Hong-li LYU，Cui-ying DONG

Department of Information Engineering，Tangshan College，Tangshan 063000，China

1.Introduction

As far as urban water supply system is concerned in our country，the leakage of water supply pipeline could cause serious water loss at each year［1］.Therefore，how to develop the instruments for pipeline to monitor and position the leakage is a new promising research subject.Since the data used to detect the pipeline leakage have a direct impact on the leakage position accuracy for the pipeline，the data acquisition system becomes very important to determine the leakage position of pipeline［2-4］.

2.Theory of correlation detection of pipeline leakage

The schematic diagram of correlation detection is shown in Figure 1［5-6］.The leakage signal travels in two directions with the velocityv.The symbolL1andL2stand for the distance from 1#and 2#sensor to the leakage position.The time difference of the signal arrival the two sensors is called time delay and is symbolizedD.The distance between the two consecutive sensors is symbolized asL.According to the similarity of the two signals，the peak value of correlation function could be obtained and the time delayDcould be evaluated and then the leakage position could be is determined.The distance from leakage position to 1#sensor isL1and it could satisfy the following formula.

Figure 1.The principle of correlation detection

3.Control scheme for signal acquisition system

Due to the different material of pipeline，diameter of pipeline and shape of leakage position，the frequency distribution of leakage signal could vary in a relative big range.The frequency range of water leakage sound signals for the metal pipeline is from 500 Hz to 3 500 Hz［7］.Since the water leakage signal is transmitted through soil to ground，the amplitude of this signal could be decreased.Based on the on-site field measurement，the frequency range of leakage sound is always between 100 Hz and 700 Hz which could be detected from the ground.Therefore，the accelerometer is needed in the system to measure the vibration signal.According to the principle of correlation detection，the signal acquisition system should simultaneously obtain these two signals.

Since the obtained signals from the two sensors are weakened and there exist some noise signals，the signal adjustment circuit is designed to amplify and filter the noise signals.And then the signals will be converted into digital signals through the A/D chip and these digital signals could be saved by singlechip microcontroller(SCM).Because there exist some distance between the consecutive two sensors and various signals are stored by the main SCM，the data transmission is necessary and it is selected as RS-485.In order to obtain the real-time analysis for this system，a DSP chip is adopted to realize the complex algorithm for correlation detection theory and the RS-485 is used for the data transmission between the main SCM and DSP.The control scheme diagram for signal acquisition system is shown in Figure 2.

Figure 2.The system control scheme diagram

4.Design of the signal adjustment circuit

The signal adjustment circuit consists of amplification circuit and filter circuit.

4.1.Design of the signal amplification circuit

The leakage signal has relatively high dynamic range and is seriously affected by the outside environment.If the fixed-gain amplifier is adopted for the signal acquisition system，there exist some problems for the signals such as insufficient amplification or saturated amplification.Therefore，the adjustablegain amplifier should be selected and the dynamic gain range is between 0 dB and 32 dB.As shown in Figure 3，it is presented the signal amplification circuit with the chip NE5532，and the gain of this circuit could be adjusted by modulating the resistance value of R4.

Figure 3.Signal amplifying circuit with NE5532

4.2.Design of the signal filter circuit

The maximum limit of high frequency components for leakage signal is 3 500 Hz and the frequency bandwidth of noise is relatively wide，therefore the low pass filter is needed for the design of the filter circuit.Since the elliptic filter is one of the low pass filters and it has the minimal pass-band and stopband ripple as compared with other filters，this kind of filer is adopted in this paper.As shown in Figure 4，the elliptic filter circuit with chip LTC1068 is presented and the cut-off frequency is 3 500 Hz and the pass-band gain is 0 dB.

Figure 4.Signal filtering circuit

4.3.Hardware design for low noise and low power consumption

The signal amplification circuit could magnify the output signal of the sensor;however，at the same time the noise could be produced.In order to control the noise signal and prevent the signal pollution during the amplification process，the low noise components is selected for the circuit design，such as metal film resistor，field-effect tube，Rail-to-Rail device for integrated operational amplifier.

In order to adapt to the water pipeline leakage detection of outdoor working environment，the signal acquisition system is needed to be powered by a battery.Therefore，the system has the requirement for the low power consumption.Due to this reason，the SCM is selected to be the processor because it has relatively low power consumption characteristics as compared with other embedded processors.In addition，the low power devices meet the requirements for the design of the peripheral circuit and Lithium-ion battery of high energy density could be used in the real industrial applications.

5.Software design

After power on and initialization，the two SCM will wait for the instructions from the main SCM through RS-485 bus.Once the data acquisition instruction is received，the A/D chip will get started and the leakage signals could be obtained and saved.The saved data will be transmitted to DSP through RS-485 bus.The program flow chart of main SCM is illustrated in Figure 5.

Figure 5.Program flow chart of data acquisition system

6.Conclusion

The leakage signals are simulated by signal generator and the peak-to-peak voltage amplitude is 500 mV and frequency is 1000 Hz.These signals will be adopted as the input signals for the signal adjustment circuit.The peak-to-peak voltage amplitude of output signals from adjustment circuit is 4 V.As shown in Figure 6，it is presented the input and output simulated waveforms for signal adjustment circuit.This demonstrates that the magnification factor of signal amplification circuit is eight.

Figure 6.The input and output simulated waveform of signal adjust circuit

Figure 7 is the model machine of signal acquisition system and Figure 8 is the waveform recorded in the field testing for water pipeline leakage.The data stored in SCM is transmitted to DSP by RS-485 and the complex filtering algorithm and localization algorithm are achieved by DSP.

Figure 7.The signal acquisition system prototype

Figure 8.The waveform recorded in the field testing

The designed leakage signal acquisition system has low noise and low power consumption.It is capable to match the outdoor working environment for leakage detection and could be applied to the petroleum，gas or other pipelines.

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