Abstract: Deflection is the most important indicator reflecting the health of a bridge. This paper introduces a photoelectric liquid level deflection sensor based on the ARM CPU LPC2132, which has advantages such as low cost, good automation performance, high accuracy, non-contact operation, and continuous online monitoring. This paper focuses on its hardware structure, principle, and performance analysis. Finally, the application of this sensor is illustrated using the safety monitoring system of the Xiaogou Grand Bridge on the Shanxi Expressway as an example. Keywords: LPC2132; bridge monitoring; deflection; sensor Abstract: Deflection is an indispensable parameter that can indicate the safety status of bridges. The photoelectric and liquid level deflection sensors (PLLD) based on the ARM CPU LPC2132 are introduced here. They are cheaper, have better automation and higher precision, and can work online continuously without contact. This paper will show the hardware, performance and theory of our new sensor in detail. Finally, a concrete application instance in XiaoGou Bridge of Shanxi Province will be given. Key words: LPC2132; bridge monitoring; deflection; sensor 1 Introduction to Photoelectric Deflection Monitoring System Bridges are large-scale civil infrastructure projects with huge investments and long service lives. Therefore, their safety plays a crucial role in the national economy. During their service life, due to the long-term effects of adverse factors such as load, fatigue effect, corrosion effect and material aging, bridge structures will inevitably experience natural aging, damage accumulation, and even sudden accidents. In recent years, the frequent bridge collapses and other accidents have caused serious damage to people's lives and property. Large bridges that have already been built and may have hidden dangers pose a significant potential threat. Therefore, it is essential to conduct safety monitoring of the operation of large-scale civil infrastructure such as bridges. Due to the large size, numerous constraint points, and complex structural deformation of bridges, a comprehensive assessment of their health requires understanding their condition from different perspectives (e.g., vibration, deflection, strain). Among the many parameters characterizing bridge health, deflection (the vertical displacement of a bridge under load) is an indispensable indicator. Currently, although some systems exist for measuring bridge deflection, devices capable of fully remote, real-time, and automatic online monitoring are still rare. The photoelectric liquid level deflection monitoring system introduced in this paper has advantages such as low cost, good automation, high accuracy, non-contact operation, and continuous online monitoring, and has been successfully applied on the Xiaogou Expressway Bridge in Shanxi Province. [align=center] Figure 1 Schematic diagram of photoelectric liquid level deflection monitoring system[/align] As shown in Figure 1, the liquid tank is located at the bridge pier. Each liquid tank can be connected to several deflection sensors through several liquid pipes. The distribution of the sensors depends on the location that the user needs to measure. Multiple sensors are connected in series to a high-speed data acquisition device (hereinafter referred to as high-speed data acquisition) via an RS-485 bus. One high-speed data acquisition device can connect to multiple such 485 bus networks [1]. Of course, multiple high-speed data acquisition devices can also be used (if there are many points to be measured). Multiple high-speed data acquisition devices can be connected to an Ethernet device, and the Ethernet device transmits the data to the monitoring center for display and analysis through external lines such as optical fibers. The deflection sensor is the core part of this monitoring system. Its main task is to interpret and execute the commands sent by the host computer and perform different operations according to different commands. The sensor consists of three parts: liquid displacement transmission part, optical liquid surface displacement recognition part, and data processing part. According to the principle of communicating vessels, if only one liquid is contained in the communicating vessel, the liquid surface in each container will always remain level when the liquid does not flow. The liquid tank is placed at the bridge pier. When the bridge deflects, the pier remains static. At this time, the liquid displacement transmission part inside the sensor will generate a displacement change, i.e., a deflection deformation value. The optical liquid level recognition part transmits the measured displacement change signal to the data processing part. The data processing part performs filtering, A/D conversion, and other operations on the received signal, and sends the final digital signal to the Ethernet device via RS-485 bus. Then, the Ethernet device converts the received data frames into TCP/IP data packets and transmits them to the monitoring center via external networks such as fiber optics. 