Fiber Optic Strain Sensor structure, working, advantages, disadvantages

This page describes structure, working operation, advantages and disadvantages of Fiber Optic Strain Sensor. It mentions Fiber Optic Strain Sensor advantages or benefits and Fiber Optic Strain Sensor disadvantages or drawbacks.

Fiber Bragg Grating used as Strain Sensor

Fiber Bragg Grating reflects wavelength of light which shifts in response to variations in temperature and/or strain. The refractive index is changed permanently according to exposed light intensity. The periodic variation of refractive index is known as fiber bragg grating.

Fiber Bragg Grating sensor working

For strain measurement, temperature effects need to be compensated on FBG. This is implemented by installing FBG temperature in close proximity to FBG strain sensor. Substraction of FBG temperature sensor wavelength shift from strain sensor wavelength shift removes any effects of temperature on the wavelength data. Hence temperature compensated strain value can be achieved using this approach. Refer fiber bragg grating sensor >>.

The change of wavelength of an FBG (Fiber Bragg Grating) due to strain and temperature can be expressed as follows.
➨ ΔλB = { [ (1 - Pe)ε + (α + ((dneff/dT)/neff)*ΔT ]}*λB
Pe = Effective photo-elastic constant
α = Thermal expansion coefficient of fiber

Plastic Fiber Optic Strain Sensor working

Plastic Fiber Optic Strain Sensor

The figure-2 depicts plastic fiber optic strain sensor. It is cost effective intensity based sensor used for strain measurement and structural health monitoring of bridges, skyscrapers, dams etc. It has many benefits such as high resistance to fracture, ease of termination and coupling etc. Refer advantages and disadvantages of plastic fiber >> for more information.

• As shown plastic fiber is bonded at lower surface of the suspended bridge using special adhesive.
• LED light source at one end and light detector (e.g. LDR) at the other end are connected with the fiber.
• Under loading condition, axial strain decreases curvature of the fiber. This reduces light intensity due to loss of the light signal through the fiber bending.
• When metal beam is subjected to transverse loading, the in-phase strain on lower surface of the beam can be expressed as follows.
➨ε = - [(12 * y) / L2 ] * δctr
y = distance of lower plane of beam from its neutral axis
L = distance between the supports
δctr = Beam deflection at its central point

Benefits or advantages of Fiber Optic Strain Sensor

It has several advantages over conventional gauges. Following are the benefits or advantages of Fiber Optic Strain Sensor:
➨It is compact in size.
➨It is highly sensitive.
➨It is easy to install and maintain.
➨It is reliable.
➨It offers longer life.
➨It is inexpensive.

Drawbacks or disadvantages of Fiber Optic Strain Sensor

Following are the drawbacks or disadvantages of Fiber Optic Strain Sensor:
➨It is thermal sensitive.
➨It is difficult to demodulate wavelength shift.
➨It is difficult to discriminate wavelength shift due to temperature and strain separately.
➨Optical fibers are fragile and prone to damage during use. Hence care should be taken while use.
➨Refer advantages and disadvantages of fiber bragg grating strain sensor >>, fiber optic pressure sensor >> and fiber optic temperature sensor >> for more specific information.

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