Prehension synergies in three dimensions.
This paper presents an external camera method for measuring fingertip forces by imaging the fingernail and surrounding skin. This method is an alternative to the pho-toplethysmograph sensor originally developed by one of the authors. A 3D model of the fingernail surface and skin is obtained with a stereo camera and laser striping system. Subsequent images from a single camera are registered to the 3D model by adding fiducial markings to the fingernail. Calibration results with a force sensor show that the measurement range depends on the region of the fingernail and skin. A Bayesian method is developed to predict fingertip force given coloration changes. Preliminary accuracy results for normal and shear force measurement are presented. In comparison to the results using the photoplethysmograph fingernail sensor, our results are more accurate and double the range of forces that can be transduced, all the way up to the saturation level.