What is temperature compensation and how does it work?

Fiber Bragg Grating (FBG) strain dependence

The strain dependence of a fiber Bragg grating is given by the expression:

 

Where:

k– k factor of the Bragg grating

Fiber Bragg Grating (FBG) temperature dependence

The temperature dependence of a fiber Bragg grating is:

Where:

 – coefficient of thermal expansion of the fiber

ζ –  thermo-optic coefficient (dependence of the index of refraction on temperature)

Temperature dependence of a fixed Fiber Bragg Grating (FBG)

If the optical strain gauge is fixed to a rigid strain free structure, the temperature may change the index of refraction of the fiber, but its expansion is fixed by the structure. This is equivalent to consider the thermal expansion of a fixed fiber as =0. The temperature dependence of a fiber Bragg grating measuring strain is:

  

When measuring strain this temperature induced wavelength change is confused with strain. The measured strain that is actually caused by temperature is:

The cross-sensitivity to temperature (TCS) is therefore given by:

 

The effective strain should be calculated from the strain sensor as the strain measured by the strain sensor minus the effect of temperature on the strain FBG:

  

This correction of the deformation does not take into account the effect of temperature on the deformation of the structure where the sensor is fixed on.

Temperature dependence of a Fiber Bragg Grating (FBG) fixed to a structure

To compensate also for the deformation of the structure due to temperature effects, the computation should be done considering the coefficient of thermal expansion (CTE) of the structure.

The total strain variation of a structure is:

The wavelength variation of a sensor fixed to a structure that is subjected to load and temperature is given by:

  

 

Meaning that to compensate the deformation of the structure due to temperature effect it is necessary to know the CTE value of the material of the structure where the sensor is fixed on.

In the case of a combined stimulation of stress and temperature, is it possible to only measure stress? Or the temperature effect only?

In order to measure only stress, the effect of temperature needs to be compensated. There are several methods for that, which include the usage of a special mechanical package or the use of an additional Fiber Bragg Grating (FBG) sensor.

Strain can only be removed from the measurement if the casing of the grating can isolate the strain of the structure from the sensing element. This can be achieved by the sensor’s mechanical design (e.g. FS63 Optical Temperature Sensors or the Optical Temperature Compensation Sensor OTC) or by using a bare FBG, terminal and not fixed. 

Can I measure bending strain on thin components with FBG sensors?

Fiber Bragg sensors have a higher layer thickness than electrical strain gauges. When measuring bending strain of thin components, there is a measurement error that must not be neglected but is easy to compensate for:

With
εOF   strain on the component's surface
εAnz  strain measured by the fiber
h    thickness of the component
d    distance of the fiber from the component's surface

There is, however, another aspect that should be addressed. Optical fibers have minimum bending radius due to optical losses. HBM FiberSensing sensors from the OP Line offer the possibility of measuring even on curved surfaces. The distance to consider with these sensors is d=0.5 mm.

Is there a specific reason for the standard Bragg wavelength distance to be 6.4nm for the FS Line sensors and 5nm for the OP Line sensors?

The Bragg wavelength of the optical sensors is defined on the instant the fiber Bragg grating is produced. To ease production, standard values were defined.

Historical reasons dictated that the standard wavelengths are currently different in the FS Line and the OP Line.

In any case, other customized wavelengths between 1500 nm and 1600 nm are available upon request.

Regarding harsh environments, where can the FBG technology be applied to?

Some examples of harsh environments where HBM FiberSensing optical systems have been successfully deployed are: high temperature, high radiation, high vacuum, high-voltage and cryogenic environments.

The following applications have also been successfully conducted:

  • Vibration and temperature monitoring in high power generators;
  • Hot spot monitoring in power transformers;
  • Wind blade monitoring;
  • Stress monitoring in airplane fuel tanks;
  • Strain, temperature and displacement monitoring in thermonuclear reactor;
  • Spacecraft monitoring, etc.

What is the maximum distance an optical fiber line can achieve?

The attenuation with distance is very small in optical fibers. In combination with HBM FiberSensing optical interrogators, optical fiber lengths can go up to tens of km.

What is the effect of pressure in an FBG?

There is a wavelength shift on the reflected Bragg peak when the grating is subjected to pressure. The wavelength variation is approximately:

 

This effect is, however, very small when compared with the wavelength variations induced by strain or by temperature changes being, therefore, commonly neglected.

