VnStylus: A Haptic Stylus With Variable Tip Compliance.

Variable stiffness tools have been shown to be advantageous for ensuring safety and improving stability, dynamic performance and energy efficiency of interaction tasks. In this article, we present the design, mathematical modeling, implementation, characterization and user evaluations of VnStylus, a stylus with hardware-based tip compliance modulation. The stiffness modulation of the stylus tip is achieved through transverse stiffness variations of axially loaded beams. This approach of mechanically-controlled stiffness variation is advantageous for a variable stiffness stylus, as it can be implemented using a fully compliant mechanism that alleviates the problem of friction forces dominating under miniaturization and allows for high fidelity stiffness display even for very low stiffness values. Thanks to its hardware-based impedance modulation, the device does not suffer from the bandwidth and the stable impedance rendering limitations of software-based impedance control strategies. Featuring a manual adjustment mechanism, the device does not necessitate actuators, sensor or electronics; hence, is lightweight, low cost, and robust. Experimental characterizations verify the large range of stiffness modulation that can be achieved and the accuracy of the equivalent stiffness model of the system. Human subject experiments provide evidence of the efficacy of the modulated stylus stiffness on human performance during several common interactions with styli.