A preliminary research on wireless cantilever beam vibration sensor in bridge health monitoring

Abstract

According to specific bridge environment, optimal design piezoelectric cantilever beam structure by using results of theoretical calculations and simulation, verify natural frequencies of piezoelectric cantilever beam and production ability of data by experiment, thus formed a complete set of design method of piezoelectric cantilever beam. Considering natural frequency of vibration and intensity of the beam body, design a new type of piezoelectric cantilever beam structure. Paper analyzes the principle of sensor data acquisition and transmission, design a hardware integration system include signal conversion module, microcontroller module and wireless transmission module, test local read and wireless transmission for the combination structure of cantilever beam and data collection card, experimental verification of the radio piezoelectric vibrating cantilever vibration response is intact, the beam produced signal by vibration, acquisition card converts and wireless transmit data, this proved a good and intuitive linear response in simulation of bridge vibration test. Finally, the paper designed a kind of new wireless sensor of vibration cantilever beam, suitable for small bridge health monitoring based on Internet of things.

Keywords

piezoelectric cantilever beam / bridge / natural frequency / wireless sensor

Introduction

Since 1980s, more and more civil engineering were built. The number of houses, bridges and roads is increasing. At the same time, people are acutely aware of the importance of engineering quality because of the frequent occurrence of active service bridge serious safety hazards and accidents, and the analysis ^[1,^2]and maintenance management level on infrastructures needs to be improved ^[3]. It is also more aware of the importance of the health monitoring for the main objective of the operation stage of the bridge in order to improve the management level and safety assurance ^[4]. At present, the sensor used in the field of industrial measurement and control especially acceleration sensor used in long-term real-time monitoring of large-scale structure, which more use inertial displacement sensor and matching amplifier. It has the disadvantages of poor anti-interference ability, easily aged, poor reliability, slowly dynamic response and high cost ^[5–^7], and it easily to cause data distortion. It is difficult to meet the requirements of long term, real time, on-line automatic perception of structural parameters in the industrial control field.

Vibration sensors are mainly used to produce the corresponding data by sense the vibration of bridge or road and. On the process of the study of piezoelectric materials, we found that through the design of the piezoelectric cantilever beam, the vibration mechanical energy can be converted into electrical energy ^[8,^9], which can not only supply power, but also can be converted into digital signal by A/D conversion, and then have the data for wireless transmission. The vibration situation of the target can be analyzed through the data collected,that will get the health status of the target ^[10].

In order to make up for the deficiency of the existing research on infrastructures ^[11], the design method of a set of wireless vibrating cantilever beam sensor applied in the bridge environment is designed, optimize design the cantilever beam structure in accordance with the vibration environment of the small bridge within 20m ^[12]. Based on the frequency of the bridge and the vehicle vibration frequency, the design target range of natural frequency of the cantilever beam is determined. Then we analyzed the mode of the cantilever beam and designed the cantilever beam. The piezoelectric cantilever beam is combined with the data acquisition, conversion, processing and transmission system, and finally a wireless vibration cantilever beam sensor is formed ^[13–^14].

Design of piezoelectric cantilever beam structure

A main application way of piezoelectric transducer is placed below the bridge in the bridge ambient, piezoelectric transducer can along with the bridge vibration and turn mechanical energy into electricity when the vehicle passes bridge. Rectangular structure of piezoelectric cantilever beam is a kind of piezoelectric transducer structure application more, this paper have a study in the design method of rectangular double-crystal piezoelectric cantilever beam, the traditional double-crystal piezoelectric cantilever vertically divided into three tiers, upper and lower layers is piezoelectric patch, the middle layer is metal substrate of the beam, it will Produce deformation When vibration, The piezoelectric patches of upper and lower layers can be formed electrical output in series ^[15,^16]. in this paper, the conventional double-crystal piezoelectric cantilever beam structure is improved, Let the metal substrate in the end of the piezoelectric cantilever beam extending a portion, Place the mass block at the end of the metal substrate extension portion, as shown in Fig. 1.

The establish of cantilever beam material and size parameters

In general, the fundamental frequency of bridge which single-hole span in range of 20~40m is much about 3-5 Hz, the fundamental frequency of simply supported beam bridge less than 20m is about 1 to 4 times to the fundamental frequency of 20-40m bridge. Therefore, considering the fundamental frequency and the load frequency of the bridge ^[17,^18], we choose 5-15 Hz as the natural frequency of the piezoelectric cantilever beam.

According to the target range of the natural frequency of the piezoelectric cantilever beam, we select the suitable materials which are used in the test. In this paper, we adopt brass as the metal substrate of the piezoelectric cantilevers and select the non-polarized piezoelectric ceramic as the mass block. PZT-5H piezoelectric ceramic is used as the material of the upper and lower layers of the piezoelectric plate, which has large dielectric constant and piezoelectric coefficient^[19,^20].

