Structural health monitoring for natural gas vehicles
With the potential added benefit of reducing hazardous emissions into the environment, the promotion of alternative fuels in the transportation industry can reduce Europe's dependence on oil. However, different factors inhibit the market growth of vehicles running on compressed natural gas, although they are considered to be the cleanest internal combustion vehicles available today. Full composite tanks used to store compressed natural gas onboard features high manufacturing costs. Furthermore, despite their extended lifespan, safety and maintenance issues remain a challenge for their widespread use in vehicles. The European project ZEM aimed to develop a monitoring system, based on fibre-optic sensors, to simplify periodical control and to facilitate detailed evaluation of the structural integrity of tanks. To offer on-line interrogation during refueling, the system has been designed to be permanently installed on natural gas tanks. In particular, advanced fibre-optic sensors were developed for embedding in the composite fuel tank material. Research work at the University of Strathclyde focused on changes in fibre attenuation, which are a significant source of inaccuracies and mainly result from microbending of the fibres during tank pressurisation. In order to overcome this problem, a well-established technique based on sub-carrier phase measurements of radio frequency modulated light was selected and introduced for strain measurements of gas tanks. Using both in-phase and quadrature mixing and taking the ratio of the outputs, the method is independent of optical power levels over a wide range of values. Since power levels can change because of components ageing and a wide range of external environmental factors, the improved fibre-optic strain sensor can be applied for different strain measurements. Moreover, they can be used in either transmissive or reflective mode to remotely interrogate units sequentially in a multiplexed fibre-optic sensors network. For the future, sensors offering precision and stability in the microstrain range and suitable for mass production at an economical cost are envisaged by the ZEM project partners.
