A Micro-Displacement Measurement Based on the Shadow Technique
Summary
High-precision micro-displacement measurement is significant in the field of aerospace engineering, ultra-precision machining, and micro-assembly. The present protocol describes measuring micro displacements based on the shadow technique.
Abstract
The precision measurement of micro displacement is important in scientific and industrial fields. However, it is a tough challenge due to the complex design and the high cost of measuring instruments. Inspired by the shadow formed from water striders walking on a water surface under sunlight, a micro-displacement measurement method was proposed. Water strider legs with superhydrophobic properties bend the water surface. The curved surface of the water refracts sunlight, creating a shadow with a bright edge at the bottom of the pond. The shadow size is generally larger than the indentation depth of the legs from the water surface. In the micro-displacement measurement system, the applied displacement is proportional to the change in the diameter of the shadow. The presented study proposes a micro-displacement measurement procedure based on this shadow technique. The displacement sensitivity can reach 10.0 nm/pixel in the range of 5 µm. This system is simple to construct, low cost, and has high precision with good linear performance. The method provides a convenient additional option to measure micro-displacement.
Introduction
Precise displacement measurements play a vital role in the fields of aerospace engineering1, ultra-precision machining2, and micro-assembly3. Structural deformation must be measured precisely for structural health monitoring4. However, micro-displacement measurements with high precision remain a tough challenge due to the complex design and high cost of the measuring instruments5.
The micro-displacement measurement technique can be divided into conventional and non-conventional methods. Conventional methods, such as magnetic, capacitive, inductive, and electric sensors, are susceptible to electromagnetic interference6. Non-conventional methods are mainly optical methods, such as the optical fiber-based method and the laser method.
Ke Tian et al. designed a balloon-shaped bent multimode fiber structure to measure micro displacement, whose displacement sensitivity could achieve 0.51 dB/µm with a measuring range of 0-100 µm experimentally7. However, the size and the cost of the fiber optic demodulator must be considered first; and it was not easy to eliminate the thermal effect. Qianbo Lu et al. proposed a sub-nanometer resolution displacement sensor based on a grating interferometric cavity, whose sensitivity could reach 44.75 mV/nm by the intensity compensation and phase modulation8. The laser interferometer is one of the commonly used micro-displacement instruments with nanoscale resolution. However, the reflector requires complicated signal processing, and the fringe resolution of the interferometry limits its applications9. Therefore, an alternative simply-constructed, low-cost, high-precision measurement system is needed.
This article proposes a micro-displacement measurement procedure based on the shadow technique10,11,12,13 that is simple, low cost, and highly precise with good linear performance. This method was inspired by water striders walking on the water surface. Water strider legs with superhydrophobic properties bend the water surface. The curved surface of the water refracts sunlight, creating a shadow with a bright edge at the bottom of the pond. The shadow size is generally much larger than the indentation depth of legs from the water surface14,15,16. In the system, the applied displacement and the change in the diameter of the shadow were proportional, which was verified by the calibration experiment. The research indicates that this method provides an alternative for measuring micro displacement precisely.
Protocol
Representative Results
Discussion
This protocol proposed a micro-displacement measurement system based on the shadow technique. The displacement calibration is the critical step within the protocol to obtain the displacement sensitivity and the measuring range. The displacement sensitivity can be improved by reducing the diameters of the cylindrical legs and that of the parallel light beam and increasing the working distance based on Equation 4. Furthermore, the pixel size and the resolution of the camera, as well as the accuracy of image processing, are…
Disclosures
The authors have nothing to disclose.
Acknowledgements
We thank the National Key Research and Development Program of China (No 2021YFC2202702) for funding this work.
Materials
| Aperture diaphragm | Processed by high precision grinding | The diameter of the aperture is 0.7 mm. | |
| Camera | Canon | EOS80D | The pixel size and the resolution of the camera are about 3.72 μm and 4000 × 6000, respectively. |
| HALCON | MVTec Software GmbH | 18.11 | MVTec HALCON is the comprehensive standard software for machine vision with an integrated development environment (HDevelop) that is used worldwide. |
| Motorized linear stage | Zolix | TSA50-C | Resolution 0.625 μm |
| Parallel light source | Oriental Technology (Shanghai) Co, Ltd. | BTPL-50G | The peak wavelength is 523 nm. |
| Polydimethylsiloxane (PDMS) | Dow Corning | Sylgard 184 | PDMS is a transparent silicon-based crosslinked polymer. |
| Vacuum pump | SHANGHAI LICHEN-BX INSTRUMENT TECHONOLOGY CO.,Ltd | 2XZ-6B | The pumping rate is 6 L/s.The ultimate vacuum is ≤1 Pa |
| Vertical precision positioner | PI | P-620.ZCD | The resolution is 0.2 nm in the range of 50 μm. |
| Workbench with three rigid cylindrical legs | Processed by high precision grinding | The diameter of legs is 0.5 mm. The legs are distributed on the trisection points of a circle with a radius of 14 mm |
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