We build an initial model of piezoelectric cantilever after defining the each part of piezoelectric cantilevers’ element types and material properties. The initial model parameters are as follows: the length of the beam (L) is 100 mm; the width of the beam (B) is 40 mm; the total thickness of the beam (H) is 1.5 mm; the thickness of metal substrate (h_m) is 0.5 mm; the thickness of piezoelectric plate (h_p) is 0.5 mm; the quality of additional mass (m) is 5 g. After improvement, the metal substrate of the initial model of the piezoelectric cantilever is extended out 20 mm on the base of the upper and lower layers of piezoelectric patches, and the extension part is included in the beam length L.

Influence of cantilever beam parameters on natural frequency

The main target of piezoelectric cantilever structure optimization designed is the natural frequency can reach the target range 5–15 Hz of small bridge environment design, So that the piezoelectric cantilever beam application to can produce accurate response application on the bridge. Modal analysis the finite element model of piezoelectric cantilever beam, and extract the first four order vibration mode, as shown Fig. 2.

The first order natural frequency of cantilever obtained according modal analysis is 71.85 Hz, second order is 343.31 Hz, third order is 382.90 Hz, fourth order is 892.37 Hz, While the range designed for the natural frequencies of cantilever beam in this paper is 5–15 Hz, even lower than first order natural frequency of vibration, and only the first order vibration mode is a standard form of bending vibration in the four order vibrations, the first order natural frequency of the piezoelectric cantilever is more easily reduced to the design target range of 5–15 Hz, the vibration mode is more suitable for the application of bridge vibration, and it can produce the corresponding voltage signal of vibration. So the first order vibration of the piezoelectric cantilever is the most suitable for the small bridge.

Every parameter of cantilever beam will have an effect on its natural frequency, it is must consider what kind of impact will the parameters of the beam on the natural frequency of vibration if we want to have a perfect linear response of cantilever beam to bridge, and choose the most suitable parameters for the manufacture of the beam from the results.

Let the beam length L from the beginning of 80 mm, every increase in 10 mm until 170 mm so far, and the other parameters remain unchanged.

Let the beam width B from 10 mm, every time add 10 mm up to 70 mm, and the other parameters remain unchanged.

Let the total thickness of the beam body from the beginning of 0.8 mm, every time to add 0.1 mm up to 2.0 mm, and the other parameters remain unchanged.

Let the quality of the mass from the beginning of 3g, every time to add 1g up to 13g, and the other parameters remain unchanged.

Have a finite element modal analysis for the four designs above, draw the relational graph of every parameter with their natural frequency(Figs. 3-6).

It can be seen from the above diagram, square of beam length is inverse proportion to the natural frequency, total thickness is proportional to vibration frequency, which is closer with natural frequency theoretical formula and no mass bimorph piezoelectric cantilever, the formula as shown in Eq. (1) [¹⁵]:

(1)f={\displaystyle \frac{{{\displaystyle \lambda }}_{\mathrm{1}}^{\mathrm{2}}H}{\mathrm{4}\pi L^{\mathrm{2}}}}\sqrt{\left\{{\displaystyle \frac{{{\displaystyle E}}_p[{\mathrm{8}\alpha }^{\mathrm{3}}-{\mathrm{12}\alpha }^{\mathrm{2}}+\mathrm{6}\alpha +\beta {(\mathrm{1}-\mathrm{2}\alpha )}^{\mathrm{3}}]}{{{\displaystyle \mathrm{3}\rho }}_p[\mathrm{2}\alpha +q(\mathrm{1}-\mathrm{2}\alpha )]}}\right\}}

In the formula, L and H are the length and the total thickness, there without these two parameters of beam width and quality of mass, and with the increase of these two parameters, the natural frequency of vibration has been largely unaffected to the last, it illustrates a smaller effect of the width of beam and mass on natural frequencies.

The establish and design of cantilever beam

From the natural vibration frequency of initial model, we can get the estimation formula which reflects the relationship between the natural vibration frequency of piezoelectric cantilever beam and the length and total thickness of beam, as shown in Eq. (2).

(2){{\displaystyle f}}_{\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}={{\displaystyle f}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}\times {\displaystyle \frac{{{\displaystyle f}}_L}{{{\displaystyle f}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}}\times {\displaystyle \frac{{{\displaystyle f}}_h}{{{\displaystyle f}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}}={\displaystyle \frac{{{\displaystyle f}}_L\times {{\displaystyle f}}_h}{\mathrm{71.85}\mathrm{H}\mathrm{z}}}

Where, {{\displaystyle f}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}——the natural vibration frequency of initial model,equal to 71.85 Hz; {{\displaystyle f}}_{\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}} is the estimation of the natural vibration frequency of piezoelectric cantilever beam, Hz; {{\displaystyle f}}_L is the natural vibration frequency corresponding to the length of beam in relation graph, Hz; {{\displaystyle f}}_h is the total thickness of the beam corresponding the natural frequency in relational graph, Hz.

