Abstract: Based on the requirements of taxi companies for vehicle monitoring and positioning, this paper describes the design of a taxi vehicle monitoring device using the MSP430F147 microcontroller as the core, integrating GPS and GPRS modules. The design details both the hardware and software solutions. This system can collect real-time location information of operating taxis and transmit the collected information to the taxi company via the GPRS module, facilitating vehicle monitoring and dispatching for the taxi company.
Keywords: microcontroller; GPS module; GPRS module
0. Introduction
With the rapid development of modern society, taxis, as part of urban public transportation, have become an indispensable part of modern urban transportation. However, the monitoring, dispatching, and safety management of taxis have become a major problem for every taxi company. To address this issue, we have developed a taxi-mounted monitoring device. This system greatly facilitates taxi companies' dispatching and management of taxis, and also allows for real-time monitoring of the taxis' location and safety status.
1. Functions of taxi onboard monitoring equipment
Taxi onboard monitoring equipment is a component of the taxi monitoring and dispatch system. It provides real-time location information of taxis to the taxi monitoring and dispatch center, ensuring safe taxi operation and enabling reasonable taxi dispatch. The functions of the taxi onboard monitoring equipment are as follows:
1) Information collection function
Vehicle-mounted monitoring equipment can accurately collect real-time information about vehicles (such as longitude, latitude, speed, direction, etc.).
2) Data storage function
It can store information such as vehicle ID number, speed information 15 minutes before parking, and continuous driving time.
3) Data transmission function
The collected real-time taxi information can be sent to the monitoring and dispatch center via GPRS, and dispatch commands from the monitoring and dispatch center can be received.
2. System Hardware Design
The system uses the single-chip microcomputer MSP430F147 as the core controller [1], and integrates a GPS module, a GPRS module, a serial memory module and a power management module. The system block diagram is shown in Figure 1.
2.1 Microcontroller Unit
The system uses TI's MSP430F147 microcontroller as the core controller [2]. This microcontroller has a 16-bit CPU integrated register and constant generator, which enables the microcontroller to achieve maximum code efficiency; it integrates JTAG and supports online programming; two general-purpose full-duplex serial synchronous/asynchronous communication interfaces; six 8-bit I/O ports; and an external interrupt input interface. The microcontroller's serial port 0 is responsible for receiving data information output by the GPS module, analyzing and extracting the received valid information data, storing the data on the one hand, and sending the information to the taxi monitoring and dispatch center through the GPRS module on the other hand.
2.2 GPS Module
2.2.1 Overview of GPS
GPS (Global Positioning System) is a new generation satellite navigation and positioning system developed by the United States. It provides continuous, real-time, and high-precision three-dimensional position, three-dimensional velocity, and time information to users worldwide. It is currently the world's most accurate navigation system and has been widely used in military, economic, geographic information measurement and control, and other fields. The GPS positioning system consists of the following three parts:
(1) GPS satellite constellation (space segment): 24 satellites (three of which are spares), distributed across 6 orbital planes, with 4 satellites evenly distributed across each orbital plane, thus ensuring that 4 satellites can be observed from any location on Earth at any time. It can transmit signals in the L1 and L2 bands.
(2) Ground monitoring system (ground control part): central control system, tracks satellite orbit determination, monitors satellite status, and corrects satellite orbit information.
(3) GPS signal receiver (user equipment section): The receiving equipment mainly consists of an antenna, power supply, data processing software, microprocessor, and terminal equipment. Its basic structure is an antenna unit and a receiving unit. The main function of the antenna unit is to capture and track satellites and receive signals transmitted by GPS satellites. The main function of the receiving unit is to record GPS signals, filter the signals, and reconstruct the navigation messages transmitted by GPS satellites to obtain useful positioning information.
2.2.2 Introduction to GPS Module
GPS uses the GS-87 module [3], which is a high-efficiency, low-power intelligent satellite receiver module or satellite receiver engine. It uses the third-generation satellite positioning receiver chip designed by SiRF Star III, a complete satellite positioning receiver with all-round functions. Its system structure is shown in Figure 2. The GPS Antenna receives satellite signals and converts weak electromagnetic wave energy into corresponding current. The current is amplified by the LNA (low noise amplifier), and then frequency-converted by the filter and sent to SIRF StarIII (composed of GRF3W and GSP3) for processing. The processed signal is connected to the microcontroller through the serial port. The user processes the information received by GPS and extracts the information useful to the user by controlling the microcontroller through software programming.
