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Strain Gauges vs Distributed Fiber Optic Sensing, Part 1 of 3: Introduction

Strain Gauges vs Distributed Fiber Optic Sensing, Part 1 of 3: Introduction

Posted on by Pierrick Vulliez

Fiber Optic Sensing (FOS) systems have an intrinsic advantage over traditional Strain Gauge (SG) technology, namely they are capable of measuring strain along the sensing fiber in multiple locations. Sensuron’s FOS system developed in collaboration with NASA is capable of measuring strain at over 2000 locations along a 13m fiber. A single fiber that provides a high-density strain profile along its length can replace tens or hundreds of strain gauges depending on the length of the fiber. For instance, 2000 SG are required to replace a 13m sensing fiber. Using SG technology to obtain such a measurement would require an enormous number of strain gauges, wire conductors, significantly more labor, and would ultimately result in higher costs as outlined in a study performed previously at Sensuron.

However, fiber instrumentation is a topic that can benefit from the vast body of knowledge and experience in SG installation technology. Sensuron recommends a surface preparation procedure analogous to the standard procedure for SG installation. The procedure includes cleaning, abrading, conditioning/neutralizing, and use of comparable bonding agents. Installation of a limited number of gauges may not appear difficult. However, extra effort is required to accomplish clean and reliable connection between strain gauges and multi-conductor wires via soldering. Management of a large number of cables with fragile soldered ends is also an issue. One typical remedy is to limit the number of SG installed by predicting the most important locations and leaving supposedly unimportant areas with no measurement at all. In addition to potentially missing critical information, this approach is inefficient in health monitoring applications where a structure’s continuous response along time and space is the essence of predictive algorithms. Due to these problems, SG technology is neither suitable nor practical for distributed sensing or effective structural health monitoring.

The distance between sensing points in all Sensuron systems can be as low as 1.6 mm. This ensures a truly high-density network of distributed strain or temperature sensors. Achieving any level of distributed sensing with SG requires significantly more time and effort, while the added total weight of wires can no longer be ignored in weight-sensitive structures. In such applications as lightweight structures and flying objects, the extra weight has traditionally been a major obstacle in using a distributed network of SG. This obstacle can now be overcome with the use of Sensuron’s FOS technology in which the signal of all the sensors is transmitted optically through the same optical fiber.
One difficulty of implementing FOS systems for potential users arises from the fact that FOS systems have different characteristics regarding their hardware (e.g., laser and fiber) and implemented interrogation schemes namely OFDR, WDM, Raman, etc. Consequently, accuracy of these FOS systems must be ideally examined by direct comparison to an established framework. While there are several established methods for strain measurement, strain gauges are undoubtedly the most frequently used tools both in lab and field applications. They have been under continuous improvement for decades and have achieved a high level of maturity regarding hardware, installation methods, and driving/excitation techniques. It is notable that most SG manufacturers advocate a truly strict installation process to significantly reduce the uncertainty in the installation process and guarantee optimal performance.

In the present study, we examine the accuracy of strain measurements obtained by Sensuron’s FOS technology in comparison to commonly used Strain Gauge technology installed on a cantilever beam as shown in Figure 1.

Figure 1. A demonstration cantilever beam with an installed fiber is part of Sensuron’s offering included with Strain Sense interrogators to provide users with a simple and effective initial experience.

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