In the finite element simulation stage, we select a higher part 10-15 Hz in the design range of natural vibration frequency as the target range, and then we choose 20 Hz as the estimation target of natural vibration frequency after the length and total thickness of beam having changed. So there is the estimation formula, as shown in Eq. (3).

(3){{\displaystyle f}}_{\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}={\displaystyle \frac{{{\displaystyle f}}_L\times {{\displaystyle f}}_h}{{{\displaystyle f}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}}={\displaystyle \frac{{{\displaystyle f}}_L\times {{\displaystyle f}}_h}{\mathrm{71.85}\hspace{0.17em}}}\le \mathrm{20}\hspace{0.17em}\mathrm{H}\mathrm{z}

Analysis the relational graph of beam length and natural frequency, choose the corresponding length of beam when natural frequency is changing slightly, so choose the beam length is 150 mm, the corresponding natural frequency of vibration is 36.88, it can obtain {{\displaystyle f}}_h\le \mathrm{38.96}\hspace{0.17em}\mathrm{H}\mathrm{z} from the Eq.(3), and according to the relational graph of total thickness and natural frequency, we can get the upper limit of total thickness of 1.2 mm.

Make the ratio of metal substrate thickness to total thickness as the independent variable parameters, measure voltage signal data generated by cantilever beam(Table 1).

Select the thickness corresponding to the voltage of the minimal data change as the design parameter of cantilever beam. It can see from the data, the thickness ratio of 0.4 was a turning point, data before 0.4 haven’t changed much, it began to fell sharply after o.4, so choose the thickness ratio of 0.4 can ensure the cantilever has a better natural frequencies and intensity of beam body.

In the foregoing analysis, according to the estimate target 20 Hz of natural frequency, obtained the length upper limit of new piezoelectric cantilever beam is 150 mm, the upper limit of the total thickness of 1.1 mm. At the same time we got the best thickness ratio is 0.4, then we can work out the thickness of piezoelectric patch and metal substrate. The beam total thickness calculated must be combined with the width of beam. We choose the beam width of 40 mm, total thickness of 0.85 mm by comprehensive consideration, and calculate with this thickness ratio of 0.4, we can get the metal substrate thickness of 0.35 mm, the piezoelectric patch thickness of 0.25mm.

The natural frequency estimated is 14.90 Hz according to type 3 after the beam length and total thickness of model is improved, Combined the simulation phase target range of 10 to 15 Hz and Fig. 6(the relational graph of mass quality and natural frequency), choose the quality of mass is 12g. Modal analysis the new model, and get the natural frequency is 10.02 Hz.

The design of wireless collection and transmission system

At present, concept of internet of things (IOT) rapidly emerging, different from the traditional wired transmission methods, the wireless sensors are normally used for data transmission and management, it is very convenient for bridge monitoring of remote and real-time. The wireless sensors not only has the ability for wireless communication, but also can process and analyze the data, Cantilever beam can be used as a measurement device, this paper designed a system of data acquisition and wireless transmission, then it is a wireless sensor by connect with cantilever beam.

The design of collection and transmission system

In this paper, the wireless collection and transmission system structure for independent design, mainly made up with the ARM microprocessor, ADXL345 sensor device, built-in 12 bit AD acquisition module, built-in RTC real time clock, CC1101 wireless communication module, a plurality of analog-digital interface, LED lights and the power supply. The system uses TI's CC1101 sub1G radio frequency chip communication with gateway. The band of using is 433.5M, modulation mode is GFSK, and communication rate is 38.4KBPS. Processor powered by LTC3331. The master controller uses STMicroelectronics’ STM32L152 ultra-low power 32-bit ARMCortexM3. CC1101 waked by electromagnetic wave to communicate with gateway. And it is a low cost wireless transceiver, designed for extremely low power wireless applications, the chip with a variety of functions such as sleep wake model and low power consumption mode, can use these functions to achieve efficient operation with low consumption(Fig. 7).

Acquisition card converted the analog signals produced by cantilever into digital signals, which were transferred to the gateway by 433 frequencies after arrangement. The gateway can receive within 100 meters of sensor data, then the sensor data via GPRS wireless transmission to the cloud from the platform. Sensor and gateway communicate freely by low power consumption of communication protocol, and Socket communication protocol was used to communication for gateway and cloud platform, achieving the data real-time transmission. Cloud platform was composed of cloud server and a website, cloud server communicated with the web side by the HTTP protocol. In the background on the visual interface, we can be a clear understanding of real-time remote monitoring of the bridge by vibration. Considering that the vibration was associated with impact load, the effect of load on the bridge can be analysis by the data of cantilever variation.