2. Application of the 32-bit LPC2132 in the photoelectric liquid level and deflection monitoring system. The LPC2132 CPU is a 32-bit high-speed processor based on the ARM7TDMI-S core, manufactured by Philips. Its operating voltage is 3.3V, while the core ARM7TDMI-S operates at only 2.5V, greatly reducing the chip's power consumption. The LPC2132 features: ① 16K of on-chip static RAM and 64K of on-chip Flash program memory, providing the necessary space for the system's programs; a 128-bit wide interface/accelerator enables operating frequencies up to 60MHz; ② An on-chip bootloader enables in-system programming (ISP) and in-application programming (IAP). Flash programming time: 1ms for 256 bytes; sector erase or positive erase takes only 400ms; ③ Embedded ICE-RT and embedded trace interfaces allow for real-time debugging (using on-chip RealMonitor software) and high-speed tracing of executed code, facilitating program debugging; ④ One 8-channel 10-bit A/D converter with 16 analog inputs, each channel having a conversion time of less than 2.44µs. This allows analog signals generated by the CCD (a photoelectric element) to be directly fed into the CPU without the need for additional analog-to-digital conversion circuitry, simplifying software programming. ⑤ Two 32-bit timers/counters (with 4 capture and 4 compare channels) meet the system's input requirements for CCD control signals. The PWM unit (6 outputs) can directly generate the pulse signals required by the CCD. ⑥ Multiple serial interfaces, including two 16C550 industrial standard UARTs, two high-speed I2C interfaces (400Kb/s), and SPI and SSP serial interfaces, meet the requirements for real-time communication with the outside world. ⑦ Up to 47 general-purpose I/O ports (capable of withstanding 5V voltage) and 9 edge- or level-triggered external interrupt pins can be used for RS-485 command reception reports, CCD status, indicator light control, etc. ⑧ The CPU operating frequency can be up to 60MHz through the on-chip PLL, and the PLL stabilization time is 100us. ⑨ Single power supply, with power-on reset (POR) and power-down detection (POR) circuits, supports two low-power modes: idle and power-down. The processor can be woken up from the power-down mode through external interrupts. Different working modes can be set as needed to reduce system power consumption [2]. Figure 2 shows the hardware connection diagram for implementing the deflection sensor function using LPC2132. [align=center] Figure 2 Hardware Connection Diagram of LPC2132[/align] 2.1 Connection between LPC2132 and CCD CCD is short for charge coupler device, which consists of a series of adjacent MOS (metal-oxide-semiconductor) memory cells. Its working principle is: when a photosensitive element is irradiated by external light, it can generate a charge, which is stored in the MOS memory cell. The amount of charge generated is proportional to the light intensity and irradiation time. Under the drive of a certain timing external voltage, the charges stored in the CCD can be sequentially removed one after another. Thus, the output terminal of the CCD generates an output voltage proportional to the stored charge. Therefore, it is mainly used for image recording and storage. In this system, the CCD belongs to the optical liquid level displacement recognition part. Its main function is to identify the change in liquid displacement in the liquid displacement conduction part. Therefore, before introducing the connection between LPC2132 and CCD, we will first introduce the working principle and structure of the optical liquid level displacement recognition part, as shown in Figure 3. [align=center]Figure 3 Schematic diagram of the optical liquid level displacement recognition part[/align] The transparent tube shown in Figure 3 is connected to the liquid tank at the bridge pier through a liquid pipe, and together they constitute the liquid displacement transmission part. In practical applications, the transparent tube is always perpendicular to the horizontal plane. When a deflection change occurs, the bridge moves the sensor vertically, so the transparent tube inside the sensor moves relative to the liquid inside. This gives us the feeling that the liquid is moving up and down in the transparent tube. The optical liquid level displacement recognition part mainly consists of a transparent tube, a light source, a lens, and a CCD. The