Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

With the development of seismic survey technology in the direction of fine measurement, it is necessary to conduct in-depth research ON seismic detection technology with bandwidth, high sensitivity, and low distortion. At the same time, a set of geophones should be placed on the survey site in a certain way, and the data of this group of geophones should be analyzed comprehensively to obtain corresponding survey results.

Authors: Zhang Cheng; Wang Jinhai; Chen Caihe; Zhang Bo; Yue Quan

With the development of seismic survey technology in the direction of fine measurement, it is necessary to conduct in-depth research on seismic detection technology with bandwidth, high sensitivity, and low distortion. At the same time, a set of geophones should be placed on the survey site in a certain way, and the data of this group of geophones should be analyzed comprehensively to obtain corresponding survey results.

Based on the principle of seismic survey, this paper proposes a scheme for constructing a seismic survey sensor network: transmitting the information of each node to a monitoring PC, using virtual instrument technology, using Labview to write measurement and control software running on the PC, and perform corresponding data analysis and processing ; Based on the all-fiber Michelson interferometer system, using AC phase tracking homodyne detection technology (PTAC) to achieve accurate detection of the signal under test and error signal compensation, reducing the impact of signal drift on the system; using C8051f020 single-chip microcomputer to demodulate The signal is sampled, and the relevant data is transmitted over the network through the UDP/IP protocol. The solution realizes an intelligent seismic survey sensor network node integrating signal processing and network communication.

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

1 System overview and working principle

The seismic survey sensor network designed in this paper is composed of various sensor nodes and monitoring master nodes distributed in the test site. The local area network is constructed based on the Ethernet structure to realize data communication based on the UDP/IP protocol. The system structure of the sensor network is shown in Figure 1. When conducting seismic surveys, each sensor node demodulates the modulated signal containing seismic acceleration information output by the interference system, and performs sampling, A/D conversion and storage of the demodulated signal, and then according to the command of the master node The demodulated information is transmitted to the master node for analysis, thereby realizing distributed monitoring and information processing.

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

The sensor node is mainly composed of three parts: detector, intelligent control unit and network interface. The detector demodulates the modulated signal output from the photodetector (PIN) based on the PTAC principle, and outputs the carrier signal and compensation signal to the phase modulator in the interference system. This part is designed with an analog Circuit method to ensure the solution Real-time tuning. The intelligent control unit is mainly composed of a single-chip microcomputer and a memory, which realizes the data acquisition and storage of the demodulated signal, and realizes the communication with the monitoring host through the network interface.

2 PTAC demodulation principle

The principle block diagram of the PTAC demodulation system based on the all-fiber Michelson interference system is shown in Figure 2. The light emitted by the laser is split into two beams by a 3dB fiber splitter through end-face coupling, and propagates in the reference arm and the signal arm respectively. After being reflected by the high-reflection film, it returns along the original path and interferes in the coupling zone. The output light intensity is :

In the formula, I

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

Use PIN to detect the light intensity signal and get the current signal. After being amplified by the pre-Amplifier circuit, the output voltage can be expressed as:

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

In the formula, kv is the coefficient determined by PIN and preamplifier.

Expansion of (3) into a Fourier-Bessel series is:

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

Through analysis, it can be known that the measured signal is applied as a sideband signal to the frequency band of an integer multiple of the carrier frequency, and the signal is filtered by a bandpass filter with a center frequency of ωc, and the following is obtained:

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

In the formula, k3 is the proportional coefficient.

Assume △φs

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

Take υ3

3 Hardware design of sensor network nodes

The hardware design of the sensor network node mainly includes two parts: the design of the demodulation circuit based on the PTAC algorithm; the design of the intelligent control unit and the network interface with the single-chip microcomputer as the core.

3.1 Demodulation circuit design

According to Figure 2, the design of the demodulation circuit is mainly divided into the following parts:

(1) Pre-amplification circuit: select OPA637 integrated operational amplifier (it has the advantages of high open loop gain, small input bias current, low offset Voltage, large input impedance, etc.) connected to the form of a DC parallel negative feedback current amplifier circuit. The current of the order of microampere output from the PIN is converted into a voltage of volts. The feedback resistance is 1MΩ.

(2) Multiplication circuit: The multiplication circuit is required to have the characteristics of no DC drift, low error, and low noise. Choose AD534 as the basic device, use AD534’s X1 as the signal input, and Y1 as the local oscillator signal input. X2, Y2, and Z2 are used as signal, oscillating signal, and output DC offset adjustment, respectively, and Z1 is used as a feedback signal to stabilize the output.

(3) Design of band-pass filter and low-pass filter 1: Use MAX274 and its peripheral circuit to realize the design of band-pass filter and low-pass filter 1 for demodulation. Because the Butterworth approximation has the largest flat amplitude, considering that no additional distortion is generated in the passband, the Butterworth approximation method is used to design the filter. Since the carrier frequency is set to 10kHz, the passband range of the bandpass filter is set to 7.5k~12.5kHz, and the center frequency is 10kHz. Since the seismic survey signal frequency band is generally within 1kHz, in order to ensure a flat spectral characteristic curve in the passband, the cutoff frequency of the low-pass filter 1 is set to 1.5kHz.

