A comprehensive review on piezo impedance based multi sensing technique

1. Introduction

Civil engineering infrastructures are growing rapidly in all corners of the construction world because of their strength, durability, versatility, and robustness. The aging of the structures causes severe inspection dilemmas due to improper maintenance. Structural Health Monitoring (SHM) is an emerging field that aims to monitor and evaluate the condition of structures such as bridges, buildings, and aerospace structures, in real-time [1]. The primary objective of SHM is to enhance the safety and reliability of structures by detecting and quantifying any damage or degradation that may occur during their lifespan. SHM is a multidisciplinary field that involves various areas of expertise such as sensors, signal processing, digital twinning, optimization techniques, data analysis, and machine learning algorithms [2]. The importance of SHM has been recognized in recent years due to the increasing demand for infrastructure and the need to maintain existing structures [3]. SHM systems can provide early warning signs of potential structural failures, thereby preventing catastrophic events and reducing maintenance costs. In addition, SHM can improve the overall performance of structures by enabling condition-based maintenance, which can lead to a longer lifespan and reduced downtime.

Over the past few decades, extensive research has been conducted in the field of SHM, and several techniques have been developed for detecting and monitoring structural damage [4]. These techniques can be broadly categorized into two types: model based and non-model based [5]. Though conventional NDTtechniques are widely used for the inspection of structures and include methods such as visual inspection, ultrasonic testing, radiography, and eddy current testing, but not highly sensitive towards detecting damage, they are limited in their ability to provide continuous, real-time monitoring of structures. SHM systems, on the other hand, are designed to provide continuous monitoring of structures and can detect and quantify damage in real-time.

Traditional SHM techniques rely upon sensors that measure structural responses, such as strain, acceleration, and displacement, to detect damage in the structure. However, these sensors have limitations in terms of cost, installation complexity, and sensitivity to environmental conditions. In contrast, the piezo impedance based electro mechanical impedance (EMI) technique has emerged as a promising alternative for SHM [6]. This technique utilizes piezoelectric materials as both sensors and actuators to detect and mitigate damage in structures. The piezoelectric sensors detect any structural anomalies (hence the piezo-coupled impedance) caused by damage, while the actuators apply electric fields to the structure to generate the surface wave [7]. Also, the impedance based monitoring has been emerged as a promising alternative tool due to its ability to detect incipient damages effectively [8]. This technique is based on the piezoelectric effect, where a change in physical parameters such as strain, temperature, and acceleration results in a change in the electrical impedance of the sensor. By measuring the electrical impedance, it is possible to obtain modal parameter of structure through signature inspection and inverse sensitive method. The past studies have mainly focused on the development and application of the piezo impedance-based sensors in specific domains. Therefore, a state-of-the-art review of the technique is necessary to bridge the gap in knowledge and provide a more holistic understanding of the piezo impedance-based multi-sensing technique. Thus, this study aims to fill this gap in the literature by providing a comprehensive review of the piezo impedance-based multi-sensing technique. This review will cover the principles, design, fabrication, and applications of the piezo impedance SHM techniques, as well as its potential limitations and challenges. By doing so, the study will contribute to the development and advancement of this sensing technique, opening up new avenues for research and applications in various fields. The novelty of this studies lies in its contribution to the existing literature by presenting a systematic approach to designing PZT sensors for piezo impedance based multisensing technique with varying EMI related parameters.

In this review article, a new learning paradigm of the piezo impedance based multi-sensing technique, including its principles of operation, sensor design and fabrication, applications, and recent advances in the field. The aim of this article is to provide a valuable resource for researchers and practitioners in the field of SHM to better understand the capabilities and limitations of piezo impedance based multi-sensing technique and to facilitate its wider adoption in practical applications.

