One of the advantages of Fiber Bragg Grating (FBG) technology is its intrinsic multiplexing capability. The sensors can have both specific and different Bragg wavelengths and can be connected in series without compromising the correct reading of the measurements as long as the sensor signals do not overlap.
Sensors can be acquired individually, with or without connectors; or as pre-assembled arrays of sensors connected by fusion splices — a permanent connection between two fibers. Upon installation, sensors and/or arrays can be linked together to one of the interrogator's optical channels, but attention must be given to the selection of wavelengths and the power losses that cable lengths and connections impose on the signals.
Several types of FBG based sensors are provided by HBM, and all of them can be combined on the same optical channel of the interrogator, considering that optical losses are within limits and wavelength signals do not overlap. The tips given below apply to strain measurements but can be easily transferred to other types of sensors:
In order to choose the most suitable type of sensor, the following requirements must be taken into consideration:
a) Measuring range
Different sensor encapsulations may limit their measurement ranges. When selecting the right sensor type, the required range of measurement can be a factor for exclusion for some sensors.
b) Mounting type
HBM offers solutions for the sensors to be glued, welded, embedded, or bolted to the specimen. The mounting type can interfere with the speed and cost of installation, it may also eliminate other options. Weldable gauges can be considered for installing sensors on metallic structures. Welding is a fast and efficient way of bonding sensors to a structure that is ready to use right after installation without the need to wait for adhesive curing. For composite materials, two options are gluing or embedding. Bolt connections are normally not advised for fiber composites, as drilling damages the fibers, but it can be a good solution for concrete or metallic structures.
There is a wide range of sensors available with varying degree of robustness. Some sensors are designed with laboratory cables, and others with indoor or outdoor cables, even dielectric cables, which will limit the sensor's usage without the need for further protection in specific environments.
d) Cable bending radius and length
HBM FiberSensing strain sensors are divided into two product lines, which differ on the characteristics of the used optical fiber. The OP Line uses a fiber with high bending capability performing products and fiber links that can be used in very narrow spaces with tight bending radius, both on the cable path with neglectable power losses, as well as on the sensor area being suitable for measuring even in bent surfaces. The FS Line shows a lower level of flexibility on the sensors and cables, but they can be freely used over several kilometers without significant optical losses.
e) Operating temperature
Strain measurements can be taken under very different environments. For high or low temperatures, only some of the optical strain gauges are suitable.
f) Temperature compensation needs
FBG-based strain sensors are sensitive to temperature changes, and correction is advised.
HBM FiberSensing offers the unique solution of an Athermal Strain Sensor, which cancels the intrinsic effect of temperature changes on the FBG wavelength (but not the thermal expansion of the specimen material). When selecting such a strain sensor no extra sensor is needed for temperature compensation of the measurement. For all other strain sensors an extra sensing element is necessary, for example:
- A temperature sensor placed under the same temperature as the strain sensor: With a temperature measurement on the same location of the strain sensor, the thermal cross-sensitivity of the sensor (as stated on the data- and calibration sheets) and the thermal expansion of the base material, the strain measurement can be corrected.
- An optical compensation element: For some strain sensors, an FBG sensor specifically designed for temperature compensation effects can be used.
- An optical strain gauge that should be applied to the same material and subjected to equal temperature conditions, but not subjected to strain: the strain measured by this sensor is the temperature induced strain.
- An optical strain gauge installed on the opposite surface of the specimen, where strain has the same value but a different signal: With sensors operating in this push-pull configuration, the effect of temperature can be nulled by combining both strain measurements.
The number of sensors needed in an array should consider the options above.
The Bragg wavelength distance between two sensors defines the maximum measuring range of both sensors, because if the signals overlap, measurements will be compromised. The used wavelength range of each sensor depends on its measurement range, within its temperature operating range, together with the sensor sensitivity and the thermal induced wavelength shift caused by the sensors’ cross sensitivity and the thermal expansion of the material.
Starting from the central wavelength of the sensor (λ0), the range of wavelength that needs to be reserved is from the minimum possible wavelength value of the sensor to the maximum:
To ease the selection, pre-selected wavelengths are available, suited to the usual sensor measurement range.
- FS line sensors exist in wavelengths spaced at approximately 6.4 nm
- OP line sensors exist in wavelengths spaced at 5 nm
The number of connections that can be used per optical channel in an FBG sensing chain depends not only on the type of connections used, but also on the interrogator, the fiber type, and length, as well as on the optical signal losses, which can be caused by the installation process (cable path, micro-curvatures, and so on).
a) Connectors vs splices
Two possible connections can be used when connecting optical strain sensors in chains: connectors and splices.
Connectors are easier to use on site, as they literally mean“plug and play”. However, they pose higher losses to the optical signal and are more prone to degradation with time.
Splices are, on the other hand, definitive connections, a fusion of the two fibers that are stable throughout time and feature low optical losses. Nevertheless, splicing requires dedicated tools, trained professionals and longer installation time.
To minimize installation time and, at the same time, increase the number of sensors that can be connected in a chain of sensors, HBM FiberSensing offers pre-assembled arrays of sensors connected by splices protected to suit the application.
b) Fiber type and length
The fibers used on the two HBM FiberSensing's sensor lines are different: the fiber used on the FS Line sensors has 9µm, core and the fiber used on the OP Line sensors has 6µm core.
Sensors and cables from the FS line can run through kilometers without compromising the sensor signals, as the 9µm core fiber has very small attenuation losses. Fibers and cables from the OP line show higher attenuation losses and hence, should not run for long distances.
When the two types of fibers are connected together, even via splices, there are also high losses on the interface that limit the number of times that different types of fiber can be used within a chain.
c) Sensor reflectivity
FBG sensor measuring principle is based on a reflected spectrum of incident light. The signal reflected back is a percentage of the incident light. FS Line sensors have a reflectivity of around 65%, and OP Line sensors have a reflectivity < 15%. On calculating the losses, the reflectivity of the sensors should also be considered.
d) Interrogators’ dynamic range
The admissible losses on an optical sensor chain are dictated by the available dynamic range of the interrogator. The dynamic range can be perceived as a measurement of the signal-to-noise ratio of the optical spectrum for peak detection. Signals with high or very close loss values to the dynamic range will not be correctly acquired by the interrogator.
HBM can offer support on the right selection of components.