Figure 2 System Structure Diagram
2.2.3 Design of GPS Module Interface Circuit
The GS-87 module has 6 pins. It primarily communicates with the microcontroller via serial communication. Therefore, when connecting them, the GS-87's data receive pin RXA and data transmit pin TXA can be connected to the microcontroller's UART0 serial port (TXD0 and RXD0) respectively. The GS-87's TXA pin is the serial data output port A, the main data transmission channel used to output navigation and measurement data; RXA is the serial data receive port A, the main receiving channel used to receive control commands from the microcontroller; VCC is an external power input port with a voltage of +3.3~5.5V. To ensure the module is not affected by external electromagnetic noise during operation, a bypass capacitor is installed between the power supply terminal VCC and the ground terminal GND. The bypass capacitor C19 is mainly used to remove high-frequency electromagnetic pulses, and the electrolytic capacitor C33 is mainly used to remove low-frequency interference and store charge; RXB is the auxiliary serial data receive port B, used for differential positioning; the Time Mark defaults to providing a 1 pulse per second output signal from the GS-87 for synchronizing one microsecond of time. In the design of vehicle-mounted monitoring equipment, only VCC, RXA, TXA, and GND are needed to meet the functional requirements. The circuit connection diagram of the GPS module is shown in Figure 3.
Figure 3 GS-87 circuit connection schematic diagram
2.3 GPRS Module
2.3.1 GPRS Overview
GPRS (General Packet Radio Service) is a wireless packet switching technology based on the GSM (Global System for Mobile Communications) system, providing end-to-end, wide-area wireless IP connectivity. It is one of the implementations of the GSM Phase 2.1 specification and can provide a higher data rate than the existing GSM network's 9.6 kbit/s. GPRS uses the same frequency bands, bandwidth, burst structure, radio modulation standards, frequency modulation rules, and TDMA frame structure as GSM. As a transitional technology from second-generation mobile communication technology (GSM) to third-generation mobile communication (3G), GPRS fully utilizes existing mobile communication network equipment without requiring changes to wireless network planning and topology, thus significantly reducing mobile network construction costs. It supports IP and X.25 protocols, providing a connection between mobile users and the data network, offering mobile users high-speed wireless IP and X.25 packet data access services. Therefore, building a GPRS system on top of the GSM system only requires adding some hardware and upgrading software. Methods for building a GPRS system: 1) GPRS is implemented on the existing GSM network, which introduces three main components: Serving GPRS Supporting Node (SGSN), Gateway GPRS Supporting Node (GGSN), and Packet Control Unit (PCU). 2) Software upgrades are performed on the relevant GSM components.
GPRS data transmission has the following characteristics:
(1) It uses time-packet switching technology for communication. (2) It charges based on data flow, not online time. (3) It has a high transmission rate. (4) It is always online. (5) GPRS network access speed is fast, providing seamless connection with existing data networks. (6) GPRS supports applications based on standard data communication protocols and can interconnect with IP networks and X.25 networks. (7) The design of GPRS allows it to support both intermittent burst data transmission and occasional large data transmission.
As can be seen from the characteristics of GPRS described above, GPRS wireless communication technology fully meets the design requirements for taxi-mounted monitoring equipment. Applying GPRS technology establishes a data communication bridge between the vehicle-mounted monitoring equipment and the monitoring and dispatch center, making remote real-time monitoring and dispatching possible. With the continuous development and improvement of GPRS technology, GPRS-based wireless data communication will be more widely used.
