Design of Data Acquisition and Processing System for IMU

Chunpeng Kang1, Zhong Su 2 1Institute of Intelligence Control，

Beijing Information Science and Technology University, Beijing, 100101, China 2 Institute of Intelligence Control，

Beijing Information Science and Technology University, Beijing, 100101, China yongren2006@126.com, sz@bistu.edu.cn

Abstract

Inertia Measurement Unit (IMU), which is mainly used to measure triaxial acceleration and angular velocity, is an important part of the navigation control system. Data acquisition for gyroscopes and acceleration in IMU is primarily explored. A design scheme based on AT91RM9200 and embedded Linux operating system is introduced. Before analogy signals from IMU are imported into A/D converter, they should be filtered by pre-treatment circuit, and then processed by Micro-computer. Lastly, direction and attitude can be exported through RS232.System driver is developed by Linux, and software compiler mainly employs U-boot 1.3. When the supply power is 5± V, compare with the data from system and Fluke 8845A with high accuracy. It’s proved that the data acquisition system is achieved the needs of design. Moreover, the system is small in size, low in power consumption, and good in stability and practicality. In the military field, it has a wide use. 1. Introduction

IMU contains three gyroscopes and three accelerometers. Input axises of these sensitive components is configured in 3D direction according to roller, pitch and yaw axis of flight carrier, which comes into being a three-dimensional inertial coordinate system. In strapdown inertial navigation systems, inertial components are riveted in the carrier, which makes the inertial coordinate system parallel with the flight carrier coordinate system. Output vector information of gyroscopes and accelerometers is attitude and course relating with inertial space. Therefore, IMU data acquisition and processing in the navigation control system is extraordinarily importance. In recent years, with the DSP, ARM technology developing, people also make more require, such as smaller size, lower power consumption, higher accuracy, reliability, and dynamic.

Data acquisition is an important means of access to data, it is a process that measured from the sensor equipment or other analog or digital units collected information automatically. Based on AT91RM9200 as the core processor, design an IMU

high-speed data acquisition and processing system with low-cost, small size, low power consumption, good stability. 2. Scheme Design

According strapdown inertial navigation system requirements, the accelerometers and gyroscopes transmits signal to A/D conversion module, and AT92RM9200 microprocessor receive input information from A/D module ,through attitude arithmetic, data integration, it gets direction and attitude, and real-time data is transmitted through the serial port. AT91RM9200 is configured an erasable memory, it can store attitude arithmetic programme debugged but also can be used to store the collected data, thus, it greatly enhances practicability of the IMU. Overall hardware structure of the system as shown in Figure 1: 3. Hardware structure 3.1 IMU pretreatment circuit

IMU output signal is very weak, and much interference. The noise contains too much random noise. In order to ensure data accuracy. So it must be filtered to weak the high harmonics or higher frequency interference and the noise before sampling.

The system uses active filter circuit which constitutes integrated operational amplifier, R and C components. So the system has advantage of no inductors, small size, and light weight. Integrated operational amplifier open-loop voltage gain and input impedance are high, the output resistor is small. So it plays a role of voltage amplification and buffer. 3.2 CPU Module

The AT91RM9200 is a complete system-on-chip built around the ARM920T ARM Thumb processor, which includes Memory Management Unit, 8-KByte Data Cache and 16-KByte Instruction Cache. Working in 180MHZ, the computing speed can up to 200MHZ. And it incorporates a rich set of system and application peripherals and standard

International Symposium on Intelligent Information Technology Application Workshops

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DOI 10.1109/IITA.Workshops.2008.82

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interfaces in order to provide a single-chip solution for a wide range of compute-intensive applications that require maximum functionality at minimum power consumption at lowest cost.

The ARM9TDMI is a member of the ARM family of general-purpose microprocessors. It supports both the 32-bit ARM and 16-bit Thumb instruction sets, allowing the user to trade off between high performance and high code density. And it also supports the ARM debug architecture and includes logic to assist in both hardware and software debug.

The AT91RM9200 incorporates a high-speed on-chip SRAM workspace, and a low-latency External Bus Interface (EBI) for seamless connection to whatever configuration of off-chip memories and memory-mapped peripherals is required by the application. The EBI incorporates controllers for synchronous DRAM (SDRAM), Burst Flash and Static memories and features specific circuitry facilitating the interface for NAND Flash/SmartMedia and Compact Flash.

Fig-1. Overall hardware structure

FLASH ROM is used to storage bootstrap program,

the kernel of operating system and applications. Select two 16 M × 8, the expansion of 32 M ROM, the room is assigned to the NCS0 address space of the AT91RM9200.

SDRAM is used to temporarily memory operating system and related data in running system.