When instead of pressure the FBG is subjected to a point lateral loading, a birefringence phenomenon occurs. This means that a new peak will appear (two peaks will coexist at the same time) and its shift can also be quantified.

What are the advantages in using optical sensors?

Optical sensors, in particular, Fiber Bragg Grating (FBG) sensors are a choice of election if the requested number of sensors is relatively high or if the distances to and between the sensors are long. Also, for particular environments the technology may be one of the existing alternatives to conventional sensing.

Take the example of an application where distances are suited for measuring with classical strain gauges. If this application requires more than 30 sensors, it becomes cost beneficial to use optical sensors rather than conventional ones.

Moreover, the advantages inherent to the technology here may turn FBG/optical sensors as the only available or the best possible solution for certain applications.

If long distances (in the order of km) or very specific application scenarios (for example, high magnetic fields, intense EMI/RFI, risk of explosion, etc.) are to be considered, optical sensors may be the only available solution, since electrical sensors would simply fail or carry numerous problems.

Such is the case of applications in cryogenic environments which require immunity to electromagnetic effects (EMI, RFI, sparks…) and electrical isolation.

Among other long recognized advantages of FBG based sensors are:

  • safe operation in potentially explosive atmospheres;
  • high multiplexing capability allowing the connection of a large number of different types of
  • sensors to a single optical fiber, reducing network and installation complexity;
  • small size and weight making them suitable to hard-to-reach locations and measurement points;
  • remote sensing: large distance between sensors and interrogator (several kilometers);
  • no mechanical failure and high resistance to fatigue;
  • ability to provide absolute measurements without the need for referencing: based on the measurement of an absolute parameter - the Bragg wavelength - independent of power fluctuations.

How many sensors may be integrated in one fiber?

One main benefit provided by optical fiber Bragg measurement technology is that several sensors can be integrated in a single optical fiber. It is a prerequisite that these sensors hold different Bragg wavelengths.

The Bragg wavelength varies as a function of the temperature and the strain affecting the sensor. Therefore, clearance distances need to be guaranteed between the sensors' wavelength peaks so that overlapping does not occur. These are required to enable the interrogator to allocate the sensors on the basis of the reflected wavelengths within the available measurement spectrum.

Another characteristic that can affect the number of sensors is the available power at the reception of the fiber Bragg grating reflection. This depends on the emission power of the interrogator, on the losses along the way (bending, connectors, splices, fiber length…) and on the reflectivity of the fiber Bragg grating.

There are so many details that can influence the number of sensors that it is difficult to state a number. However, a recommended value is 13 or 14 sensors per fiber, which corresponds to the standard central wavelengths available at HBM FiberSensing.

Are there specific wavelenghts pre-defined for each type of optical sensor?

The Fiber Bragg Grating (FBG) wavelength is defined during the sensor’s production and can be tuned to be any value between 1500 nm and 1600 nm. All types of sensors (temperature, strain, tilt, displacement, etc.) can be produced with any wavelength. Nevertheless, there are pre-defined wavelengths that ease the production process by making it repetitive. However, these are transversal and coexisting in all sensor types. The constraint in selecting the sensors wavelengths is that it is not possible to have two FBG sensors in series (in the same fiber) reflecting the same wavelength. If multiple FBG sensors are in the same optical fiber, the only requirement in terms of their Bragg wavelength is that they do not overlap (each FBG should have a unique Bragg wavelength and should not overlap with each other within the measuring range). Sensors may reflect the same wavelengths as long as they are being measured in different optical channels of the interrogator (data acquisition system). Usually, the wavelength of the sensor is defined by the customer on request or by the engineering team during the project’s design.

How is an optical sensor attached to the specimen?

The optical sensor may be attached to the specimen in different ways. Optical sensors produced by HBM FiberSensing may be glued, spot welded to metallic structures, embedded (into concrete, for instance), inserted in composite material, fixed with screws…

What is the response time of the temperature sonsors?

The influence of temperature on the sensor is immediate. It only depends on the heat transfer through the material.

 

 

Are temperature sensors delivered with its own polynomial calibration function?

All temperature sensors are delivered with a calibration sheet including polynomial calibration function.

What active grid length may be assumed on an optical strain sensor?

An optical strain sensor does not have an active grid length as it can be defined for electrical strain gauges.

The gauge length depends on the sensor encapsulation and is stated on the sensors datasheets. In case a bare FBG is used, or a sensor where the full length of the grating is attached, the gauge length corresponds to the grating length that is approximately 6 mm.

