Design of a wireless low-power geomagnetic vehicle detection sensor

　　This wireless geomagnetic vehicle detection sensor solution is mainly composed of Freescale's miniature, low-power, 3-axis digital geomagnetometer MAG3110 and TI's low-power microprocessor CC430F5137 with wireless RF transceiver core. This sensor can be used in parking lot intelligent parking guidance, intelligent traffic management and other projects. It has the characteristics of high accuracy, little damage to the ground, and easy installation [1].

　　1. Introduction to the Principle of Parking Lot Intelligent Guidance System

　　The wireless geomagnetic vehicle detector uses a magnetic sensor to detect the impact of vehicles on geomagnetism in real time to determine whether there is a vehicle parked in the parking space, and transmits the detection data wirelessly to a wireless detection receiver.

　　The wireless vehicle detection receiver can receive the information sent by the wireless geomagnetic vehicle detector and then upload it to the control center via wireless or wired. The wireless detection receiver can receive signals from 30 wireless vehicle detectors within the communication distance. The topology diagram of the parking lot intelligent guidance system is shown in Figure 1. The structural block diagram of the wireless geomagnetic vehicle detector is shown in Figure 2. The structural block diagram of the wireless geomagnetic vehicle information receiver is shown in Figure 3.

　　2.1 Hardware Design of Wireless Geomagnetic Vehicle Information Receiver

　　The wireless geomagnetic vehicle information receiver is designed with Zigbee Pro module based on TI's CC430F5137 processor and ember's Em357. It provides RS232 interface to connect to the server. The outer shell uses a metal shielded chassis, which can be mounted on the wall, and the antenna is external.

　　2.2 Hardware Design of Wireless Geomagnetic Vehicle Detector

　　The wireless geomagnetic vehicle detector is designed with the geomagnetic detection chip MAG3110 combined with the CC430 processor. There are two common ways to judge the presence of vehicles through geomagnetic field detection: judging by the distortion of magnetic field lines when the vehicle passes by; or judging by the change of magnetic field strength in the vertical and horizontal directions when the vehicle passes by [3]. The focus of this paper is to judge whether there is a vehicle in a predetermined area (parking space), using the latter judgment method. As a ferromagnetic body, a car will cause changes in magnetic field lines when it approaches the geomagnetic sensor. By monitoring these tiny changes, it can be judged whether there is a car in the parking space. In order to improve the accuracy of judging whether there is a vehicle in the parking space, this study uses a three-axis magnetoresistive sensor, combined with a photoresistor and a reflective photoelectric sensor for a hybrid design.

　　The geomagnetic vehicle detector uses the open ISM microwave band and 433MHz wireless sensing technology. It is powered by a built-in high-performance lithium thionyl chloride battery and uses a low-power design to work continuously for 3 to 5 years.

　　The device shell is designed to be similar to a circular spike structure, and is injection molded with high-strength ABS and PC materials. The overall dimensions are: diameter 126mm × height 32mm. It is waterproof, resistant to acid and alkali corrosion, and resistant to collision.

　　2.2.1 Core Processor Module

　　The geomagnetic vehicle detector also uses the CC430F137 main controller, and the wireless part processing circuit is the same as the receiver. The external interface circuit also includes battery voltage detection, geomagnetic sensor, photoresistor measurement, reflective photoelectric sensor measurement and reed switch disconnection detection circuits.

　　2.2.2 Geomagnetic measurement interface

　　The geomagnetic measurement is made up of Freescale's geomagnetic signal sensor MAG3110 [4]. This device is a 3-axis sensor, placed horizontally on the side of the circuit board 120 mm away from the CPU to reduce the impact of electromagnetic interference generated by the PCB (printed circuit board). The peripheral circuit is shown in Figure 4.

　　2.2.3 Photoresistor and reflective photoelectric sensor detection circuit

　　Photoresistor detection is used to improve the accuracy of vehicle identification. This study selected a doped photoresistor with a resistance value of 10 kΩ, which responds to visible light of (0.4~0.76) μm and a diameter of 5 mm.

　　In order to avoid the interference of oblique light sources as much as possible, a plastic light guide device is installed on the light sensing surface of the photoresistor during the design, and a hole is opened in the detector shell to embed a flat light-transmitting mirror. The installation simulation diagram is shown in Figure 5.

　　The reflective photoelectric sensor detection circuit consumes a large amount of power and is designed only for manual irregular calibration of whether there is a vehicle in the parking space, which is achieved through server operation. The design uses a diffuse reflective photoelectric sensor device with a detection distance greater than 800mm.