Stephen van der Velden

Development of structural health monitoring systems for composite bonded repairs on aircraft structures

The application of bonded composite patches to repair or reinforce defective metallic structures is becoming recognized as a very effective versatile repair procedure for many types of problems. Immediate applications of bonded patches are in the fields of repair of cracking, localized reinforcement after removal of corrosion damage and for reduction of fatigue strain. However, bonded repairs to critical components are generally limited due to certification concerns. For certification and management of repairs to critical structure, the Smart Patch approach may be an acceptable solution from the airworthiness prospective and be cost effective for the operator and may even allow some relaxation of the certification requirements. In the most basic form of the Smart Patch in-situ sensors can be used as the nerve system to monitor in service the structural condition (health or well-being) of the patch system and the status of the remaining damage in the parent structure. This application would also allow the operator to move away from current costly time-based maintenance procedures toward real-time health condition monitoring of the bonded repair and the repaired structure. TO this end a stand-alone data logger device, for the real-time health monitoring of bonded repaired systems, which is in close proximity to sensors on a repair is being developed. The instrumentation will measure, process and store sensor measurements during flight and then allow this data to be up-loaded, after the flight, onto a PC, via remote (wireless) data access. This paper describes two in-situ health monitoring systems which will be used on a composite bonded patch applied to an F/A-18. The two systems being developed consists of a piezoelectric (PVDF) film-based and a conventional electrical-resistance foil strain gauge-based sensing system. The latter system uses a primary cell (Lithium- based battery) as the power source, which should enable an operating life of 1-2 years. The patch health data is up- loaded by the operator using an IR link. The piezoelectric film-based sensing system is self-powered and has been designed to operate using the electrical power generated by an array of piezoelectric films, which convert structural dynamic strain to electrical energy. These transducers power the electronics which interrogate the piezoelectric film sensors, and process and store the patch health data on non-volatile memory. In this system the patch health data is up-loaded by the operator using a magnetic transreceiver. This paper describes the development and evaluation of the two systems, including issues such as system design and patch health monitoring techniques.

Advanced Instrumentation for Acousto-Ultrasonic Based Structural Health Monitoring

Structural health monitoring (SHM) systems using structurally-integrated sensors potentially allow the ability to inspect for damage in aircraft structures on-demand and could provide a basis for the development of condition-based maintenance approaches for airframes. These systems potentially offer both substantial cost savings and performance improvements over conventional nondestructive inspection (NDI). Acousto-ultrasonics (AU), using structurallyintegrated piezoelectric transducers, offers a promising basis for broad-field damage detection in aircraft structures. For these systems to be successfully applied in the field the hardware for AU excitation and interrogation needs to be easy to use, compact, portable, light and, electrically and mechanically robust. Highly flexible and inexpensive instrumentation for basic background laboratory investigations is also required to allow researchers to tackle the numerous scientific and engineering issues associated with AU based SHM. The Australian Defence Science and Technology Group (DST Group) has developed the Acousto Ultrasonic Structural health monitoring Array Module (AUSAM+), a compact device for AU excitation and interrogation. The module, which has the footprint of a typical current generation smart phone, provides autonomous control of four send and receive piezoelectric elements, which can operate in pitch-catch or pulse-echo modes and can undertake electro-mechanical impedance measurements for transducer and structural diagnostics. Modules are designed to operate synchronously with other units, via an optical link, to accommodate larger transducer arrays. The module also caters for fibre optic sensing of acoustic waves with four intensity-based optical inputs. Temperature and electrical resistance strain gauge inputs as well as external triggering functionality are also provided. The development of a Matlab hardware object allows users to easily access the full hardware functionality of the device and provides enormous flexibility for the creation of custom interfaces. This paper discusses the impetus for the concept, and outlines key aspects of the hardware design and the module capabilities. The efficacy of the system is demonstrated through the results of first-of-class testing, as well as laboratory AU studies on a flat plate using an array of piezoelectric elements.

