5-D force control system for fingernail imaging calibration

This paper presents a low-cost automated system that is able to apply a 5-degree-of-freedom (DOF) force on a human fingertip with high precision. It is designed to be used as a calibration platform for the previous proposed fingernail imaging system, and as a haptic system. The system is composed of two Novint Falcon devices linked by two universal joints and a rigid bar to provide 5-DOF motion and force, with feedback from a 6-DOF force sensor. A force controller is designed with an inner position control to meet the calibration goal and requirement. Experiment result and analysis showed that the system was capable of controlling the force with a settling time of less than 0.25 seconds. Two force trajectories are designed for fast and sufficient calibrations. A calibration experiments demonstrated that the system tracked the trajectories with an interval of 0.3 seconds, and step sizes of 0.1 N and 1 N·mm with root-mean-squared errors of 0.02 - 0.04 N for forces and 0.39 N·mm for torque.