Theoretical and experimental characterization of an active ferrofluid pad bearing for nanopositioning

The use of actively controlled smart fluids for high-precision manipulation holds significant promise. This paper introduces a novel active ferrofluid pad bearing capable of controlling motion of a platform with nano-scale accuracy. The actuation force and stiffness of the bearing are generated through the fluid magnetization pressure, which can be controlled precisely by adjusting the current through an electromagnetic coil. The combination of passive and active flow properties of the ferrofluid enable the system to achieve fast and precise motion without the need for complicated control strategies or system design, thereby providing a simple and cost-effective solution. A theoretical model of the active bearing system, including both viscous and magnetic pressure fields, is derived from first principles and validated through experimental testing. Based on the modeling results, an optimized PI control system is proposed to achieve a suitable balance of position error minimization and noise attenuation. The experimental results show the capability for motion control within 5 nanometers resolution. The results also show that matching the system and controller design with the viscosity of the ferrofluid is crucial for achieving high performance, as the passive damping effects from the fluid can be leveraged to enhance stability and disturbance rejection.