Defence Science and Technology Organisation (DSTO) has developed a low power RS485 sensor network that can be
hardware configured at design time from a number of modules, depending on its final application. The core predesigned
module includes network communications, microprocessor control and digital input/output. A number of analogue
sensor interface modules can easily be added to this core. In addition, the software is also of modular design consisting
of a set of core operating routines and a set of routines for controlling sensor operations that can be downloaded or
upgraded in the field. Prime consideration in this development has been given to the need for small size, low weight, low
power and versatility of operation. The hardware is based around the Texas Instruments MSP430® micro-controller. This
paper will present some of the considerations leading to the design and examples of applications of the sensor network.
Defence Science and Technology Organisation (DSTO) is engaged in the development of sensor systems to monitor the
environment and condition of high value structures and machinery. The development of this technology promises to
contain escalating costs associated with the through-life support of major capital platforms, including high-rise
buildings, bridges, aircraft, ships and offshore oil/gas structures. As part of this work a laser micromachining process for
fabricating thin foil sensors has been developed. Laser micromachining has some inherent advantages over other
processes such as metal deposition and chemical etching for the production of thin foil sensors. A chief advantage of the
process is the ability to make relatively thick (100 µm) micro-patterned sensors (20 µm features) out of a very wide
variety of metals with only minor changes to the process. This last feature makes feasible the manufacture of sensors
out of the same material as the bulk structure that is being monitored. This paper presents results for some laser micromachined
thin foil corrosion and environmental sensors and compares these with similar sensors made using different
fabrication processes.
Recent advances in UV laser machining have allowed the development of accurate and rapid prototyping of micro scale
devices. Two examples are presented of how modern UV laser micro machining may be applied to novel applications in
engineering research and the development of state of the art micro sensors for structural health monitoring. We also
show how micro device development is now a rapid process where novel procedures allowing the manual handling of
work pieces may be used to make two layer laser machined devices with alignment to tens of microns.
A conductive polymer strain gauge was screen printed to produce an active area of 3mm × 4mm. The graphite and titanium dioxide loaded thermoplastic device was found to have a resistance of 4.3kΩ and a gauge factor of up to 20. The higher resistivity and gauge factor result in a lower power consumption and higher sensitivity when directly compared to metal foil strain gauges. However, a substantial hysteresis of approximately 80με was identified in a complete strain cycle from 0me to 730με. The source of this hysteresis was considered to be the thermoplastic matrix. Subsequently the viscoelastic nature of the polymer matrix was analysed using the gauge's resistive signal as it changed under applied strains, and this output was then compared to the standard linear solid (or Zener) model from linear viscoelastic theory. This model was applied to the data and with some limitations was found to make an improvement to the reported hysteresis.
Thin metal foil sensors for corrosion monitoring are being developed for applications under paints, sealants and in lap joints. These sensors are two electrode electrochemical devices. A fundamental consideration for these sensors is that they reflect the corrosion activity of the structure to be monitored. To this end, fine milling, chemical etching and laser micromachining are used to fabricate the sensors from the same material as the structure of interest. Details of the fabrication, characterisation and micro-circuit instrumentation of the sensors will be presented.
UV Laser micro-machining of metals for the production of micro devices is generating significant interest. This approach to micro-fabrication has the distinct advantage of being non-contact, rapid, flexible and precise. At present, the laser micro-manufacture of new micro device designs, or the introduction of untried materials requires exploration of laser parameters to achieve acceptable yields. We have undertaken a quantitative study of UV laser micro-machining of 2024 Aluminium, Copper and marine grade steel. These metals are of broad application in the manufacture of sensors for structural health monitoring and are of particular interest to defence. The UV laser micro-machining study was carried out using a frequency tripled Nd YAG laser. In this paper the influence of the number of passes of the laser over the metal surface are compared with cutting profiles, and ablation or melt behaviour as a function of cut depth.