1.1. Nondestructive evaluation and techniques (NDE&T)

Nondestructive evaluation (NDE) has gained significant attention as a cost-effective and efficient tool for identifying structural defects. Nondestructive testing (NDT), a crucial component of NDE, entails the detection and characterization of surface-level damages or defects in an object without causing harm to it [9]. In traditional NDE, the testing device is located outside of the structure, and the data is collected after a thorough examination. The measured data is then processed to assess the system or part of system. This non-destructive approach offers numerous advantages such as accident prevention, reduced maintenance costs, and increased overall safety and reliability of the structure. However, it also has many limitations. For instance, NDE may not detect defects that are deeply embedded within a material or structure, requiring more invasive methods such as destructive testing. Moreover, the accuracy and sensitivity of NDE techniques can be limited by the type of material being tested and the equipment used. Some materials such as composites or ceramics have unique properties that require specialized techniques and equipment, rendering some NDE methods unsuitable. To bridge this gap, a combination of both NDE and SHM techniques can be employed. This approach allows for the early identification of potential issues, which can be addressed promptly through timely repairs or replacement, resulting in an increase in the safety and reliability of the structure or component.

1.2. Structural health monitoring (SHM)

In recent years, Structural Health Monitoring (SHM) has experienced rapid growth due to numerous technological advancements that provide new and improved methods for monitoring the health and integrity of structures. Through the collection, validation, and analysis of scientific information, SHM can improve life cycle strategic decisions for infrastructure sector of country. Fig. 1 graphically illustrates the concept of SHM. Overall, SHM is a reliable system for detecting and understanding alterations in structures, and the use of vibration-based methods has shown promising results in identifying structural concerns and evaluating deficiencies, which can inform maintenance strategies and improve overall structural integrity.

Research has explored the use of various smart materials and sensors in SHM, including piezoelectric sensors, fiber optic sensors, and smart composites [10]. Among these approaches, vibration-based methods have been the most extensively utilized [11]. The vibration behavior of a structure is a crucial aspect in determining its physical characteristics. Changes in the modal parameters of a system due to structural damage can serve as a valuable indicator of the structure's overall integrity, as noted in previous studies [12]. The information collected from sensing devices can be used to determine the vibration characteristics of the system. The ability of sensors to detect damage can be leveraged to identify and evaluate corresponding changes [13]. Various specifications and codes for vibration-based SHM have been established and utilized in industrial applications, building on these methodologies and newly developed IT and monitoring technologies [14].

Despite the remarkable strides made in technology and methodologies, SHM research faces limitations. One major limitation involves the complexity and noise of the data obtained from sensors, which pose difficulties in interpretation. Additionally, the accuracy and reliability of sensors may be affected by environmental factors such as temperature, humidity, and noise. Another challenge lies in the high implementation cost of SHM systems, which could make it inaccessible to some structures. Also, certain structures may not be suitable for monitoring, and some types of damage may be undetectable. Furthermore, vibration-based methods may not be appropriate for structures with low natural frequencies. Addressing these limitations and weaknesses is essential to harnessing the full potential of SHM systems in enhancing the safety and durability of structures. Electromechanical Impedance (EMI) technique can be a solution to overcome some of the limitations of local inspection approaches and complex geometry of the structure. EMI technique can provide information on both local and global structural health, making it a versatile tool for damage detection.

1.3. Electromechanical impedance (EMI) technique

The EMI Technique is a well-established NDE method for determining the structural integrity and detecting damage in materials. Liang's 1D impedance model, as shown in Fig. 3, is a mathematical representation of the electrical impedance of a material along a one-dimensional axis. In similar vein, Sun et al. [15] proposed a modal analysis technique that utilizes the measurement of electric admittance of collocated actuator/sensors for the characterization of the mechanical impedance of structures. An SDOF model governing the electromechanical interaction is derived and used to extract the mechanical impedance of the structures from the measured electrical admittance. Park et al. [16] presents an interesting methodology for detecting and locating structural damage in one-dimensional structures. The authors provide a clear and concise explanation of the methodology and its underlying physical principles. The use of piezoelectric materials and frequency response functiondata is a novel approach that can potentially lead to more accurate and efficient damage detection. However, the paper lacks a detailed evaluation of the accuracy and reliability of the approach, which leaves room for further exploration and improvement. Xu and Liu [17] were the first to identify the significance of the adhesive layer in the impedance model, which was depicted as a 1-D system of spring-mass-damper connected in series with the structure. Fig. 2 shows the most typical size of the PZT, which is a piezoelectric material commonly used in various applications such as sensors, actuators, and transducers. The size depicted in the figure is representative of the dimensions used in many PZT-based devices, and it plays a critical role in determining their performance characteristics. Understanding the size and properties of PZT is essential for designing and optimizing these devices for specific applications.

