A Novel Haptic Interface for Musical Keyboards Jos´ e Lozada *† Moustapha Hafez * Xavier Boutillon † * CEA-LIST, Sensory Interfaces Laboratory, 18 route du panorama, 92265, Fontenay aux Roses, France † Laboratory for Mechanics of Solids, Department of Mechanics, ´ Ecole Polytechnique, F91128 Palaiseau,France Abstract—A novel haptic interface for musical keyboards is presented. The piano key has an embedded active damping system based on magneto-rheological (MR) fluids that allows to reproduce the dynamic behavior of traditional pianos. The MR fluid is encapsulated into a mechanical sealing to avoid leakage. A slider is attached to the back of the key and shears the MR fluid. When a magnetic field is applied across the MR fluid, chains of particles are aligned along the magnetic field lines. The strain of the chains will create a resistant force that can be controlled on real time. An analytical model of the whole system is presented and compared to experimental results. Index Terms—Magneto-rheological fluids, haptic interface, an- alytical model I. I NTRODUCTION The traditional acoustic piano action mechanism is com- posed of many different parts of wood, wool felt, leather, metal and spring steel . These parts form a multi-degree-of-freedom system that transmits the key motion to the hammer. The action mechanism generates a specific tactile rendering that is felt by pianists during play. The tactile feedback is essential for a precise control of timing and loudness. The action mechanisms used in numerical pianos are much simpler. The result is a poor tactile feedback. In the last few years, many developments have been carried out by keyboard manufacturers in order to improve the touch feedback of their products. Most of these systems are not actively controlled and are based on simplified models of the dynamic behavior of traditional pianos. The traditional piano action mechanism resistant force varies from 0.5 N, which correspond to the minimum force needed to begin the key motion, to 15 N at fortissimo nuance [1]. Extensive measurement of kinematics of a grand piano ac- tion mechanism are presented in [2] [3] . The key mouvement lasts from 20 to 250 ms depending on the nuance whereas key velocities varies from 0.1 m.s -1 to 0.6 m.s -1 . Active systems capable of reproducing the traditional piano dynamics have been developed by researchers [4] [5] and in the industry [6]. These systems use electromagnetic actuators which size is not suitable for a keyboard implementation and are based on simplified models or pre-recorded behaviors that do not match the traditional piano dynamic behavior. Improvements are still required in terms of size, performances and realism of the device. We present in this paper a novel system aimed at reproduc- ing the dynamic behavior of the piano action. The traditional mechanism is passive and generates a resistant force to the * jose.lozada@cea.fr, moustapha.hafez@cea.fr † lozada,boutillon@lms.polytechnique.fr player’s motion; the system introduced in this work, controls a resistant force in real-time and dissipates the mechanical energy given by the pianist through the use of magneto- rheological fluids. Magneto-rheological (MR) fluids are suspensions of ferrous particles of a few microns size (typically, 10 to 50 μ m). The fluid behaves as a Newtonian fluid which viscosity depends on the carrying fluid.. In the presence of a magnetic field, the particles align them- selves with each other and form chains along the magnetic flux lines. These cohesive chains resist to the fluid flow. The amplitude of the magnetic field controls the apparent viscosity. Since the magnetic field can only increase viscosity, systems based on MR fluids are intrinsically dissipative. Their intrinsic stability makes them appealing for use in haptic interfaces and for emulating a piano haptic interface. MR fluids are used in two basic operational modes : “valve mode” and “direct shear mode” [7]. Active dampers usually operate in the valve mode in order to develop high stiffness and damping. The drawback of systems based on this mode of operation is the high frictional forces induced by the piston configuration (even in the absence of magnetic field). Direct shear mode is often used in rotating machines such as brakes or clutches that do not need to produce large forces [8]. In most cases, the rotor is magnetic and the inertia of the system is large. Such mode is more suitable for a haptic piano key. Rather than shearing the fluid with the magnetic poles, a thin slider is introduced between them and shear the MR fluid located into the gap. II. THE RHEOLOGY OF MRF SURROUNDING A MOVING SLIDER A. Proposed model According to [9], the rheological model of the direct shear mode is given in (1) where η is the fluid viscosity, V the relative velocity, A the shear area, g the gap, and τ y yield stress which is magnetic field H dependent. F s = F η + F τ = ηVA g + τ y (H )S (1) In this expression, the shear force F s depends on I and V. The mechanical and the magnetic (or electrical) behaviors can be addressed independently. This behavior is based on Bingham rheological model. When the shear stress applied is lower than the yield stress τ y (H ) for a given magnetic field H , the fluid behaves like an elastic solid. Therefore, when the shear stress applied to the 1-4244-1264-1/07/$25.00 ©2007 IEEE