This is the final accepted version of the paper (uncorrected proof), uploaded at Research Gate as part of the privileges available to the authors as part of the copyright agreement. Please visit : http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1747-1567 for the final authentic version of the paper. © Dr. Suresh Bhalla
A COST EFFECTIVE APPROACH FOR TRAFFIC MONITORING USING PIEZO-TRANSDUCERS
Suresh Bhalla1 and Swapan Kumar Deb2 Department of Civil Engineering, Indian Institute of Technology Delhi Hauz Khas, New Delhi 110016 (INDIA)
ABSTRACT This paper presents the development and testing of a low-cost system for traffic monitoring based on piezoelectric ceramic (PZT) transducers. The PZT transducer is embedded inside a concrete beam, 100x100x500 mm in size, in turn buried inside the carriage way width. The sensor generates voltage signal when the axle of a vehicle presses against the beam. The embedding configuration imparts higher sensitivity due to the simultaneous invoking of d31 and d33 piezoelectric effects. The response of the sensor is tested against a pedestrian, a cycle and a car crossing over the buried beam. The sensor response could successfully detect all the traffic types. In addition, the peak voltage response provides a basis for classification of the vehicle type. Not only the proposed approach employs low cost and easily available PZT patches, the hardware and the data processing requirements are also minimal.
KEYWORDS: Traffic, monitoring, piezoelectric-ceramic (PZT) patch, stress.
INTRODUCTION 1
Assistant Professor (corresponding author) Email:
[email protected], Tel: (91-11) 2659-1040, Fax : (91-11) 2658-1117.
2
Assistant Professor, Email:
[email protected].
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Traffic management is key to proper functioning of any big metropolitan city. This in turn is dependent on the real time assessment of traffic volume as well as its composition at key locations of the city. In general, traffic planners are interested in vehicle count, speed, class, gap, occupancy and weight. A reliable and cost effective system to measure these parameters in real time, with no or minimal disruption to the traffic, is very much desirable. The following techniques, currently in vogue, are based on sensors located above ground : video image processing, microwave/ laser radar, infrared, ultrasonic and acoustic techniques. Of these, the vision based video image techniques have received some acceptance from transportation engineers and the research community. For example, Messelodi et al1 recently proposed an algorithm employing support vector machines for vehicle classification using video image processing. However, the image based techniques are not only expensive, but involve complex processing of the measured data. In addition, location of the sensors above the ground renders them susceptible to vandalism and frequent maintenance and warrant special protective measures. The associated hardware costs as well as the operational and maintenance costs are typically exorbitant.
Apart from the above ground sensing techniques, there are techniques which require the sensors to be buried below the pavement. These include magnetometers, microloop probes, pneumatic road tubes, fibre optic sensors and other weigh-inmotion sensors. These sensors might warrant disruption of traffic for installation and repair. A fraction of such sensors fail to work properly after installation. However, being buried, the sensors are less prone to vandalism and to some extent from protected against environmental degradation.
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This paper presents the development and testing of a new piezoelectric-ceramic lead zirconate titanate (PZT) based low-cost traffic monitoring sensors warranting minimum hardware, data processing and maintenance/ operational costs. The sensor’s signal response is much improved due to simultaneous piezoelectric effects along the two axes. The next sections present the physical principles associated with the proposed sensor followed by fabrication, installation and testing.
PHYSICAL PRINCIPLES The PZT patches, which are the key elements of the technique proposed in this paper, are made up of materials which exhibit the phenomenon of piezoelectricity2. The characteristic feature of the ‘piezoelectric’ materials is that they generate surface charges in response to an applied mechanical stress, which is called the direct effect; and conversely, undergo mechanical deformations in response to an applied electric field, which is called the converse effect. This unique capability of the piezoelectric materials to exhibit ‘stimulus-response’ characteristic qualifies them to be members of the group of so-called smart materials3.
