Photo-plethysmograph nail sensors: for measuring finger forces without haptic obstruction: modeling and experimentation

A new type of touch sensor for detecting contact pressure at human fingertips is presented. A fingernail is instrumented with miniature LEDs and photodetectors in order to measure changes in the nail color pattern when the fingertip is pressed against a surface. Unlike traditional electronic gloves, in which sensor pads are placed between the fingers and the environment surface, this new sensor allows the fingers to directly contact the environment without obstructing the human´s natural haptic senses. The finger touch force is detected by measuring changes in the nail color, hence the sensor is mounted on the fingernail rather than on the fingertip. Photo-reflective plethysmography is used to measure the nail color. Haemodynamic modeling is used to investigate the dynamics of the change in blood volume under the fingernail. The model is simulated and then evaluated by comparison with the experimental dynamic response of the sensor. Applications to human-machine interaction are discussed