Nezih Mrad

Fiber Optic Sensors for Structural Health Monitoring of Air Platforms

Aircraft operators are faced with increasing requirements to extend the service life of air platforms beyond their designed life cycles, resulting in heavy maintenance and inspection burdens as well as economic pressure. Structural health monitoring (SHM) based on advanced sensor technology is potentially a cost-effective approach to meet operational requirements, and to reduce maintenance costs. Fiber optic sensor technology is being developed to provide existing and future aircrafts with SHM capability due to its unique superior characteristics. This review paper covers the aerospace SHM requirements and an overview of the fiber optic sensor technologies. In particular, fiber Bragg grating (FBG) sensor technology is evaluated as the most promising tool for load monitoring and damage detection, the two critical SHM aspects of air platforms. At last, recommendations on the implementation and integration of FBG sensors into an SHM system are provided.

Measurement of Microwave Frequency Using a Monolithically Integrated Scannable Echelle Diffractive Grating

A novel approach to the measurement of microwave signal frequency is studied and demonstrated. The approach is based on a monolithically integrated echelle diffractive grating (EDG). The microwave signal is converted to an optical signal of two sidebands using an optical carrier and a Mach-Zehnder modulator. One of the sidebands is then filtered out by a fiber Bragg grating, while the other sideband is characterized by an EDG-based interrogator. Due to the better than 1-pm interrogation resolution of this interrogator, the center wavelength of the sideband tested is capable of being accurately measured. Combining this data with the wavelength of the optical carrier used, the frequency of the microwave signal can be calculated. The results obtained are found to be in good agreement with those of the microwave signals.

A Methodological Review of Piezoelectric Based Acoustic Wave Generation and Detection Techniques for Structural Health Monitoring

Piezoelectric transducers have a long history of applications in nondestructive evaluation of material and structure integrity owing to their ability of transforming mechanical energy to electrical energy and vice versa. As condition based maintenance has emerged as a valuable approach to enhancing continued aircraft airworthiness while reducing the life cycle cost, its enabling structural health monitoring (SHM) technologies capable of providing on-demand diagnosis of the structure without interrupting the aircraft operation are attracting increasing R&D efforts. Piezoelectric transducers play an essential role in these endeavors. This paper is set forth to review a variety of ingenious ways in which piezoelectric transducers are used in today’s SHM technologies as a means of generation and/or detection of diagnostic acoustic waves.

Miniaturized Optical Fiber Sensor Interrogation System Employing Echelle Diffractive Gratings Demultiplexer for Potential Aerospace Applications

To enable the application of optical fiber sensors to aerospace vehicles, the sensor interrogation or data acquisition system has to meet small size and low weight requirements. This paper presents the developmental work of an echelle diffractive gratings (EDGs) demultiplexer-based optical fiber sensor interrogation system. The operation principle of this system and its application to fiber Bragg grating (FBG) sensor interrogation are presented. The experimental results have shown that the developed interrogator (not including the electronic controller) weighs less than 60 g and provides better than 1 pm measurement resolution and better than plusmn10 pm measurement accuracy.

Broadband generation of ultrasonic guided waves using piezoceramics and sub-band decomposition

Classically, damage detection or dispersion curve determination using piezoceramic-generated guided waves has been based on analysis of propagation properties of multiple narrowband excitation signals. However, dispersion and multimodal propagation impair the determination of propagation properties. More recently, it has been proposed to consider broadband excitations for both damage imaging and group velocity estimation. Among existing transducer technologies, although laser excitation is prone to practical limitations in terms of dimensions and generated amplitudes, it allows generation of noncontact, point-like broadband displacement. Thus, broadband generation of guided waves using piezoceramics can be envisioned. However, direct impulse response measurements are limited by the generated amplitude, leading to low SNR measurements. For this purpose, chirp excitations have been proposed using variable-frequency bursts, leading to phase and amplitude variations with respect to the frequency, such that this approach is not suitable for precise estimation of time of flight (ToF) or modal amplitude. In this paper, a sub-band decomposition technique that allows high-SNR measurements of impulse response in a given frequency range is proposed. Broadband excitation is decomposed over a given number of frequency sub-bands, generated by a piezoceramic element and measurement is performed using a laser Doppler vibrometer (LDV) or a piezoceramic sensor. Application to experimental estimation of group velocity and damage detection in pitchcatch configuration is proposed. It is shown that the proposed method allows damage estimation without a priori knowledge of the damage size, whereas narrowband techniques can fail at specific wavelengths.