2.3.2 Introduction to GPRS Module
The GPRS module uses Siemens' MC52i module [4], which is an important part of establishing communication between the vehicle monitoring equipment and the taxi company's monitoring and dispatch center. Through the GPRS module, the vehicle positioning information (such as time, longitude, latitude, speed, direction, etc.) collected by the GPS module can be sent to the server of the taxi monitoring and dispatch center in real time; on the other hand, it can also receive dispatch commands from the taxi monitoring and dispatch center. This realizes two-way communication between the vehicle monitoring equipment and the monitoring and dispatch center.
2.3.3 GPRS Interface Circuit Design
The MC52i module is connected to the MSP430F147 via a serial port and signal control pins, as shown in Figure 4. Pins 26-30 of the MC52i are the power supply input terminals, with an input voltage range of 3.3V to 4.8V and a peak load current of 2A. The TXD0 and RXD0 pins of the MC52i module are the data receive and data output ports, respectively, and are connected to the TXD1 and RXD1 pins of the microcontroller's serial port (UART1). Pins 1-6 of the MC52i provide a standard interface for an external SIM card. CCGND and CCVCC provide the operating voltage for the SIM card; the CCCLK signal provides the clock pulse signal for the SIM card; CCIO is the serial data input/output interface; the CCRST signal is the SIM card reset signal; and the CCIN pin is mainly used to detect whether the SIM card is inserted into the SIM card slot.
Figure 4 MC55 connection schematic diagram
0. System Software Design
3.1 Main Program Functions
The main program is responsible for initialization, enabling interrupts, analyzing and parsing GPS data, and displaying the data to guide the system into various corresponding working states. The flowchart of the main program is shown in Figure 5a.
3.1 GPS Data Receiving Program Design
Vehicle-mounted terminal equipment needs to extract useful information from GPS positioning data and display it, so it also needs to understand the meaning of various NMEA data. Below is only an example of the GPS Fixed Data Output Statement (GPRMC), and the meaning of each data point is shown in Table 1. Output Example:
$GPRMC,025812.487,A,3752.5236,N, 11229.1768,E,0.13,309.62,181211, ,*70
Table 1 Meaning of GPRMC Data
Tab. 1 GPRMC data meaning
When the GPS module outputs data, the microcontroller enters the serial port interrupt routine. The program first checks if it contains the data header flag "$GPRMC". If the data is correct, the program enters data receiving mode until it receives an asterisk (*) indicating successful data reception; otherwise, it exits the interrupt routine and waits for the next data arrival. The following is a partial interrupt routine code.
While ((IFG1 & URXIFG0) == 1);
{ UART0_RX_BUF[num++]=RXBUF0;
if (UART0_RX_BUF[0]!='$') num=0;
if (num == 5)
{ if (UART0_RX_BUF[2]!='P') num=0;
if (UART0_RX_BUF[3]!='R') num=0;
if (UART0_RX_BUF[4]!='M') num=0;
}
if (num==56)
{ for (i=56;i<73;i++)
{ if (UART0_RX_BUF[i]!='*') num=0;
else k=i; }
}
}
}
3.3 GPRS Data Transmission Program Design
The taxi onboard monitoring equipment can transmit real-time vehicle information to the taxi monitoring and dispatch center via a GPRS module so that the center can monitor the taxi's operational status. After receiving GPS data and parsing valid information, the microcontroller stores the information (latitude, longitude, speed, etc.) to be sent to the monitoring and dispatch center in a data transmission buffer for later retrieval during data transmission. This design uses a timer interrupt for data transmission, with a transmission interval of 5 seconds. When the timer overflows after 5 seconds and triggers an interrupt, the data transmission program executes, retrieves the data from the transmission buffer, and sends the data to the taxi monitoring and dispatch center via the GPRS module. The flowchart of the data transmission program is shown in Figure 5b.
4. Conclusion
Currently, vehicle-mounted GPS positioning systems are applied in various civilian and military industries. In the design of the taxi vehicle-mounted monitoring equipment presented in this paper, both the hardware and software adopt a modular design, facilitating upgrades and maintenance. After actual vehicle-mounted testing, the taxi vehicle-mounted monitoring equipment demonstrated stable performance, with the accuracy of data collection and the correctness of data transmission meeting the pre-design requirements. It provides a sound operational platform for taxi company monitoring and dispatching, and the system has significant promotional value and application prospects.
