Design of a passive alternative for long stroke linear aerostatic stages based on ferrofluid bearings

Abstract The objective of this research is to demonstrate the capability of a long stroke linear ferrofluid (FF) stage. This stage is a passive alternative to existing linear aerostatic stages and can be used in low loaded CNC devices, pick and place machines, microscopy or scanner applications. To compete with aerostatic stages the bearing must be repeatable and achieve sufficient stiffness for the application. The effects of ferrofluidtrail formation are countered with the use of a ferrofluidreservoir located on the mover. To increase stiffness a specially designed magnet configuration is used. A linear guidance was built with outer dimensions of 180x600x80 mm (WxLxH), a mover of 1.8 kg without actuator and payload having a 430 mm stroke. The load capacity of the stage was measured to be 120 N, with a stiffness of 0.4 N/ μ m. The maximum height delta after a stroke with 1 kg payload and a mover velocity of 0.25 m/s was measured to be less than ± 3 μ m , and with 1.75 kg payload and a velocity of 0.5 m/s the height delta was within ± 7 μ m . Using a rheometer, it was shown that the effects of evaporation in ferrofluidcan be reversed, within certain limits of mass loss, by adding carrier fluid. The damping is shown to be a function of payload and velocity and was measured to be between 2 and 4 N ⋅ s/m for velocities between 0.2 and 0.5 m/s. In comparison to a linear aerostatic stage it can be concluded that while the linear ferrofluidstage is outperformed in stiffness and out-of-plane repeatability, the ferrofluidstage doesn’t require a continuous supply of air and has lower fabrication tolerances due to the higher fly height. Thus, the linear ferrofluidstage is a cost-effective alternative to a linear aerostatic stage when the stiffness and straightness are of less importance.