Magneto-Rheological Technology for Human-Machine Interaction

Abstract

Human Machine Interfaces aim to give feedback to the user according to a given mechanical model or behavior. These systems are in constant interaction with the users senses. Haptic interfaces are mechatronic systems designed for providing force and tactile feedback to the operator while immersed into a virtual environment. The user inputs a motion or force into the system that reacts to this mechanical energy according to the virtual environment dynamical behavior. The mechanical behavior needed to give correct feedback is then directly related to the mechanical impedance of human being. The system should be capable to display free motion as well as a rigid contact (for virtual reality applications). Traditional actuators, DC motors for example, are limited due to stability issues for high forces and usually their weight and size do not allow free motion. Magneto-rheological fluids can be used to design semi-active systems (controlled damping or braking forces) that dissipate the mechanical energy applied by the user, this ensures their stability over all the working range. Moreover, the fluid exhibits a ~1 ms response time that permits real time control in the human interaction bandwidth. Two basic operational modes have been reported for MR fluids: the "valve mode" and the "direct shear mode" Jolly et al. [1999]. Active dampers usually operate in the valve mode in order to develop high stiffness and damping. The potential drawback of systems based on this mode of operation is the high level of frictional forces induced by the piston configuration, even in the absence of magnetic field. Current limitations for the magneto-rheological devices are often due to the design which is either in a brake configuration or a piston design. These limitations are mainly high uncontrolled forces due to friction and high inertia because of heavy moving parts. We propose in this work, a novel operating mode that reduces uncontrolled forces as well as the inertia of moving parts. This operating mode is based on shearing the fluid by a thin element (plate or cylinder), rather than shearing with the magnetic poles, increasing the performances and reducing the overall size. We present the modeling and experimental characterization of the novel operating mode through two haptic interfaces. 12