Zhou et al. [18] extended the formulations of Liang et al. [19] to model the PZT element coupled to a two-dimensional (2D) host structure shown in Fig. 4. The main limitation of Zhou's model is that, it is difficult to determine structure's mechanical impedance experimentally. Giurgiutiu and Rogers [20] developed a theoretical model for the EMI response of a damaged composite beam interrogated by a PZT patch. The effective axial force and bending moments induced by the PZT wafer into the beam were considered. Park et al. [21] modelled piezo bonded 1D structure using the wave propagation approach. Direct frequency response function data were utilized to detect and locate damage in the structure. They derived spectral formulation for a dynamic finite element, referred to as the spectral finite element or the dynamic stiffnessmatrix formulation, suitable for high frequency analysis.

Table 1 outlines the characteristics of the PZT patch commonly utilized by various researchers. Annamdas and Soh [22], proposed a new 3D model that utilizes 3D actuation and a PZT, addressing the limitations of previous 1D and 2D models. Additionally, Bhalla and Soh [23] established equations to determine the coupled complex admittance signature of bonded or embedded PZT patches.(1) $\overline{Y} = G + B j = 4 ω j \frac{l^{2}}{h} [\overline{ε_{33}^{T}} - \frac{2 d_{31}^{2} \overline{Y^{E}}}{(1 - μ)} + \frac{2 d_{31}^{2} \overline{Y^{E}}}{(1 - μ)} (\frac{Z_{a, e f f}}{Z_{S, e f f} + Z_{a, e f f}}) \overline{T}]$ where the dimensions of a PZT material are represented by l, w, and h, and its conductance and susceptance are designated by B and G, respectively. The PZT patch's and the structure's effective impedances are represented by $Z_{a, eff}$ and $Z_{S, e f f}$ , respectively. The Poisson's ratio is referred to as μ. $\bar{ε_{33}^{T}}$ and k are the complex electrical permittivity and wave number are expressed in Eq. (1). $\bar{Y^{E}}$ denoted by the complicated Young's modulus of elasticity. Additionally, the complex tangent ratio is denoted by $\bar{T}$ and the piezoelectric strain coefficient is represented by d31. The symbol for excitation's angular frequency is ω.

Table 1. Characteristics of the PZT patch employed in numerous literature sources [32].

Property

unit

value

Poison's ratio,μ

0.300

Mechanical loss factor

0.032

Dielectric loss factor

0.022

Thickness

0.0003

Electric permittivity

Farad/m

2.124 \times 10^{- 8}

Piezoelectric strain coefficient

m/V

- 2.100 \times 10^{- 10}

1.3.1. Overview of basics and technological aspects

In EMI technique, the impedance analyzer is used to evaluate the system's performance, where the PZT patch acts as an actuator, causing axial vibrationsand interacting with the host structure. The dynamic PZT-host structure system is formed by the intricate connection between the PZT's electrical response and the host material's mechanical impedance. However, the high cost of these devices has driven the need for more cost-effective methods of implementing Electromechanical Impedance (EMI) techniques. Peairs et al. [25] introduced the integration of an FFT analyzer into the EMI approach. This innovation facilitated the detection of structural damage and allowed for a comprehensive evaluation of the strength progression of the structures. Despite ongoing efforts to develop a low-cost EMI approach, the task of monitoring large structures remains a significant challenge, given the high cost and complexity of using many piezoelectric transducers. The advent of wireless sensor technology has thus become increasingly relevant [26]. EMI methodology offers the advantage of utilizing piezoelectric transducers as both accelerators and detectors, providing a cost-effective solution for structural monitoring. The EMI technique has found widespread application across engineering fields, including strength analysis, corrosion assessment, and pipeline monitoring. Furthermore, EMI techniques were also utilized to detect cracks or notch in buildings. This includes identifying damage to various structures, such as plates, prestressed concrete, and building infrastructures. According to several recent studies, closer and more extreme damage will result in a higher predictive statistical score [[27], [28], [29], [30], [31], [32]].

