Optical Fibre Strain Sensors

Avago Technologies has announced an optical fibre strain sensing method for industrial applications, based on plastic fibre and a patented optical phase interrogation (OPI) technique.

The technique enables plastic optical fibre to be used as a highprecision strain sensor on par with current fibre Bragg grating strain sensors for applications including wind-blade load management and structural health monitoring, the firm said. According to Avago fibre product manager Martin Weigert, inspiration for the sensor came from years of supplying plastic optical cables and communication modules for industrial networking and data communication.

In the sensor, fibre is meandered in a series of loops (pictured) which, when strained, will alter the phase of an optical signal injected into the fibre.

The electronics box may be mounted several metres from the sensor, connected to it by a cable with four fibres. Within the cable, one fibre carries the optical signal to the sensor, and another brings it back. The two other cores do the same thing for a reference signal.

The reference signal is used to temperature-compensate the fibres in the cable and sensor - at the sensor there is a length of un-strained fibre to complete the reference loop.

Within the electronics, the same optical signal (from a 650nm (red) LED) is injected into the sensing and reference loops, and the difference in phase between the signals on their return is converted into a strain measurement.

Optical detection is through a silicon photodiode, or photodiodes.
Little information has been released about phase detection, apart from Avago saying: "Signal processing algorithms process the received signals to evaluate the time and frequency domain of the received signal, and that allows the designer to draw conclusions regarding the structural condition (structural health) and correlate it to other performance-related aspects of the system."

Operation appears to have been tested with modulation between 100MHz and 2.7GHz. With no conductors, the array is immune to EMI and EMC, and is electrically isolated.
"Made out of polymer, the fibre offers a comparably high elasticity. Most can be stretched more than 5% and remain within the elastic range," said the firm.

Sensors are being installed in "a multi-megawatt conceptual turbine for both wind blade and tower load monitoring", said the firm.

A developer's kit with fibre, optical phase interrogator, and documentation, will be available in January 2014.

From a press release issued by:
Avago Technologies - www.avagotech.com

November 2013


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