Designation of gateway system

Gateway connects the cloud platform with sensors, and plays an important role in remote monitoring(Fig. 8). GPRS DTU module is used for the communication between the gateway and the cloud platform, containing high-performance industrial-grade with 8/16/32 bit communication processor and industrial-grade wireless module. TI CC1101 is used for data transmission between the gateway and sensors. The main controller is STM32F103RCT6 processor, which has the high performance RISC core of ARM® Cortex™-M3 32-bits, and the built-in high-speed packet is SRAM. The power supply voltage of the gateway is 2.0V – 3.6V, supplied by external power and solar energy power, coordinating with each other for power supply. The solar panels adopt PVM-050 type of SINFO, which is of stable performance, high conversion rate and automatic intelligent charging management. Gateway has multiple standards, advanced communication interface yet, which can measure temperature and humidity in surrounding environment with sensors.

The design and test of wireless vibration cantilever sensor

Making the sample of piezoelectric cantilever beam and data acquisition card of the above design, And test the connection system of cantilever beam and acquisition card, in this article, the piezoelectric cantilever beam was firstly designed and then manufactured by a company, the material properties of each of the piezoelectric cantilever structure in line with the finite element simulation of above-mentioned.

The electric signal can flow out along the wire which is link with the surface the piezoelectric patch, then connect the output line of signal with input line of acquisition card, The main test devices consist of AFG-3022B signal source, HEAS-20 power amplifier, HEV-20 high energy vibrators, cantilever beam and acquisition card, gateway with computes, There are two kinds of acquisition card used in the experiment, one kind is the data transmission through the gateway to a cloud platform, another kind is local read the data. The specimen and experimental structure diagram as shown in Figs. 9, 10.

In Fig. 10, the two devices of No. 1 are the acquisition card by local read and computer data, the No. 2 are the acquisition card through gateway and cloud platform, No. 3 is the cantilever beam, No. 4 is the gateway, Fig. 5 is the vibrator.

Adjust the HEV-20 high energy vibrators against the piezoelectric cantilever to produce simple harmonic vibration frequency of 20 Hz, considering the amplitude of span midpoint of small bridge in the process of vehicle traveling, the adjust the output power of HEAS-20 power amplifier to keep the amplitude of the fixed end in piezoelectric cantilever at about 1 mm.

When the cantilever beam generated amplitude of 20 Hz, we collected the data through wireless collection card as shown in Fig.11, the acquisition card be setting as every millisecond to a data, then collected data within 1 second and upload, as can be seen by the figure data within one second of a total of twenty wave crest, it proved that the wireless vibration cantilever beam sensor is accurate, in the application of the bridge, wireless vibration cantilever beam sensor can real-time and remote monitoring the bridge through the gateway and cloud platform, when the bridge vibration exceeds the threshold for early warning, and to supervise and manage the bridge.

After the experiment, this paper chooses a 20 meters small bridges for field test, the cantilever beam packaged with 3 d printing box, then field test after wiring, the field experiments photo the test data as shown in Figs. 12, 13.

Filtering the measured data by Matlab software, it can be seen from the field test data, it produced vibration of 10 Hz against the bridge when vehicle passing, and happened some obviously amplitude attenuation after the vehicle passed. We can see wireless cantilever beam vibration sensor can effectively response the vibration of the bridge intuitively through the field test. The contents of this paper are paving the way for the next step of bridge health monitoring based on IOT.

Conclusions

This paper propose the theory analysis, simulation, the design and manufacture of collection and transmission card around the piezoelectric cantilever beam and complete the design of the wireless vibration of cantilever sensors which not only can be more intuitive measuring the vibration of the bridge but also can achieve remote monitoring through wireless transmission.

(1) It is clear that piezoelectric cantilever beam in bridge health monitoring system has great application value and its power can supply for wireless sensor system. According to the shortage of the recent research, we put forward a set of design method of piezoelectric cantilever beam in the bridge environment, and design the structure of piezoelectric cantilever beam in accordance with it.

(2) The development of wireless vibration of the cantilever beam is convenient for the structure of bridge health monitoring system based on Internet of things which makes the collection of data have the advantage of remote transmission, long-term periodic detection and real-time monitoring of reading and processing the data online. It is more convenient and intuitive to monitor the change of the bridge structure.

In the design, the sensor design of wireless vibration of cantilever is novel and reliable, but it remains some problems in technology. In the later research, we would design and do experiment on the cantilever beam power for power supply of wireless cantilever beam vibration sensor, and establish the architecture of a multifunctional intelligent bridge health monitoring system.

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