Development of a miniature tunable stiffness display using MR fluids for haptic application

For realistic haptic interaction, both tactile and kinesthetic information should be simultaneously conveyed to users. Haptic display units generally require miniaturization of tactile and kinesthetic modules, particularly, in small consumer electronic products (such as hand-held devices). While minimizing tactile modules is relatively easy, it is quite challenging to minimize the size of kinesthetic actuators for hand-held device applications. In an effort to address this issue, this study investigates a miniature tunable stiffness display. Actuated by MR fluids, the stiffness display is intended to provide kinesthetic information to users in small electric devices. In this study, a prototype stiffness display was designed and constructed, which consists of three parts: (1) an elastic returning part (which generates an elastic force), (2) a stiffness tuning part (which creates a resistive force), and (3) a PDMS membrane reservoir part (which serves as a repository for the MR fluids). In designing the prototype, the three operating modes of MR fluids (i.e., flow mode, direct shear mode, and squeeze mode) are used in order to maximize the resistive force generated by the fluids in a given size of the device. The stiffness display was then fabricated by using the MEMS technology as well as precision machining. For performance evaluation of the prototype, a micro stage, which is equipped with a precision load cell, was used. Using the micro stage, the stiffness changes of the display were measured by varying indented depth discretely (0.5, 1, and 1.5 mm). The results show that the stiffness change rate is nearly 30%, which is sufficient to create a stiffness sensation, for all indented depths considered in this study. The results further show that the proposed display can offer a range of stiffness change that can be conveyed to human operators.