An Advanced Multi-Sensor Acousto-Ultrasonic Structural Health Monitoring System: Development and Aerospace Demonstration

A key longstanding objective of the Structural Health Monitoring (SHM) research community is to enable the embedment of SHM systems in high value assets like aircraft to provide on-demand damage detection and evaluation. As against traditional non-destructive inspection hardware, embedded SHM systems must be compact, lightweight, low-power and sufficiently robust to survive exposure to severe in-flight operating conditions. Typical Commercial-Off-The-Shelf (COTS) systems can be bulky, costly and are often inflexible in their configuration and/or scalability, which militates against in-service deployment. Advances in electronics have resulted in ever smaller, cheaper and more reliable components that facilitate the development of compact and robust embedded SHM systems, including for Acousto-Ultrasonics (AU), a guided plate-wave inspection modality that has attracted strong interest due mainly to its capacity to furnish wide-area diagnostic coverage with a relatively low sensor density. This article provides a detailed description of the development, testing and demonstration of a new AU interrogation system called the Acousto Ultrasonic Structural health monitoring Array Module+ (AUSAM+). This system provides independent actuation and sensing on four Piezoelectric Wafer Active Sensor (PWAS) elements with further sensing on four Positive Intrinsic Negative (PIN) photodiodes for intensity-based interrogation of Fiber Bragg Gratings (FBG). The paper details the development of a novel piezoelectric excitation amplifier, which, in conjunction with flexible acquisition-system architecture, seamlessly provides electromechanical impedance spectroscopy for PWAS diagnostics over the full instrument bandwidth of 50 KHz–5 MHz. The AUSAM+ functionality is accessed via a simple hardware object providing a myriad of custom software interfaces that can be adapted to suit the specific requirements of each individual application.

Autonomous stress imaging cores: from concept to reality

The historical reliance of thermoelastic stress analysis on cooled infrared detection has created significant cost and practical impediments to the widespread use of this powerful full-field stress measurement technique. The emergence of low-cost microbolometers as a practical alternative has allowed for an expansion of the traditional role of thermoelastic stress analysis, and raises the possibility that it may in future become a viable structural health monitoring modality. Experimental results are shown to confirm that high resolution stress imagery can be obtained from an uncooled thermal camera core significantly smaller than any infrared imaging device previously applied to TSA. The paper provides a summary of progress toward the development of an autonomous stress-imaging capability based on this core.

Reconfigurable multivariable MEMS sensor array

Research into operational aspects of mini unmanned aerial vehicles (UAV) and structural health monitoring systems (SHM) is being conducted at the Defence Science and Technology Organisation and La Trobe University. A fundamental area of interest is investigating the problems associated with miniaturization of the control and health monitoring sensors for such systems. While many technologies for UAV and SHM systems can be, and have been, adapted from those currently available in large manned aircraft; cost, weight, and size constraints have prevented mini UAVs from including many of the robustness mechanisms common to larger aircraft. Moreover, the ubiquitous nature of the sensing requirements for SHM systems has limited their uptake, due mainly to the same issues of cost, weight and size. This paper details the design of a reconfigurable multivariable MEMS (Micro Electro Mechanical System) sensor array to address these issues. This array is comprised of multiple instances of identical sensors, which can be dynamically reconfigured to achieve the desired measurand(s) with tradeoffs against accuracy. The available measurands include such items as; accelerations, rotational rates, magnetic fields (all in X, Y and Z directions), temperature and pressure. This paper presents the design of a reconfigurable multivariable MEMS sensor array together with simulation results.

Design and implementation of a highly efficient piezoelectric power harvesting and vibration damping system

Over the last few years, piezoelectric elements have gained popularity as a convenient and relatively inexpensive interface between the electrical and mechanical domains of power harvesting and vibration damping systems. Power harvesting is commonly performed by placing a bridge rectifier across the piezoelectric element and feeding the output into a capacitor and matched load, in much the same manner as used in a standard power supply circuit. However, the overall efficiency of the electrical power harvesting system using this approach can be quite low. Therefore, there is a continued search for circuit architectures and techniques to enhance the efficiency and performance of such systems. It is shown that using piezoelectric devices for electrical power harvesting is closely related to vibration damping using the same devices. This paper proposes that focusing on the reflected mechanical power could produce more efficient systems than focusing on electrical power transfer alone. In exploring this proposition an attempt was made to identify important parameters in the design of such systems. This exploration has demonstrated the importance of maximizing the voltage across the piezoelectric element as the primary means of maximizing the reflected mechanical power. Complexity and cost are often issues when operating piezoelectric devices at high voltages, which led to the development of a relatively simple charge polarity reversal mechanism. Such a mechanism has been demonstrated to improve the efficiency of energy harvesting and/or vibration damping. Simulation of this concept shows a substantial improvement over the bridge rectifier concept. Whilst the magnitude of improvement is dependent on how high the voltage across the piezoelectric element can be raised, the scenario shown in detail gives an improvement of approximately two orders of magnitude.

