Haptic Interfaces based on Smart Materials: towards 3D free-hand exploration of virtual environments

The aim of this Thesis is to investigate the possibility of mimicking haptic perception by using new technologies. Innovative solutions are needed to address multi-channel tactile perception. One avenue is to use “smart fluids”, such as e.g. Magneto-Rheological Fluids. Their rheological behaviour could be exploited to implement innovative haptic interfaces, which are capable of reproducing shape and compliance of virtual objects. Unlike kinaesthetic displays present in literature, this type of haptic interfaces would allow a direct contact with a compliant object. In this case both kinaesthetic and cutaneous channels of the fingerpads are stimulated during manipulation thus tactile perception is augmented. To justify this approach some design considerations are discussed and several preliminary prototypes based on Magneto-Rheological Fluids are proposed and designed. A new concept of haptic display for free-hand exploration is envisioned, the Haptic Black Box, which can be imagined as a box where the operator can poke his/her bare hand and interact with a virtual object by freely moving the hand without mechanical constraints. The first implementation of Haptic Black Box (HBB-I) consisted of a volume of Magneto-Rheological Fluid placed within a plexiglas box into which a hand, wearing a latex glove, can be introduced through a sealed opening to interact with the fluid. Starting from the analysis of the previous prototypes, some detailed magnetic simulations by means of numerical code are provided in order to identify a new device with improved performance. A 3D numerical analysis, taking into account the material non-linearity, is achieved to predict the distribution of the magnetic field and the rheological behaviour of the fluid. In this way some technical difficulties related to the non-linearity of the fluid and magnetic saturation, to the thermal behaviour of the coils and to the magnetic hysteresis, have been overcome. The new design is performed acting both on the characteristics of the fluid and on a shrewd constructive approach. These improvements led to a new 3D implementation of Haptic Black Box. The HBB-II is described in terms of technical design and mechanical arrangements. This new device enhanced performance in terms of magnitude and accuracy of magnetic field as well as increased spatial resolution. An analysis of interactions between the operator hand and the fluid specimen is proposed with the aim of reducing the artifacts and the discontinuities on the workspace containing the fluid. To evaluate the possibility of mimicking biological tissues softness, a descriptive model of the Magneto-Rheological Fluid able to emphasize its viscoelastic behaviour is formulated. Furthermore, some psychophysical tests and experimental results on excited MRF specimens, in terms of softness and/or shape recognition are illustrated and discussed.