Application of Fiber Optic Sensors in Measurement Technology

1. Introduction-Fiber optic sensors and measurement technology are new development directions in the field of instrumentation

Since fiber optic sensors and technologies have unparalleled characteristics compared to other sensors, in recent years, fiber optic sensors and measurement technology have become a new development direction in the field of instrumentation. New fiber optic sensors have the following characteristics:

*Fiber optic sensors have excellent light transmission performance and very small light transmission loss. Currently, the loss can reach ≤0.2dB/km.

* Fiber optic sensors have wide bandwidth, can perform ultra-high-speed measurements, and have good sensitivity and linearity.

* Fiber optic sensors are small in size and light in weight, and can perform non-contact, non-destructive and long-distance measurements in harsh environments.

It also has the characteristics of high sensitivity, good reliability, rich raw material silicon resources, anti-electromagnetic interference, corrosion resistance, high voltage resistance, good electrical insulation performance, bendability, explosion-proof, wide frequency band, low loss, etc. At the same time, it is also easy to connect to the computer to realize intelligent and long-distance monitoring. It plays the role of expanding and improving traditional sensors, and in many cases can complete tasks that the former is difficult to complete or even impossible to complete.

It is precisely because fiber optic sensors have many unique advantages and can solve many problems that traditional sensors cannot solve. Therefore, since their advent, they have been widely used in various fields such as medical care, transportation, electricity, machinery, petrochemicals, civil construction, and aerospace. On this occasion, the application of fiber optic sensors in the field of petrochemicals will be discussed, that is, the design scheme of the real-time monitoring system of the liquid level and temperature signal of the oil tank in the oil depot (see Figure 0). Because this monitoring system uses fiber optic liquid level sensors, fiber optic temperature sensors and fiber optic liquid level alarms. For this reason, the relevant fiber optic sensor technology is first introduced.

2. Composition and types of optical fiber sensors

Fiber optic sensors are generally composed of light sources, interfaces, optical fibers, optical modulation mechanisms, photodetectors, and signal processing systems. The light from the light source enters the optical fiber through the interface, and then the detected parameters are modulated into amplitude, phase, color or polarization information, and finally the microprocessor is used for information processing. In summary, fiber optic sensors are generally composed of three parts. In addition to the optical fiber, there must be two important components: the light source and the photodetector, as shown in Figure 1.

Fiber optic sensors are generally divided into two categories: one is the light transmission type, also known as non-functional fiber optic sensors; the other is the sensing type, or functional fiber optic sensors. The former mostly use multimode optical fibers to transmit more light; while sensing optical fiber sensors use the measured object to modulate or change the characteristics of the optical fiber, so they can only use single-mode optical fibers.

3. Fiber optic sensing technology for measurement

3.1 Fiber Optic Temperature Sensor - Light Transmitting Fiber Optic Temperature Sensor

Figure 2(a) is a schematic diagram of a semiconductor light-absorbing (light-transmitting) optical fiber temperature sensor. A cut optical fiber is installed in a thin steel tube, and a semiconductor temperature-sensitive sheet (such as GaAs or InP) is sandwiched between the two end faces of the optical fiber. The intensity of light transmitted by this semiconductor temperature-sensitive sheet changes with the measured temperature. Therefore, when a constant light intensity is input into one end of the optical fiber, the light intensity received by the receiving element at the other end of the optical fiber also changes with the measured temperature because the transmission capacity of the semiconductor temperature-sensitive sheet changes with temperature. Therefore, by measuring the amount of electricity output by the light detector, the temperature at the temperature-sensitive probe 2(b) can be remotely measured.

In the probe, the characteristic curve of the transmittance and temperature of the semiconductor material is shown in Figure 2(c). When the temperature rises, its transmittance curve moves toward the long wavelength direction. Obviously, the absorptivity of the semiconductor material is related to its bandgap width Eg, which changes with temperature. The bandgap width Eg of most semiconductor materials decreases almost linearly with the increase of temperature T, corresponding to the wavelength λg at the edge of the semiconductor's transmittance characteristic curve shifting toward the long wave direction with the increase of temperature T. When light emitted by a light source with a radiation spectrum consistent with λg passes through this semiconductor, the intensity of the transmitted light decreases with the increase of temperature T. So what is a light transmission type fiber optic sensor?

The optical fiber in the light transmission type optical fiber sensor is only used as a medium for transmitting light and plays the role of transmitting light waves. The "sensing" function of external information is completed by sensitive elements of other substances. Therefore, other sensitive elements must be installed on the end face or in the middle of the optical fiber to form a sensor. In this way, the optical fiber in the sensor is interrupted and discontinuous in the middle, and the interrupted part must be connected to sensitive elements of other media, as shown in Figure 1.

The modulator is a sensitive element placed between the incident optical fiber and the receiving optical fiber. Under the action of the object to be measured, the optical path of the sensitive element is blocked or the light transmittance of the sensitive element is changed. In this way, the amount of light received by the photodetector becomes the modulated signal of the object to be measured. After amplification and demodulation, the object to be measured can be obtained.

3.2 Fiber Optic Liquid Level Sensor

Based on the principle of total internal reflection, it can be designed into a fiber optic liquid level sensor. The fiber optic liquid level sensor consists of the following three parts:

*The device for detecting the amount of light reflected after contact with liquid is the light sensitive element;

*Dual-core optical fiber for transmitting optical signals;

*Receiving device for emitting light, receiving light and processing signals.