1.3.2. Merits and demerits of EMI techniques

Structural health monitoring (SHM) is an essential part of ensuring the safety and reliability of mechanical, aerospace and civil engineering structures. Electromechanical impedance (EMI) techniques have emerged as a promising SHM tool due to their non-destructive nature and high sensitivity to small changes in the mechanical properties of a structure. The merits and demerits of EMI techniques are discussed below.

1.3.3. Merits

I.
Non-destructive: One of the significant advantages of EMI techniques is that they are non-destructive. EMI measurements do not cause any damage to the structure being monitored, making them ideal for long-term monitoring.
II.
High sensitivity: EMI techniques are highly sensitive to small changes in the mechanical properties of a structure. This sensitivity makes them useful for detecting early signs of damage, which can prevent catastrophic failure.
III.
Real-time monitoring: EMI techniques can provide real-time monitoring of structural health, allowing for immediate action in case of damage. Real-time monitoring enables continuous data collection, which can improve the accuracy of the damage detection process.
IV.
Low cost: EMI techniques are relatively low-cost compared to other SHM techniques. The simplicity of the instrumentation required and the ease of installation makes EMI techniques cost-effective.
V.
Easy installation: Piezoelectric transducers used in EMI techniques are small and easy to install. This feature makes them ideal for monitoring complex structures such as bridges, dams, and pipelines.

1.3.4. Demerits

i.
Limited to specific structures: EMI techniques are most effective for monitoring metallic, concrete and composite structures with a high degree of symmetry and uniformity. Therefore, they are less suitable for monitoring irregular or heterogeneous structures.
ii.
Limited range: EMI techniques are most effective for detecting small changes in mechanical properties. Their sensitivity decreases for larger damage or structural changes, limiting their ability to detect significant damage events.
iii.
Environmental factors: EMI measurements can be affected by environmental factors such as temperature and humidity. These factors can lead to false readings if not correctly accounted for.
iv.
Complex analysis: The analysis of EMI data can be complex, requiring specialized knowledge and software tools. The interpretation of data can be challenging, which can lead to errors in the damage detection process.

2. Single sensing vs multisensing

The main limitation of single sensing EMI techniques is their inability to detect multiple damage locations within the large structural member. This limitation is particularly problematic for large structures where damage may occur in multiple locations simultaneously and can be unnoticed due to actuation and sensing range, particularly for composites. Finally, single sensing EMI techniques suffer from difficulties in discriminating between different types of damage. The changes in impedance due to different types of damage may be similar, making it difficult to distinguish between different types of damage using a single sensor. Multisensing EMI techniques overcome the limitations of single sensing EMI techniques by using multiple sensors to detect changes in impedance at multiple locations simultaneously. This technique enables the detection of damage in multiple locations, which is particularly important for large structures. In addition, multisensing EMI techniques can detect high-frequency damage modes, which are important for certain types of structures.

The forthcoming section focuses on the piezo impedance-based multi-sensing technique. The methodology for this review involved a thorough literature search and analysis of relevant studies in the field. The past relevant studies were filtered based on their suitability and the quality of the research methodology employed. The selected studies were then analyzed and synthesized to provide a comprehensive understanding of the piezo impedance-based multi-sensing technique. It covers principle of the piezo impedance-based multi-sensing technique, its advantages and limitations, applications in various fields, and recent advancements in the technique.

3. Multi sensing approach

The multisensing based electromechanical impedance (EMI) techniques has gathered significant interest in recent years due to their several advantages over single sensing EMI techniques. Although EMI-based monitoring of structures has proven to be effective in strength and damage identification, there is still room for further improvement. One potential solution to minimize the amount of information obtained through multiple sensor designs that detect the deterioration within a system or part of a system. Also, the multi-sensing approach can be introduced in the form of hybrid transducer design for piezoelectric sensors, either in series or in parallel [33]. Effective implementation of sensors is a critical aspect in improving the efficiency of infrastructure. Annamdas and Soh [34] presents a semi-analytical model that considers the mass of the PZT transducers and does not impose restrictions on their shape, size, and electrical properties is a significant contribution to the field of structural health monitoring. They proposed the experimental verification of lab-sized aluminum plate instruments with multiple PZT patches. Fig. 5 depicts the three-dimensional piezoelectric structure interaction model. The model addressed the limitations of previous existing models and is experimentally verified the proposed model for 3D modelling of PZT-structure interaction.

