FBG Optical Sensing: A New Alternative for Challenging Strain Measurements

Publish Date: Feb 11, 2016 | 8 Ratings | 4.25 out of 5 | Print


There have been many advances in the methods used to perform strain measurements. The three most prevalent technologies today are electrical foil gages, electrical vibrating wire, and fiber Bragg grating (FBG) optical sensors.

For most standard strain sensing applications, electrical sensing has been and will continue to be the best and most effective solution. However, optical sensors can offer an important alternative in traditionally challenging applications. Harsh environments, distributed systems, or long-term deployments are a few examples where the characteristics of an optical sensing system can make it a more effective solution compared to conventional electrical sensors.

Applications often require measurement solutions that encompass the benefits and attributes of multiple sensing technologies. It is therefore important to consider a hybrid approach. This paper provides a brief overview of each technology and weighs their benefits and drawbacks.

Table of Contents

  1. Technology Introduction
  2. Application Considerations
  3. Conclusion

1. Technology Introduction

Electrical Sensing: Metal Foil Gages

Foil strain gages use the relationship between electrical resistance and conductor length to measure changes in strain. As the foil is stretched, its length is increased, which translates into a minute increase in resistance. To accurately measure these small changes in resistance, additional signal conditioning is necessary, often in the form of a Wheatstone bridge resistance network. A constant voltage is applied across the resistance network, and the varying proportional drop in voltage across the foil can be translated to strain.

Figure 1. A Typical Foil Strain Gage

Electrical Sensing: Vibrating Wire

Vibrating wire sensing, as the name implies, uses the known relationship between the tension experienced by a wire and the frequency at which it oscillates. The most common method to stimulate an oscillation is to drive a current through a coil that is close proximity to the wire. This in turn induces a magnetic field that repels or attracts the wire based on the polarities. Once the wire has reached steady-state, its oscillation is measured by using the same vibration-inducing coil. The frequency data is then analyzed and converted into strain.

Figure 2. Layout of a Typical Vibrating Wire Sensor

Optical Sensing: Fiber Bragg Grating

Optical sensing uses the properties of light to measure a physical phenomenon. The FBG is a series of localized changes in the refractive index of the glass fiber. The optical fiber is connected to a light source and when the light encounters the strain FBG sensor, a specific wavelength is reflected based on the properties of the gratings. As the FBG expands or contracts, so does the gap between these gratings, therefore changing the reflected wavelength of light. The reflected light is then measured and the shift in wavelength can be converted to a strain value.

Figure 3. An Expanded View of an FBG

To learn more about optical sensing technology, see Fundamentals of FBG Optical Sensing.

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2. Application Considerations

Media Technology Electrical Noise Immunity Measurement Speed Sensor Configuration Mounting
Electrical Foil Gages Low Up to ~100 kHz Single-Point:
One Sensor/Ch
Vibrating Wire Moderate ~1 Hz Single-Point:
One Sensor/Ch
Optical Fiber Bragg Grating (FBG) Complete Typically < 1 kHz Distributed:
and Embeddable

Table 1. Strain Sensing Technology Attributes Summary

Foil Gages

In most controlled environment applications, foil gages provide a familiar and cost-effective measurement solution. Foil gages are a well-established technology, so they have a large product ecosystem including sensors, signal conditioning circuitry, and data acquisition hardware. In addition, foil gage measurement systems have become more efficient by combining the signal conditioning and data acquisition system components. Examples include NI C Series and SC Express modules.

For applications that require distribution across a large geographic area, it becomes more difficult to implement a foil gage system. It can be challenging to install and maintain up to 10 lead wires devoted to each foil gage sensor over long distances. When spanning long ranges the voltage drop due to lead wire resistance may also make it prohibitive to implement a foil gage system. In addition, foil gages generally do not have a long life span and therefore may require replacement in longer term test and monitoring applications.

In uncontrolled or harsh environments, it may become infeasible to deploy a foil gage measurement system. As changes in strain on a foil gage typically result in voltage differences on the order of millivolts, the sensors and wiring are highly vulnerable to electromagnetic interference (EMI). Installing the measurement system in close proximity to the gage and reducing the length of the lead wires can reduce the impact of external noise. However, depending on application requirements, this approach may not always be possible.

