Calibration of fingernail imaging for multidigit force measurement

Fingernail imaging has been used to measure fingerpad force on the index finger of the right hand. This paper extends the procedure to the thumb, middle and ring fingers, in preparation for two-, three- and four-fingered grasping experiments where the individual finger forces are measured using fingernail imaging. These results demonstrate the effectiveness of fingernail imaging on the first four fingers of the right hand. The force prediction accuracy of all four fingers (thumb, index, middle and ring) is shown to be equivalent. In the x-direction, the force prediction error is 0.52N±0.02N, 0.52N±0.03N in the y-direction and 0.56N±0.04N in the z-direction. In addition, the four fingers register with equal accuracy. Given these results, experiments may be performed using fingernail imaging to measure the individual finger forces during grasping, provided a finger-tracking system is implemented to allow a camera to keep the fingers in view.