Hey et al. [35] proposed a parallel architecture to optimize the interrogation of multiple PZT patches o reduce the noise in the measured signals coupled with electrical noise and structural defects. They also designed a protection circuit to prevent PZT damage by limiting the voltage and current applied to the PZTs. To improve accuracy and reduce interrogation time, parallel multiplexing techniques of the PZT patch were depicted in the proposed study (see Fig. 6). This approach can be highly beneficial in situations where multiple PZT patches need to be monitored simultaneously, such as in structural health monitoring applications. Additionally, the authors did not provide any quantitative results to support their claims of improved accuracy and reduced interrogation time. Chen et al. [36] made a significant contribution to the field of bolt looseness detection by introducing a novel approach that combines EMI and multi-sensing techniques to improve the accuracy and reliability of the detection process. The above past studies focuses mainly on the theoretical aspects of the proposed technique, and there is a lack of empirical evidence to support the claims made in the paper. Secondly, those past investigations does not provide a detailed discussion of the limitations and challenges associated with the proposed technique, which could affect its practical implementation. Fig. 7displays the easy model multisensing parallel PZT arrangements. The reliability of this purpose parallel easy model circuit makes it an ideal solution for various sensing applications. Furthermore, Saravanan et al. [37] explored the utilization of a series connection of PZT patches for nondestructive testing of metallic structures and monitoring concrete hydration through the utilization of plates with varying thicknesses. Guo et al. [38] presents an interesting and potentially useful approach to monitoring grouting compactness in tendon ducts. Fig. 8illustrates the multisensing EMI-based PZT patch connected serially. The series connection of these PZT patches results in the formation of multiple frequency peaks. The changes can be observed in the impedance spectra at various frequency peaks, providing evidence for the aforementioned modifications. Fig. 9 depicts the schematic diagram of multisensing EMI-based grouting compactness monitoring. A series configuration of three PZT sensors with varying sizes was employed in an empty duct to evaluate their impedance responses.

In summary, the study provides a comprehensive investigation into multi-sensing techniques. Hence, the utilization of multi-functional sensors that can significantly reduce the number of sensors needed for sustainable development may gain prominence. However, there is a need for further research to investigate the effectiveness of these sensors and their potential limitations.

Building upon earlier research, the utilization of multi-sensing techniques has been extended to predict damages in reinforced concrete structures. In the field of civil engineering, monitoring the strength and durability of concrete structures is of utmost importance to ensure their safe and sustainable use. Various researchers have explored the potential of smart aggregate-based multi-sensing piezoelectric transducers (PZT) for real-time monitoring of concrete structures. Song et al. [39] proposed a sensing technology based on smart aggregates integrated with PZT for monitoring the health of concrete structures. The technology was tested extensively on various civil engineering structures to evaluate its effectiveness and suitability. In a similar vein, Priya et al. [40] demonstrated the significant potential of smart aggregate-based sensing technology for continuous monitoring of structural strength and damage detection. They found that multi-sensing techniques were more efficient in detecting concrete specimens at an early age than direct bonded Piezo sensor readings. The authors evaluated the effectiveness of PZT arrays by examining their performance in both series and parallel connections and observing changes in frequency peaks across various age ranges to determine the optimal configuration of PZT arrays for maximum efficiency. Karayannis et al. [41] used piezo sensors for real-time monitoring of structural damage. Their research highlights the importance of real-time monitoring for identifying and assessing damage to structures. However, continuously monitoring a significant amount of data can be tedious and challenging to manage. Overall, the contributions of these studies were the development and evaluation of smart aggregate-based sensing technology and the use of PZT arrays and piezo sensors for continuous monitoring of structural health and damage detection.

Moharana and Bhalla [42] explored the feasibility of utilizing non-bonded piezo sensors to track the effect of hydration on concrete specimens. They evaluated the effectiveness of three distinct multi-sensor setups, namely rebar bonding, concrete vibration sensors, and metal foil-based PZT bonding. Their research illustrated the effectiveness of the suggested arrangements in monitoring the hydration process of concrete samples. In a similar vein, Raju et al. [43] examined the potential of a modified non-bonded setup to evaluate pipeline corrosion via EMI methods. The novel, reusable multi-sensor configuration was found to be highly efficient in accurately detecting pipeline corrosion.

