Development of a nano-scale displacement sensor based on the shadow method

Practical applications in many engineering fields, including micro-fabrication, micro-assembly and aerospace, require precision measurement of micro-displacement. There are several techniques for measuring displacement. Among them and the most popular method is the use of displacement sensors. The sensors provide the displacement measurements of the target objects based on other parameters like acceleration, deflection and vibration. There are different types of displacement sensors, and the choice of one depends on the type of displacement being measured and the properties of the sensor.

Conventional displacement sensing methods face some restrictions that limit their applications. For instance, most of these sensing methods cannot be effectively used for micro- and nano-scale displacement measurements due to low resolution, small measuring range and nonlinearity, among other limitations. In an attempt to overcome these restrictions and enhance their measurement capabilities, several types of displacement sensors have been developed. However, the performance effectiveness of these sensors varies, and each of them has different problems, such as occlusion due to object shape.

Notably, developing displacement sensors based on a coaxial displacement measurement approach has been identified as a feasible solution as it has both the displacement and measurement light path on the same axis. Although the performance of displacement sensors based on this approach are remarkable, the complicated signal processing, the size of the light and the reflector requirements limit its practical applications. In addition, an equal laser coherence length and measured displacement affect the fringe resolution of the interferometry. These drawbacks make such sensors inconvenient for measuring micro-displacement.  

On this account, a team of researchers at Tianjin University: PhD candidate Yong Yang, Professor Meirong Zhao, Dr. Yinguo Huang, Dr. Dantong Li and Professor Yelong Zheng together with Dr. Yu Tian from Tsinghua University developed a bionic coaxial micro-displacement sensor. The sensor design was based on the shadow method and was inspired by the shadow formed when a water strider walks on the water surface. The water served as a sensitive element for measuring the micro-displacement by amplifying the displacement of the target object (water strider legs in this case). Their work is currently published in the research journal, Applied Optics.

The research team observed that subjecting the superhydrophobic circular plate to a coaxial displacement excitation resulted in the formation of a meniscus. The effect of the refraction when the parallel light illuminated the meniscus further formed the desired shadow. It was experimentally shown that the coaxial displacement excitation was proportional to the shadow diameter. A maximum sensitivity of 62 nm/pixel over a displacement measurement range of 50 µm with a linearity error of 1.58% was reported. It was worth noting that the resolution of this sensor could be improved further via sub-pixel edge division. Furthermore, other kinds of transparent liquids with different properties could be used in place on water in specific situations.

In summary, the authors reported the successful development of a novel bionic nano-scale displacement sensor. Besides having a simple and easy to construct structure, this coaxial displacement method achieved remarkably high precision displacement measurement and good linear performance with only water, camera and light source. In a statement to Advances in Engineering, the lead and corresponding author Professor Yelong Zheng stated that the advantages of the presented method make this sensor a promising device for high-precision micro-displacement measurement in many fields.