What influence does humidity/water have on the bonding strength of adhesives?

Cold-curing adhesives used in the bonding of optical sensors do not offer long-term stability at increased relative humidity. This applies for cyanoacrylate adhesive (Z70) in particular.

Epoxy resin systems (X280), however, are resistant against the influence of humidity.

Please note that humidity affecting optical sensors results in swelling of the materials used. In the case of the optical strain gauges, this generates forces affecting the Bragg grating. This has a negative effect on the measuring point's stability.

At any rate, we recommend using a covering agent similar to those used with electrical strain gauges.

Regarding the tilt sensor, two FBGs are needed to eliminate temperature influences. Why?

The tilt sensor includes two FBGs and both are needed to get the angle measurement without temperature influences. The two fiber Bragg gratings operate in a push-pull configuration which means that when one sensor is being tensioned due to the sensor position the other is being compressed with the same strain value. With this configuration we can identify the angle variation by the wavelength variation that is equal in value but with opposite signals. The temperature effect causes equal wavelength variations to the FBGs and is, therefore, removed.

Does the tilt sensitivity depend on the initial positioning angle of the tilt sensor?

The sensitivity of the tilt does not depend on the initial angle of the sensor. However, the sensor operates as a pendulum, so the angle measurement range of ±5 deg is centred at the vertical and the sensor will not operate outside these boundaries.

Nearly all sensors have an operation temperature range from [-20,80]°C, why?

The temperature limitation of -20ºC to 80ºC is true for FS line sensors. The limits are given by the acrylate coating of the fiber.

What is the reason for using optical, i.e. FBG sensors on wind turbine blades instead of traditional strain gauges?

There are many reasons to select FBG sensors for the monitoring of wind turbine blades. The following table compares FBG sensors to conventional strain gauges and to other types of optical technology.

Vs Conventional Technology Vs Other Optical Technology
Passive sensors - lightning immune Internal spectral reference – long term accuracy
Fatigue resistant Auto adjustment in each 10 ms - no need for recalibration
Multiplexing - different variables using the same interrogator Wide wavelength range available  – large sensor count
Less cabling - several sensors in the same line Immunity to polarization effect – signal stability
No welding procedures High reflectivity sensors (>70%) – no limitation in distance between them

You may find more information on HBM FiberSensing optical system for monitoring and testing applications on Wind turbines here.

What is the maximum sampling rate of an FBG sensor?

We can say that the fiber Bragg Grating alone (bare fiber) has “no limit”. The limitation on the sampling rate is on the interrogator side. The interrogation technology will dictate the maximum samples per second that can be measured. HBM FiberSensing standard interrogators are currently limited to 500 S/s, but the company has in house technology for acquisition rates up to 10 kS/s. For encapsulated sensors, i.e. with some mechanical apparatus around, the virtual limit might not be true as the mechanical interface limits the strain transfer to the grating.

What is the difference between static and dynamic interrogators?

The fundamental difference between the static and dynamic interrogators lays in their sampling rate: while the first is used for static applications, the second – with higher acquisition rates is for quasi-static or dynamic applications. Please check HBM FiberSensing interrogators page and the available options.

Due to their design there are also some differences in terms of performance. The static interrogators provide a better accuracy and resolution and can operate as stand-alone, while the dynamic have a slightly bigger accuracy and resolution values and do not have internal memory to store data without an additional PC.

There is one feature that is unique for the static interrogators that is the SPD. Learn more about it here.

Is the BraggMONITOR software compatible with both static and dynamic BraggMETER interrogators?

Although the interface is quite similar, HBM FiberSensing has available two distinct applications: BraggMONITOR SI for the static interrogator FS22 Industrial BraggMETER SI and BraggMONITOR DI for the dynamic interrogator FS22 Industrial BraggMETER DI.

When using the BraggMONITOR software, which value should I use on the Central Wavelength (CWL, λ0) field?

The central wavelength value that must be filled during the sensor edition corresponds to the wavelength (λ0 in nm) value from which the wavelength variation (x=Δλ, in nm) is calculated. This means that the result for the wavelength variation of a sensor in an instant t (xt, Δλt) is:

 

Where λt is the wavelength of the sensor measured at the instant t.

If the user’s purpose is to “zero” the measurements from an instant, the value to insert on the CWL field should be the value measured at that particular instant. On the other hand, if the user needs absolute measurements (as, for instance, on temperature sensors) the CWL to use should be the one defined on the calibration Sheet that is provided with the sensor.

