MEMS force sensor with DDS-based position feedback and tunable sensitivity

This paper presents a capacitive Micro ElectroMechanical System (MEMS) force sensor coupled with a properly-designed interface circuit based on Direct Digital Synthesizer (DDS) that allows tunable sensitivity exploiting an electrical servo-assisted position-feedback mechanism. The position of the probe tip is kept fixed exploiting a feedback loop that includes a variable-gap capacitive position sensor and a pair of variable-area electrostatic actuators. Signals provided by the DDS are used to excite and read the capacitive position sensor and to perform a synchronous demodulation used for the position control and the electrostatic compensation of the external applied force Fext. By adjusting specific loop parameters, the force sensitivity and the tip working point can be finely tuned electrically. Experimental results show that the proposed system is able to measure the applied force in nanonewton range, making the system promising for measuring forces generated by living biological cells.