Abstract: Vehicle detection sensors can accurately and in real-time acquire various traffic data (including traffic flow, vehicle speed, vehicle density, headway, occupancy rate, etc.), and are one of the most important traffic data acquisition devices in intelligent transportation systems (ITS). Based on the development of vehicle detector technology at home and abroad, this paper adopts miniature coils and introduces single-chip microcomputer control technology and the novel short-range wireless access technology ZigBee into the design of vehicle sensors, thereby effectively reducing the installation workload and developing a more versatile vehicle sensor. Keywords: Intelligent Transportation System; Vehicle Detector Sensor; ZigBee Abstract: Vehicle detection can collect traffic data (such as vehicle's flow, speed, density and occupancy ratios). It is one of the key technologies in ITS (intelligent traffic system). In this paper, the design and reality of a new recharge inductive loop vehicle detection sensor with ZigBee technology is presented. This sensor is more feasible with miniature inductive loop, easy maintenance and installation, and intelligent controller. Keywords: ITS; Vehicle Detector Sensor; ZigBee I. Introduction At present, highway and urban traffic problems are common problems in countries around the world, which directly affect economic development and people's lives. Intelligent Transportation System (ITS) is an effective way to solve the increasingly serious highway and urban traffic problems, and vehicle detector is one of the most important traffic data acquisition devices in ITS. Induction coil vehicle detection sensor has the advantages of good stability and high cost performance, so it is the most widely used in engineering[1]. However, the existing induction coil vehicle sensor still has some disadvantages, such as the induction coil being too large (1m×2m), requiring the installation of feeders, resulting in a large amount of installation work, requiring traffic to be blocked during installation, the induction coil being easily damaged, difficult to repair after damage, and having a short service life, etc. [2]. In addition, in order to avoid missing vehicles, multiple vehicle detectors need to be installed at intersections to transmit the detection data to the host computer via cable and realize network transmission. In order to solve these problems, as early as the late 1970s, JF Scarzello [3] proposed a magnetic detector that uses radio frequency communication to save the installation of feeders and reduce the amount of installation work. Based on the development of vehicle detector technology at home and abroad, this paper adopts a micro coil and introduces single-chip microcomputer control technology and the new short-range wireless access technology ZigBee into the design of vehicle sensor. This not only saves the installation of communication cables and reduces the amount of installation work, but also safely and reliably realizes data transmission and network interconnection, thereby developing a more applicable vehicle sensor. II. Hardware Design The induction coil vehicle detection sensor consists of a detection module, a power supply module, a control module and a communication module. Its principle block diagram is shown in Figure 1. The detection module is a frequency selection module consisting of a detection coil and an inverter forming an oscillator; the control module uses a low-power microcontroller MSP430F149 with intelligent control function to measure the change of oscillator frequency to determine whether a vehicle has passed, and to manage and coordinate the work of each part of the system; the power supply module uses a rechargeable battery to power the vehicle sensor; the transmission module is used to communicate with the outside world and send the relevant information detected. The design of the main circuit is described below. [align=center] Fig. 1 The block diagram of the inductive loop vehicle detector[/align] 2.1 Design of the detection module The detection module is a frequency selection module consisting of a detection coil and an inverter forming an oscillator[4]. Since the oscillation frequency of the sensor contains the measured information, maintaining the stability of the short-term frequency is very important; moreover, a good oscillation waveform is beneficial for the microcontroller to measure the frequency and reduce the occurrence of misjudgment and missed judgment. Therefore, considering the working frequency band, frequency stability and oscillation waveform, this paper adopts a capacitor three-point oscillation circuit[5-6]. In addition, to facilitate microcontroller measurement, a shaping circuit was added to the design. Its function is to shape the sinusoidal signal generated by the oscillation circuit into a square wave signal of the same frequency. The specific circuit is shown in Figure 2. In the design, we minimized the volume of the detection coil and added a ferrite core to the coil. The reduction in coil volume greatly reduced the amount of installation work, while making the coil less susceptible to damage and easier to maintain; the addition of a ferrite core can increase the stability of the oscillator frequency and the sensitivity of detection. [align=center] Figure 2 The detection circuit[/align] 2.2 Power Module Design The power module uses a rechargeable battery to power the vehicle sensor. The power module is managed by a power management chip and a control module. When the power management chip detects a voltage drop, the transmitting module sends a signal indicating that charging is needed, notifying the staff to charge the vehicle sensor battery. When the battery is fully charged, the transmitting module sends a signal to stop charging, notifying the staff to stop charging the vehicle sensor battery. The power protection circuit provides overcharge protection, overcurrent/short circuit protection, and over-discharge protection. The charging scheme design is shown in Figure 3. [align=center] Fig. 3 The scheme for Battery Charge[/align] In addition, the charging control module uses the bq2000 chip core from TI. This chip can be used for programmable fast charging of nickel-cadmium, nickel-metal hydride, and lithium-ion batteries. It has the function of detecting the battery type and optimizing charging accordingly and stopping charging. It can avoid damage to the battery due to undercharging or overcharging, thereby achieving safe and reliable fast charging control[7]. 