This miniature or micro-sized semi-conductor sensor design provides direct, nonintrusive measurement of skin friction or wall shear stress in fluid flow situations in a two-axis configuration. The sensor is fabricated by microelectro-mechanical system (MEMS) technology, enabling small size and multiple, low-cost reproductions. The sensors may be fabricated by bonding a sensing element wafer to a fluid-coupling element wafer. Using this layered machine structure provides a truly three-dimensional device.
The sensor design (see figure) includes a shear-force collecting plate (fluid-coupling element) with dimensions tailored to application-determined resolutions (spatial, temporal, and force). The plate is located coplanar to both the sensor body and flow boundary. This plate is coupled to a biaxial gimbal structure provided with piezoresistors on its torsional hinges, and, located parallel to but some distance from the force collection plate, with a connecting column. This design thus allows a nonintrusive method to qualitatively measure the shear force vector on aerodynamic bodies.
The sensors themselves are typically made in single crystal silicon with the piezoresistor elements formed by doping the silicon (by ion implantation or other means) to a suitable type and level of conductivity that provides the desired sensitivity depending on the crystal orientation. Metallic electrical leads on the back face of the device are provided to route excitation currents and output signal voltages from these sensors to the external world.
Subjecting the plate of the device to a shear force, by mounting it on an aerodynamic surface exposed to flow, will result in a moment acting on the hinges of the biaxial gimbal structure that is proportional to the shear stress on the plate, the arm, and torsional hinge dimensions. This moment creates a mechanical torsional shear stress within the hinges and, thereby, an output signal proportional to the shear stress on the plate from the piezoresistive sensor. The shear stress at the fluid-sensor interfaces is thus initially converted to a mechanical shear stress in the hinge that is sensed with a piezoresistive sensor. The two orthogonally located hinges and sensors enable measuring the shear stress existing on the plate in both directions. This configuration of the sensor device enables a large moment and stress level to be generated at the hinge from relatively small shear stress acting on a small plate, thereby enabling high spatial and stress resolution capability.
This work was done by Sateesh S. Bajikar of Goddard Space Flight Center and Michael A. Scott and Edward E. Adcock of Langley Research Center. GSC-15431-1