(4) Design of low-pass filter 2: The function of this filter is to filter out part of the low-frequency components in the output signal of the multiplier as a compensation signal. Because this part of the low-frequency component is caused by the unequal length of the two interference arms and temperature changes, it is a slow variable, and the frequency of this component is usually not greater than 0.1Hz, so the second-order Butterworth approximation method is also used to design the low-pass filter , Its schematic diagram is shown in Figure 3.

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

(5) Design of local oscillator: PTAC demodulation algorithm requires a sinusoidal signal for demodulation and output as a carrier signal to the phase modulator on the reference arm. ICL8038 is selected to generate the sinusoidal signal, and the peripheral resistance is adjusted. Make its output frequency 10kHz.

3.2 Design of intelligent control unit and network interface

This part is mainly divided into the design of data acquisition module and network transmission module. Use C8051F020 microcontroller as the core of this part of the design.

Utilize the on-chip A/D converter of C8051F020 one-chip computer to transform the analog signal into the digital signal. The accuracy of the converter is 12bit, and the conversion speed can reach 100KSps, which can meet the sampling requirements of the demodulated signal. Using the internal integrated A/D converter not only reduces the complexity of the design, but also reduces the noise interference.

The network transmission module selects the 10Mbps RTL8019AS chip as the network interface. In order to reduce the connection between RTL8019AS and C8051F020, the address/data line multiplexing method is adopted, and 74LS373 is used for address latching. The interface circuit of RTL8019AS and C805/F020 is shown as in Fig. 4.

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

In the figure, ALE is the latching permission control signal of 74LS373. The /RD and /WR pins of C8051-F020 are directly connected with IORB and IOWB of RTL8019AS to control RTL8019AS to read and write external data. Map the relevant register address of RTL8019AS to the storage address of C805/F020, and set the register of RTL8019AS by reading and writing external storage address instructions. RTL8019AS works in query mode, and its reset is directly controlled by the P5.2 pin of C8051F020, thereby improving the reliability of reset. Through controlling the relevant register of RTL8019AS, realize the transmission of network data.

4 Software design and porting of network protocol stack

This design realizes the transplantation of UDP, IP, ARP and other protocols on the C8051F020 microcontroller. Based on the idea of ​​embedded system design, the UDP/IP protocol is tailored. According to the design of the monitoring network system structure is not very complicated, and the network traffic is not very large, remove the network layer related routing and other protocols, customize a streamlined UDP/IP protocol stack suitable for this design. Based on the client/server model, the monitoring PC is used as the server, each sensor node is used as the client, and the UDP protocol is used for communication. According to the principle of UDP/IP protocol, four sub-Modules of ETH layer, network layer, transport layer and application layer are designed. When testing is needed, the monitoring PC sends out sampling commands to each sensor node in the form of broadcast. After the sensor node receives the command, it starts sampling and stores the sampled data in the buffer. When the buffer is full, the node identification information and sampled data are encapsulated into a UDP message and sent through RTL8019. Use the timer interrupt mode to sample the demodulated signal to avoid the influence of the sequential execution mode on the network communication program. The data buffer is shared between the sampling module and the communication module, and communication is carried out through the semaphore, which improves the execution efficiency of the program. After the monitoring PC receives the message, it analyzes and processes the data of different sensor nodes according to the node identification information. When the monitoring is over, a stop command is sent to each sensor node in the form of broadcast again to make each node stop sampling. The main program flow of the system is shown as in Fig. 5.

5 Experimental results

The experiment is divided into two parts: demodulation circuit test and network communication test. In the experiment of testing the demodulation circuit, the testing technology of virtual instrument is adopted to make the testing process easier. Use NI’s Labview software and 6221 data acquisition module to simulate the output of PIN as the input signal of the demodulation circuit, and use Labview software to collect the demodulated signal and observe the demodulation result. For the convenience of testing, a sinusoidal signal is used as the signal to be tested, and a temperature drift signal of 0.1 Hz is introduced. The carrier signal frequency is 10 kHz, and the phase change caused by it is π. The multi-group demodulation situation with the phase change caused by the measured signal in the range of 0.1-πrad and the frequency in the range of 10-1000 Hz was tested. Figure 6 shows the demodulation result when the measured signal frequency is 100Hz and the phase change caused by it is 0.1rad. It can be seen from Figure 6 that the phase consistency between the demodulated signal and the measured signal is better, and the distortion is small.

Design of seismic survey sensor network based on C8051f020 single-chip computer and UDP/IP protocol

Figure 7 is a comparison diagram of the temperature drift signal and the compensation signal under the experimental conditions. It can be seen from the figure that the phase of the compensation signal is opposite to the temperature drift signal, which verifies the suppression effect of the compensation feedback on temperature drift.

Through experiments, the demodulation circuit can demodulate the measured signal whose phase change is 0.1-πrad and the frequency is 10-1000 Hz.

In order to conduct network communication experiments, a simple Labview software is designed to receive data from sensor nodes and perform spectrum analysis on the data. Figure 8 shows the FFT analysis of the waveform demodulated by the experimental sensor node by Labview software, which verifies the network communication function of the sensor node and the feasibility of the distributed processing.

Through experimental tests, the seismic detection sensor node designed in this paper can demodulate acceleration signals from 10 to 1000 Hz, and has the characteristics of low distortion and strong anti-electromagnetic interference capabilities. It can be easily networked to realize distributed information processing based on sensor networks.

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