 

Using the temperature sensor example

On the Calibration Sheet of an HBM FiberSensing temperature sensor, the temperature is described
as a second order polynomial of the wavelength variation:

Where:

S2 is the second order sensitivity,

S1 is the first order sensitivity,

S0 is the temperature offset.

The S0 value corresponds to the reference temperature at the calibration procedure, so to get absolute temperature values the x has to be calculated using the same central wavelength from the calibration: the used CWL in the measurements has to be the same as the stated on the calibration sheet of the sensor.

 

Using the strain sensor example

The FBG based strain sensors dependence to deformation is:

 

Where k is the k factor of the strain sensor, and S is the strain sensitivity indicated on the calibration sheet.

This deformation will always be registered from an instant that is defined as “zero” meaning that the x value will always be calculated relatively to the wavelength that the sensor was exhibiting at the “zero” moment after the sensor installation.

Is it possible to connect multiple FBG sensors, if the range of the interrogator is limited?

There is a limit to the number of sensors that can be addressed with an interrogator that has a limited range, let’s say [1500; 1600] nm. However, it is still possible to have multiple sensors in a single optical fiber (tens or even hundreds of sensors), as long as the Bragg wavelengths of each sensor are distinct and do not overlap within their measuring range.

As an example, the interrogator range is [1500; 1510] nm and one would like to have 3 sensors measuring in this range.

If the Bragg wavelengths of the sensors are as follows:

sensor1 = 1502 nm

sensor2 = 1505 nm

sensor3 = 1508 nm

and all the sensors have a wavelength shift of +/- 1 nm during measurement, there will be no overlap during measurement.

If for instance, the measurands cause the wavelength shift of the sensors to be +/- 3 nm, overlap of the Bragg wavelength within the measuring range would occur and one would be limited by that.

In order to measure strain, it is necessary to compensate for temperature using another sensor. In this case should I use two channels?

It is good practice to join a temperature sensor to a strain sensor in order to compensate for the temperature effect on the strain sensor.

However, there’s no need to spend two channels to measure temperature and strain.

FS22 Industrial BraggMETER interrogators from HBM FiberSensing (with up to 8 channels) may measure sensor arrays with several sensors.

HBM FiberSensing athermal strain sensors are able to measure strain regardless of the temperature with only one FBG.

Is the wireless transfer of the measurement data to a PC possible?

HBM FiberSensing interrogators communicate via Ethernet. With this, it is possible to connect them via Wi-Fi or GSM using  dedicated equipment connected to the interrogator. Care has to be taken, though, to the reliability of the transferred data. For example, on the dynamic interrogators with higher acquisition rates the needed bandwidth to ensure flawless data transfer is high.

Optical sensors do not comprise any communication as the sensors do not have any electronics involved. Optical sensors are electrically passive and rely only on a bare optical fiber.

Are HBM FiberSensing data acquisition systems portable?

HBM FiberSensing interrogators are small and can easily be carried around. Nevertheless, FS22 Industrial BraggMETERs need power supply and a PC for an interface, which makes the equipment harder to move.

The FS42 Portable BraggMETER interrogators are designed for on-the-move applications. These are stand-alone interrogators that can provide measurements in disperse sites and be used with any type of FBG sensors (temperature, strain, tilt, etc.). They include batteries, a touch screen interface and a built-in software in the internal PC. It is very common to use these portable interrogators in the field during the installation of networks of sensors, or to perform in-situ live measurements.

Do the interrogators need calibration?

The FS22 Industrial BraggMETER and the FS42 Portable BraggMETER interrogators are calibrated upon production and include a traceable internal reference that ensures their accuracy over time.

Nevertheless, it is possible to perform a certified calibration for all interrogators from time to time.

Is it possible to embed an HBM FiberSensing optical interrogator in other systems?

Yes, in the case of the FS22 Industrial BraggMETER interrogators. These interrogators communicate via Ethernet with TCP/IP interface.

In the case of the FS42 Portable BraggMETER interrogator the answer is no.

Are there drivers available to control the interrogators?

Drivers are supplied with the interrogators’ support material.

What is the storage capacity available in the FS22 Industrial BraggMETER SI interrogator?

The storage capacity of the static interrogator is 2 GB. If you consider 25 sensors/channel and an 8-channel interrogator, 2 GB will be sufficient to store about 3000 hours of data.

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