Alan R. Wilson

A small low-power networked and versatile sensor interface

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.

Thin metal foil sensors

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.

Philosophy, fabrication, and characterisation of thin metal foil corrosion sensors

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.

In-Situ Sensing of Surface and Atmospheric Moisture

Water is one of the most important materials on the Earth, being necessary for all forms of agriculture and many industrial processes. Water is also a critical element of many crop diseases and the prime cause of corrosion, rot and decay. The monitoring of atmospheric and surface water contributes to the understanding of these adverse effects and can be used to mitigate them. Sensors for both atmospheric and surface moisture have been in use for a long time but it is only recently that miniaturised and cheap sensors have become available so that in-situ, multi-point monitoring of water is feasible.

Multi-walled carbon nanotubes of 200 nm diameter and carbon micro-balloons

The growth of multi-walled carbon nanotubes (MWNTs) with a large diameter of 200 nm on nickel (Ni) coated silicon wafer at different growth times up to 30 min was investigated. CNT cobwebs were found after a short growth time (7 min) and developed into ordered forests at higher growth times (> 10 min). One of the important formation features of the CNTs in this study was the coaxial structure of the nanotubes, ie. a MWNT within a MWNT. Its formation was due to secondary nanostructures of Ni particles. Unexpected growth of carbon micro-balloons occurred when high flow rates of carbon feedstock was used or when growth time exceeded 15 min.

Australian Defence Applications of Advanced Smart Materials Research

This paper overviews some recent S&T innovations in smart materials and structures at the Australian Defence Science and Technology Organisation (DSTO) under a Corporate Enabling Research Program (CERP) on Signatures, Materials and Energy. The CERP program includes development and transitioning of technology across the maritime, air and land domains, with the major focus of the smart materials program component being to increase the safety, availability and maintainability of Defence assets. Three specific examples are provided of the smart materials and structures program, ranging across the spectrum of technology readiness from new concept phase to technology transitioning, viz.: (i) Advances in smart sensing for prognostics-based platform management; (ii) Fabrication of nanostructured and ultrafine grained materials through top-down severe plastic deformation processing of bulk materials; (iii) Innovative application of carbon nanotubes/conducting polymers as artificial muscles for low-power propulsion and control of small autonomous underwater systems. In each case, the DSTO effort is underpinned by strong university or industry linkages to deliver challenging interdisciplinary S&T.

Hysteresis and drift in a carbon-polymer composite strain sensor

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 gauges 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.

Macro to micro laser machining: A case study

Laser cutting of metals for industrial applications is now a mature field. Significant effort is currently being undertaken to extend this technique into the realm of micro machining of metal components.A quantitative study has been performed of the laser micro-machining of slots in thin metal foils for the production of micro sensors for structural health monitoring using a frequency tripled Nd:YAG laser. The effects of defocus, the number of laser passes and the angle of the laser to the sample on the cut profile have been investigated. The results for the effect of defocus and number of laser passes are presented and discussed. A simple side wall re-deposition model is proposed to explain the results.Laser cutting of metals for industrial applications is now a mature field. Significant effort is currently being undertaken to extend this technique into the realm of micro machining of metal components.A quantitative study has been performed of the laser micro-machining of slots in thin metal foils for the production of micro sensors for structural health monitoring using a frequency tripled Nd:YAG laser. The effects of defocus, the number of laser passes and the angle of the laser to the sample on the cut profile have been investigated. The results for the effect of defocus and number of laser passes are presented and discussed. A simple side wall re-deposition model is proposed to explain the results.

Adhesive bond degradation sensor

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.