liquid in the transparent tube is opaque. The uniform light emitted by the line light source composed of several LEDs illuminates the background of the transparent tube. Due to the different refractive indices of light in different shaped media, the image of the glass tube after passing through the lens forms a dark band in the middle part of the CCD. The transmitted light at the upper and lower edges is relatively strong, forming a bright band. The width of the dark band in the middle is the image of the liquid column in the glass tube on the CCD. The size of the image is used to obtain the height of the liquid level, thereby achieving the purpose of liquid level recognition. The CCD used in this system is the TCD1208AP linear CCD product manufactured by TOSHIBA Corporation of Japan. It has 2160 pixels, a pixel size and spacing of 14mm x 14mm, high sensitivity, low dark current, and a single operating voltage of 5V. It is a two-phase output linear CCD device, an improvement on the earlier TCD142D, and is inexpensive, highly sensitive, and widely used. Using an ARM series microprocessor greatly simplifies its drive circuit and enhances its operational stability. The TCD1208AP operates using two-phase drive pulses. The timing pulse drive circuit provides four working pulses: the light integration pulse SH, charge transfer pulses F11 and F12, and the output reset pulse RS. In the specific implementation, one PWM is used as RS, and after processing, it is divided by two by a 74HC74 to generate F11 and F12. The SH signal is generated by controlling the general-purpose I/O port through a timer. The level of the drive pulse is controlled by a 74HC04 with pull-up resistors. Since the typical reset pulse of a linear CCD is 1 MHz, there is a minimum speed requirement for the microcontroller. Therefore, to implement this driving method, a microcontroller with an instruction cycle of less than 1 µs must be used. ARM microcontrollers can operate at clock speeds up to 60 MHz, which fully meets the requirements. The analog signal obtained from the CCD is directly fed into the internal A/D converter of the LPC2132 for processing. 2.2 Connection between LPC2132 and SN65HVD3082 Because the system needs to detect deflection at dozens of points, and each deflection sensor is only a basic unit of the entire system, requiring both external input of necessary information and output of its own operating parameters and status, network control technology was adopted to meet the equipment control requirements. This involves organically connecting numerous devices to ensure the safe and reliable operation of the entire system. The system uses an RS-485 bus to form the entire sensor network, enabling multi-machine communication within the system. Since the 485 bus is an asynchronous half-duplex communication bus, that is, at a certain moment, the bus can only present one state. Therefore, this method is generally applicable to the communication method of the host to the extension. There must be a device on the bus that is always in the position of the host and is inspecting other extensions. The high-speed data acquisition device in this system plays the role of the host. The RS-485 driver in the system uses Texas Instruments' 485 driver chip SN65HVD3082. This chip is small in size and has extremely low power consumption. Under 1/8 load, it can drive up to 256 devices with the same interface at the same time, and the driving rate can reach 200kb/s. In the on-site construction of the specific application engineering system, since the communication carrier is generally a twisted pair, its characteristic impedance is about 120Ω. Therefore, in the line design, a 120Ω matching resistor should be connected at the beginning and end of the RS-485 network transmission line to reduce the reflection of the transmitted signal on the line [3]. This is because there is also a distributed capacitance between the conductor and the ground. Although it is very small, it cannot be ignored in the analysis. 2.3 Connections between LPC2132 and Other Components Other components that connect to the LPC2132 include: the system clock circuit, reset circuit, and JTAG interface circuit. These components are relatively simple. The LPC2100 series ARM7 microcontroller can use an external crystal oscillator or external clock source. The internal PLL circuit can adjust the system clock, making the system run faster (the CPU's maximum operating clock is 60MHz). If the on-chip PLL function and ISP download function are not used, the external crystal oscillator frequency range is 1MHz-30MHz, and the external clock frequency range is 1MHz-50MHz; if the on-chip PLL function or ISP download function is used, the external crystal oscillator frequency range