The hottest rechargeable micro vehicle sensor base

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Design of rechargeable micro vehicle sensor based on ZigBee

at present, highway and urban traffic problems are common problems all over 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 is the most widely used in engineering because of its good stability and high cost performance [1]. However, the existing induction coil vehicle sensor still has some shortcomings, such as the volume of the induction coil is too large (1m, 2m), the feeder needs to be installed, the installation work is large, the traffic needs to be blocked during installation, the induction coil is very easy to be damaged, it is difficult to repair after damage, short service life, etc. [2]. In addition, in order to avoid missing vehicle detection, multiple vehicle detectors need to be installed at the intersection to transmit the detection data to the upper computer through cables and realize network transmission. In order to solve these problems, as early as the late 1970s, J.F. scarzello[3] proposed a magnetic detector using radio frequency communication, so as to eliminate the installation of feeders and reduce the amount of installation work. According to the development of vehicle detector technology at home and abroad, this paper adopts micro coil, and introduces single-chip microcomputer control technology and new short-range wireless access technology ZigBee into the design of vehicle sensor, which can not only save the installation of communication cable and reduce the amount of installation work, but also safely and reliably realize data transmission and network interconnection, so as to develop a more applicable vehicle sensor

II Hardware design

induction coil vehicle detection sensor is composed of detection module, power module, control module and communication module. Its principle block diagram is shown in Figure 1. The detection module is a frequency selection module of oscillator composed of detection coil and inverter; The control module adopts the low-power MCU MSP430F149 with intelligent control function, which is used to measure the change of oscillator frequency to judge whether there is a vehicle passing, and manage and coordinate the work of each part of the system; The power module uses rechargeable batteries to supply power to the vehicle sensors; The transmitting module is used to communicate with the outside world and send the detected relevant information. Next, only the design of circuits that mainly focus on accelerating the transformation and upgrading of high-end high molecular composites will be introduced in 4 Loading speed: 5 ± 0.5 kg/s

Figure 1 principle block diagram of the inductive loop vehicle detector

fig. 1 the block diagram of the inductive loop vehicle detector

2.1 design of the detection module

the detection project adopts advanced electrophoresis technology. The detection module is a frequency selection module consisting of a detection coil and an inverter [4]. Since the oscillation frequency of the sensor contains and adds the measured information into the concrete, it is very important to maintain the stability of the short-term frequency; Moreover, a good oscillation waveform is conducive to the frequency measurement of single-chip microcomputer and reduces the occurrence of misjudgment and missed judgment. Therefore, considering the working frequency band, frequency stability and oscillation waveform, this paper adopts the capacitive three-point oscillation circuit [5 ~ 6]. In addition, in order to facilitate the measurement of single-chip microcomputer, a shaping circuit is added in the design, whose 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 reduce the volume of the detection coil as much as possible and add a ferrite core into the coil. The reduction of coil volume greatly reduces the amount of installation work, and makes the coil not easy to be damaged and easy to maintain; Adding ferrite core can increase the stability of oscillator frequency and the sensitivity of detection

Figure 2 detection circuit

Fig. 2 the detection circuit

2.2 power module design

the power module uses rechargeable batteries to supply power to vehicle sensors. The power module is managed by the power management chip and the control module. When the power management chip detects that the voltage of the power supply is low, the transmitting module sends a signal requiring charging to inform the staff to charge the battery of the vehicle sensor. When the battery is fully charged, the transmitting module sends a signal to stop charging, informing the staff to stop charging the battery of the vehicle sensor. The power protection circuit protects the power supply from overcharge, overcurrent/short circuit and over discharge. The charging scheme design is shown in Figure 3

Figure 3 charging scheme

Fig. 3 the scheme for battery charge

in addition, the charging control module adopts ti's bq2000 chip core. The chip can be used for programmable fast charging of cadmium nickel, nickel hydrogen and lithium-ion batteries. It has the function of detecting the type of battery and optimizing the charging and stopping the charging accordingly. It can avoid the damage of under charging and overcharging to the battery, so as to realize 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 choose the cc2420em RF module based on ZigBee protocol of Chipcon company. ZigBee is a new short-range wireless access technology. Compared with Bluetooth, it has the advantages of low rate, low cost, low power consumption, convenient group and so on [8]. CC2420 is the first RF transceiver based on ZigBee Technology launched by Chipcon. It requires only a few external components, has stable performance and extremely low power consumption, and can ensure the effectiveness and reliability of short-distance communication. The wireless communication device developed with this chip supports data transmission rate up to 250kbps, and can realize multipoint to multipoint fast group [9]. Cc2420em module integrates CC2420 and its required peripheral circuits. MSP430F149 configures and controls cc2420[10] through high-speed SPI bus, and its interface circuit is shown in Figure 4

Figure 4 interface circuit between MSP430F149 and CC2420

fig.4 interface of MSP430F149 and CC2420

msp430f149 controls and sets the working mode of the chip through 4-wire SPI bus (ste1, simo1, somi1, uclk1), and realizes read/write cache data, read/write status register, etc. The transmit/receive buffer can be set by controlling the status of the FIFO and fifop pin interfaces, and the fifop pin must be connected to the interrupt pin of the MCU. Idle channel estimation can be obtained through CCA pin status. The timing information of transmitted and received frames can be obtained through the state of SFD pin, so as to judge the working state of the system. SFD pin should be connected to the clock capture pin of MCU

III Software design

according to the functional requirements of the vehicle detection sensor, combined with the hardware circuit structure, the system software mainly realizes the following functions:

(1) measure the change of frequency, and determine the scheme of measuring the change of frequency to ensure the sensitivity of the system. Due to the drift of the system frequency itself, the frequency value when there is no car needs to be corrected to eliminate the system error

(2) configure RF module, design data frame and vehicle sensor group design

the overall software design flow chart is shown in Figure 5. The whole program is divided into system initialization program, frequency measurement program and communication program

Figure 5 main program flow

Fig.5 flow diagram of main program

the system initialization program mainly includes the setting of MSP430F149 clock, the setting of each communication port, the setting of timer, etc. After the system is set, enter the frequency measurement program. The frequency measurement program mainly includes the measurement of frequency variation, the calibration of threshold, and the processing of anti-interference. If no vehicle is detected, the system enters the low power consumption mode. If the measured frequency value is greater than the threshold value, enter the transmission procedure and transmit the signal of the detected vehicle. After the launch, the system enters the low-power mode. At this time, the watchdog timer (WDT) is started. When the watchdog timer overflows, the system wakes up for the next measurement

IV Conclusion

the author's innovations are as follows: first, ZigBee, a new short-range wireless communication technology, is applied to the design, so that the feeder is omitted, and the installation of the sensor is fast and convenient. In addition, the new single-chip microcomputer control technology is used to improve the sensitivity and reliability of the sensor, reduce the false detection rate, and make the sensor intelligent, which can significantly reduce power consumption and realize self inspection and power management; Second, the power module is composed of new power control and charging control chips, which greatly extends the service life of the sensor and simplifies management. The sensor has the advantages of small volume, convenient installation, little damage to the road surface and easy maintenance. It can be used not only for road traffic vehicle detection, but also for intelligent parking lot parking space detection, and has a wide application prospect

Copyright © 2011 JIN SHI