Optical-based sensors have a number of advantages over electrical sensors. Fiber optics are insensitive to and do not produce electromagnetic interference (EMI), and they suffer virtually no signal degradation even over vary long (100′s of meters) cable lengths. They can be used at higher temperatures than many conventional electrical sensors, and are safe for use with combustible materials and in HERO (Hazards of Electromagnetic Radiation to Ordnance) applications.
Principle of Operation
RealShear™ sensors are direct measurement force sensors employing a floating element that is brought in contact with the flow and a mechanical cantilever system which bends in response to shear stress applied to the sensor’s surface. This bending is detected by two optical strain gauges called Fiber Bragg Gratings (FBGs), attached to either side of the cantilever beam. Bending causes strain in the FBGs which shifts their optical resonance frequencies.
Micro-Optical strain gages
Fiber Bragg Gratings are resonant periodic structures inside optical fiber in which the index of refraction varies along the fiber. Like other optical resonators, light of a particular wavelength is reflected from the grating while light of other wavelengths passes through forming a characteristic absorption or reflection spectrum. The FBG is attached to the side of the cantilever and experiences longitudinal strain (or stress) when the cantilever is deflected, which alters the spatial structure of the grating causing a change in the wavelength of light that is reflected from it. By illuminating the fiber with light and detecting the reflected spectrum, the shift in the optical resonance wavelength, Δλ, can be measured and related to strain of the FBG. Therefore, shear force on the floating element can be measured by tracking the shift in the resonant wavelength. This optical detection of the force applied to the cantilever allows for small footprint of the sensor, provides high sensitivity combined with ruggedness, durability and insensitivity to electromagnetic noise.
Temperature compensation and temperature measurement
Two micro-resonators are housed inside the sensor so that temperature effect can be compensated for. Strain shifts FBG spectrum, but so does temperature. To get information about strain independently from temperature, and measure temperature independently from strain, two FBGs are attached to opposite sides of the cantilever. The differential signal (shift of FBG 1 spectrum less shift of FBG 2 spectrum) is independent from temperature, while the average signal (shift of FBG 1 spectrum plus shift
Strain shifts FBG spectrum, but so does temperature. To get information about strain independently from temperature, and measure temperature independently from strain, two FBGs are attached to opposite sides of the cantilever. The differential signal (shift of FBG 1 spectrum less shift of FBG 2 spectrum) is independent from temperature, while the average signal (shift of FBG 1 spectrum plus shift of FBG 2 spectrum) is independent from the strain:
Spectrum shift measurement and wall shear stress computation
Spectrum shift Δλ is measured using an optical circuit that is a part of the controller. Wall shear stress is found from τw = GΔλ , where G is the calibration coefficient. Sensors are calibrated by applying a varying mechanical force F to the tip of the cantilever and measuring Δλ: while shear stress is calculated from the known applied force as τw = F/A (A is the area of floating element).