Distributed data acquisition unit based on GPS and ZigBee for ...

Distributed Data Acquisition Unit Based on GPS and ZigBee for Electromagnetic Exploration Chen Rujun, He Zhangxiang, Qiu Jieting, He Lanfang

Cai Zixing

Non-seismic Survey, Bureau of Geophysical Prospecting, China National Petroleum Corporation Zhuozhou, P. R. China m [email protected], [email protected]

School of Information Science and Technology, Central South University, Changsha, P.R. China [email protected]

Abstract—The design and implementation of the distributed data acquisition unit (DDAU), which is based on GPS synchronization and ZigBee, are described. The DDAU is used in threedimensional electromagnetic exploration targeted for oil and gas (hydrocarbon) detection. It is composed of data acquisition and DSP module, embedded control module, GPS sync and timing module, and power supply module. The data acquisition and DSP module, which owns 4 signal channels, amplifies, converts, and processes weak signals from electric field and magnetic field sensors. The embedded control module, which includes ZigBee OEM board, temperature sensor, 10M bps Ethernet, 4 UARTs, 4 SPIs, 2 SSCs, 8 GB NAND flash and 8 MB NOR flash, is based on AT91RM9200 and Linux 2.6. The GPS sync and timing module, which ensures precise synchronized data acquisition in field, generates sync signal and precise clock for A/D convertor and digital signal processing. A large-scale distributed data acquisition system can be built by combining large amount of DDAUs to a wireless sensor network based on ZigBee. The testing result showed that the RMS noise of each analog channel was less than 0.63 uV, and the THD is less than -111 dB. Keywords-distributed data acquisition unit (DDAU); electromagnetic exploration; GPS synchronization; ZigBee; data acquisition; signal conditioning

I.

INTRODUCTION

As an important geophysical exploration method, the electromagnetic exloration method includes natural resource electromagnetic method (e.g. magnetotelluric method and audio frequency magnetotelluric method) and artificial source electromagnetic method (e.g. controlled source audio frequency magnetotelluric method and controlled source electromagnetic method). It plays an important role in oil and gas (hydrocarbon) exploration [1] [2] [3]. Some successful case histories were reported in recent years [3]. Because hydrocarbon is buried at large depth as 3~4 km in underground, and located in few places, which are very hard to find, a largescale data acquisition system, which can cover an area of more than 100 km2, is needed to receive weak electromagnetic signal reflected from the target which contains hydrocarbon [2][3]. Current data acquisition systems, which are based on high speed wire or wireless communication, cannot offer satisfactory area coverage, because mountain area in field let reliable long-distance communication become very difficult. For example, long-range wireless communication can be blocked by mountains; and the distance between two data

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acquisition units is also limited in plain because the height of their antenna is less than 1 m usually. Therefore, we propose the design and implementation of the distributed data acquisition unit (DDAU), which is based on ZigBee and GPS OEM boards, to build a large scale sensor network that can satisfy the requirement of three-dimensional electromagnetic sounding method targeted for hydrocarbon exploration. II.

HARDWARE DESIGN AND IMPLEMENTATION

A. Overview of DDAU Design TABLE I.

SUMMARY OF UNIT SPECIFICATIONS

Signal channels:

4

Sensor interface:

3-component electric sensor (Ex, Ey, and Ez) 3-component magnetic sensor (Hx, Hy, and Hz)

Input singal range:

±1 mV ~ ± 2000 mV

Input impedance:

2 MΩ

Frequency range:

DC ~ 500 Hz

Amplifier: Gain: filter: RMS input noise: Sample frequency: Sync method: Sync error: Commnucation interface: NAND flash: Embedded system:

two-stage chopper-stabilized amplifier 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, independently selectable for each channel differential radio frequency filter, anti-alias lowpass filter, and digital filters < 0.7 uV @ 0.5~240 Hz, 0dB gain 2400, 1200, 600, 300,150, 75, 60, 30, 24, 15, 12, 6, 3, 0.6 Hz selectable GPS disciplined oven controlled oscillator plus UTC offered by NMEA message < 1 uS RS232, ZigBee, and 10M Ethernet 8~16 GB AT91RM9200 CPU with Linux 2.6

Three major factors, which are signal conditioning, precise synchronization, and sensor network, are considered by making good traded off. Table I lists specifications of the DDAU. According to above specifications, we divide the DDAU to 4 modules, which are data acquisition & DSP module (DADM), embedded control module, GPS sync & timing module, and

