the superstructure movement.The tested results of amplification effect of the rack and gear are in good agreement with those of theoretical analysis and numerical simulations.The available maximum damping force of ECD-RGD can be significantly increased by decreasing the thickness of the conductor plate and the air gap,increasing the thickness of the back iron plate and the number of the permanent magnets.The nonlinear theoretical model is considerable consistent with the numerical results of electromagnetic finite element.（2）A novel rack gear type axial eddy current damper（RGA-ECD）was developed and studied.A 50-ton prototype of the RGA-ECD was manufactured and tested,and its nonlinear damping force was investigated using prototype tests and electromagnetic finite element numerical simulations.Furthermore,the effectiveness of RGA-ECD in mitigating seismic responses was studied by comparing it with fluid viscous dampers.The results show that the maximum damping force of the prototype is 507 k N at the velocity of 0.509 m/s,which achieves the design goal.The hysteretic curves of the prototype are considerable plump,and its repeatability is satisfactory,which indicates that the mechanical properties of the RGA-ECD are outstanding.Note that there is a force-limiting characteristic for the RGA-ECD,which can prevent the damage of damper and controlled structure caused by excessive impact load.（3）The effectiveness of two kinds of apparent mass system（TVMD and TID）,compared with the TMD,was studied to mitigate seismic responses of base isolated structures.The structural parameter optimization of TVMD,TID and TMD was carried out with three optimization indexes,and their robustness was evaluated.In addition,the control effect and start-up sensitivity of them were investigated by considering far-field and near-fault seismic excitations.The results show that when the mass ratio is not greater than 0.4,the effectiveness of TVMD on mitigating the seismic responses is better than TID and TMD,and when the mass ratio is 0.05,TVMD has the outstanding performance,including excellent effectiveness,small stroke,strong robustness and high start-up sensitivity.TVMD can be made by connecting rack gear type eddy current damping device developed in this study with a spring in series.As for the TID,with the increase of its apparent mass,the effectiveness and robustness increase,and the stroke decreases.However,the larger apparent mass requires higher requirements for amplification devices.（4）A mechanical model of the variant tuned mass damper inerter（VTMDI）was proposed,and its performance for vibration control was systematically studied.The optimal frequency and optimal damping of VTMDI were derived using fixed-point theory.Considering three kinds of loads,including harmonic load,random load and natural seismic load,the effectiveness for vibration control and robustness of VTMDI were compared with those of three classical tuned mass dampers（TMD,VTMD and TMDI）.The results show that the vibration reduction efficiency of TMD and VTMD can be significantly improved by setting the apparent mass,and the VTMDI performs better effectiveness for vibration control than TMDI on same apparent mass under harmonic load and random load.Furthermore,VTMDI shows better robustness for damping detuning,and poorer robustness for frequency detuning,but it is still far superior to TMD and VTMD.Compared with TMDI,the robustness of VTMDI on positive frequency detuning is better than that of TMDI,while the robustness of VTMDI on negative frequency detuning is weaker than that of TMDI.（5）All structural parameters of rotational inertia double tuned mass damper（RIDTMD）are numerically optimized with H_∞and H₂ methods,and the robustness of RIDTMD is evaluated.Compared with the TMD,the effectiveness of RIDTMD in reducing the wind-induced responses of a solar tower was investigated.The results show that when the tuned mass is the same,the vibration reduction effect of RIDTMD optimized by H_∞method is about 20%higher than that of TMD,and the vibration reduction effect of RIDTMD optimized by H₂ method is about 13%higher than that of TMD.The order of structural parameters affecting the robustness of RIDTMD is the frequency of tuned mass,the frequency of apparent mass,the apparent mass and the damping of tuned mass,and the robustness of these four structural parameters on positive detuning is better than that on negative detuning.When the tuned mass is the same,RIDTMD is more effective than TMD in mitigating the wind-induced responses in the cross-wind and along-wind directions of high rise structures.