Floating Element Shear Stress Sensors

Direct measurement of tangential force exerted by the fluid on the sensor element is possible by floating element shear stress sensor. Therefore, floating element-based shear stress sensor is classified as direct measurement device. Schematic of a floating element shear stress sensor 

No assumption regarding the flow field is required for the floating shear stress sensor. The displacement (5) of the floating element sensor as a function of shear stress (r ) can be derived from Euler-Bernoulli beam theory  

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Schmidt MA et al (1988) Design and calibration of a microfabricated floating-element shear-stress sensor. IEEE Trans Electron Devices 35(6) 750-757... 

The schematic of a floating element shear stress sensor (a) top view and (b)... 

Padmanabhan A, et al (1996) A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes. J Microelectromech Syst 5(4) 307-315 Jiang F, Tai Y-C, Huang J-B, Ho C-M (1995) Polysilicon structures for shear stress sensors. In TENCON 95. IEEE Region 10 International Conference on Microelectronics and VLSI Qiao Lin YX, Tai Y-C, Ho C-M (2005) A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633 Rouhanizadeh M, et al (2006) MEMS sensors to resolve spatial variations in shear stress in a 3D blood vessel bifurcation model. IEEE Sens J6(l) 78-88... 

Due to numerous benefits of MEMS-based shear stress sensors, the following shear stress measurement techniques having great promise for future MEMS applications are discussed in the following sections velocity measurements, thermal sensors, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal. 

The shear stress sensor for turbulent flow needs to accurately capture the complete turbulent fluctuation spectrum. Therefore, the shear stress sensor should possess a large bandwidth with flat and minimum frequency-phase relationship. For direct measurement, i.e., floating point sensors, the resonant frequency of the floating element and the fluidic damping determines the usable bandwidth. For the thermal sensor, the thermal inertia of the sensor element and the frequency-dependent heat conduction to the substrate influence the usable bandwidth. It is complicated to analytically predict the frequency response of the thermal sensor. Therefore, dynamic calibration is essential to characterize the frequency response of the sensor. 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry and shear stress sensitive liquid crystal. The Stanton tube is a rectangular shaped pitot tube located very close to the boundary wall and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. These shear stress measurement techniques are not ideal MEMS-based techniques. 

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