The physical electro-mechanical model
associated with PZT materials is illustrated in Fig. 1. The patch undergoes strain in direction ‘1’ when an electric field E3 is applied in direction ‘3’. The patch has length l, width w and thickness h. The behaviour of the patch is governed by the following constitutive relationships2, 4 T D3 = ε 33 E3 + d 31T1
S1 =
T1 Y11E
3
+ d 31 E3
(1) (2)
where S1 is the strain in direction ‘1’, D3 is the electric displacement (charge density) over the PZT patch, d31 (first subscript indicating the direction of the electric field and second the direction of the strain) is the piezoelectric strain coefficient and T1 the axial stress in the PZT patch . Y11E = Y11E (1 + ηj ) is the complex Young’s modulus of the T T = ε 33 (1 − δj ) is the complex electric PZT patch at constant electric field and ε 33
permittivity of the PZT material at constant stress, with η and δ respectively denoting the mechanical loss factor and the dielectric loss factor of the PZT patch. Eq. (1) describes the direct effect and Eq. (2) the converse effect associated with piezoelectric materials. The direct effect has been utilized in numerous sensing applications of practical importance4. During the last decade, the PZT patches have also emerged as impedance transducers for detecting practically all types of
damages5-7 for civil/
mechanical/ aerospace structures.
In pure sensing applications, such as the present context of traffic monitoring, the external electric field is absent, i.e., E3 = 0. Using the theory of parallel place capacitors8, the charge density can be related to the potential difference V across the terminals of the PZT patch as D3 =
ε 33T V
(3)
h
Hence, from equations (1) to (3), noting the fact that E3 =0,
with
⎛ d hY E ⎞ ⎟ S = kS V = ⎜ 31 1 ⎜ εT ⎟ 1 ⎝ 33 ⎠
(4)
⎛ d hY E k = ⎜ 31 ⎜ T ⎝ ε 33
(5)
4
⎞ ⎟ ⎟ ⎠
where the constant k depends upon the parameters of the PZT patch. Thus, the output voltage across the terminals of the PZT patch is proportional to the strain of the host structure. The output voltage can be easily measured by oscilloscopes supported by conditioning circuit4. In the present study, a low cost interrogation system, namely the Agilent 34411A digital multimeter9, which can directly measure the voltage across high impedance devices (thus no requirement of conditioning circuits) has been employed. This can be controlled remotely and can make real-time measurements using the VEE PRO9 easy to program software environment.
SENSOR FABRICATION, INSTALLATION AND TESTING Fig. 2 shows the PZT patch, 10x10x0.3mm in size, manufactured by PI Ceramic10 , conforming to grade PIC 151, used as the basic sensing element. The key feature of this particular patch is that both the electrodes are available on the same face, which facilitates hassle free bonding of the other surface to the component to be monitored. In order to be used as traffic monitoring sensor, wherein rugged conditions are encountered, the patch was embedded inside a disk of concrete of diameter 25mm and thickness 12mm, as shown in Fig. 3(a). Fig. 3(b) shows the voltage signal generated by the application of radial hand pressure on the disk three times in succession. Typically, a peak voltage of the order of 1 volt is generated by the mere act of pressing the sensor by hand, thereby demonstrating the high sensitivity of the sensor. Two such disks were then embedded inside a concrete beam of dimensions 100x100x500mm, as shown in Fig. 4(a), at the time of casting. The procedure of embedding the PZT patch in the disk first rather than directly in the beam was followed to protect the patch from damage during the concreting procedure. The
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beam was supported on two bricks at the ends and buried inside ground, as shown in Fig. 4(a). Fig. 4(b) shows the condition at the ground surface after burying the beam.
In the present condition, under an external load, such as the one produced by the wheel of a vehicle, stresses are generated inside the beam. The PZT element is subjected to multiaxial stresses as shown in Fig. 5, represented by the T1 and T3 along the two principal directions. Hence, d33 effect is also introduced, modifying Eq. (1) as (note that E3 = 0)
D3 = d 31T1 + d 33T3
(7)
Thus, the voltage output across the PZT patch is expected to be higher than in the idealistic 1D case illustrated in Fig. 1.
