附件外文翻译
Optic fiber-based dynamic pressure sensor for WIM system
ShenfangYuan a, , , FahardAnsari b, XiaohuiLiu a and Yang Zhao b
a The Aeronautical Key Laboratory for Smart Materials and Structures, Nanjing University of Aeronautics and Astronautics, 29 YuDao Street, Nanjing 210016, China
b Department of Civil and Materials Engineering, University of Illinois at Chicago, Illinois, IL 60607, USA
Received 16 August 2004;
accepted 10 November 2004.
Available online 15 December 2004.
Abstract
An optic fiber-based dynamic pressure sensor is described here to measure weight-in-motion of vehicles. In the research reported herein,    a Michelson interferometer with specially designed hardware
and software were developed and experimentally subjected to dynamic compressive loads of different magnitudes, and loading rates. Experiments showed that both output fringe number and fringe period could be used to indicate the dynamic load. A calibration technique has been put forward to calibrate the sensor. Both the dynamic weight and static weight of the vehicle passed can be obtained. The findings that resulted from these studies developed an understanding for the behavior of interferometer sensor under dynamic compressive states of stress and are fundamental to the application of fiber optic sensors for the monitoring of truck vehicle weights while in motion.
Keywords: Optic fiber sensor; Dynamic pressure; Weight-in-motion; Hardware and software
Article Outline
1. Introduction
2. The sensor design
2.1. Sensor setup
2.2. Sensor principle
3. Experimental procedures and results
3.1. Experimental setup
3.2. Experimental data
3.3. Repeatability of the sensor
3.4. Calibration of the sensor
3.4.1. Calibration of the static weight
3.4.2. Calibration of the dynamic weight
4. Conclusion
References
Vitae
1. Introduction
The need to weigh vehicles in motion, applied especially to traffic control, has
grown substantially in the past decades. Several techniques for weighting vehicles in-motion are now used including piezoelectric cables, capacitive mats, hydraulic and bending-plate load cells [1]. Hydraulic and bending-plate load cells offer high accuracy (1–5%) and dynamic range, yet suffer from high installation costs and size constraints. The piezoelectric and capacitive mat techniques are substantially lower in cost, yet are less accurate (5–15%) and do not function properly at speeds lower than 20 km/h [2] and [3]. To offer the required accuracy at reduced installation and maintenance costs, optic fiber-based WIM sensors are now being developed to improve, complement or even replace the ones currently in use.
Based on the effect of polarization coupling between two orthogonally polarized eigenmodes of polarization-maintaining fiber, Ansari et al. report on using highly birefringence polarization-maintaining (HiBi) fiber for dynamic measurement of pressure with practical ramifications to the determination of weigh-in-motion of trucks [3]. Navarrete and Bernabeu report a multiple fiber-optic interferometer consisting of a Mach-Zehnder interferometer configuration with one of its arms replaced by another Mach-Zehnder interferometer [4]. Cosentino and Grossman developed a dynamic sensor using the microbend theory to test weight-in-motion [5].
The present work describes the development of a dynamic pressure sensor based on the Michelson in
terferometer, which has simple structure, is cost effective and can potentially offer the high accuracy required for many applications. Special hardware and system software based on Labview WINDOWS/CVI are designed to implement the sensor functions, such as eliminating environmental noise, self-triggering of the test procedure and the fringe number and fringe period simultaneous count. Responses of the dynamic sensor are studied when subjected to dynamic compressive loads with different magnitudes and loading rates. Data calibration method is also researched to calibrate the sensor.
2. The sensor design
2.1. Sensor setup
Fig. 1 illustrates schematically the proposed dynamic pressure sensor system. Single-mode optical fiber is used as a sensing element to form a Michelson interferometer. The optoelectronics components of the interferometer consist of a laser operating at wavelength of 1550 nm, a laser isolator and a photodiode. The sensor is made of communication grade optical fiber (Corning SMF28). The output signal from the detector-amplifier is first fed to a special hardware circuits including a two-order high pass filter, a zero-point detection circuit and a Schmitt Trigger circuit. The hardware circuits
are designed to implement the following functions: (1) self-diagnose the arrival time of the vehicle to self-trigger the measurement process;
(2) provide function to eliminate the low frequency disturbances, such as temperature influences and slow changes of the element's performances; (3) provide function to reduce the noise to a frequency band similar to the useful fringe output of the Michelson interferometer. One possible source of this noise is caused by vehicles passing in a near-by lane. Since the output of the Michelson interferometer under pressure is fringe which can be considered as high frequency signal comparing to the noise caused by temperature changes, laser and diode performance change and other
low frequency environmental influences, a two-order high pass filter was adopted to eliminate those low frequency components. A zero-point detection circuit is designed to change the sine-form fringe to pulse signal for the counter in the computer data acquisition system to count the fringe number and measure the fringe period. The self-trigger function is accomplished by the Schmitt circuit. The threshold voltage of the Schmitt circuit is set according to experiments to distinguish the real fringe signal caused by vehicles and the pseudo fringe signal caused by small vibration in the test environment. In practical application, this could be caused by the passing vehicles in adjoining lanes. System software based on Labview Windows/CVI is designed to set measurement parameters, control
the test procedure and display results.
