Active vibration isolation of large load structures typically requires significant actuator energy. With a zero stiffness support, which can be realised with a permanent magnet system, reactive forces may be applied to the structure with comparatively little effort. This paper presents results for such a magnetic spring arrangement which is stabilised by a non-linear controller. Transmissibility less than unity is demonstrated over the entire frequency spectrum. In a large load vibration isolation system, significant forces are required to counter the effects of gravity. In order to bear this load, a conventional linear isolator requires either a high stiffness (k) if the allowable static displacements (x) are small, or large displacement if the stiffness must be low, via force F = k(x x0) with x0 the static deflection. While low stiffness is advantageous for vibration isolation, there is a practical lower limit on obtaining a stiffness to support a large load. As a spring compresses, its stiffness tends to increase, which has the disadvantage of increasing the system’s resonant frequency !n = p k/m, for mass m. A high resonant frequency has the general disadvantage of poor passive vibration isolation, as attenuation occurs only above p 2 · !n. With these points in mind, a method of supporting large loads while keeping the stiffness low is desirable. For an active system, further advantage is gained from reduced control effort in actuating the device. In this paper, a permanent magnetic configuration is demonstrated that can reduce the passive stiffness to zero at unstable equilibrium while still providing a supporting force. The magnetic design is scalable, which provides for the capability of large load bearing. Non-linear control laws are proposed to stabilise the system and are demonstrated via simulation. Finally, implementation issues are discussed for a practical system. 2 SIMPLE MAGNET LOAD-BEARING The simplest form of magnetic suspension is the vertically attractive pair shown in Figure 1a, in which a fixed upper magnet supports the lower in an unstable manner. Due to the inherent instability resulting from negative stiffness and the non-linear forces involved, this system is often used for demonstrations of the efficacy of active control techniques. While all permanent magnet levitations are unstable by nature [1, 2], instability in a direction other than the supporting direction is desired for a completely passive spring (for example, see the spring of Puppin and Fratello [3]). A similarly simple (in terms of geometry) configuration is a vertical pair of magnets in repulsion, with the lower magnet
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