Tham khảo tài liệu 'the mems handbook (1st ed) - m. gad el hak part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | FIGURE SEM image of the UCLA Caltech shear stress sensor. The shear layer separating from the edge of the delta wing is thin order of 1 mm for the UCLA Caltech work and very sensitive to minute changes in the geometry. Therefore as discussed earlier in this chapter the use of microactuators to alter the shear-layer and ultimately the vortical-structure characteristics has good potential for success. Furthermore when the edge of the wing is rounded rather than sharp the specific separation point location will vary with the distance from the wing apex the flow velocity and the position of the wing relative to the flow. Therefore a distributed sensor actuator array is needed to cover the area around the edge of the delta wing for detection of the separation line and actuation in the immediate vicinity of it. Sensing To detect the location of the separation line around the edge of the delta wing the UCLA Caltech group utilized an array of MEMS hot-wire shear sensors. The sensors which are described in detail by Liu et al. 1994 consisted of 2- am wide X 80-ựm long polysilicon resistors that were micromachined on top of an evacuated cavity an SEM view of one of the sensors is provided in Figure . The vacuum cavity provided thermal isolation against heat conduction to the substrate in order to maximize sensor cooling by the flow. The resulting sensitivity was about 15 mV Pa and the frequency response of the sensors was 10 kHz. For more comprehensive coverage of this and other MEMS hot-wire sensors the reader is referred to Chapter 26. Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the separation line it was not possible to identify the location of separation from the instantaneous shear-stress values measured by the MEMS sensors. Instead Lee et al. 1996 defined the location of the separation line as that separating the pressure- and suction-side flows in the vicinity of the edge of the wing. The distinction .
