Using a cylindrical tactile sensor for determining curvature

This paper shows how contact curvature can be determined from a single contact with a cylindrical tactile sensor. When the tactile finger touches an unknown smooth convex surface, contact location, principal curvatures, and normal force are determined from a 4 x 4 window of strain measurements. In principle, contact properties can be determined by a linear inverse-filter process, but this approach is inherently doomed by low sampling density and sensor noise. We use a nonlinear model-based inversion from strain measurements back to contact type. Sensor strains are predicted by convolving the spatial impulse response of the rubber skin with the assumed surface pressure distribution derived from a Hertz contact model. Gradient search finds the parameters of the convex second-order shape and the force that best fit the sensor data. Experiments under laboratory conditions show radius estimation within 10070, orientation within 3", and subtactel (tactile element) localization to 3% of the element spacing. Using a linearized model, we predict error bounds due to sensor noise on the inversion process.