US Navy places major order for fibre optic current sensors

Citation

(2005), "US Navy places major order for fibre optic current sensors", Sensor Review, Vol. 25 No. 4. https://doi.org/10.1108/sr.2005.08725daf.003

US Navy places major order for fibre optic current sensors

US Navy places major order for fibre optic current sensors

Keywords: Fibre optic sensors

An effect first observed by Michael Faraday in 1845 is now being exploited in a family of optical fibre current sensors for monitoring critical ship-board electrical equipment. In March 2005, Airak, Inc., a developer of optical sensors for electrical monitoring and control, announced the award of a 2.3 million dollar contract from the US Navy for sensor systems that will provide ship-wide monitoring of mission-critical electrical systems and power plant. This contract reflects the major technological changes that are underway within the American Navy's fleet, notably the development of the “all electrical ship”. The first such vessel will have her keel laid in 2008 and will be commissioned in 2011. A critical issue is monitoring, regulating and ensuring uninterrupted power flow to the multitude of electrical and electronic systems and one estimate suggests that as many as 10,000 sensors will be required to achieve this. This would be impossible with conventional Hall effect or wire- wound current sensors due to weight and size restraints and the safety considerations associated with the higher voltages involved. Accordingly, alternative sensor technologies are required. Airak's optical current and voltage sensors and measurement systems will be delivered to an American aircraft carrier in early 2006 through the American Competitiveness Institute's (ACI) ManTech programme and will concentrate on busbar monitoring within electrical switchgear and the provision of performance analysis data.

Fibre optic current sensors are a recent technological innovation and utilise the phenomenon now known as the Faraday effect. Faraday noted that when linearly polarised light travelled through flint glass that was exposed to a magnetic field, its plane of polarisation rotated. This effect is widely used in the fibre optic telecommunications field to prevent reflected light energy from coupling back into a light source and changing parameters such as frequency or power output. In sensor systems that exploit this effect, the fibre sensor is placed in the magnetic field associated with the current flowing in a conductor. By monitoring the rotation of the light, a direct measurement of the magnetic field intensity and the current can be made, as Ampere's law guarantees that if the light is uniformly sensitive to the magnetic field along the sensing path, and the path defines a closed loop, then the total rotation of the light's plane of polarisation (u) is directly proportional to the current flowing in the wire. The amplitude of the effect is governed by two variables: the length of the sensing fibre (L) and the Verdet constant (V) of the sensing material, i.e. θ = 14;V·L·H; where H is the magnetic field strength associated with the current. Thus, to optimise the effect, materials with high Verdet constants are used, in this case optically transparent, crystalline ferromagnetic materials with a V of around 6×105°=cm–Oe:

Plate 6 The optical current sensor head (Airak, Inc.)

Weighing in at only 28g, the Airak sensors have a linear measuring range of ,4-4,000 Amps RMS and a dynamic range of at least 80 db. They are intrinsically safe for use in medium voltage applications (38.0 kV insulation class and below), there are no ground loops and they are 98 per cent lighter and 97 per cent smaller than an equivalent 15 kV current transformer with the same measurement range (Plate 6). This makes them well suited to monitoring electrical equipment in the stringent space constraints imposed by a military shipboard environment. They are claimed to be the world's smallest fibre optic current sensors and their size and construction permits fitting to planar busbars (Plate 7) without the need for disassembly of the busbar, resulting in less down-time and lower installation costs. A further benefit is that, to vary the current range, the sensor's overall size does not alter, as range is governed by the thickness of the sensing crystal. The sensors are used in conjunction with an optoelectronics unit (Plate 8), which offers analogue and digital output options, a 6-3,000 Hz bandwidth and a full-scale error of < 10 per cent. The deployment of these sensor systems on Naval vessels will allow the adoption of condition-based maintenance programmes by alerting, in real-time, problems with electrical equipment. This will improve overall system reliability and reduce downtime. Other potential applications include monitoring magnetic fields and the currents in overhead power transmission lines and underground electrical vaults. Later in 2005, Airak expects to provide systems for the electrical utilities which will contribute to improving the reliability of load monitoring, reduce costs and assist in avoiding the catastrophic power failures that have plagued the US in the recent past.

Plate 7 The sensor mounted on an insulated planar busbar (Airak, Inc.)

Plate 8 The optoelectronics control module (Airak, Inc.)

The ACI and the EMPF

The ACI was established in 1992 as a scientific research corporation dedicated to the advancement and integration of leading edge technologies in electronics manufacturing and related engineering applications. Since 1995, ACI has managed multi-million dollar research projects and the Electronics Manufacturing Productivity Facility (EMPF) under a cooperative agreement with the Navy's Office of Naval Research (ONR) under the ManTech programme. The EMPF was established in 1984 to aid the electronics industry in improving electronics manufacturing processes required in the production of military systems. Today, the EMPF operates as a National Electronics Manufacturing Centre of Excellence (NEMCE) deriving its resources from a consortium of industry, university and government participants led by the ACI.

Contact: Paul Duncan, President, Airak, Inc., 21641 Beaumeade Circle, Suite 300, Ashburn, Virginia 20147-6027, USA. Tel: +1-703-858- 9401; web site: www.airak.com, E-mail: contactus@airak.com

Rob BogueAssociate Editor, Sensor Review