Factors affecting the design of untethered magnetic haptic interfaces

This paper presents an analysis of the factors affecting the performance, workspace, and stability of untethered magnetic haptic interfaces, with the goal of informing the design of future devices. We differentiate untethered magnetic interfaces, which use a stylus with an embedded permanent magnet to interact with magnetic fields projected into space by electromagnets, from the more well-known Lorentz-force magnetic interfaces, which work on different principles. We show that even though untethered magnetic interfaces have little to no damping, they still exhibit inherent stability properties as part of a sampled-data system. Although a ferromagnetic core often enables increased forces to be rendered at distances farther from the electromagnet, we show that there are cases in which larger forces can be rendered and cases in which stiffer virtual walls can be rendered by removing the ferromagnetic core.