power supply module. Figure 1 shows signal connection among these modules. Hx/Ex

Ez

Hy/Ey

Hz

FREQUENCY_LOCK CH1 CH2 CH3 CH4

GPS_LOCK

Data acquisition and DSP module

19.6608MHz 12.288MHz 1Hz/SECOND

ESSI SPI

1/60Hz /MIN

SCI

+/-3.3V ZigBee 10M LAN UART RS232

GPS

SSC SPI UART0 UART2

Embedded control module

RS232

GPS sync and timing module

UART1 +5V

RS232 12V Battery

Isolated UART

Power supply module

+/-15V +12V +/-3.3V

signal, the DADM can acquire signal with time aligned to GPS. This ensures synchronized data acquisition when a large amount of DDAUs are deployed in field for hydrocarbon exploration. The DADM can acquire signals generated by electric field and magnetic sensor. It supports search coil and fluxgate sensor [4]. The embedded control module is the brain of the DDAU. It controls the power supply module, and receives the voltage measurement of a battery, which offers energy in the field, by its UART1. It uploads DSP program to DADM by UART0. The SPI interface is used to control the status of the DADM. The SSC is used to receive acquired data from the DADM. The debug UART is used to debug and update boot program of the embedded control module. The 10M Ethernet interface is used to retrieve stored data inside the DDAU, and to test the DDAU at high speed. The ZigBee interface plays key role in field operation. It receives the command from the host, and sends the status and some data of the DDAU to the host. For threedimensional electromagnetic data acquisition, the spacing between two adjacent DDAU is less 100 m, so it is adequate to build to wireless network based on ZigBee. All modules of the DDAU share the same size to ensure easy installation and high reliability in field operation. Figure 2 shows the 4 modules of the DDAU under test. B. Data Acquisition & DSP Module (DADM)

Figure 1. Signal connection and interface of modules of DDAU Ex/Hx

Ey/Hy

Ez

Hz

CH1 diffamp & filter CH2 diffamp & filter

I/O expander 2-ch ∆-∑ modulator 4-ch decimation filter

CH3 diffamp &filter CH4 diffamp & filter

The power supply module supplies low-noise power for other 3 modules and magnetic sensors. It converts 12 V voltage of a battery to other voltages required by other modules. The GPS sync & timing module sends NMEA message to the embedded control module by RS232, and generates precise minute pulse, second pulse, 12.288 MHz clock, 19.6608 MHz clock, GPS lock indication, frequency lock indication to the DADM. With the support of precise timing signal and clock

SPI 19.6608MHz

2-ch ∆-∑ modulator

ESSI Calibration subsystem

Figure 2. The modules of DDAU in stack. (From top to bottom: digital data acquisition & DSP module, embeded control module, GPS sync & timing module, and power supply module)

GPIO

DSP subsystem

SCI 12.288MHz

Figure 3. Simplified block diagram of data acquisition and DSP module

Figure 3 shows the simplified block diagram of the DADM. The main ICs for signal conditioning are specially designed for geophysical data acquisition by Cirrus Logic. These ICs are adopted in seismometer frequently [5] [6]. The amplifier, ∆-Σ modulator, test DAC, and decimation filter are CS3301A, CS5372A, CS4373A, and CS5376A, respectively [7]-[10]. The 24-bit fixed-point DSP56309 is used in DSP subsystem. The tasks of the DSP subsystem include FFT, decimation, filtering, correlation, and custom data processing required by

electromagnetic exploration. With the support of the DSP, the DADM can conduct different data acquisition method of electromagnetic exploration. The decimation filter CS5376A is controlled by its SPI port. The SPI port can control 13 GPIO ports of CS5376A. We use 11 GPIO ports to expand the number of control signals to 64. Therefore, the gain and operation mode of each CS3301 can be controlled independently. Calibration signal From sensor

data acquisition. There are two tasks mainly for device driver development. The first task is the device driver for SSC, which realizes high speed data communication between the DSP and ARM core. The second task is to change driver setting of UART0 to activate clock signal output for serial communication, because the SCI of DSP56309 requires serial clock to upload DSP program.

Gain and mode control

Input protection & radio frequency filter

Amp1 (1,2,4,8, 16,32,64)

RC filter

Amp2 (1,2,4,8, 16,32,64)

DEBUG UART

64 MB SDRAM

UART0~3

8 MB NOR FLASH

GPIO

8G NAND FLASH USB Host

SPI AT91RM9200 To ∆-∑ modulator

RC filter

Figure 4. Simplified block diagram of signal conditioning

Figure 4 shows simplified block diagram of the signal conditioning circuit for each channel. There are two CS3301s for each channel, and they can be controlled independently. Differential RC filter and radio frequency filter are adopted for good interference immunity.

POWER

USB Device

RESET

RTC

10/100Mbps Ethernet

Temperature sensor

SSC

ZigBee OEM board

Figure 6. Block diagram of embedded control module

Figure 5. An implementation of the DADM

Figure 7. An implementation of the embedded control module

Figure 5 is the picture of an implementation of the DADM. Eight-layer PCB is adopted for good EMI suppression.

Figure 7 shows an implementation of the embedded control module. Sixlayer PCB is adopted the for this moudule.

C. Embedded Control Module Based on AT91RM9200 AT91RM9200 is a RISC controller of ARM9 series. It has enough resources to control peripherals. We custom the embedded control module by making some improvement compared to the evaluation board issued by ATMEL. A temperature sensor, a large storage NAND flash, a real time clock, and an interface for ZigBee OEM board are added in this module (See Figure 6). Furthermore, the porting of Linux 2.6 and the development of some device drivers are required for

D.

