Concise answers to the most frequently asked questions about optical strain gages: procedure, advantages, installation

A fiber Bragg grating comprises many reflection points that reflect particular wavelengths of incident light. Such a point is created by intense UV light affecting the fiber core. This process is also called "writing". Writing very many such reflection points into the fiber at regular intervals creates a grating.

Fig. 1: Basic structure of a fiber Bragg grating

The distance between the reflection points of a fiber Bragg grating is always equal. The wavelength that exactly matches the distance between two reflection points is reflected by the grating. All other wavelengths are transmitted through the grating without being reflected or damped. Interference of the light of the individual reflection peaks creates a reflection peak determined by the distance of the grating points from each other.

If all reflections are in phase - in this case the wavelength corresponds to the distance of the reflection points -, this results in constructive interference at this point. The wavelength of such a reflection peak is determined in the interrogator. As soon as a fiber Bragg grating is subjected to strain, the distance of the reflection points changes and a different wavelength is reflected. This enables the Bragg wavelength variation to be determined. In analogy to the relationship for the metal strain gage, the following applies:

With

λ  base wavelength of the fiber Bragg grating (wavelength at the start of measurement)
Δλ  wavelength variation sith strain applied to the grating
k    gage factor
ε   strain

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

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

Optical strain gages from HBM have a distance of d=0.5 mm.

An optical strain gage comprises a fiber Bragg grating enclosed in a special structure. Optical strain gages from HBM are designed such that they can be bonded like conventional strain gages. Users thus do not have to cope with the difficult handling of optical fibers.

Another advantage is that negative strain, too, can be easily measured without having to prestress the Bragg grating - a feature that is only available with patented Bragg gratings from HBM.

All optical strain gages from HBM have been tested to the VDI/VDE2635 standard for electrical strain gages with regard to characteristics like fatigue life, gage factor and maximum elongation.

Damping components such as plugs and couplings used for connecting the interrogators as well as differing optical fibers can cause optical losses. By matching gain and noise threshold in the catman^®Easy optics add-on module, the sensors' spectrum can be adjusted in the tolerance range of the signal strengths that can be evaluated.

Gain is used for increasing signal strength. Noise threshold is used for adjusting the noise level to suppress noise. In this case, it is essential that the intensity of the reflection peaks is about equal to the noise level. Optical strain gages from HBM installed as strain gage chains have level differences of maximal 4 dB - a value that does not negatively affect measurements. For more information on how to correctly adjust gain and threshold please refer to the corresponding operating manuals.

An optical strain gage does not have an active grid length as it can be defined for electrical strain gages. In general, optical strain gages from HBM have a length of 30 mm.

A fiber Bragg grating is embedded in a special plastic in the center of the strain gage (light area in the image). Strain is applied to the fiber Bragg grating by the optical strain gage's "endpoints" (the two darker areas in the image). These endpoints have a length of 5 mm each. The optical strain gage needs to be bonded all over its total length.

The light special plastic material guides the fiber, in particular, in the event of negative strain. The optical strain gage's strain signal that is fed to the interrogator is the average of all strains in the area of the light special plastic material.

One main benefit provided by optical fiber Bragg measurement technology is that several sensors can be integrated in one and the same fiber. It is prerequisite that these sensors have different Bragg wavelengths.

As described in the paragraph "How does a fiber Bragg grating work?", the Bragg wavelength varies as a function of temperature and strain affecting the sensor. Peaks must not interfere.

The sensors cause a wavelength variation that depends on the mechanical load of the sensor. The total of these variations must not exceed the interrogator's wavelength bandwidth.

It must be pointed out that clearance distances need to be allowed for between the sensors' wavelength peaks. These are required to enable the interrogator to allocate the sensors on the basis of the reflected wavelengths. A recommended value is 13 sensors per fiber. A four-channel interrogator thus enables up to 52 sensors to be connected.

In general, optical strain gages are installed using cold-curing adhesives that do not offer long-term stability at increased relative humidity. This applies for cyanacrylate adhesive (Z70) in particular.

Epoxy resin systems (X280), however, are resistant against the influence of humidity. Please note that humidity affecting optical strain gages results in swelling to a very limited degree of the materials used.Inside the optical strain gage, this generates forces affecting the Bragg grating. This has a negative effect on the measuring point's stability of zero. At any rate, we recommend using a covering agent similar to those used with electrical strain gages.

All covering agents from HBM's range of products (except for SL450) can be used with optical strain gages.

Static and dynamic interrogators differ in their sampling rate. Static interrogators operate at a sampling rate of 1 to 10 Hz. Dynamic interrogators operate at a sampling rate of 100 to 1,000Hz.

• HBM part numbers for static interrogators: SIxxx
• HBM part numbers for dynamic interrogators: DIxxx

Optical multiplexers can be connected to 4-channel interrogators. They multiply the number of optical measurement chains to eight or 16 respectively. This enables measurement systems with up to 320 optical sensors to be built up. For more information, please see the corresponding data sheets at hbm.com

The values measured by the interrogator are the wavelengths of the signals reflected by the sensors. When strain at the optical strain gage causes the wavelength to change, this variation is proportional to the strain. To obtain the corresponding strain values, the wavelengths have to be converted to strains.

The gage factor specified on the sensor packaging is used as the proportionality factor. Another option is to use data acquisition and analysis software that is suited for optical measurement technology.

Required settings such as the parameters for spectrum display and strain measurement as well as zero adjustment values can be permanently stored in the software. Temperature variations have a significant effect on the measurement results. Measurement data acquisition software enables temperature effects to be compensated for. This can additionally be done using a compensating sensor or by correction through a temperature channel.