Later, Naskar and Bhalla [44] presented an EMI approach that utilizes metal wires for the purpose of detecting damage in two-dimensional structures. This non-bonded and multi-sensing configuration holds potential for application in situations where direct bonding of PZT patches is not feasible. In another study, Luo et al. [45] proposed a simple and effective approach for designing and producing a smart modulation unit by attaching PZT patches with four different thicknesses of the magnetic disc. Fig. 10 demonstrates the smart modulation transducer-based multi-sensing series connection, in which the thickness of the permanent magnet disc can linearly change the transducer's impedance frequency response. This feature simplifies transducer design and ensures each one has a significantly different frequency.

The piezo impedance-based multisensing connection with protective cover is a very promising sensing approach that provides high sensitivity, non-intrusive sensing, and multiple sensing capabilities in a single device. Its wide range of applications, including medical, environmental, structural health monitoring, and aerospace applications, make it a versatile sensing approach that can meet the needs of various industries. Table 2 shows various civil Engineering structures and their assement using multi piezo configurations.

Table 2. Various civil structures and their aim of assessment using piezo impedance based multi sensing technique.

Structure	Aim of Assessment	Piezo Impedance Based Multi Sensing Technique
Bridges	Determine the structural health and integrity of bridges.	Detect defects, damage, and cracks in the bridge deck and superstructure using piezo impedance sensors to monitor the dynamic response of the bridge.
Buildings	Evaluate the performance of building structures.	Measure the dynamic response of buildings using piezo impedance sensors to determine the structural health, identify any damage or deterioration, and monitor changes in structural behavior over time.
Dams and Embankments	Assess the stability and integrity of dams and embankments.	Detect seepage, leakage, and internal erosion using piezo impedance sensors to monitor changes in water pressure and soil behavior.
Tunnels	Evaluate the safety and integrity of tunnel structures.	Monitor the dynamic response of the tunnel using piezo impedance sensors to detect any deformation, cracking, or collapse.
Wind Turbines	Assess the structural health and performance of wind turbines.	Monitor the dynamic response of the turbine blades using piezo impedance sensors to detect any damage, deterioration, or fatigue.

3.1. State of art for multisensing based SHM

This study provides a review of the utilization of multisensing EMI techniques in diverse industries and fields [46]. Traditional piezo sensors have limitations such as sensing region, environmental degradation, noise etc. However, multi-configuration piezo sensors offer an added advantage as they can be configured in different ways. A novel multisensing approach coupled in series and parallel can reduce time and manpower, resulting in a single impedance signature [47]. The overlapping zone of the sensor units can be determined using physically coupled series and parallel connections collected from a pair of sensor units with both mechanical and electrical coupling. This method can be applied in a complex structure where the attachment of PZT is inaccessible. Piezo sensors with multi configurations can be employed to convert mechanical energy to electrical energy, making them useful in various applications such as harvesting energy from ocean waves, wind, and other sources [48]. In summary, multi-configuration piezo sensors are considered an efficient tool for monitoring and measuring various physical parameters [49].

Multisensing EMI techniques offer numerous advantages, such as improved accuracy, increased sensitivity, reduced sensing uncertainty, improved signal-to-noise ratio, and increased robustness. Consequently, these techniques are a valuable tool for ensuring the safety, performance, and reliability of various systems in aerospace, defense, automotive, consumer electronics, and industrial applications [50,51].

3.2. Factors affecting multi sensing approach

The efficiency of multisensing approach for damage evaluation is heavily dependent on the frequency range used. However, these techniques are primarily focused on detecting damage rather than precisely locating it. Most research in the field of electromechanical impedance analysis has centered on the frequency domain approach [52]. The frequency range selected for analysis is crucial and is determined by identifying the highest peak in the impedance signatures. Previous studies have shown that the accuracy of sensing is significantly impacted by the frequency range chosen [53]. When a piezoelectric transducer (PZT) is positioned near damage, the conductance signatures tend to be in the higher frequency range, while signatures from a PZT positioned at a distance appear to have lower frequencies. The sensing zone plays a significant role in EMI techniques, but its size depends on various factors, such as the host structure's material, dimensions, material properties, vibration, and structural discontinuities. The typical sensing radius of a standard PZT patch is 2 m for simple beams and 0.4 m for composite structures [54], but it is challenging to quantify the energy dissipation at such high frequencies, making it difficult to precisely identify the sensing zone. Structural discontinuities function as numerous reflections, leading to the most significant attenuation of propagating waves. Several factors can affect the effectiveness of a multisensing approach, such as sensor selection, signal processing, environmental factors, integration of sensors, and cost [55]. According to Park et al. [56], adding random ambient factors to the structural system has no impact on the monitoring of airplanes in operation, which is crucial since the signatures obtained from PZT sensors are sensitive to thermal changes. In practical situations, a combination of damage and thermal conditions is often present.

