Amplitude Fiber-Optic Vibration Sensor
Vibration sensors are required to determine the presence of a dangerous level of vibration during seismic monitoring of mine and mine openings, monitoring of construction sites and finished facilities and transport infrastructure facilities. The use of electrical sensors is not always possible due to the presence of electromagnetic fields, e. g., near existing electrical generators, or fire and especially explosion-proof requirements.
The material of fiber-optic sensors (FOS) is a dielectric. FOSs do not require electrical power and grounding , therefore they can be used in explosive environments without the risk of an electric spark; they offer good accuracy and performance; they may have a small size (up to 0.1 cm2 for a Bragg sensor); they have low cost, with the possibility to be placed at a far distance from the recording equipment.
Amplitude modulation of the optical signal in the proposed sensor occurs through the violation of total internal reflection by bending.
In FOS on bending of a fiber, a change in external factors leads to a change in sensor parameters. Depending on the design of the sensing element, these changes lead either to periodic bending of the fiber (bends can be divided into micro- and macro-bends) or to a decrease in the diameter of the bent fiber, which leads to additional losses of radiation power.
A modulator based on a curved fiber is applicable for operation at frequencies up to several kilohertz, which is enough to measure the vibration that occurs, e. g., when the motors rotate (1500 rpm = 25 Hz). In sensors of this type, conventional optical fibers can be used; however, to obtain a higher sensitivity, it is necessary to design special optical fibers.
When an optical fiber bends along a small radius, the condition of total internal reflection is violated, as a result of which radiation from the core enters the fiber cladding, propagates through it, or scatters into the surrounding space. Then the power of the transmitted signal drops. There are two reasons for such losses :
• displacement of the mode spot from the fiber axis at the beginning of the bend;
• in a bent fiber, the peripheral part of the mode propagates at a speed exceeding the speed of light in the cladding. This part of the mode is radiated into the cladding and is lost.
The magnitude of the power loss caused by the first cause depends only on the radius of curvature of the fiber, and the second on the radius of curvature and the length of the bent portion.
In , various issues related to the existence of modes propagating in a cladding in optical fibers were considered. Point and approximate methods for calculating the distribution profiles of cladding modes are introduced. In , the dependences of losses on micro- and macrobends are introduced. The microbend method was used in the construction of position, force, pressure, mechanical stress, acceleration, and vibration sensors. This group of sensors also includes a fiber-optic accelerometer . An example of a FOS on macrobending is a fiber-optic device for measuring the refractive index .
The purpose of this work was the development and quality control of the vibration sensor.
CONCEPT OF AMPLITUDE FIBER-OPTIC VIBRATION SENSOR
The developed vibration sensor, is a point amplitude fiber-optic sensor. Its functioning is based on the violation of the law of total internal reflection. The sensing element is a section of the optical fiber, made in the form of a loop (4) attached to the base of the sensor body (2). The housing, in turn, is attached to the controlled surface. The weight (3) is fixed on the loop of the optical fiber (Fig. 1). The breakers (5) are installed along the sensitive fiber to dampen horizontal vibrations.
The principle of operation of the device is as follows. Radiation from the source is fed to a sensitive element along fiber 1, resistant to bends. The presence of vibration causes vertical oscillations of the load, which in its turn, leads to a change in the diameter of the bent fiber (Fig. 2). Part of the radiation passing through the curved fiber 4 leaves the fiber due to the violation of total internal reflection. Therefore, the power losses of the radiation passing through the sensing element increase. The radiation propagates further through the fiber and hits the photodetector.
An experimental installation was implemented to measure power losses (Fig. 3. a). The flow-chart of the setup (Fig. 3.b) includes a radiation source (L), Thorlabs SFL1550S laser with an operational wavelength of 1550 nm; sensitive element (SE), implemented from Corning SMF‑28 Ultra optical fiber in the form of a loop with a diameter of 18 mm; a weight with a mass of 70 mg; photodetector (PD), Thorlabs RM‑200 (S146C (InGaAs), ∆P = 10 µW−20 W); personal computer (PC) and source of vibration (VS), portable speaker.
The automated data collection from the proposed vibration sensor was implemented using the developed software for the photodetector with the subsequent software processing.
To test the performance of the proposed sensor design, the dynamics of changes in power during the action of the vibration source were studied, the results are shown in Fig. 4. It can be seen from the plot that the average power of the transmitted radiation under the influence of vibration changes by ΔР, and a variation of the magnitude of power δР appears. Peaks during vibration are caused by switching the sound file to the beginning. After the vibration is stopped, the value of the power value returns to the previous value.
The dependences of the power change on the amplitude are shown in Fig.5. Analyzing the plot, it can be found that the experimental lines obtained by repeating the experimental conditions overlap each other, i. e., the results are consistent and change in magnitude of the power clearly depends on the change in the amplitude of oscillations.
An optical fiber (OF) was examined for the presence of microcracks at the time preceding the series of experiments. Then, this study was repeated after the experiments. In this case, the OF was examined after its use for 60 minutes and after its use for a month. During operation, microcracks appear in the optical fiber of the sensing element. However, it should be noted that this does not affect the performance of the sensor, since with prolonged use of the OF, the sensitive element does not fail during operation. Obviously, the life of a particular SE will depend on the external conditions, the amplitude of vibration and the type of fiber used.
In the course of the experiments carried out, it was established that the created vibration sensor and the measurement system register the vibration, deliver consistent results, and the SE does not fail during operation. Thus, the proposed design has confirmed its performance.
The sensitivity of the sensor can be estimated using the data shown in Fig. 5. With the vibration amplitude A ≈ 1 mm and the frequency f = 80 80 Hz, the vibration acceleration is ав = A (2 π f)2 = 25 g, where g is the acceleration of gravity. The change in power ΔР0.1 with an amplitude of 0.1 mm corresponds to a vibration acceleration of 2.5 g, with 0.3 mm – 7.5 g. The sensitivity of the amplitude sensor in this case (ΔP0.3 – P0.1) / ΔaB = 0.18 mW / g.
A device has been created that allows continuous recording of vibrations of the controlled object. The sensor eliminates the need to pre-set the operating point before operating the device. Thus, the arsenal of technical means for registering vibrations is expanding. A distinctive feature and a possible advantage over piezoelectric sensors, is the possibility of operating the SE with large (up to several tens of centimeters) vibration amplitudes.
Currently, a prototype FOS of vibration has been created, laboratory tests have been conducted in the range of vibration frequencies 5–100 Hz, vibration amplitudes 1–100 mm, the sensor sensitivity has been 0.18 mW / g. A patent has been obtained for the proposed sensor design .
The use of the sensor in the systems for measuring the level of vibration makes it possible to evolve the development of measuring systems in the following directions: the improvement of the SE; the development of an optoelectronic device providing input / output of stabilized radiation and processing (amplification and integration); the software improvements; the planning of vibration level measurements on models of construction objects and on natural construction objects.