Harsh and Hazardous Conditions
Applications requiring sensor measurements in demanding environments can prove very problematic for traditional sensor systems. In particular, corrosive and wet conditions require careful protection of electronics and sensors. Exposing electrical current-carrying sensor wires to explosive media is potentially hazardous and requires special consideration if it is allowed at all. These conditions are present in a wide range of applications, from sensors located in combustion engines to refineries, containment tanks, or outdoor structures.
Because fiber optic sensors are made of glass instead of copper, they are chemically inert and electrically passive. As such, they are unaffected by such conditions and provide stable, reliable measurements in harsh and hostile environments. Because fiber optic sensors carry no electrical current, they can be used in hazardous and explosive environments. You can even use radiation-hardened optical fibers and sensors in radioactive environments where electrical sensors and components would not survive.
Instrumenting Large Structures
A common concern with traditional sensing systems is cabling, particularly when installing many sensors on large structures such as bridges or pipelines. With each sensor requiring a wired connection back to the DAQ system, the cabling quickly becomes cumbersome and costly. Some applications, such as structural measurements of an airframe, cannot tolerate long, heavy cables because they apply loads and moments to the structure, resulting in test errors.
FBG-based fiber optic sensors dramatically reduce the volume and weight of cabling because multiple sensors can be multiplexed on a single lightweight optical fiber. Fiber optic sensors also work very well on large structures and over large areas because the optical signals can be reliably transmitted over very long distances with no signal degradation.
Sensing in Tight Spaces
Installing conventional electrical sensors within a material or in small spaces can be a daunting task. Sensor size, along with the necessary cabling, often limits the number of sensors that can be applied to the object of interest. Applications that require sensors to be embedded within a material can also cause issues, as the cluster of sensors and cables can actually change the properties of the material itself.
Fiber optic sensors are extremely small and lightweight—unpackaged FBG sensors reside within optical fiber, which is less than 0.15 mm wide. In addition to supporting multiple sensors per fiber, multiple FBG optical sensors can be applied to a material or delicate structure with minimal intrusion. A common example of this is embedding FBG optical sensors within composite materials. Because the sensors and optical fibers are smaller than the smallest particle of many materials, they have minimal effect on the structure of the material.
FBG Sensors in Practice
Understanding the science behind why fiber optic sensors are better suited for some applications that confound electrical sensors may be convincing, but seeing these benefits in action is the ultimate proof. The Milan Cathedral, or Duomo di Milano is undergoing a major restoration for which very large scaffolding was constructed around the main spire (see Figure 3).
Figure 3: Duomo di Milano cathedral monitored with optical sensors during restoration
Concerned about the added weight of the scaffolding and resulting wind forces, the engineering firm contracted the Politecnico di Milano to design a continuous monitoring system for the health of the Duomo spire and the scaffolding. They chose FBG optical strain gages for their long term reliability, sensitivity, as well as immunity to lightning strike, and NI PXI as a platform that gave them the modularity to combine their optical strain gages with traditional electrical sensors, such as IEPE accelerometers .
The PXI system, located in the bell room near the base of the cupola, includes the NI PXIe-4844 optical sensor interrogator, which acquires data from fiber-optic strain gages and temperature sensors. Currently, the system includes an NI CompactRIO chassis with NI 9234 dynamic signal acquisition (DSA) modules to acquire data from the electrical accelerometers and linear variable differential transformer (LVDTs). However, these sensors will eventually be ported over to the PXI system as well. The integrated PXI system is powered by LabVIEW Real-Time Module software.
1. For more information on this application, read the case study located on ni.com
About the Author
Kellis Garrett, NI product marketing engineer - SC Express for optical sensing
Kellis Garrett is a National Instruments (NI) product marketing engineer responsible for the fiber optic sensing product line. Additionally, Kellis has contributed to customer support, sales enablement, and marketing project work as an applications engineer, as well as a NI LabVIEW software instructor. She graduated from the Georgia Institute of Technology with a bachelor's degree in biomedical engineering and has worked with fiber optic technology for 6 years.