Full-size image (14K)
Fig. 1. Dynamic fiber optic pressure sensor setup.
2.2. Sensor principle
One arm of the Michelson interferometer is subjected to a distributing dynamic load L d(t). The generalized stress–optic relationship between the optical path change Δl and the strain induced over the gauge length can be derived as Eq. (1)[6]:
(1)
and
(2)
where P11and P12are the Pockels constant; l the length(gauge length) of the optical fiber within the pressure field; t x, t y and t z correspond to the mechanical and geometrical property of the optical fiber and the host material-epoxy.
By measuring the deformation of the fiber, the strain in the host material can be measured. The strain is linearly proportional to the external applied pressure p. Consider αas a constant of proportionality between the pressure and the change in length of the fiber Δl, then
(3)
and
(4)
Thus, the output fringe number of the Michelson interferometer is
(5)
where N f is the output fringe number; λ the wavelength of the laser light.
The fringe period can be deduced by Eq. (6):
(6)
where T f is the fringe period; t the loading time, approximately equal to the duration time of the optic fiber fringe.
3. Experimental procedures and results
3.1. Experimental setup
The experimental setup is shown in Fig. 2. A steel chamber is designed to contain the optical fiber. The optic fiber is sandwiched between two 1 cm thick stiff rubber pads and glued to one of the two pieces. Rubber pads are necessary for the protection of fiber from damage. The cross-sectional area of the loading surface for the steel chamber is 247.1 mm × 24 mm. The gauge length of the optical fiber pressure sensor is 247.1 mm. A closed-loop materials testing system (MTS) is used for the application of pressure. Ramp function load with different loading rates and magnitudes are chosen as dynamic loads to simulate the weight loads caused by moving vehicles on road.
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Fig. 2. Experimental setup.
Fig. 3 shows a typical loading profile and fringe output of the interferometer during the duration of the applied ramp function load. As can be seen, the number of fringes at left corner of Fig. 3 is so great that it is hard to distinguish each fringe. Thus, another figure on the right is used. The figure is the enlargement of the circled area to show clearly the fringes. Because of the polarization effect, the ampl
itudes of fringes slightly vary. But will not influence the fringe count and period, thus neglected in this paper. In the experiment, the pressure load is applied to the chamber at 7.5 MPa increments up to the maximum pressure level of 30 MPa, corresponding to load increments from 44.5 kN to a maximum of 178 kN. The loading time is applied starting from 1 to 6 s with an increment of 1 s.
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Fig. 3. Typical loading procedure and fiber optic sensor output waveform.
3.2. Experimental data
Fig. 4 shows the experimental results of the fringe number and fringe period readouts of the sensor output under different loads. Lines with different signs represent relations between the sensor's outputs and the maximum amplitudes of the load under different loading rates.
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Fig. 4. The experimental results: the fringe numbers and fringe periods vs. loads.
The fringe number has a linear relationship with the static load, while the fringe period has a non-linear one. Note that, the relationship differs under different loading rates, since with increasing loading rate, the same maximum amplitude load will turn out to a bigger dynamic load, which causes the increase in fringe number and the decrease in fringe period. Though both the fringe number and the fringe period are sensitive to the dynamic load, their sensitive ranges are different. The sensitivity of the fringe number to load is a constant in the whole testing range. When load is low, the small change of dynamic load may not be recognized by the fringe number. On the other hand, the sensitivity of the fringe period to load is not a constant. When the load is low, the small change of dynamic load corresponds to big change of fringe period. These two parameters can be used together to give a more precise indication of the load tested.
Considering Eqs.(5)and(6), the functions between loads and fringe number and fringe period can be approached as Eqs. (7) and (8), respectively:weigh翻译
(7)
L s=k sn1N f+k sn2
(8)
where k sn1, k sn2, k st1 and k st2 are the parameters approached.
3.3. Repeatability of the sensor
Three experimental results under the same loading conditions are compared in Fig. 5 demonstrating that the dynamic fiber optic pressure sensor has good repeatability.
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Fig. 5. Illustration of the repeatability of the optic fiber sensor.
3.4. Calibration of the sensor
According to the fringe number and period of the optic fiber sensor output, the

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