2.3 RF module design The transmitting module is used to communicate with the outside world and send the detected relevant information. The receiving module uploads the received information to the toll station or control center. In this design, we selected the CC2420EM RF module based on the ZigBee protocol from Chipcon. ZigBee is a new short-range wireless access technology. Compared with Bluetooth, it has the advantages of low speed, low cost, low power consumption, and convenient networking[8]. The CC2420 is the first RF transceiver based on ZigBee technology launched by Chipcon. It requires very few external components, has stable performance and extremely low power consumption, and can ensure the effectiveness and reliability of short-range communication. Wireless communication devices developed using this chip support data transmission rates up to 250kbps and can achieve fast multi-point to multi-point networking [9]. The CC2420EM module integrates the CC2420 and its required peripheral circuits. The MSP430F149 configures and controls the CC2420 via the high-speed SPI bus [10], and its interface circuit is shown in Figure 4. [align=center] Fig.4 Interface of MSP430F149 and CC2420[/align] The MSP430F149 controls and sets the chip's operating mode via the 4-wire SPI bus (STE1, SIMO1, SOMI1, UCLK1) and implements read/write buffer data, read/write status register, etc. The transmit/receive buffer can be set by controlling the state of the FIFO and FIFOP pin interfaces. The FIFOP pin must be connected to the microcontroller's interrupt pin. The idle channel estimate can be obtained through the CCA pin state. The timing information of the transmitted and received frames can be obtained through the SFD pin status, thereby determining the working status of the system. The SFD pin should be connected to the clock capture pin of the microcontroller. III. Software Design According to the functional requirements of the vehicle detection sensor and combined with the hardware circuit structure, the software of the system mainly implements the following functions: (1) Measure the frequency change and determine the scheme for measuring the frequency change to ensure the sensitivity of the system. Due to the drift of the system frequency itself, the frequency value when there is no vehicle needs to be corrected to eliminate system error; (2) Configure the radio frequency module, design the data frame and the vehicle sensor network design. The overall software design flowchart is shown in Figure 5. The entire program is divided into system initialization program, frequency measurement program and communication program. [align=center] Fig.5 Flow diagram of main program[/align] The system initialization program mainly includes setting the MSP430F149 clock, setting each communication port, setting the timer, etc. After the system is set up, it enters the frequency measurement program. The frequency measurement program mainly includes measuring the frequency change, calibrating the threshold, and handling anti-interference. If no vehicle is detected, the system enters the low power mode. If the measured frequency value exceeds the threshold, the system enters the transmission procedure and transmits the signal indicating the detected vehicle. After transmission, the system enters a low-power mode, at which point a watchdog timer (WDT) is started. When the watchdog timer overflows, the system is woken up for the next measurement. IV. Conclusion The author's innovations are twofold: First, the novel short-range wireless communication technology ZigBee is applied to the design, eliminating the need for feeders and making sensor installation quick and convenient. New microcontroller control technology is used to improve sensor sensitivity and reliability, reduce false detection rates, and enhance sensor intelligence, significantly reducing power consumption and enabling self-testing and power management. Second, a new power control and charging control chip is used to construct the power module, greatly extending the sensor's lifespan and simplifying management. This sensor is small in size, easy to install, causes minimal damage to the road surface, and is easy to maintain. It can be used not only for road traffic vehicle detection but also for intelligent parking lot space detection, showing broad application prospects. References: [1] Guo Min, Comparative analysis of vehicle detection technology and vehicle detector, Technology and Application 2003.55-56 78-79 [1] Liu Dawei, Wang Qi, Chen Jianji. Ring coil vehicle sensor and its application [J] [3] Scarzello JF, Lenko DS, Brown RE, Krall AD, SPVD: A Magnetic Vehicle Detection System Using A Low Power Magnetometer, IEEE TRANSACTIONS ON MAGNETICS, 1978, 14(5): 574-576 [4] Sun Guodong, Jiang Yonglin, Liang Qi, Design and implementation of intelligent ring coil vehicle detector, Microcomputer Information, 2003.19(9): 54-57 [5] Ding Mingfang, Inductor and capacitor circuits and their applications, University of Science and Technology of China Press, 1995 [6] Yang Jianping, Design method to improve the frequency stability of oscillator, 081 Technology, 2004: 31-34 [7] Texas Instruments, bq2000 Programmable multi-chemistry fast-charge management IC data sheet [8] Jin Chun, Jiang Xiaoyu, Luo Zuqiu, Analysis and comparison of ZigBee and Bluetooth, Standards and Technology Tracking, 2004, 6: 17-20 [9] Chipcon, CC2420 2.4GHz IEEE 802.15.4/ZigBee-ready RF Transceiver Data Sheet [10] Texas Instruments, IEEE 802.15.4 (TM) and ZigBee (TM) Hardware Platform using MSP430F1612