FIBER OPTIC STRAIN SENSORS: FIBER OPTIC STRAIN SENSORS Beril Bicer University of Illinois at Urbana-Champaign Content: Content Optical Fiber
Fiber Optic Sensors
Basic Components of FOS Setup
Classification of FOS
Fiber Optic Strain Gages
Products Optical Fiber: Optical Fiber a filament of transparent dielectric material, glass or plastic
usually cylindrical in shape
a guidance system for light
Optical fiber is : Optical Fiber:
n1 sin J = n2 sin J
where n is refractive index
Optical Fiber Guidance is achieved through multiple reflections at the fiber walls.
Core, transparent dielectric material, surrounded by another dielectric material with a lower refractive index called cladding. (n1 >n2)
In practice, there is a third protective layer called jacket. Ray Transmission through an Optical Fiber: Ray Transmission through an Optical Fiber Critical angle of reflection
(sin Jc = n2 /n1) Fiber Optic Sensors: Fiber Optic Sensors Basic Components: source of light
a length of sensing
(and transmission) fiber
a photo-detector demodulator
processing and display optics
Fiber Optic Sensors: Fiber Optic Sensors Measured Parameters: Measured Parameters Light intensity
strain (rotation and displacement)
magnetic and electrical fields
velocity, acceleration and vibration
force and stress Main Advantages: Main Advantages Non-electric (immune to electromagnetic and radio-frequency interference)
withstand high temperature and harsh environments (corrosion)
High shock survivability (explosion or extreme vibration)
high accuracy and sensitivity
light weight and small size
high capacity and signal purity
Can be easily interfaced with data communication systems Basic Components of FOS Setup: Basic Components of FOS Setup Applications: Applications Real-time monitoring of civil engineering structures.
Structural monitoring of aircraft, both in-flight and on-ground
Instrumentation of robots used on board in the International
Testing and analysis of solid rocket motors
Smart structures instrumentation
Fiber Aerospace guidance and control
Damage localization in civil, mechanical, and aerospace
Embedment in concrete structures
Classification of FOS: Classification of FOS A. Based on application areas:
physical sensors (measurement of temperature, stress, etc)
chemical sensors (measurement of pH content, gas analysis, spectroscopic studies, etc.)
biomedical sensors (measurement of blood flow , glucose content, etc.) Classification of FOS: Classification of FOS B. Based on modulation and demodulation process:
compare the phase of light in a sensing fiber to a reference fiber in a device called interferometer.
Light is not required to exit the fiber at the sensor (no optical loss)
more complex in design
better sensitivity and resolution Classification of FOS: Classification of FOS Example:
Mach-Zehnder Interferometric sensor Classification of FOS: Classification of FOS B. Based on modulation and demodulation process:
Light is required to exit the fiber at the sensor (optical loss)
simpler in design
widespread in application Classification of FOS: Classification of FOS B. Based on modulation and demodulation process:
measures the changes in the wavelength of the light due to the environmental effects.
Slide17: Classification of FOS C. Based on sensing characteristics of fibers
a coating or a device at the fiber tip performs the measurement.
Slide18: Classification of FOS C. Based on sensing characteristics of fibers
fiber itself performs the measurement.
Fiber Optic Strain Sensors: Fiber Optic Strain Sensors A. Intensity Modulated Strain Gages
One bundle is used to transmit the light to a reflecting target
Other collects the reflected light and transmits to a detector
Any movement of the target will effect the intensity of the reflected light. Fiber Optic Strain Sensors: Plain reflective displacement sensors have a limited dynamic range of about 0.2 in.
Can be improved by a lens system to 5 in.
sensitive to the orientation and contamination of the reflective surface Fiber Optic Strain Sensors Slide21: Fiber Optic Strain Sensors A. Intensity Modulated Strain Gages
If a fiber is bent, a portion of the trapped light is lost through the wall. Slide22: Fiber Optic Strain Sensors B. Phase Modulated Strain Gages
Fabry-Perot Interferometers (FPI)
light source is conveyed via an optical fiber to two mirrors (reflectors).
When the displacement between the mirrors has changed due to strain, optical spectrum changes
absolute distance between the mirrors gives the strain. Slide23: Fiber Optic Strain Sensors Extremely sensitive
provides point-sensing capability
excellent mechanical properties
output is easy to process
difficult to make rugged enough for harsh construction env. (embedding in concrete) Product: EFO Embedded Strain GageFISO Technologies: Product: EFO Embedded Strain Gage FISO Technologies 70 mm long sensor
can be embedded in concrete
intrinsic Fabry-Perot strain gage is bonded in a very small hole in the center of the steel body.
can be cast directly into the wet mix
can be encapsulated into a concrete briquette, then cast into wet concrete can be placed into a pre-drilled hole and grouted.
Diameters are 3mm and 30 mm.
range, +/- 1000, 1500micro strain
resolution <0.01% full scale
Temperature range, -55 oC to 85oC
Product: Embeddable EFPI Strain GageLuna Innovations Inc. : Product: Embeddable EFPI Strain Gage Luna Innovations Inc. 2-10 mm length, 350 micrometer outer diameter
sensitivity, +/- 5000 micro strain
Temperature range, -100 oC to 350oC
Measurement cycle, 100kHz Product:Wide Sensing Fiber Optic CableSunX-Ramco Inc.: Product:Wide Sensing Fiber Optic Cable SunX-Ramco Inc. 11 mm wide sensing area
long sensing distance
freely cuttable fiber cable
2 m. lenght