The authors have developed the finger-shaped tactile sensor which consists of an optical waveguide, an elastic cover, eight LEDs to provide light to the waveguide, a PSD for obtaining current outputs from a light spot, and lens system for forcussing the light emitted from the contact point. The use of a PSD enables us to execute a real time sensory feedback control using the finger-shaped tactile sensor. This is an essential advantage to use a PSD compared with a CCD. However, since a PSD can only provide four current components depending on both the intensity of the total light emitted and the location of the light spot focused on the PSD, it can not decompose multiple contacts into individual contact points. Assuming that multiple contacts are allowed, we challenge the issue on how to estimate individual contact points from the PSD outputs. In this paper we consider this issue under the assumtion that multiple contacts never happen at the same time. We show that the change of contact phase from single contact to double contact can be evaluated by differentiating the PSD output signals and the multiple contact points can be estimated by considering the time history of the output signals.
