Study Of Quasi-distributed Liquid Level Sensor Based On Plastic Optical Fiber

As the optical fiber sensor technology is not as fast as optical fiber communication technology in the industrial production. A number of key technology also are at laboratory stage, and it is a long term away from commercialization. So it is important to research the optical fiber sensor technology. As the liquid level measurement is widely used in industry, the liquid level measurement is also the focus of optical fiber sensor technology. Meanwhile, the plastic optical fiber has many advantages, so the study based on plastic optical fiber is bound to play a significant role in the liquid level measurement field.The liquid level sensor based on plastic optical fiber and sensor system are Designed, and a good experimental result is achieved.1. The designing of quasi-distributed liquid level sensorThe quasi-distributed liquid level sensor is shown in Figure 1. The refractive index changed based on the curved side profiling POF, which will lead to the attenuation of optical power. If we process the side profile after multi-turn winding, it will cause a process of several optical power attenuation due to the changed refractive index. To improve the sensitivity of plastic optical fiber sensor, this paper ues the method of tightly wound multi-turn. Because it is closely wound, the measurement of the sensitivity can be improved to the length of its diameter. The length of diameter is also the minimum measurement value of the sensor. We named this kind structure of sensor"quasi-distributed liquid sensor". To make the liquid level sensor useing this method, the diameter of 1.0mm and tightly wound method will also make the sensor sensitivity improved.In this paper, the diameter of 1.0mm of PMMA POF, the designing of quasi-distributed plastic optical fiber sensor is shown in Figure 2.2 Automatic stepping motor side profiling control technology and the determination of best depth value(1) Automatic stepping motor side profiling control technologyFigure 3 shows the side profiling process plants: Stepper motor can be connected to the computer and the parameter include: running speed, the distance of running forward, the distance of running backward, the current memory lacation and returning to the original point. After setting the parameters, it will drive the stepper motor in some operation rules. Using the stepper motor side profiling control process can be achieved automatic side profiling technology, and the uniformity of the sensor will be guaranteed. So that the sensitivity of the standard is guaranteed. At the same time, it solve the uneven depth of each unit, and it will save time.(2) The determination of best depth valueWe measure the depth value through the microscope, the different depth corresponds to the two consecutive changing level in height (the height variation of a uniform standard, the paper selected this range 10mm-11mm). The voltage difference is related to the best side profile depth value about the minimum sensitivity. The experimental data are in Table 1. 3. Scattered light affect on the experiment result This experiment produced the scattered light source consists of two parts: In the communication process of the winding optical fiber, it will leak light in cladding, this is defined as the winding scattering; the other part is in the side profiling control part it will leak light, which is defined as the side profiling control scattered light. The two parts of light affect the external environment, some of which will re-enter the side profiling control part and continue propagating in POF. This process is main reason of the opposite result. In this paper, two experiments are designed to prove whether the two parts of the scattered light will play a role to the results.(1) The wound scattering light do not affect the experiment resultsThis paper designs an experiment to verify whether the wound scattering light is the main factor of the result, the designing principle is shown in Figure 4.The uniform standard in the experiment: the side profile depth for the POF sensor is 200μm and voltage value is 100mv, we just determined to the voltage change of placing the same water level(24.0mm). Experimental data in Table 2. After the measurement, we get that the voltage value is 168mv. The voltage value of the sensor that was not blocked in the same parameters and incident power is 168mv after placing the same water level. The two voltage value does not change. We can, therefore, come to the conclusion that winding scattering light has no effect on this measurement.(2) Side profiling control scattered light affect the experiment resultsMeasurement process: First, placing the sensor in the container, the voltage value corresponding to optical power is 100mv. Open the external source of scattering light , the voltage value is 112mv. Turn off the scattering light source, and inject water to the container for 24.0mm, the voltage value is 168mv. And then open the scattering light source, the voltage value is 188mv. Experimental data is shown in Table 3.From the experimental results, the voltage value is increased for 12mv by adding scattering light, which is because part of the scattering light enter the fiber and propagate in the fiber. But the voltage value is 188mv by adding the scattering light after inject water in the container. There is a difference of 8mv in the two results. It proves that the Rayleigh scattering effect in the water is more active than the effect in the air. In the paper, the greater part is 8mv. (3) ConclusionThe conclusion can be drawn through these two experiments: the impact on the results is the scattering light for the side profile. And the side profiling control scattering light is the main reason for the measurement results. We can come to a further conclusion that the Rayleigh scattering effect in the water is more active than the effect in the air. 4.The integral plastic optical fiber liquid level sensor systemTaking the experiment into account, there are many factors that influence the results, such as the light source: the aging, the different output power caused by the instability of output power; Optical path: the optical fiber bending, mechanical vibrations or loss caused by changing in ambient temperature. These two experimental results will affect the accuracy of experiment, so we will consider the intensity compensation to improve the experimental precision.The integral plastic optical fiber liquid level sensor system is shown in Figure 5Figure 6 shows the curve of voltage value and water level, as long as we get the voltage value, we can achieve the corresponding water level. As the intensity compensatory measures we have taken, the experimental error has reduced. And the vertical axis (water level) value vary from 1.0mm to 5.0mm, covering the maximum value 24.0mm. The curve will make sense because of the improvement of the precision.