Inductive Micro Tri-Axial Tactile Sensor Using a CMOS Chip With a Coil Array

This letter presents a simple approach to implement the inductive-type micro tri-axial tactile sensor by integrating a complementary metal-oxide-semiconductor (CMOS) chip with sensing coils and a stainless steel sheet (as the sensing interface) using polymer encapsulation. In addition to the component integration, the polymer layer is exploited as the spring for the tactile sensor. The proposed CMOS-MEMS inductive tri-axial tactile sensor presents several features and advantages as follows: 1) the gap-closing inductive sensing approach is used for normal force (Z-axis) detection; 2) the area-change inductive sensing method is adopted for shear forces' (X-axis and Y-axis) detection; 3) the compact sensing coil array is implemented using the standard CMOS process; and 4) the polymer is employed as the encapsulation layer to cover and integrate the rigid stainless steel sheet sensing interface and the CMOS chip, while also acting as the spring to avoid the fragile suspended thin-film structure. To demonstrate the concept, a CMOS sensing chip with a 2 x 2 coil array is fabricated using the Taiwan Semiconductor Manufacturing Company standard process and is further integrated with a stainless steel sheet prepared by lasermachining. The measurements indicate that the typical fabricated tri-axial inductive tactile sensor exhibits a normal load sensitivity (Z-axis) of 2.9 nH/N, an X-axis shear force sensitivity of 17.4 nH/N, and a Y-axis shear force sensitivity of 15.3 nH/N.