Analatom Inc. in conjunction with the DSTO (Defence Science & Technology Organisation) has been developing a micro Linear Polarization Resistance (LPR) system for assessing the integrity of high value structures. The device operates on the principle that as a metal corrodes, the oxide formed effectively creates an anodic cell. Hence, if the metal can be separated into two sections, a potential and resistance can be measured between each section. These values can be used to compute the effective mass loss of the device. By matching the material properties of the device with that of the structure whose "health" is being monitored, it is possible to establish a corrosion rate of the structure. Previous research at DSTO has shown that such a system can be fabricated and operated on the micro scale. The task has now been to develop the device into a commercially viable system; it is this development that is examined in this paper. In the original system, a potentiostat is used to evaluate the device for data relating the mass loss during corrosion. This system is now replaced with simplified electronics to reduce both the cost and size of the device. Signal conditioning into the LPR is critical as potentials over 20mV across the terminals can be a source of corrosion of the device. Micro controllers and small board computers are used to run this signal conditioning process and the LPR interface circuit.
Early detection of adhesive bond degradation using sensing elements embedded within the 100um bond-line of aluminium epoxy adhesive joints has been demonstrated. Sensing elements of varying heights were fabricated at the ends of narrow conductors on a flexi-circuit carrier. This construction simulates the active sensing region on a patented silicon adhesive bond degradation sensor and has been used to characterize the sensing elements without the expense and time associated with fabricating the complete integrated silicon sensor. The highest elements on the flexi-circuit serve both as electrical pickup studs, providing a circuit from the flexi-circuit to the top aluminium plate, and as spacers to ensure that the shorter sensing elements do not contact the aluminium plate. The non-contacting sensing elements are thus arranged to be close to the metal/adhesive interface and are sensitive to any change in conductivity in this region due to release of ions as the interface is degraded by the environment. Accelerated aging tests were performed on flexi-circuit sensors embedded in the bond-line of double cantilever beam specimens. The specimens were immersed in 50° C water and pre-loaded to just initiate a crack. Load on the specimen was then maintained by applying a constant load point displacement with a very low velocity to ensure that the environment would degrade the bond-line in advance of the crack front. The change of load and the conductivity measured by the sensing elements were then logged with time. The onset of bond degradation was detected approximately 10-20 mm ahead of the crack tip.
The Australian Defence Force is increasingly facing escalating costs on through-life support for major platforms (ships, aircraft and land vehicles). The application of smart materials and structures technologies in platform management systems is seen as a very promising approach to reduce these costs and to potentially achieve significant enhancement of platform capability. A new DSTO Key Initiative, 'Smart Materials and Structures', has been recently developed and funded to address these technologies. The Initiative will build on and grow the current activities within DSTO and promote collaboration with external Australian institutes and industry. This paper will present an overview of the Initiative and the generic sensor and system issues inherent in the 'whole-of-platform' and 'whole-of-life' monitoring and management of major defence platforms. Examples for some particular elements of this will be drawn from current work in DSTO. Other presentations in the conference will cover the technical and scientific aspects of these in more detail.
Structures have been electroplated directly to metal (Al) pads on Si wafers using only the metal pad geometry to define the position and shape of the electroplated pad. No further processing steps are required after the electroplating. These pads have been used to form the sensing elements in an adhesive bond degradation sensor. Other uses could include the electroplating of high density materials to increase inertial/accelerometer sensor mass for higher sensitivity. The electroplating has been performed on Al pads formed in a CMOS compatible process on whole Si wafers. Multiple 'bus bars' connected to different sensor pads have been used to simultaneously electroplate structures with different heights. Both zincating and evaporation of a Cu seed layer on the Si substrate followed by electroplating have been investigated to determine the parameters to achieve optimum Cu cover and the best adhesion of the electroplated Cu to the Al/wafer. This involved the development of a test to measure the adhesion of 100 micrometer X 100 micrometer electroplated Cu studs on a silicon wafer.
The problem addressed in this paper is the through-life, non-destructive monitoring of corrosion and disbonding damages in airframes. The concept presented here is to produce a MEMS smart sensor consisting of a number of small, independent, wireless sensors within the structure of the aircraft. The MEMS smart sensor can be installed during repair and in particular when the specific platform goes through a complete tear down during the Life Extension Program (LEP). The sensors are permanently installed and can be permanently monitored and contain bond degradation sensing elements and CMOS circuits. Each sensor has an independent address and can perform measurements and communicate over a true 2-wire bus to an external interface unit. US ( 09/501,798) and international (PCT/US00/03308) patent applications have been lodged for this technology.
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