Design of digital north finder based on ARM9 and MMC212xMG

Introduction

The geomagnetic field is the basic physical field of the earth system. People have used geomagnetic information for navigation since ancient times. The geomagnetic field provides a natural reference system for aviation, aerospace, and navigation, and can be used for the positioning, orientation, and attitude control of spacecraft or ships. The magnetic navigation technology that uses the spatial distribution of the earth's magnetic field is simple, efficient, reliable, and has strong anti-interference capabilities. It has always been an indispensable basic positioning method for developed countries in the world.

MMC212xMC is a 2-axis MEMS geomagnetic sensor with integrated signal processing module and I2C bus launched by MEMSIC. This paper designs a digital north finder based on embedded system ARM9 and geomagnetic sensor MMC212xMG.

1 System Solution

In order to ensure the superior performance of the north-seeker, its control and processing module adopts an embedded system. Embedded systems are generally used in the design of handheld instruments and portable systems. They are convenient and flexible, powerful, and highly embedded. They can realize functions such as calculation, processing, storage, and control. Embedded systems include hardware and software, namely embedded microprocessors and embedded operating systems. The north-seeker designed in this paper adopts S3C2440A microprocessors and WinCE 4.2 operating systems.

1.1 MMC212xMG geomagnetic sensor

MMC212xMG is a 2-axis MEMS (Microelectromechanical System) geomagnetic sensor based on AMR (Anisotropic MagnetoResistive) design, integrated with signal processing module and I2C bus (400 kHz fast mode), which can easily communicate with other controllers without A/D conversion and timer. The sensor can measure the magnetic induction intensity range of -2 to +2 Gs (1 Gs = "10-4T"), and its sensitivity is 512 counts/Gs at 3.0 V operating voltage and 25°C room temperature.

1.2 S3C2440A microprocessor

Samsung's S3C2440A is a 32-bit RISC embedded microprocessor based on the ARM920T core. Its main frequency is 400MHz, it adopts a 5-stage pipeline structure, and has a high-speed cache and memory management unit; it provides rich external resources, including 130 general I/O interfaces and 24 external interrupt sources, NAND Flash interface, SDRAM interface, SD card interface, 100M Ethernet interface, USB interface, touch screen interface, CMOS image sensor interface, SPI interface, I2C interface, I2S interface, PWM function, embedded LCD controller, 8-channel 10-bit ADC, power management, etc.

1.3 WinCE 4.2 operating system

WinCE is an embedded real-time multitasking operating system launched by Microsoft. It extends the external features of the desktop Windows operating system, uses the technology of embedded real-time operating system to implement a subset of Win32 API internally, takes concise and efficient fully preemptive multitasking as the core, supports powerful communication and graphic display functions; and its platform customization tool PlatformBuilder and application software development tool Embedded Visual C++ are both very powerful and practical development tools, providing a standard platform for software development of embedded portable instruments.

1.4 Solution Design

The system block diagram of the north finder is shown in Figure 1. The S3C2440A is used as the control core, and communicates with the geomagnetic sensor MMC212xMG through the I2C bus. In the WinCE 4.2 embedded operating system environment, the data analysis, processing and display program is developed.

2 Hardware Design

The operating power supply range of MMC212xMG is relatively wide, with the analog circuit operating power supply VDA ranging from 2.7 to 5.25 V and the digital circuit operating power supply VDD ranging from 1.62 to 5.25 V. The operating power supply of S3C2440A is 3.3 V. In order to ensure reliable communication of the I2C bus, the system is uniformly powered by +3.3 V; at the same time, +3.3 V is divided into analog power supply and digital power supply to improve the system's anti-interference ability.

The pin functions of MMC212xMG are listed in Table 1. Pins 1 and 8 are factory test pins and can be left unconnected when in use. Pin 2 is connected to a 10μF capacitor, and a 1μF capacitor is connected between pins 4 and 5 to implement the chip's set/reset function. Pins 3 and 7 are the analog power supply and digital power supply of the chip, respectively. Pin 6 is the ground pin of the chip. Pins 9 and 10 are the clock pin and data pin for I2C communication, which are pulled up to the digital power supply.

The working circuit of MMC212xMG is shown in Figure 2. I2C bus pins 9 and 10 are connected to S3C2440A. During I2C communication, S3C2440A is the master device and MMC212xMG is the slave device. The master device communicates with a specific slave device through the slave device address to perform read and write operations.

3 Software Design

In the embedded operating system WinCE 4.2 environment, EVC is used to develop geomagnetic measurement information acquisition, processing and display programs, mainly including measurement data reading based on I2C bus, data processing of geomagnetic measurement signals and virtual instrument interface design.

3.1 Reading measurement data based on I2C bus

In order to facilitate the connection of multiple geomagnetic sensors in I2C bus communication, MMC212xMG has 4 different device addresses (factory defined), namely 60H, 64H, 68H and 6CH. The device address used in this system is 60H. The internal register of MMC212xMG has 5 bytes. The first byte is the device control command register, which can be set with a command value to start the corresponding action. The definition of each command control word is listed in Table 2.

The 2nd and 3rd bytes of the internal register are the X-axis magnetic field measurement values, and the 4th and 5th bytes are the Y-axis magnetic field measurement values. The command flow for starting the MMC212xMG for measurement is shown in Figure 3.

After the start measurement command is completed, a delay of 10 ms is performed to read the measurement data. The process is shown in Figure 4.

3.2 Data processing of geomagnetic measurement signals

In order to eliminate the jitter on the display, mean filtering can be used, that is, collecting geomagnetic measurement signals 5 times, calculating the azimuth, and taking the average value for display.

In order to improve the pointing accuracy, nonlinear calibration technology can be used. At intervals of 45°, let the geomagnetic sensor module point to eight directions: due north, northeast, due east, southeast, due south, southwest, due west and northwest. After measuring the geomagnetic azimuths of these eight directions multiple times and taking the average value, the azimuth is stored as a calibration table. In the actual measurement process, after obtaining the measurement data, the calibration value is used to perform linear interpolation on it to obtain the azimuth.

3.3 Virtual instrument interface design

The display interface of the North Finder is designed to simulate a mechanical compass to achieve an intuitive display effect. EVC provides a rich interface drawing API function, which can easily realize the design of the virtual instrument interface. During the operation of the software platform, multi-threading technology is used to read the measurement information of the geomagnetic sensor in real time, and after data processing, it is sent to the virtual instrument interface for display.

4 System Demonstration

The actual picture of the north finder is shown in Figure 5. The system consists of two parts: an embedded system module composed of S3C2440A and a geomagnetic sensor module composed of MMC212xMG. Figure 5(a) shows the effect of the geomagnetic sensor module pointing to the north, and Figure 5(b) shows the effect of pointing to any direction.

In actual use, the embedded system module and the geomagnetic sensor module can be installed as a whole to form a portable north finder; or they can be installed separately, and the embedded system module can simultaneously obtain information from other sensors (such as acceleration sensors, GPS sensors, etc.) to form a combined navigation system.

Conclusion

This paper designs a digital north finder using the embedded microprocessor S3C2440A and the MEMS geomagnetic sensor MMC212xMG. Compared with the mechanical compass, the system has the advantages of user-friendly human-machine interface, no inertia influence, and data storage. After long-term operation test, the north finder works stably and can meet the requirements of high-precision north finding.