Distributed Strain and Temperature Sensing – A Thousand Points of Light

Organizations that put a premium on safety and reliability have been using test and measurement solutions for years to validate structural integrity (strain), moderate temperature, and gain insight into places not visible with the human eye.

Traditionally, strain and temperature were measured using electrical sensing technologies, such as foil (strain) gauges and electrical vibrating wire. In the past few years, new advances in technology have challenged those traditional measurement methods by requiring more stringent accuracy, reliability, and durability, as well as the need to measure on a continuum, rather than being limited to a single sensing point.

Following a UAV prototype crash a few years ago, NASA realized that it had innovated beyond its capability for monitoring and keeping airborne assets safe. A paradigm shift in sensing technology was sorely needed to ensure that such an event would not occur again.

Since then, NASA and Sensuron have jointly developed and brought to market a new technology that afforded them the ability to take more accurate measurements in order to validate an aircraft’s structural integrity before it ever leaves the ground.

That technology is now commonly known as fiber optic sensing – the gold standard of strain and temperature measurements. But how does this distributed strain and temperature sensing platform work and what are its uses?

Photo courtesy of NASA

With fiber optic sensing, a single fiber spanning 10 meters can perform the measurements of thousands of individual strain gauges or thermocouples simultaneously. The technology utilizes Optical Frequency Domain Reflectometry (OFDR) to achieve this process.

With OFDR, a wavelength source (continuously tunable laser) is used to spectrally interrogate a multitude of sensors (also known as FBGs) along a fiber. The reflected light from these sensing elements is then detected, demodulated, and analyzed.

Because the optical path difference between the reference reflector and each individual sensor is different, the reflected signal from each sensor is modulated by a unique frequency that is directly dependent upon the grating’s location in the fiber.

This technique enables detection from hundreds to thousands of discretized points along a single fiber, allowing it to collect data from a dense array of spatially distributed sensors that is impossible to replicate with other techniques currently available.

Distributed fiber optic sensing can measure continuous strain, temperature, deflection, 3D shape, liquid level and more. It is ideally suited for applications that are exposed to harsh environments, require long-range, long-term deployments, and measurements that are in close proximity to noise sources (such as power transformers, electric motors, antennas, etc.). Moreover, it enables new sensing capabilities.

One of these is 3D Shape Sensing, which benefits medical applications by guiding devices safely within the human body during invasive procedures.

Next month, we’ll explore some exciting applications currently utilizing fiber optic sensing for distributed strain and temperature measurement.
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