Filter design and calibration for fingernail sensors to measure fingertip forces and finger posture

When the human fingertip is pressed against a surface or bent, the hemodynamic state of the fingertip is altered due to mechanical interactions between the fingernail and bone. Normal force, shear force, and finger extension/flexion all result in different patterns of blood volume beneath the fingernail. This phenomenon has been exploited in order to detect finger forces and finger posture by creating a photoplethysmograph "fingernail sensor," which measures the two-dimensional pattern of blood volume beneath the fingernail. In this paper, a filter is designed to predict the normal force, lateral shear force, longitudinal shear force, and bending angle based on readings from the fingernail sensor. Linear, polynomial and neural network models are proposed as candidates for the predictor framework. A method is developed to uniformly calibrate the predictor for each user. Calibration experiments are performed to train and validate the predictor for seven human subjects. Results indicate that a simple linear model performs best, predicting shear forces with an average of 0.5 N r.m.s. error and normal force with an average of 1.0 N r.m.s. error.