DEVELOPMENT & FLIGHT TESTING OF AN INTELLIGENT, AUTONOMOUS UAV CAPABILITY

UAV/UCAV technology is at a key stage of its development with the introduction of truly autonomous operations. The DARPA/Air Force/Navy J-UCAS program is a joint effort to demonstrate the technical feasibility, military utility and operational value for a networked system of high performance, weaponized unmanned air vehicles. The first demonstration of this program promises truly autonomous operations, coordinated multi-vehicle operations and dynamic tasking. The DARPA/Army UCAR program will demonstrate the enabling technologies and system capabilities required to perform the mobile strike concept of operations within the Army’s Objective Force system-ofsystems environment. Specific objectives include autonomous multi-ship cooperation and collaboration, and autonomous low-altitude flight.

An Enhanced Experimental Facility for Durability and Performance Characterisation of Piezoelectric Transducers for Structural Health Monitoring

With any structural health monitoring (SHM) system, verification of the health of the sensing elements is essential in ensuring confidence in the measurements furnished by the system. In particular, SHM systems utilised for structural hot spot monitoring applications will conceivably require transducers to operate reliably after sustained exposure to severe mechanical loading. Consequently, a good understanding of the long term mechanical durability performance of structurally integrated piezoelectric transducers is vital when designing and implementing robust SHM systems. An experimental facility has been developed at the Australian Defence Science and Technology Organisation (DSTO) capable of performing an autonomous long-term mechanical durability test on piezoceramic transducers. The Autonomous Mechanical Durability Experimentation and Analysis System (AMeDEAS) incorporates a general purpose data acquisition program controlling up to three 8-channel relay multiplexers and two instruments. AMeDEAS is highly flexible, allowing user-specified channel configurations and automatic interrogation of selected instruments. The system also interfaces with the uni-axial mechanical testing machine to provide control of the load sequence allowing transducer elements to be interrogated under stable load-free conditions after being subject to a predefined loading regime. AMeDEAS was used to investigate the fatigue characteristics of a low-profile layered piezoceramic transducer package developed by DSTO. A total of 16 transducers were tested under tension-dominated cyclic loading with peak-to-peak strain amplitude increasing from 400 με to a maximum of 3000 με, with periodic acoustic transduction efficiency and electromechanical impedance measurements taken throughout the test. This paper details the AMeDEAS and includes preliminary results which confirm the efficacy of the new facility.

Dynamically Reconfigurable Multivariable MEMS Sensor Array for Unattended Systems

This paper presents a dynamically reconfigurable multivariable Micro-Electro-Mechanical Systems (MEMS) sensor array, capable of reconfiguration in real time, to meet the sensing demands of unattended systems operating in highly variable environments, with an emphasis on maintaining operation of these systems in the presence of structural damage. This array is comprised of multiple instances of identical sensors which can be dynamically reconfigured to target a variety of measurands including acceleration, rotational rate, magnetic fields, temperature, air pressure and density. A simulated environment is used to illustrate how the array can be dynamically reconfigured to respond to variations in several of these parameters. Also shown are simulations that demonstrate the ability of such a sensor array to continue operation in the presence of structural damage.

Ten ways to destroy a prototype MEMS device

Prototyping a Micro Electro Mechanical System (MEMS) device is a very different process to that employed for a standard Integrated Circuit (IC) or Printed Circuit Board (PCB). While the manufacturing methods for MEMS devices largely derive from the IC industry MEMS present many unique manufacturability challenges. These challenges typically relate to two distinct features, specifically; mechanics of the device and the packaging of the device. This paper discusses some of the potential pitfalls in the manufacture of a MEMS prototype; more specifically the paper considers issues leading to low yield rates in a MEMS prototype developed by the authors and then discusses possible improvements to enable a better chance of success. This discussion first identifies some of the more significant MEMS sensor design features that contributed to a low yield and then presents design improvements that could significantly increase the yield. Following this is the identification of several issues involved in packaging the sensor, which had the effect of reducing the yield further; in this case improvements in the packaging are suggested. Also discussed are some general prototyping problems researchers may face that with careful planning may be avoided.