Simultaneous Interrogation of a Hybrid FBG/LPG Sensor Pair Using a Monolithically Integrated Echelle Diffractive Grating

A simultaneous interrogation technique of a hybrid fiber Bragg grating (FBG) and long-period grating (LPG) sensor pair is proposed and demonstrated using a monolithically integrated echelle diffractive grating (EDG). The operation principle that is based on the monotonic temperature dependence of the EDG transmission wavelengths is presented. Initial results show that a 1-pm resolution and 24-nm interrogation range are achieved by using the proposed interrogation technique, which can effectively be implemented to interrogate hybrid FBG/LPG-based sensor pairs for the discrimination of refractive index (RI)/temperature in RI measurement. The specially designed EDG-based interrogator has the added features of low cost and compact size.

Validation of Strain Gauges for Structural Health Monitoring With Bayesian Belief Networks

The application of structural health monitoring (SHM) often employs multiple sensors to monitor the state of health and usage of the structures. The fault of any sensor may lead to an inaccurate or even incorrect inference with the collected sensor data, which will accordingly create a negative impact on higher-level decisions for maintenance and services. Thus, sensor validation becomes a critical process to the performance of the whole SHM system. This paper presents the use of Bayesian belief network to validate the reading of strain gauges on an aluminum plate for loading monitoring. The Bayesian belief network is constructed with the training data. The factors investigated in this paper, which may affect the validation process, include sensor configuration, sensor redundancy, and sensor data range for the discretization. The feasibility of using a Bayesian belief network for SHM sensor validation is demonstrated with the experimental results.

On the use of a compact optical fiber sensor system in aircraft structural health monitoring

Structural Health Monitoring (SHM) has been identified as an area of significant potential for advanced aircraft maintenance programs that ensure continued airworthiness, enhanced operational safety and reduced life cycle cost. Several sensors and sensory systems have been developed for the implementation of such health monitoring capability. Among a wide range of developed technologies, fiber optic sensor technology, in particular fiber Bragg grating based emerged as one of the most promising for aircraft structural applications. This paper is set to explore the suitability of using a new Fiber Bragg Grating sensor (FBG) system developed for operation in two modes, low and high speed sensing modes, respectively. The suitability of the system for potential use in aircraft load monitoring and damage detection applications has been demonstrated. Results from FBG sensor system were in good agreement with results from conventional resistive strain gauges, validating this capability for load monitoring. For damage detection, the FBG sensor system was able to detect acoustic waves generated 52 inches (1.32 m) away. The initial results, obtained in a full stale experimentation, demonstrate the potential of using FBG sensors for both load monitoring and damage detection in aircraft environment.

Metallic Environmental Effect on RF-Based Energy Transmission

Radio frequency (RF)-based energy transmission enables controllable and simultaneous power delivery to wireless sensors. In such applications, RF energy transmission is likely to be affected by highly conductive (metallic) environments. Understanding metals influence on power delivery will help improve practical implementation. This letter studies the behavior of RF energy transmission in selected scenarios by measuring return loss and peak received power. Visualized energy flow is simulated to obtain the direct picture of such scenarios. The simulation results are consistent with conclusions drawn from experimental results measured using return loss and peak received power.

Wavelength Interrogator Based on Closed-Loop Piezo-Electrically Scanned Space-to-Wavelength Mapping of an Arrayed Waveguide Grating

We demonstrate a novel technique for the interrogation of grating-based fiber optic sensors. The proposed technique is based on space-to-wavelength mapping using an arrayed waveguide grating (AWG). The beam position along the AWG input coupler is controlled by a closed-loop piezoelectric motor. By employing a real-time position feedback encoder, the absolute position of the input light beam can be accurately obtained, which would yield a precise interrogation of the wavelength due to a fixed relationship between the beam position and the transmission wavelength of the AWG channel. The proposed system for the interrogation of fiber Bragg grating (FBG) sensors and a tilted-FBG sensor is experimented. An interrogation resolution of 3 pm and an interrogation range of 18 nm are demonstrated as well as the multichannel measurement capability. Initial results show that the proposed interrogation system has the potential of being packaged into a compact, light weight, and cost-effective interrogator with good performance.