In order to improve the speed, two 32 M × 8, the expansion of the 64 M SDRAM, the room is assigned to the NCS1 address space of the AT91RM9200. 3.3 A / D Converter

There is much more data needed to acquire, and the real-time requirement is relatively high, so the MAXIM MAX1168 chip is employed to design. The MAX1168 low-power, multichannel, 16-bit analog-to-digital converters (ADCs) feature a successive-approximation ADC, integrated +4.096V reference, a reference buffer, an internal oscillator, automatic power-down.

AIN0~7: Analog input. DIN: Serial Data Input. Use DIN to communicate with

the command/configuration/control Register. In SPI/QSPI/MICROWIRE mode, the rising

edge of SCLK clocks in data at DIN. DOUT: Serial data output. When it was high, DOUT is

the high resistance state. DSEL: Data-Bit Transfer-Select Input. Logic low on

DSEL places the device in 8-bit-wide data transfer mode. Logic high places the device in 16-bit-wide data-transfer mode. Do not leave DSEL unconnected.

SCLK: Serial clock input. When the device work in the external clock mode, SCLK control entire conversion process and the output data.

COE : End-of-Conversion Output. In internal clock mode, a logic low at EOC signals the end of a conversion with the result available at DOUT. In external clock mode, EOC remains high.

Connect SCLK port to IRQ4SPCK/PA2 pin of ARM. AT91RM9200 produce crystal to drive the MAX1168 chip. DOUT and DIN pin connect with MOSI and MISO pins of AT91RM9200 respectively. The chip-select signal is generated by the PCSO, Schematic is as follows (Fig-2): 3.4 Serial Communication Interface

RS-232, called asynchronous ports or a COM (communications) ports, is used to communicate with the PC. On the one hand, PC real-time receives parameters, such as direction and attitude. On the other hand, the direction and attitude of the working parameters can be set through the PC. Owing to the output voltage of S3C44B0X is from 0 to 3.3 V, it doesn’t match with RS232 standards voltage, so it’s necessary to do electrical conversion. The system employs MAX3232 chip to achieve this function in the paper, Schematic is as follows (Fig-3):

8bit

8bit

Attitude

Direction

Gyroscope

Pre-treatment Circuit

16-bit A / D converter

FLASH

RS232

Temperature Compensation

AT91RM9200

SDRAMIMU

Acceleration

586

Fig.-2 A / D Converter Circuit

Fig.-3 Serial Communication Interface Circuit

4. Some Common Mistakes 4.1Flow chart of the main software system

The flow chart is showed as in Fig-4. (1) Design of transplantation main program and

operating system The design of the system main program is developed

object / host mode, compile in environment of cross compiler of host, generate executable binary files in target board, download to the target board through serial ports and implement. Development environment uses ARM-Linux cross compiler environment (cross-2.95.3), Linux kernel is Linux-2.4.19, System cross-compiler environment establish under the host usr/local/arm/2.95.3/bin.

(2) U-boot Compile U-boot.1.3 is used to design this system, because it

comes with drivers of Ethernet interface chip and serial ports. It needs to make configuration modified in structure directory accord with hardware configuration, and then performance make clean; make proper; make at91rm9200dac-config; make all, the system can be generated the binary image file U-boot. bin adapted with this system. 4.2 Data Acquisition Software Design

Fig.-5 Flow chart of Data Acquisition

Fig.-4 Flow chart of the main software system

5. System Testing

Start up

Initial ports and register

/CS=0?

Data Acquisition

Wait

NCheck Normally

Start up

Induct from ROM

Load Linux EMS memory，drive

program，Run main process

User Setup

Data Acquisition

Data Storage

End

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There is some problems in debugging the circuit, such

as the A/D conversion result is nonlinear and one channel disturbing another channel. It is find that these problems were caused by too short delay of control signal, so these problems can resolved in software.

The power supplies of data acquisition system is 5± V. Serial port program of upper computer is programmed by MFC.It’s used to receive 6-way data from the gyroscopes and accelerometers. At the same time, employ a high accuracy multimeter to receive the data, and the multimeter is adopted FUKE 8845A, which has 6.5 digital resolution. Then compare the data with PC data to correct the system to zero. We can see the data from TABLE I. 6. Conclusion

From the table we obtained, the data are high accuracy, and real-time is fully satisfied the requirements of attitude measurement. At the same time, replacing the A/D converter, digital filtering and increased compensation algorithm can further improve the system accuracy.

TABLE I Data Compared Fluke

8845A System

Acquisition

1 0.02123 0.021656

2 0.50374 0.504125

3 1.00343 1.003813

4 1.5043 1.504680

5 2.0010 2.001271

6 2.5050 2.505280 7 2.9981 2.998257

8 3.4988 3.498840

9 4.0067 4.006650

Acknowledgements

This work is supported by Fund of Beforehand Research of National Defense.

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