3-D force control on the human fingerpad using a magnetic levitation device for fingernail imaging calibration

This paper demonstrates fast, accurate, and stable force control in three axes simultaneously when a flat surface is pressed against the human fingerpad. The primary application of this force control is for the automated calibration of a fingernail imaging system, where video images of the human fingernail are used to predict the normal and shear forces that occur when the fingerpad is pressed against a flat surface. The system consists of a six degree-of-freedom magnetic levitation device (MLD), whose flotor has been modified to apply forces to the human fingerpad, which is resting in a passive restraint. The system is capable of taking simultaneous steps in normal force and two axes of shear forces with a settling time of less than 0.2 seconds, and achieves a steady-state error as small as 0.05 N in all three axes. The system is also capable of tracking error of less than 0.2 N when the shear force vector rotates with a frequency of 1 rad/s. This paper also demonstrates the successful tracking of a desired force trajectory in three dimensions for calibrating a fingernail imaging system.