Table 3 summarizes recent literature related to multi-sensing techniques, providing further insights into the research field. Overall, these studies demonstrate the potential and versatility of multi-sensing techniques in various applications such as predicting damages in concrete structures, evaluating pipeline corrosion, and detecting damage in two-dimensional structures. various multi-sensing techniques have been proposed and evaluated for real-time monitoring of concrete structures, including smart aggregate-based PZT, piezo sensors, and EMI. These listed studies shows how multisensing technique has several advantages over traditional methods of monitoring concrete structures, which can help ensure their safe and sustainable use in the future.

Table 3. Literatures related to techniques and specimens used in multi sensing EMI techniques.

Author	Sensing techniques	Specimens	Descriptions	References
Hey et al. (2006)	PZT patches attached parallel	Aluminum plate	Damage detection	[35]
Madgav and Soh (2008)	Single input, single output, and multi input multi output (MIMO) sensing of PZT	Aluminum plate	Uniplexing and multiplexing capabilities of PZT transducers	[57]
Annamdasand Soh (2008)	3D EMI model for multi-piezo structure interaction	Lab-sized aluminum plate	Formulated 3D multi sensing EMI model	[58]
Yang et al. (2010)	Aluminum enclosure based PZT setup	Concrete cube	Consistency of reusable PZT setup	[59]
Tawie and Lee (2011)	Reusable PZT plate	Cement mortar	Setting time, moisture, and damage monitoring of cement mortar	[60]
Na and Lee (2012)	PZT attached in a metal plate	Composite plate	Damage detection	[61]
Na and Lee (2013)	Multi sensing EMI method	Aluminum plate	Multiple area damage detection from a single frequency sweep	[62]
Talakokula and Bhalla (2014)	Surface bonded and embedded (CVS) piezo sensor	Reinforced concrete cube	Comparison of corrosion assessment using both the sensor	[63]
Naskar and Bhalla (2015)	Metal wire based EMI techniques attached serially	Steel plate	Investigated MWBEMI techniques for damage detection	[44]
Na and park (2017)	Circular metal dish based bonded PZT patch in series	Metal plate	Multiple area damage detection of plate	[64]
Srivastava et al. (2017)	Non-bonded aluminum clamp based attachment of PZT patch	Artificial human bones	Implementation of non-bonded configuration in bio medical application	[65]
Huo et al. (2017a)	PZT based smart washer	Steel plate with nut and bolt connection	Bolt looseness monitoring using smart washer	[66]
Priya et al. (2018)	Serial sensing of PZT patch bonded on plate	Concrete beam	Novel serial sensing using PZT patch for different curing conditions of concrete beam	[40]
Tawie et al. (2019)	Conventional PZT, Steel wire PZT, Metal dish PZT	Glass fibre composite plate	Delamination and crack detection using proposed techniques	[67]
Balamonica et al. (2019)	PZT based Smart sensing unit (SSU) with multiplexed parallel/series connection	RC beam	Damage detection through multiplexing	[68]
Moharana and Bhalla (2019)	Embedded (CVS), PZT attached to metal foil, surface bonded	Concrete cube	Concrete hydration monitoring using reusable sensor	[42]
Raju et al. (2020)	Modified non-bonded and surface bonded piezo configuration	Metal pipe	Corrosion detection in pipeline structures	[43]
Guo et al. (2020)	Multiple PZT connected serially	Post-tensioned tendon duct	Monitoring grouting compactness	[38]
Chen et al. (2020)	Multisensing smart washer based PZT patch	Steel plate connected with bolt	Detection of bolt looseness	[36]
Wang et al. (2021)	Smart crawfish based PZT patch	Ice cube	Detection of underwater multi bolt looseness	[39]
Luo et al. (2020)	Multisensing smart modulation transducer (SMT)	Steel plate with bolted connection	Bolt looseness monitoring using multisensing SMT based sensor	[45]
Parpe and Saravanan (2021)	Multisensing smart PZT	Concrete beam	Parallel connection of smart sensing unit for damage detection	[70]
Gayakwad and Thiyagarajan (2022)	EMI and wave propagation techniques using SSU patch	Concrete beam	Damage detection of the concrete beam using combined EMI-WP techniques	[71]

