Reduction of contact force dependence on the MEMS hardness sensor using reference plane to detect human body hardness

In this study, stable detection principle of MEMS hardness sensor using "reference plane" structure is theoretically analyzed and demonstrated with experimental results. Hardness measurement independent to contact force instability is realized by optimum design of the reference plane. Fabricated devices were evaluated, and “shore A” hardness scale was obtained as reference in the range from A1 to A54 under a stable contact force. Contact force dependence was effectively reduced by 96.6% using our reference plane design (i.e. very stable to the contact force change). Below 1N contact force, maximal signal error of hardness is suppressed to A4. This result corresponds to the detection ability of fat´s hardness, even if the contact force is instable. Thorough the experiments, stable detection of human body hardness has been demonstrated without any control of the contact force.