Stiffness control of magnetic suspension by local current feedback

A control method of adjusting the stiffness of suspension is proposed for magnetic suspension systems whose bias flux is provided by permanent magnets. In such magnetic suspension systems, zero-power control has often been applied. Since the zero-power magnetic suspension system behaves as if it has negative stiffness, it has been applied to vibration isolation system. However, the amplitude of negative stiffness cannot be adjusted. It is one of the obstacles to the industrialization of the vibration isolation system. In this paper, a proportional feedback is incorporated into the minor current feedback loop of the zero-power controller. It is shown that the stiffness can be adjusted by the gain of the added current feedback. The characteristics of the designed control system are studied both analytically and experimentally. In addition, the controller is modified to switch the stiffness from negative to positive when excessive disturbance acts on the suspended object. This function is achieved with a saturation element added to the integrator of the minor current loop. It is demonstrated experimentally that the modified controller enlarges the operation range of the suspension system.