A New Fiber-optic Multipoint Sensing System for Condition-Based Maintenance

Toshiba developed a fiber-optic sensing system that allows multipoint measurement of various physical quantities with one set of sensing fibers, and has inherent resistance to heat, radiation, and electrical noises.

To measure physical quantities such temperature, strain, or vibration, the system uses Fiber Bragg Grating (FBG), which is a periodic variation of the refractive index in a fiber cable core, and reflects light of the specific wavelength. The variation of the physical quantities causes expansions and contractions of the grating, which are sensed in a variation of the light reflected and transmitted back from the FBG. Thus, the system measures and transmits physical quantities of remote devices without using electricity.

Figure 1 shows a typical structure of the fiber cables. The fiber cables are configured in a tree structure, where the two trunk cables can be expanded more than hundreds of meters, and branch cables can be expanded about a hundred meters. Each branch fiber cable can place up to 16 FBGs (colored box). The system uses time division and wavelength division techniques for multipoint measurements.

Lastly, the system measures slow varying quantities, such as temperature or strain, and fast varying quantities, such as vibration or dynamic strain. This can be achieved using different methods; (a) sweeping the wavelength of the light pulses and measuring the intensity of the reflective light pulse; (b) keeping the wavelength constant and measuring the intensity of the reflective light pulse.

 

 

 

(1) Components
This system has been developed aiming to monitor nuclear power plants devices, such as pumps motors, pipes, or tanks. This system can monitor similar devices in other plants, such as thermal power plants, or chemical plants without modification.

(2) Materials
This system can be applied to measure the temperature, strain, or vibration of devices made of various materials, though typical materials originally considered are carbon steel and stainless steel.

(3) Condition
This system is designed to operate in the nuclear power plants primary containment vessel, where the maximum temperature is 300˚C, and a radiation field exists.

 

(1) How FBG Works
Figure 2 shows the working principle of FBG, which is a periodic variation of the refractive index in a fiber cable core. FBG reflects light of wavelength equal to twice the grating period. Therefore, the reflected light wavelength varies as the grating period varies due to expansions and contractions of the fiber. Because the variation of the temperature, strain of the fiber, or vibration of the fiber changes the grating period, these physical quantities can be measured as variations of the reflected light wavelength.

 

 

(2) Multipoint Sensing
As shown in Fig. 1 the fiber cables are configured in a tree structure. The light pulses generated from the light pulse source (drawn left) propagate through the entire branch, and are reflected back by FBGs of the specific grating period. The reflected light pulses are captured by the photo-detector (drawn right). The system determines which FBG reflects the light pulse from the wavelength and the return time of the light pulse.

Figure 3 explains the method in detail. The FBGs numbered "λ1" through "λ16" have different grating periods, and reflect different wavelength light. The FBGs marked "λ1-1," "λ1-2," "λ1-3," and "λ1-4" have the same grating period, and reflect the same wavelength light, but the return times of the light varies proportional to the distances of the FBGs, as shown on the right hand side figure.

 

 

 

(3) Measuring Slow Varying and Fast Varying Quantities
Temperature and strain are slow varying quantities, which varies in a time scale of minutes, or hours; while vibration or dynamic strain are fast varying quantities, which varies in a time scale of milliseconds. The system measures the two types of quantities using two methods.

Figure 4 explains the method to measure slow varying quantities. The spectrum from the light pulse source has a triangle shape, and the wavelengths of the reflected light from the FBG are in a narrow band. When the light pulse source sweeps the wavelength from low to high, the intensity of the reflected light increases as the light pulse wavelength approaches to the band of the reflected light, and decreases as the light pulse wavelength becomes longer than the band. As a result, an intensity distribution, shown on the right hand side of Fig.4, is obtained. The peak of the distribution indicates the grating period, from which the strain can be measured.

Figure 5 explains the method to measure fast varying quantities. In this case, the light pulse wavelength is unchanged, but placed so that the wavelength of reflected light comes on the slope of the spectrum. When the wavelength of the reflected light varies according to expansion and contraction of the grating, the intensity of the reflected light varies. As a result, an optical intensity distribution shown on the right hand side of Fig.5 is obtained. Applying Fast Fourier Transform analysis to this distribution, we obtain a frequency and amplitude of the vibration or dynamic strain.

 

 

 

 

 

(4) Feature Summary
The system has the following features:
a) Measures up to 192 points, including slow varying quantities and fast varying quantities.
b) Insusceptible to electrical noises.
c) Resistance to heat, keeping 80% or higher reflective index for more than 3000 hours in 335 ˚C atmosphere.
d) Resistance to radiation, keeping 90% or higher reflective index against 10⁶Gy dose.

Please refer to Reference [3] for more details.

 

(1) In nuclear power plants, environment around the nuclear reactor is harsh. Sensors placed near the reactor must withstand high-temperature (300 ˚C), and radiation. In addition, electrical equipment around the reactor makes large electrical noises.

Considering the inherent resistance of FBG to heat, radiation, and electrical noise, we plan to use the FBG sensing system as a strain sensor to measure a very small dynamic strain of piping around the nuclear reactor. Our resolution target is measuring 0.1 micro-strains (one expansion or contraction length against ten million lengths) on pipes.

We verified this level of resolution by an experiment using a mock-up piping. An example of the result is shown on Fig. 6, which shows good agreement between the FBG strain sensor and a strain gauge for 250 hertz dynamic strain.

 

 

 

(2) We hope the FBG sensor system can be applied for monitoring of various quantities, such as temperature, strain, or vibration in various industrial plants.