Figure 3(a) shows the basic structure of the optical fiber liquid level sensor. The sensitive element and the optical fiber that transmits the signal are both made of glass fiber, so it has the advantages of good insulation performance and resistance to electromagnetic noise.

The working principle of the fiber optic liquid level sensor is shown in Figure 3(b). The light emitted by the light emitting device is sent to the sensitive element through the transmission fiber. On the spherical surface of the sensitive element, a part of the light is transmitted, while the rest of the light is reflected back. When the sensitive element is in contact with the liquid, the light transmission amount of the spherical surface increases and the reflection amount decreases compared to the contact with air. Therefore, the amount of reflected light can be used to know whether the sensitive element is in contact with the liquid. The amount of reflected light depends on the refractive index of the glass of the sensitive element and the refractive index of the measured material. The larger the refractive index of the measured material, the smaller the amount of reflected light. The reflected light from the sensitive element is output after photoelectric conversion by the phototransistor of the light receiving device through the transmission fiber. The change in the amount of reflected light of the sensitive element, if based on the amount of light in air, is -6-7dB in water and -25-30dB in oil. It can distinguish materials such as water and oil with large differences in reflected light.

The optical fiber liquid level sensor that uses low light to detect liquid level has the following characteristics:

*Can be used in facilities with flammable and explosive materials;

* The sensitive element is small in size and can be used to detect trace amounts of liquid;

* Short response time from liquid detection to detection signal output;

* The sensitive element is made of glass, so it is resistant to chemical corrosion;

*Can detect the interface between two liquids (oil, water, etc.):

* Low price.

In practical applications, it should be noted that optical fiber liquid level sensors should not be used to detect substances adhering to the glass surface of the sensitive element. How to detect liquid level?

In a tank filled with liquid, install the sensitive element at a predetermined detection height below the liquid level. When the liquid level is lower than this height, the amount of reflected light generated by the sensitive element increases, and the liquid level position can be detected based on the signal generated at this time. If the sensitive element is installed at different heights, the height of the liquid level can be detected.

4. Design of a real-time monitoring system for the liquid level and temperature signals of oil tanks in oil depots composed of optical fiber sensors and CAN bus network.

4.1 Composition of monitoring system.

As shown in Figure 0, the monitoring system consists of a central control central monitoring unit and a field acquisition unit (or data acquisition unit). The field acquisition unit collects data on the liquid level and temperature signals of the oil tank in real time, and completes data statistics and storage at the same time; the central monitoring unit can obtain data from the field acquisition unit regularly or irregularly and complete image monitoring, data statistics, reports, printing and database management. The central monitoring unit and the field acquisition unit are connected together through the CAN bus. In this network, the central monitoring unit is in the master control position, and the field acquisition unit can respond to the command of the central monitoring unit at any time. Its field acquisition unit consists of a single-chip microcomputer 8C552 and acquisition, storage, display, remote control and communication modules. Each field acquisition unit can be connected to 16 devices such as optical fiber liquid level sensors, optical fiber temperature sensors and optical fiber liquid level alarms. The reason for applying the CAN bus network structure is that the CAN network has the characteristics of convenience, flexibility, outstanding characteristics, anti-interference, reliability, real-time performance, etc. All fiber optic liquid level sensors, fiber optic temperature sensors and fiber optic liquid level alarms can be connected in series through a pair of twisted pair cables, saving space and simplifying wiring.

4.2 About data collection

4.21 The detection of oil tank level signal adopts optical fiber level meter. The optical fiber level meter is used in oil field oil depot processing, mainly relying on certain sensing functions of optical fiber itself and the characteristics of optical transmission information:

*Can realize non-contact gun testing, no interference and no pollution to the object being tested;

* It realizes non-electric detection, intrinsic safety, and explosion-proof, and solves the safety detection and production process of multiple parameters in flammable and explosive dangerous places such as petroleum and petrochemical;

*High precision, high insulation, anti-electromagnetic interference and anti-lightning interference.

4.22 uses optical fiber temperature sensor (or optical fiber Bragg grating temperature sensor is better, but the cost is high), which is suitable for temperature measurement in various harsh environments. It has high measurement accuracy, is safe and explosion-proof, has good reliability and high sensitivity. It is an ideal temperature measuring instrument. When working, the continuous broadband light emitted by the light source is transmitted to the site through the transmission optical cable. The measuring grating in the temperature sensing probe selectively reflects the broadband light and a narrowband light is transmitted to the modem through the same transmission optical cable. It is demodulated by the receiving system, and the central wavelength of the narrowband light is measured, thereby measuring the temperature on site.

4.23 uses a fiber optic liquid level alarm, which is a fixed-point liquid level detection instrument developed based on the principle of light refraction and reflection. It is suitable for fixed-point liquid level detection and control, is resistant to high pressure, anti-pollution, safe and convenient, and has a long service life.

Regarding software, its operating system can adopt Windows series platform; the monitoring software can use configuration monitoring software.

The monitoring system realizes automatic monitoring, display, alarm, printing and other functions, laying a foundation for the modern management of oil tank farms. The system has convenient and concise human-computer dialogue, flexible and easy to expand system structure, and high system reliability. It should be said that the system has high reliability and good promotion value.

By making full use of the characteristics of CAN network and optical fiber sensors, the entire system can be expanded to a real-time monitoring system for the liquid level and temperature signals of the oil tanks in the remote oil depot.

5. Conclusion

Because fiber optic sensors and measurement technology are the new development direction in the field of instrumentation today, there are many types of fiber optic sensors used for measurement and many working modes. You should carefully consider the specific application requirements of the project and make a good choice.