Sensors 2015, 15, 8266-8283; doi:10.3390/s150408266 sensors
ISSN 1424-8220 www.mdpi.com/journal/sensors
A New Method of Using Sensor Arrays for Gas Leakage
Location Based on Correlation of the Time-Space Domain of
Xu Bian, Yu Zhang *, Yibo Li, Xiaoyue Gong and Shijiu Jin
State Key Laboratory of Precision Measurement Technology and Instrument, Tianjin University,
Tianjin 300072, China; E-Mails: email@example.com (X.B.); firstname.lastname@example.org (Y.L.); email@example.com (X.G.); firstname.lastname@example.org (S.J.) * Author to whom correspondence should be addressed; E-Mail: email@example.com;
Tel.: +86-135-1220-0288; Fax: +86-22-2789-0026.
Academic Editor: Vittorio M.N. Passaro
Received: 25 February 2015 / Accepted: 3 April 2015 / Published: 9 April 2015
Abstract: This paper proposes a time-space domain correlation-based method for gas leakage detection and location. It acquires the propagated signal on the skin of the plate by using a piezoelectric acoustic emission (AE) sensor array. The signal generated from the gas leakage hole (which diameter is less than 2 mm) is time continuous. By collecting and analyzing signals from different sensors’ positions in the array, the correlation among those signals in the time-space domain can be achieved. Then, the directional relationship between the sensor array and the leakage source can be calculated. The method successfully solves the real-time orientation problem of continuous ultrasonic signals generated from leakage sources (the orientation time is about 15 s once), and acquires high accuracy location information of leakage sources by the combination of multiple sets of orientation results.
According to the experimental results, the mean value of the location absolute error is 5.83 mm on a one square meter plate, and the maximum location error is generally within a ±10 mm interval. Meanwhile, the error variance is less than 20.17.
Keywords: continuous ultrasound; gas leakage; location; real time; dispersion; sensor array
Sensors 2015, 15 8267 1. Introduction
Gas leakages in pressure systems are serious faults that can affect the tightness of vacuum structures, reduce the system operational safety coefficient, and can cause economic losses. Thus, a reliable real-time detection method to quickly identify the source of leakages is very necessary.
In recent years, the demand for real-time gas leakage location has increased yearly  and how to quickly and effectively locate leakage sources has become an urgent problem to be solved. According to different theories, current leakage detection technology mainly includes four methods: optical methods [2–4], the pressure change method , the resistance change method  and the acoustic emission (AE) method. The AE method detects the position of leakage holes by analysis of the leakage acoustic signal, which is collected by an AE sensor. Compared to the other methods, the AE method is easy to implement, the structure of the detected object does not need to be changed, it has fast location speed, and high immunity from interference. However, the ultrasonic leakage signal is a continuous signal without time domain features and the propagation characteristics are complicated [7–11], thus the position cannot be determined using the traditional Time Difference of Arrival (TDOA) technique .
Therefore, the traditional AE method presents some shortcomings in locating the continuous ultrasonic signal and it needs to be further explored. Mostafapour et al.  proposed a leak-locating algorithm in pressurized gas pipes based on a wavelet transform, filtering and cross correlation techniques; the error in leak location was less than 5%. Meng et al.  performed an acoustic experimental study on leak detection and localization for gas pipelines, conducted on a high-pressure and long-distance leak test loop. The researchers found that most acoustic leak signals were within the 0–100 Hz range, and they used different de-noising methods for different noise signals to improve the leakage location formula considering the pressure and temperature. Li et al.  used the cross-time–frequency spectra of leakage-induced acoustic vibrations to obtain the leak location in gas pipelines. Some ultrasonic leak detection equipment like UL101 [16,17] are used to locate leakages. The equipment locates the leakage source by collecting the ultrasonic leakage signal from the air surrounding the leakage holes. However, its detection range is small, and the equipment needs to be manually scanned in every suspicious area, so the method is time-consuming. Sedlak et al.  compared the first-arrival determination results for thin plates obtained from a two-step AIC picker with Kurz’s method, STA/LTA method, and standard threshold crossing technique. Kitajima et al.  determined the leakage source position by considering that the AE signal attenuates with the distance. However, this method is easily affected by the structure of the detected object and background noise, so it has a large location error under normal circumstances.
Holland et al. proposed an 8 × 8 sensor array to collect ultrasonic signals of orbiting spacecraft leaks [20–23], and calculated the intensity distribution of the wave number diagram (k-domain) of the collected signal, to estimate the direction of the sound source. Meanwhile, two sets of array orientation results are used to locate the leakage. However, this method requires a large number of sensors in the array (at least 64), and the location accuracy is poor (the biggest location error is 20 mm in a one square meter plate), therefore its applications are limited.
This paper proposes a new method for the location of continuous leakage sound sources. This method analyzes the continuous gas-leakage generated ultrasonic signal, and creates a mathematical model of the continuous ultrasound signal that propagates in the plate. Meanwhile, when the continuous broadband ultrasound propagates in the plate, it also solves the problem of location results affected by