Conventional foil based electrical strain gauges have been around for more than 70 years. They are very widely used - in structural testing rigs or as permanent structural integrity monitors, in weighing systems and in sensors measuring parameters as diverse as torque and pressure. A relative newcomer is the optical strain gauge, based on glass fibre Bragg grating sensors (exploiting a phenomenon discovered by Nobel laureates, Sir William Henry Bragg and his son, William Lawrence Bragg in the early part of the 20th century). And while this method of strain measurement has been in continual development for about 14 years since the first sensors were introduced by 3M and Photonetics back in the mid-1990s, it remains an immature measurement technology and, as yet, is not defined in any standards. Despite this slow progress, optical strain measurement does have several advantages over its electrical foil based counterpart.
Fibre Bragg gratings are cut directly into specially doped optical fibres using a laser, and many separate gratings can be inscribed on a single fibre strand. Thus, individual strain measuring points - several thousands in some cases - can be monitored in series via a single fibre connection. This compares very favourably with traditional electric strain gauge test installations that require each individual strain gauge to be wired back to the data acquisition system.
Fibre gratings are attached to measurement locations just like their electric counterparts - usually with an adhesive. Stretching a fibre optic Bragg sensor causes a change in the grating period, which results in a change in the wavelength of the reflected light. The wavelength changes relate to the measured strains and these values are discriminated one from another by an optical ‘interrogator’, working in conjunction with a tunable laser, which form part of a somewhat complex data acquisition system.
There is only a handful of specialist companies providing fibre Bragg grating sensors in the world today. One of them is the German company, HBM, which considers the technology to be important in certain strain measurement applications - particularly the testing of airframes and other aerospace structures fabricated from composite materials, or large civil engineering structures involving many thousands of measuring points and long transmission distances. However, HBM does not regard it as a likely successor to the well-established electrical bridge type strain gauge, which remains a superior device in terms of precision and price.
But despite this reservation, the company does emphasise certain advantages of the optical approach to strain measurement. Briefly, these include an ability to measure very high strains in excess of 10,000um/m; total immunity to electromagnetic interference; suitability for use in a hazardous environment, since there is no electrical supply to the sensor; long term stability, excellent resistance to corrosive and the aforementioned drastically reduced cable burden compared with an electrical strain gauge installation.
Of course, there are disadvantages of the technique, not least being its sensitivity to temperature variation, often requiring separate compensating measurements, and the complexity and high cost of equipment, such as optical interrogators that are required to discriminate between individual gratings on a common fibre.
HBM is now able to supply optical strain gauges with improved fatigue life - up to ten million load cycles at an alternating strain of ±3.000µm/m can be attained. This makes the sensors unbeatable compared with available alternatives when sustained loading is tested, for example, on fibre-reinforced plastics subjected to high alternating strain. The relative wavelength variation resulting from a mechanical strain is linear over a strain range of more than ±10.000µm/m. The company is also able to supply supporting systems for the technology, including its Catman AP data acquisition and analysis software, which provides an interface to the optical interrogator.
Water detection using lasers
Moving away from strain measurement, my attention was recently drawn to an interesting development from the Japanese company, IDEC.
Accurate and consistent detection of liquids can prove very difficult, particularly if the detection method must be non-contacting, such as the sensing of liquid levels in containers moving quickly on a conveyor, or checking the continuity of automatically dispensed beads of adhesive on a substrate. The usual approach is to adopt a form of vision inspection, the complexity (and cost) of which will depend upon the demands of the application.
However, so long as water or trace moisture is present in the liquid, paste or gel, IDEC’s new SA1W series of laser based moisture detector is able to sense its presence. Moreover, it will detect any water/air interface, even if the liquid is contained within a thick-walled, transparent or coloured/translucent vessel.
IDEC has exploited the optical absorption properties of water in the near infra red to develop a novel water detection system using semiconductor laser diodes. The wavelength dependence of optical transmittance in water is directly related to the stretching and vibration of the water molecule’s OH bond. It is thus feasible to tune the laser system to this ‘resonance’ to provide reliable and sensitive non-contact detection of moisture.
The sensors, which are available in through-beam and diffuse sensing formats, are no bigger than standard proximity devices, and like these, the SA1W can be sited remotely from the target via a fibre optic cable for those difficult-to-access applications. A great advantage of this sensing technique, apart from its compactness and elegant simplicity, is its ability to ‘see’ through any clear or translucent container – even when labels are applied or the container is made from coloured materials.
This recent development from IDEC follows work the company undertook for the plastics recycling industry, which needed to discriminate between PET, PVC and PS waste streams. The method of material identification is the same as that used for water detection, but is determined by the CH bond rather than the OH bond.