The sensor system thus installed at the site was tested with different traffic types. Fig. 6 shows the voltage response of the sensor to a pedestrian walking past the beam. Voltage response was measured using the Agilent 34411A digital multi meter (DMM) with a sampling interval of 200µs. It is observed that a peak voltage of 0.06 volts is registered by the DMM. Fig. 7 shows the response of the sensor for a bicycle passing over the beam. The two wheels can be easily identified and the peak voltage across the sensor is higher than the pedestrian, at 0.0858 volts. Fig. 8 show the test conducted by a car, Maruti 800 model, with the sensor response depicted in Fig. 9. Again, the front and the rear wheels can be easily identified and the peak voltage response was measured as 0.171 volts. The results of the tests demonstrate the success of the sensor in not only counting the axels but also in differentiating different traffic types. This can be done on the basis of the peak voltage generated across the PZT patch. The
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necessary calibration can be done by field tests on the sensor and a suitable classification system devised. Trials are currently underway and the results will be published in subsequent publications.
In general, the PZT sensors have a prolonged fatigue life11-13, negligible ageing, fast dynamic response and large range of linerarity. These features make the proposed sensor a desirable one for traffic monitoring. Typically, PZT material has large failure strains of the order of 0.00614. At the same time, the material is chemically inert. The only possible deterioration is that due to water absorption, which could alter the T , though there is no possibility of corrosion. piezoelectric constants d31 and ε 33
Protection against water and strain compatibility is ensured by proprietary encapsulation15. Hence, compared to the conventional sensors, the proposed sensors are expected to be more rugged and at the same time cost effective.
CONCLUSIONS This paper has presented the fabrication, installation and testing of a new type of traffic monitoring sensor based on the piezoelectric effect. Compared to other traffic sensors available in the market, the proposed sensor is very cost effective and warrants minimal data processing efforts and hardware costs. The PZT sensors have well known competitive characteristics such as low cost, fast dynamic response and negligible decay rate. The proposed sensor has successfully passed the test under different traffic types. From field tests, the sensor can be easily calibrated for action under real situations. With the active research and development efforts under way across the world in embedded systems, it could be a possibility that the PZT sensors
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harvest their own power and wirelessly transmit the signals over long distances16. Thus, the proposed sensor holds the promise of widespread use as traffic monitoring sensor.
REFERENCES 1. Messelodi, S., Modena, C. M. and Cattoni, G. “Vision Based Bicycle/ Motorcycle Classification.” Pattern Recognition Letters, 28: 1719–1726 (2007). 2. Ikeda T. Fundamentals of Piezoelectricity, Oxford University Press: New York; (1990). 3. Gandhi M. V., Thompson, B. S. Smart Materials and Structures, Chapman & Hall: London; (1992).
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4. Sirohi, J. and Chopra, I. “Fundamental Understanding of Piezoelectric Strain Sensors.” Journal of Intelligent Material Systems and Structures, 11: 246-257 (2000). 5. Bhalla, S. and Soh, C.K. “Structural Health monitoring by Piezo-Impedance Transducers: Modeling.” Journal of Aerospace Engineering, 17(4): 154-165. 6. Bhalla, S. and Soh, C.K. “Structural Health monitoring by Piezo-Impedance Transducers: Applications.” Journal of Aerospace Engineering, 17(4): 166-175. 7. Soh, C.K., Tseng, K.K. H., Bhalla, S., Gupta, A. “Performance of Smart Piezoceramic Patches in Health Monitoring of a RC Bridge.” Smart Materials and
Structures, 9(4): 533-42 (2000). 8. Halliday D. and Resnick R. Fundamentals of Physics; 3rd ed., John Wiley and Sons: New York (1988). 9. Agilent Technologies. Test and Measurement Catalogue, USA (2009). 10. PI Ceramic, http://www.piceramic.com, Lindenstrabe: Germany, (2009). 11. Winston, H. A., Sun, F. and Annigeri, B. S., “Structural Health Monitoring with Piezoelectric Active Sensors”, Journal of Engineering for Gas Turbines and
Power, ASME, 123(2): 353-358 (2001). 12. Giurgiutiu, V., Reynolds, A. and Rogers, C. A. , “Experimental Investigation of E/M Impedance Health Monitoring for Spot-Welded Structural Joints”, Journal of
Intelligent Material Systems and Structures, 10(10): 802-812 (1999). 13. Bhalla, S., Veljkovic, M. And Vittal, A. P. R., “Residual Fatigue Life Assessment of Bolted Steel Joints Using Electro-Mechanical Impedance Technique”, Working
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Paper, Department of Civil Engineering, Indian Institute of Technology Delhi (2009).
