Sensitivity Analysis of Metamaterial Surface Plasmon and Waveguide for Monitoring Metal Surface Profile and Concrete Crack/ Separation using Robotic-Arm Based Structural Health Monitoring

doi:10.1038/protex.2018.073

Method Article

Sensitivity Analysis of Metamaterial Surface Plasmon and Waveguide for Monitoring Metal Surface Profile and Concrete Crack/ Separation using Robotic-Arm Based Structural Health Monitoring

https://doi.org/10.1038/protex.2018.073

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Many electromagnetic (EM) metamaterial inspired designs such as localized surface plasmons (LSPs), have been extensively adopted for structural health monitoring (SHM) in the recent past. These LSPs, in contact with the ‘engineering structure under inspection’ generate a diagnosable surface plasmons/ wave as output, when subjected to input EM waves in the near fields. This paper, an extension to recently published article in Springer, presents a procedure to analyse the sensitivity of the output surface wave. The Springer article primarily presented a novel method of transferring ‘surface waves’ from within the confined boundaries of the LSP sensor to the far-off distance along a guided path. In experiments, the input - EM waves and the output - surface waves were handled by a vector network analyser via a 2D robotic arm. Signals generated by LSP sensor; and, LSP sensor plus waveguide was analysed for monitoring (a) metal shell's surface profile and (b) separation of two adjacent concrete blocks. Further, in this article, the sensitivity of the output signals using a unique root mean square deviation (RMSD) index is presented.

Materials science

Physics

Optics and photonics

Electromagnetic radiation (EMR) · Metamaterials · Localized surface plasmons (LSP) · Structural health monitoring (SHM) · Surface wave · Robot

Metamaterial inspired designs have been successfully tested in several applications such as gradient negative index lenses, invisibility cloaking, metasurface sub-wavelength imaging [1-4], and many more. Their entry in structural health monitoring (SHM) of Aerospace, Civil and Mechanical (ACM) engineering [5-7] started attracting engineers because of their strong electromagnetic (EM) response to the geometrical changes in the unit element or assemblies of elements [8], making it suitable for ‘strain’ monitoring, which is a common issue for most ACM structures.

Metamaterial inspired LSP sensors are highly non-fragile, which are manufactured as bi-material composites, made of metal inscriptions on flexible and ultra-thin Rogers materials \(RT5880) using PCB technique. These are bent and twisted-able; and do not require fragile packaging, unlike FBG sensors and moreover their ‘waveguides that transfer EM waves’  do not need any protection unlike optical fiber cables. 





   Most of the present designs are mostly based on the generation of \(A) ‘progressive wave’ that travel long distances in the ‘far fields’ \(air medium); and \(B) the measurement of the scattered parameters in the far fields. Thus, EM radiation leakages or reflections are common, which forces experimentation in special anechoic chambers. Thus, they were used mostly for biological or medical applications, but not for ACM applications \[9].

These EM radiations in the microwave range are harmless, and their applications for ACM engineering structure can be considered without any isolation. Hence, ‘near field’ based LSP sensor using the robotic arm and waveguide technique is adopted in our study published in Springer's Journal of Nondestructive Evaluation [10].

This technique was evaluated in terms of sensitivity for a couple of ACM engineering specimens as demonstrated in Figs 1-6, that shows its potential applications for SHM.

These LSPs, in contact with the ‘engineering structure under inspection’ generate a diagnosable surface plasmons/ wave as output, when subjected to input EM waves in the near fields. This paper is an extension of our article [10], presents a procedure to analyse the sensitivity of the output surface wave.

  The Springer article primarily presented a novel method of transferring ‘surface waves’ from within the confined boundaries of the LSP sensor to the far-off distance along a guided path. Our several articles in the last few years explored the spoof LSP sensor \[6, 10 -12], first developed a few years ago by Shen and co-researcher group \[13] which resembles a piezoelectric diaphragm \[14], but it is ultra thin and bendable unlike piezo. In experiments, the input – EM waves and the output – surface waves were handled by a vector network analyser via a 2D robotic arm. Signals generated by LSP; and, LSP plus waveguide were analyzed for monitoring \(A) metal shell’s surface profile and \(B) separation of two adjacent concrete blocks.

Further, in this article, the sensitivity of the output signals using a unique root mean square deviation (RMSD) index is presented.

In the present article, we employed

(1) Vector Network Analyzer (VNA)

(2) Hollow and confined input and output co-axial cables of 6 m each, which respectively supplied input EM waves and acquired output surface wave from and to VNA.

(3) LSP Sensor: It consists of a ground plate (copper) attached to a 254 μm PCB (Rogers RT5880) with a relative permittivity of 2.2 and loss tangent of 0.0009. Where this plate composed of periodic grooves (period of 1.256mm with 0.628mm air gap between neighboring copper strips) etched on an 18 μm thick copper disk, arranged in a circular pattern in the X-Y plane

(4) We also used a ‘conductive and solid’ lead cable of circular cross sectional diameter 1.5 mm as waveguides with their lengths ranging from 10 to 30 cm based on experimental types as carried-out in our article [10]. The present short article is limited to demonstration of the (1) Sensitivity of the slightly bent LSP sensor and (2) Sensitivity of combined LSP sensor and lead cable waveguide.

(5) Yamaha robotic arm, assembled in Nanyang Technological University

(6) Software to handle robotic arm. Co-axial cables with dipole antennas were mounted on the 2D robot arm as illustrated in Figures 1 and 2. The important component of the robot scanner (X-Y table) is a horizontal sliding arm, which can move in the X and Y directions rapidly (<1 sec for each move). In our study [10], this robotic arm was used to hold and move one coaxial cable that supplied either input EM waves at intermittent predefined locations or acquired output surface waves at instructed locations, but not for both simultaneously. Thus, the robotic arm facilitates scanning of (a) LSP sensor for a metal shell (Figure 1), and, (b) lead metal waveguide for monitoring the separation of two adjacent concrete blocks (Figure 2) rapidly and effectively with minimal human errors.

(7) Metallic shell made of aluminium (AL) with a Poisson ratio of 0.33, a density of 2715 Kg/ cubic. m and Young’s modulus of 66.67 GPa

(8) Two non-metallic i.e concrete blocks of characteristic strength 30 N/ m square.

(1)