Pervasive sensor networks and ubiquitous connectivity are among the ‘edge’ technologies required to unleash the era of efficient manufacturing proposed by Big Data. The most likely suitor for the connectivity problem is optical fibre, which first emerged as a viable telecommunications medium in the late 1970s. Today, some 80% of the world’s long-distance voice and data traffic is carried over fibre optic cables. Now, it seems, the technology can also be used to revolutionise the sensor industry in any process where the measured parameter changes the transmission properties of the fibre material in some fundamental way.
As it turns out, light travelling through the core of the fibre is modulated by changes in the surrounding medium. The neat thing is that these modulation-inducing changes are associated with many commonly measured process parameters such as temperature, pressure, strain, humidity, vibration, pH, etc. Also, the extent of the modulation – in the form of changes in the shape or wavelength of the light waves – is correlated to the extent of the change in the measured parameter.
Commercially, two implementation methods are under research: point sensing, where the active portion of the fibre is less than 10 mm; and distributed sensing, where the entire fibre, perhaps tens of kilometres long, is the sensor. The point sensors are more complex to produce since the fibres incorporate tiny wavelength-dependent optical filters, which must be implanted using nanotechnology. They are, however, extremely precise and provide high-speed data acquisition. For the distributed sensors, standard fibre optic materials can be used, but at the expense of lower signal fidelity and slower data acquisition.
Perhaps the most obvious benefit of the technology is that the optical fibre acts as both the sensing element and the communication medium – ideal in an IIoT context. When one adds to this the advantages of being electrically passive (low power, Ex applications), wide dynamic range and freedom from electromagnetic interference, the prospects look bright for future sensor networks communicating at high data rates over long distances without the need to convert between electronics and photonics along the way.
One of the difficulties however, which also applies to non-optical sensors, is interference from multiple effects i.e. a sensor intended to measure pressure may also be temperature dependent. In the case of mechanical or electronic sensors, the nature of this dependence is well understood and can therefore be compensated either in hardware or software. In the case of optical fibre technology, intense R&D efforts over the last five years have yielded significant progress, but more insight is still required for some applications. In other cases, the limitations imposed by cross-effects do not cause problems and the technology is ready for use, particularly the capability to implement distributed temperature sensing over long distances.
Hot roller detection in mine conveyors is a good example. For this application, a passive fibre optic cable can provide accurate temperature measurement along the entire length of the belt, which enables effective monitoring over even the longest conveyor runs. The cable acts as a distributed type sensor that allows hotspots to be detected within a few degrees Celsius, and localised with an accuracy of about one metre. This is more than enough information for a manager to use as input to a reliability-centred maintenance strategy, and it also acts as a potential fire hazard warning. See the article ‘Fibre optic sensors for detecting hot rollers on conveyors’ for more on AP Sensing’s innovative use of this new technology.
Technews Industry Guide
Speaking of maintenance strategy, the 2018 edition of the Technews Industry Guide: Maintenance, Reliability & Asset Optimisation will be posted with next month’s magazine. This auxiliary publication is designed to provide maintenance professionals with a one-stop definitive resource that covers everything from in situ sensor-based solutions for condition monitoring, through handheld portable devices for periodic maintenance-related checks, through software solutions for analysis and reporting, and on to customised services like reliability management consulting and training. Our hope is that the information we gather will help personnel to solve maintenance-related problems that they may be struggling with on their own particular plants. Contact Jane (jane@technews.co.za) or Laura (laura@technews.co.za) if you would like to participate.
Steven Meyer
Editor: SA Instrumentation & Control
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