14. Cheng, B. L. and Reece, M. J., “Stress Relaxation and Estimation of Activation Volume in a Commercial Hard PZT Piezoelectric Ceramic”, Bulletin of Material Science, Indian Academy of Sciences, 24(2): 165–167. 15. Bhalla, S. and Gupta,. A. “A Novel vibration Sensor for ConcreteStructures”,
Invention Disclosure, 26 February 2007, Foundation for Innovation and Technology Transfer (FITT), Indian Institute of Technology Delhi, (2007). 16. Overly, T. G. S., Park, G., Farinholt, K. M. and Farrar, C. “Development of an Extremely Compact Impedance-Based Wireless Sensing Device.”
Smart
Materials and Structures, 17, article no 065011(2008).
LIST OF FIGURES Fig. 1 A PZT patch under mechanical stress and electric field. Fig. 2. A typical commercially available PZT patch. Fig. 3 (a) PZT patch encased in concrete disk. (b) Voltage response to hand pressure applied on disk encasing PZT patch. Fig. 4 (a) Placement of disk in concrete beam. (b) Embedding of beam below ground and connection to DMM. Fig. 5 The PZT patch under multiaxial mechanical stress. Fig. 6 Sensor response to a pedestrian.
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Fig. 7 Sensor response to a bicycle. Fig. 8 Test vehicle (car) passing over beam. Fig. 9 Sensor response to a car.
3
E3
2 1
w T1
ha
l
Fig. 1 A PZT patch under mechanical stress and electric field.
11
10mm
10mm
Top electrode film Bottom electrode film wrapped to top surface
Fig. 2. A typical commercially available PZT patch.
12
(a)
0.8 0.6
13 0.4
(b) Fig. 3 (a) PZT patch encased in concrete disk. (b) Voltage response to hand pressure applied on disk encasing PZT patch.
Axle of vehicle
Carriageway
300mm 500mm
Encapsulated PZT Sensor
100mm 100mm 70mm
300mm
(a)
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(b) Fig. 4 (a) Placement of disk in concrete beam. (b) Embedding of beam below ground and connection to DMM.
3 2 1
T
w
T1
h T3
l
Fig. 5 The PZT patch under multiaxial mechanical stress.
15
0.06
0.055 volts
Voltage (V)
0.04 0.02 0 ‐0.02 ‐0.04 0
0.5
1
1.5 Time (s)
Fig. 6 Sensor response to a pedestrian.
16
2
2.5
3
0.1
0.06
Voltage (V)
0.0858 volts
Rear wheel
0.08
Front wheel
0.04 0.02 0 ‐0.02 ‐0.04 ‐0.06 0
0.5
1
1.5
2
Time (s)
Fig. 7 Sensor response to bicycle.
17
2.5
3
Fig. 8 Test vehicle (car) passing over beam.
18
0.15 0.1
Voltage (V)
0.05 0 -0.05 -0.1 -0.15
Front wheel -0.2 0
1
Rear wheel 2
-0.171 volts
3 Time (S)
19
4
5
6
Fig. 9 Sensor response to a car.
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