On force generation in electro-fluidic linear actuators with ferrofluid

Abstract

In ferrofluid actuation systems, forces are generated by actively controlling pressure and flow within the fluid using an applied magnetic field. There are multiple contributing factors in force generation involving complex non-linear couplings between electromagnetic and fluid pressure fields. This brings significant challenges in theory-based design and optimization. In this paper, a theoretical model of pressure transmission between a ferrofluid and solid is derived starting from Maxwell’s stress tensor and accounting for magnetic saturation within the fluid. This model shows that linear actuator designs based on orthogonal mode operation, where the field direction through the fluid is perpendicular to the motion direction, can provide the highest force capacity for a given field strength from the actuator coil. This is verified by theoretical analysis of some basic linear actuator topologies. The results are applied in the design and analysis of a novel piston-type linear actuator with sealed chamber and two internal electrical coils for bidirectional operation. Experimental measurements of both static and dynamic behaviour are shown to validate the described principles. The actuator produces smooth and precise flow-regulated motion, has zero intrinsic stiffness, and exhibits very low friction due to the suspension effect from ferrofluid layers within the actuator.