3.3. Comparative assessment between single piezo sensing and piezo based multisensing

Single-sensing based EMI involves the use of a single sensor placed on a structure to detect and localize damage. However, the utilization of single-sensing technique for damage detection has its limitations as it is unable to differentiate the changes in electrical impedance due to damage from those resulting from other environmental factors such as high temperature, humidity, or creep. In response, the multi-sensing approach has been proposed, which involves the placement of multiple sensors at various positions on the structure. This approach provides improved accuracy in damage detection and localizationcompared to the single-sensing approach. Additionally, the use of multiple sensors also allows for the creation of a baseline electrical impedance signature for the structure, which can be used for comparison with future measurements to detect changes in the structure's health. In the context of EMI approaches, the selection of a Single-sensing or Multi-sensing approach will depend on the specific demands of the application. Both approaches have distinct pros and cons, and the ultimate choice must align with the needs of the situation. Table 4clearly illustrates the distinct contrast between the single and multisensing approaches used in various structures.

Table 4. Single piezo sensing and piezo based multisensing techniques.

Sensing Methods	Requirements	Pros	Cons
Single piezo sensing	✓ Suitable for various types of structures. ✓ It is useful for structures in the fields of civil engineering, mechanical engineering, and aerospace engineering.	✓ Sensitive to incipient damages. ✓ High frequency excitation. ✓ Reasonably simple to analyze and interpret the data. ✓ Economical. ✓ Monitors wide range of faults, debonding damages, and corrosion in structural systems. ✓ Post processing of data is easy.	✓ Connected wires. ✓ Frequency range is reduced due to high frequency excitation.
Piezo based Multi sensing	✓ Commonly useful in shells, complex structures, Rail networks, and composite structures. ✓ Reusable and non-bonded configurations useful for structures having complex geometry.	✓ The utilization of an array of sensors renders the system highly susceptible to all forms of damage, thereby increasing its sensitivity. ✓ Useful to sense more area. ✓ Multisening can be used in railways, piers, cables, arches, and offshore structures. ✓ Interpretation of data can be used to visualize structural system identification. ✓ Economical	✓ Large network of wired element of sensor and transducers. ✓ Having multiple sensor nodes required energy (hence the voltage booster) in the sensor network system. ✓ The standard power back would not sufficient for major structural/environmental accidents

3.4. Challenges

Piezo impedance-based multi-sensing approach is a technique used to measure multiple parameters such as pressure, temperature, and flow using piezoelectric sensors. While this technique offers many advantages, it also poses several challenges, including:

•
Cross-talk: The signals from different sensors can interfere with each other, leading to cross-talk. This can be particularly problematic when the sensors are closely spaced or share a common electrode.
•
Calibration: Accurate calibration of each sensor is crucial to ensure reliable measurements. However, the calibration process can be time-consuming and expensive, especially when dealing with multiple sensors.
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Environmental sensitivity: Piezoelectric sensors can be sensitive to environmental factors such as temperature and humidity. This can lead to inaccurate measurements if these factors are not taken into account.
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Signal amplification: The output signals from piezoelectric sensors are often very small, requiring amplification before they can be accurately measured. However, amplification can also introduce noise and distortion into the signals.
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Sensor placement: The placement of the sensors can affect the accuracy of the measurements. For example, if the sensors are not evenly distributed, some crucial and stressed areas may be over-sampled while others are under-sampled.
Overall, while piezo impedance-based multi-sensing approach offers many benefits, careful consideration must be given to the challenges posed by this technique to ensure reliable and accurate measurements.