is 10MHz-25MHz, and the external clock frequency range is 10MHz-25MHz. This system uses an external 11.0592MHz crystal oscillator. This allows for more accurate serial port baud rates and also supports the LPC2132 microcontroller's internal PLL and ISP functions. Due to the characteristics of ARM chips such as high speed, low power consumption, and low operating voltage, their noise margin is low, which places higher demands on many aspects such as power supply ripple, transient response performance, clock source stability, and power supply monitoring reliability. In this system, the reset circuit uses the CAT809T power monitoring chip, with a reset delay of up to 250ms, and the reset pulse generated by RESET fully meets the timing requirements. In addition, the system also reserves a JTAG interface circuit for convenient debugging and program downloading. 3 System Performance Analysis and Experimental Data 3.1 System Deflection Measurement Accuracy The circuits of the sensing system are all digital circuits, and the transmission system transmits numerical values. The CCD sensor is digitized in space, so the system accuracy is determined by the optical imaging part. Currently, the optical resolution of the system is 0.098mm, which is better than 0.1mm, which is sufficient for testing bridge deflection deformation. 3.2 Frequency Response Characteristics of the Sensing System Static Response Characteristics: When the liquid surface in the pipe is stationary, the stationary liquid surface causes the sensor to output a fixed value, meaning the sensor has a zero-frequency response characteristic. Numerous experiments and field tests have proven that the sensor indeed has a 0Hz DC response. Therefore, this characteristic of the sensor can be used to monitor the static deformation of the bridge. Dynamic Response Characteristics: We tested the dynamic characteristics of the sensor through numerous experiments. The experimental method involved using a vibration table to drive the liquid tank to undergo vertical periodic vibration. The displacement of the water tank was transmitted to a fixed liquid level sensor through a 50m pipe. The output data of the sensor was read in the software on the host computer. The data obtained at different frequencies are shown in Table 1. Table 1: Dynamic Response Table of Liquid Level Sensor (In this experiment, the water tank travel: 47.5mm-38.4mm, totaling 9.5mm.) From Table 1, the system accuracy can be calculated to be between 9.5/101 and 9.5/110, with a resolution within 0.1mm. This meets the monitoring standard for bridge deflection values. 4 Specific Application Examples The photoelectric liquid level deflection sensor has been successfully applied on the Xiaogou Grand Bridge of Xinyuan Expressway in Shanxi Province. It has been running continuously for nearly a year, proving the stability and reliability of the system. Figures 4 and 5 are the bridge deflection waveforms when vehicles pass over the bridge, captured from the LabVIEW software[4] in the monitoring center. . [align=center] Figure 4 Deflection waveform of the bridge when only one vehicle passes over the bridge Figure 5 Deflection waveform of the bridge when multiple vehicles pass over the bridge[/align] The curvature of the curve in the figure represents the deflection of the bridge when the vehicle passes over it. By saving these data in real time, the long-term deflection change of the bridge can be obtained. Regular analysis of these data can help understand the current health status of the bridge. 5 Conclusion The innovation of this paper is that the principle of the connecting pipe is applied to the deflection sensor, and the high performance of the ARM CPU is used to realize the advantages of the system such as good automatic performance, high precision, non-contact, and continuous online monitoring. The photoelectric liquid level deflection monitoring system of the bridge is a relatively large system. This paper only involves its core part, and that is the hardware part. The workload of the software part is also relatively large. The LPC2132 with ARM7TDMI-S as the core can be programmed, debugged and simulated in standard C or C++ language on the ADS1.2 debugging platform, which greatly shortens the software development cycle. References. [1] Yan Yan, Chen Baoping, Ma Zengqiang, et al. Design of networked remote bridge health status detection system [J]. Microcomputer Information, 2005, 8 [2] Zhou Ligong, Zhang Hua. ARM7-LPC2132 in depth. Beijing: Beijing University of Aeronautics and Astronautics Press, 2005 [3] Zhang Jianxin, Li Xuemin. Encyclopedia of Electronic Circuits. Beijing: Science Press, 1989 [4] Wang Minsheng. LabVIEW Basic Tutorial [M]. Beijing: Electronic Industry Press, 2002