Design of a piezoresistive surface micromachined three-axis force transducer for microassembly

One of the challenges facing microrobotic manufacturing is the ability to sense interactions for force-guided assembly of small devices. There is a need for a force transducer with the ability to sense forces in multiple degrees-of-freedom in the mN range with resolution on the order of 10 mu N for microassembly applications. This paper presents theoretical studies for developing a surface micromachined piezoresistive force transducer that can measure normal force in the z-direction and moments about the x and y-axes. The devices proposed here are based on a compliant platform design with integrated piezoresistive sensing elements fabricated in a modified SUMMiT process. Various configurations and sensor element layouts are explored to determine the relationship of the applied forces and moments experienced during assembly and the corresponding strain. Structural and finite element analysis is used to determine the elastic response of the device and establish the best locations and orientations of the sensing elements to effectively utilize the piezoresistive effect of the polysilicon sensors. Initial experiments show the polysilicon piezoresistors to have a gauge factor of approximately 25. The expected sensitivities for these devices are presented.