Dynamic systems utilizing negative-stiffness structure or bistable structure for vibration control, wave propagation and robotic functionalities.
Acoustic/elastic metamaterials for manipulating wave propagation properties, such as bandgap, nonreciprocity and acoustic black hole.
Smart structures/materials such as magnetic systems and piezoelectric systems.
Exploring the benefits (new functionalities, better performance, breaking limitations et al.) of nonlinear properties in dynamic systems.
Project 1: Low-frequency isolators using negative-stiffness magnetic spring
Low-frequency vibration isolation is a key issue for precision engineering. In order to improve low-frequency isolation performance, the negative-stiffness mechanisms have been utilized in vibration isolators so as to reduce the resonance frequencies. In this project, we harness negative-stiffness magnetic springs, which are compact and frictionless and thus appropriate for precision isolation, to implement low-frequency isolators. In the following, low-frequency isolators in the vertical direction, torsional direction and six directions are introduced respectively.
A low-frequency isolator in the vertical direction
The negative-stiffness magnetic spring (NS-MS) is composed of two ring magnets that are magnetized radially and oppositely. Since the two magnets are in a repulsive configuration, it is known that it can produce negatve stiffness in the axial direction, which is utilized to cancel the positive stiffness of the mechanical spring. By carefully designing parameters of the NS-MS, the resonance frequency of the isolator can be reduced to quite a low level.
A low-frequency coupling (torsional isolator)
Many translational negative-stiffness mechanisms have been proposed to overcome the restriction between low-frequency isolation and load-bearing capacity of linear isolators. The couplings of rotor systems undertake the functions of transmitting static driving torque and isolating disturbing torque simultaneously, which thus creates the demand of torsional negative-stiffness mechanisms. However, torsional isolators using negative-stiffness mechanism have been rarely studied. Here a torsional magnetic spring composed of two coaxial ring magnet arrangements in repulsive configuration is proposed to produce negative torsional stiffness and used to formulate a low-frequency coupling.
Fig. (a) Schematic of the low-frequency coupling (the components in the dashed box compose the coupling); (b) A-A view is the cross-section view of the rubber spring; (c) B-B view is the cross-section view of the TMS (torsion magnetic spring): 1-outer magnet arrangement of the TMS; 2-inner magnet arrangement of the TMS; 3-rubber spring; 4,5-connectors; 6-bearing; 7-driving shaft; 8-driven shaft; 9-motor; 10-rotor.
A low-frequency Stewart isolation platform
Six-direction vibration isolation is essential in some engineering areas. The Stewart isolation platforms with active-control strategy are usually used to achieve 6-DOFs isolation. The main factor that limits the application of passive Stewart isolators is the poor isolation performance in low-frequency range. Here the negative-stiffness magnetic spring (NS-MS) is employed to reduce strut stiffness of the Stewart platform and hence improve its low-frequency isolation performance in six directions.
(c)
Fig. Model of the Stewart isolation platform: (a) 3D model of the platform; (b) configuration of the strut with a negative stiffness magnetic spring (NS-MS); (c) the experimental prototype
Project 2: Piezoelectric metamaterials shunted with bistable circuits
The nonlinear metamaterial has proved to be an effective way to realize nonreciprocity. Previous investigations are all relying on mechanical metamaterials, which are cumbersome to fabricate and integrate physically. Also for most of the approaches, the nonlinear characteristics of system constituents cannot be easily adjusted once they are fabricated and as a result the wave transmission properties of the system cannot be conveniently adjusted. In this project, we explore a piezo-metastructure shunted with bistable circuits. Since nonlinearity is induced by circuits, the proposed nonlinear piezo-metastructure can be easily realized based on the mature circuit integrating and fabrication technology. Also the nonlinear properties of the metastructure can be easily adjusted. We demonstrate theoretically and experimentally that the proposed piezo-metastructure can achive adaptive nonreciprocal wave transmission. The system creates a paradigm shift for manipulating elastic wave transmission with unprecedented programmability, which would inspire more research activities in the field of nonlinear piezo-metastructures.
Fig. (a) Schematic of the nonlinear piezo-metastructure; (b) zoom in view of a beam segment; (c) the bistable circuit diagram