Three-axis force-torque sensor with fully differential capacitive readout

We present a 3-axis force/torque sensor fabricated in a single SOI wafer. The sensor allows fully differential capacitive readout also for the normal force measurement, without the need for complicated lever mechanisms or wafer bonding. Fabrication is straightforward, with only two masks for deep reactive ion etching and one release etch. Furthermore, out-of-plane displacements are limited by the buried oxide layer thickness which allows for overload protection. The chip has a diameter of 5 mm, a normal force range of 1 N and a torque range from -4 to +4 Nmm and is designed for measuring the interaction forces involved in eye surgery.