Differential Fiber Grating Vector Flow Velocity Sensor Based on Strain Amplifying Cantilever Beam Structure

A vector flow velocity sensor with a stainless steel equal-strength cantilever beam as the elastic element and dual fiber-optic gratings as the sensitive element has been proposed and experimentally demonstrated. And it uses differential measurement to eliminate the effect of temperature. To improve the sensitivity and stress transfer efficiency, we focus on the structural design of the cantilever beam through theoretical derivation and simulation. The cantilever beam structure combines two lug bosses on both sides, a rectangular beam, and an enlarged thrust surface. Three sensors A–C with cantilever beams that are 0.3, 0.4, and 0.4 mm thick with double size, are selected for experiments. The displacement sensitivities of the three sensors at static stress tests are 1.4405, 1.6455, and 0.6662 pm/ $\mu \text{m}$ , respectively. For practical measurements of underwater velocity, the quadratic term coefficients of the flow velocity sensitivity fitting curves as 210.2, 48.9, and 354.6, the linear term coefficients as −6.67, +5.05, and −19.76, and the ${R}^{{2}}$ as 0.9963, 0.9887, and 0.9981, have been calculated respectively. The error between the forward and reverse displacement sensitivities of sensor C is 0.404%, and its average error between the forward and reverse flow velocity measurements is 4.98%. Temperature-controlled experiments are further conducted and verify that the sensors can resist the effect of temperature. Experiments show that the sensor with large size and small thickness of cantilever beam has better velocity sensitivity. And experiment results prove that the sensors have application prospects in 3-D ocean current measurements.