Foil gages are the most common sensors used for dynamic strain measurement applications, including wind tunnels and impact testing. This is primarily due to the wide availability of measurement hardware that can sample at rates exceeding 1 kHz. When considering the implementation of these high-speed applications, it is vital to have a data acquisition platform capable of handling the large data rates. Technologies such as those offered in the PXI Express platform provide the bandwidth and synchronization necessary to capture and transfer large amounts of acquired data. Therefore in typical short-range, controlled operational environments, foil gages are a good, cost-effective, and well-supported sensor technology for both static and high-speed dynamic measurements.

Vibrating Wire

Vibrating wire systems are often used for structural monitoring applications because of their reliability for long-term deployments.

Once a vibrating wire is calibrated to the material in which it is mounted or embedded, it provides a reliable long-term reference. Because vibrating wire technology measures a change in the frequency of an electrical signal, it has a certain amount of inherent electromagnetic noise immunity. However as vibrating wires use electrical conductors, high voltages such as lightning strikes can pose a danger to both the sensor and connected data acquisition system. In addition, vibrating wires are slow sensors (in the order of 1 Hz) because the sensor must first be stimulated, reach steady-state, and then measured.

For accurate measurements, temperature compensation should be implemented at each sensor. This is typically done with an additional thermistor. Vibrating wire systems typically require two- or four-lead wires per sensor depending on whether temperature compensation is implemented. Multiplexing architectures have been introduced in which a single set of wires can be shared between multiple sensors. Although this multiplexing reduces the sampling rate, for most long-term strain monitoring applications rates on the order of samples per minute or less is sufficient. However, even when implementing a multiplexed configuration for large distributed systems, the amount of cabling infrastructure required between the sensors and data acquisition hardware may still be prohibitive.

Vibrating wire systems are well suited for long-term deployment applications that may experience limited EMI, are distributed on the order of hundreds of meters, and sample at rates slower than one sample per second.

FBG Optical Sensing

FBG optical strain sensors provide many advantages over conventional electrical systems particularly in applications that are exposed to harsh environments and require long-range, long-term deployments.

FBG optical sensing systems do not use electrical conductors and are therefore completely immune to EMI and high voltages. This is ideal for applications requiring sensor measurements in close proximity to noise sources such as power transformers, electric motors, antennas, and more. Open-air and harsh environments can also greatly benefit from optical sensing due to its immunity to lightning and resistance to metallic corrosion, potentially reducing the long-term maintenance costs of the system.

For applications that span large geographic areas, FBG optical sensing systems are a well-suited alternative to electrical sensors. Dozens of FBG sensors, including temperature, strain, and pressure, can be daisy chained along a single optical fiber. As a result, the required cabling maintenance and installation is substantially reduced, making system deployments on the order of kilometers cost-effective. Furthermore, FBG optical sensing uses frequency rather than amplitude modulation and therefore can reach very long distances (more than 10 km) without the need for signal conditioning.

Figure 4. Two FBG optical strain gages from Micron Optics Inc. that can be glued or welded onto the structure under test. 

Similar to conventional electrical sensors, both temperature and strain affects an FBG optical strain gage. One must compensate for the temperature effects on the FBG. This can be achieved by installing an FBG temperature sensor in close thermal contact with the FBG strain sensor. A simple subtraction of the FBG temperature sensor wavelength shift from the FBG strain sensor wavelength shift yields a temperature compensated strain value.

The thermal expansion of the test material can also be compensated, similarly to foil strain gages. A number of manufacturers provide sensor scaling equations with a provision for the coefficient of thermal expansion (CTE) of the material under test.

Applications that are distributed across a wide area are also often required to withstand long-term deployment. FBG optical sensors are reliable for long-term installation and do not require calibration after the initial postinstallation nulling. In addition, sensor interrogators such as the NI PXIe-4844 require no external calibration for the life of the device.

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3. Conclusion

Electrical sensing technologies have been successfully implemented in many applications. When a dynamic short-range system is deployed in a controlled environment, foil gages provide a cost-effective and familiar option. Vibrating wire systems are generally well suited for longer-term deployments that may have exposure to limited EMI.

With the introduction of optical sensing, scientists and engineers can perform measurements that were previously impractical or, in some cases, impossible with vibrating wire or foil gages. These application areas include those that experience EMI or high voltages as well as long-term deployments that span long ranges.

Many real-world applications will require a hybrid approach using the benefits of both electrical and optical measurement technologies. National Instruments modular PXI hardware offers the capability to meet the mixed I/O requirements of these systems necessary to acquire the data. Furthermore, NI LabVIEW graphical programming provides the flexibility and power needed to acquire, analyze, and present the data.

Learn More

Fundamentals of FBG Optical Sensing
Fiber-Optic Sensing Components: Optical Fiber General Use and Maintenance


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