GPS Sync & Timing Module In GPS sync & timing module, we use PPS to discipline an OCXO to ensure precise clock signal generation. Two clock signals of 12.288 MHz and 19.6608 MHz are aligned to PPS. The minute pulse, second pulse, GPS lock indication, and frequency lock indication signals ensure all DDAUs to acquire data at given time simultaneously. Because two clock signals are aligned to PPS, DDAUs can last data acquisition for several

months with sync error < 1 uS. This is the key to realize distributed data acquisition with precise synchronization. E. Power Supply Module 12V input

Regulator & +/-3.3V LC filter

DC/DC & LC filter

DC/DC & +/-15V LC filter GPS power DC/DC & +5V LC filter Regulator & LC filter

+5V

Control signal

Battery voltage measurement

89C51 uC system UART

Voltage Converter RS232

Control logic

Multiplexer UART

Power supply

Figure 8 shows block diagram of the software for DDAUs. Most of the software modules execute on embedded control module, including the main control module. The main control module receives request from remote monitor and control module that execute on the host, and activates different task according the request. The digital signal processing module executes on the DSP, and the power control and monitor module executes on the 89C51 microcontroller. The remote monitor and control module can send commands through wired network or ZigBee network. High speed data transfer is required for indoor testing, so we use wired network. In the field environment, acquired data is stored inside DDAU, we use ZigBee network just to monitor the status of DDAUs.

Power control

Optoisolator

Figure 8. Block diagram of power supply module

For large-scale electromagnetic exploration, more than 1000 DDAUs are deployed in field. Because each DDAU is powered by a battery, and the deployment of all DDAUs will spend one month or more, it is necessary to manage power supply. When a DDAU is in idle state, most of the power supplies are off. So the life of a battery is saved. Figure 8 shows the block diagram of the power supply module. We use an 89C51 microcontroller to control power supply and to monitor the voltage of a battery. When the voltage of a battery is less than threshold value, the DDAU can send an alarm to the host; and the battery can be replaced by people in the field.

IV.

We carried out a series of tests to evaluate the performance of the DDAU. The tests include gain test, noise test, total harmonic distortion (THD) test, ZigBee communication test, clock error test, and so on. The test result shows that the performance of the DDAU reaches the requirement of the design. When the amplifiers and test DAC CS4373 were not calibrated, the maximum gain error is 0.7%. This error is less than the error limit given by handbooks [7] [9]. Another important test is noise test. According to Figure 4 and the handbook of CS3301A, there are several ways to conduct noise test. When a signal passes all analog parts of the signal conditioning section, the input noise is maximum. Table II is input noise test result in above condition. The following is test condition: the sample frequency is 600 Hz; the total sample number is 1200 points; and the offset is removed before the calculation of RMS value. Table II shows that the input noise of the DDAU is lower than that of seismometer given by [6].

LC filters are added to the output of DC/DC modules and regulators to suppress power supply noise. III.

DDAU calibration module GPS control & monitor module

Data acquisition module for electromagnetic exploration

Magnet sensor calibration module

Time series acquisition module Self-testing module

Main control module

Telemetry module by TCP/IP

TABLE II. Gain Setting Amp1 Amp2

SOFTWARE DESIGN

Remote monitor and control module

THE TESTING OF THE DDAU

TEST RESULT OF INPUT NOISE Test Result in RMS (uV) Channel 1

Channel 2

Channel 3

Channel 4

1

1

0.58

2

1

0.31

0.31

0.32

0.31

4

1

0.17

0.17

0.18

0.17

8

1

0.11

0.11

0.11

0.12

16

1

0.09

0.09

0.09

0.09

32

1

0.08

0.08

0.08

0.08

64

1

0.08

0.08

0.08

0.08

TABLE III. Digital signal processing module

Power control and monitor module

Telemetry module by ZigBee

Figure 9. Block diagram of the software

Sample Frequency

0.60

0.61

0.62

RESULT OF THD TEST

Channel 1 (dB)

Channel 3 (dB)

Channel 4 (dB)

600 Hz

-112.73

Channel 2 (dB)

1200 Hz

-112.31

-111.76

-113.44

-111.44

2400 Hz

-112.46

-111.86

-113.46

-111.85

-111.96

-113.62

-112.27

REFERENCES THD is another important factor which determines the performance of the DDAU. We carried a series of THD test under different sample frequency, gain, and channel structure. Table III shows THD test result at different sample frequency when the gain of Amp1 and Amp2 is set to 0 dB. Because the THD of signal generated by CS4373A ranges from -112 dB to -116 dB, the test result is acceptable. V.

CONCLUSION

The design and the test of the DDAU are described in this paper. The DDAU’s data acquisition is synchronized to GPS; and a large amount of DDAUs can be organized as a large wireless network with the support of ZigBee OEM board. The test shows that the input RMS noise of the DDAU is less than 0.63 uV at 0 dB gain, and the THD of the DDAU is less than -111 dB at 0 dB gain. This can satisfy the requirement of threedimensional data acquisition for electromagnetic exploration for hydrocarbon. ACKNOWLEDGMENT We thank Mr. Liu Xuejun, Mr. Qiu Kailin, Mr. Lu Xianghong and Mrs Wan Haizheng for their conbribution in the project.

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