Intelligent optical fiber sensor systems may help reduce the number of mining disasters.
Tongyu Liu, Shandong Micro-Sensor Photonics Ltd., and Eric Bergles and William Yang, BaySpec Inc.
According to the latest official mining statistics, from 2003 to 2006, more than 20,000 people died from coal mining accidents in China. Death tolls are at levels of four persons per billion tons mined per year compared with the US level of 0.04. As a result, a mine safety program in China is turning to fiber sensing technologies to gauge gases, to detect mine shaft collapses and more.
Previously, digital methane-monitoring systems were used to sense gases, but they suffer from cross-sensitivity and from small dynamic range, and they require recalibration every one to two weeks. For mine pressure monitoring, displacement sensors or electronic resistance strain gauges were used; however, these devices suffer from drift and electromagnetic field interference.
Collapsing mine shafts can be detected at an early stage using electrical or piezoelectric actuator seismic gauges, but these devices have small travel distances and are vulnerable to environmental conditions and intrinsic nonlinearities. For mine fire warnings, the most commonly used technique is remote analysis of gas samples drawn from the mine via long-distance tubing, which is limited critically by time delay up to one or two hours.
Not only do all of these techniques have their drawbacks but they also are intrinsically incompatible and require a large number of local signal-conditioning units, substations and dedicated data transmission links to take the information to the monitor center on the ground.
A fiber optic sensors-based intelligent coal mine system monitors the parameters of real-time critical safety conditions. These systems are intrinsically safe underground and are immune from electromagnetic interference, which eliminates the need for the underground substations required by electrical-based sensor systems for power supply and signal transmission relay. In addition, fiber optic systems can multiplex sensors for multipoint data processing and alarming.
In temperature sensing, which is based on the Raman scattering effect, when a pulsed light is injected into a standard optical communications fiber, the light interacts with the thermally dependent molecular oscillation inside the fiber. This results in scattering optical signals that have their optical frequency shifted.
The Raman scattering signal has the property that the ratio between the intensities of the frequency upshifted signal (anti-Stokes) and of the down-shifted signal (Stokes) is dependent on the ambient temperature of the optical fiber. The location of the measurement is determined using a time-of-flight technique known as optical time-domain reflectometry. For most coal mine applications, the measurement distance of 10 km is adequate. Typical applications require temperature and spatial accuracy of ±1.0 °C and ±2 m, respectively.
Table 1. Fiber optic sensors are available for a wide range of applications.
The variety of fiber optic sensors means that there is one to suit almost any type of monitoring (Table 1). A fiber Bragg grating, for example, which is made of single-mode optical fibers, has been used to fabricate most of the physical and mechanical sensors. The periodic refractive index profile along the fiber axis is formed in the core by illuminating it with UV interference beams. The fiber Bragg grating is essentially a narrow-band reflective optical filter whose wavelength is dependent on ambient temperature and on the strain applied to the fiber axis.
Fiber Bragg grating high-voltage temperature sensors are packaged such that they are isolated from external stress. The packaging material is polyimide, which has a high dielectric constant. This sensor has been applied to monitoring the temperature of high-voltage switches and power cable connections. It is made of steel-armored cable and can be used in coal mines.
Fiber Bragg grating strain and mining pressure sensors can be measured by embedding a fiber Bragg grating in a slot on a standard mining steel bolt. The effect of temperature can be compensated for with a fiber Bragg grating temperature sensor. The major advantages of the fiber Bragg grating mining pressure sensor over its conventional counterparts made of resistance strain gauges include: negligible drift over time, ease of multiplexing, intrinsic safety and long-distance monitoring.
A fiber Bragg grating water pressure sensor consists of a fiber optic Fabry-Perot interferometer cavity between a steel alloy diaphragm and a single-mode fiber end. The cavity length can be measured via the channel spectrum method. A fiber Bragg grating in a strain-free condition is housed in the same package for temperature compensation.
Fiber Bragg grating-based seismic sensors consist of a pair of cantilevers that are designed to have resonant frequency up to 200 Hz. During testing, two fiber Bragg gratings were mounted on the surfaces of two cantilevers with adhesives. Whereas the sensing cantilever is free of movement, the other is restrained and used to compensate for the ambient temperature. The sensing cantilever will vibrate when subjected to acceleration and will result in strain change. A high-speed detection method has been developed using a matched fiber Bragg grating scheme.
Gas Sensors
A fiber optic methane gas sensor is based on a single-mode fiber and on graded index lens collimators. The length of the gas cell is 200 mm, and it was packaged with cast metallic microball filters to block dust. The operation principle of the methane sensor is based on NIR absorption at 1.650 mm. The optical wavelength of the laser diode was a sawtooth signal. Gas concentration is measured from the absorption intensity.
Light sources typically are distributed feedback laser diodes or semiconductor LEDs operating in the NIR spectrum. New semiconductor LEDs feature high output power, large bandwidth, and cost-effective housing and very low ripple values.
New cost-effective fiber Bragg grating analyzers made by BaySpec Inc. of Fremont, Calif., have integrated wide-bandwidth spectral engines using a volume phase grating as the spectral dispersion element and an ultrasensitive InGaAs array as the detection element. Response time frequency is 0.2 kHz, and the dynamic range is 60 dB. An athermal design for the first time enables low-power consumption, battery-operated portable operation. Display spectral resolution is ±1 pm. Devices are hermetically sealed with no moving parts.
Field results
A multiparameter intelligent coal mine safety-monitoring system based entirely on fiber optic sensors was installed in Linzi Coal Mine in Shandong, China. Four fiber optic methane sensors, two mining pressure sensors, two seismic sensors, one water pressure sensor and two temperature arrays combined with five fiber Bragg grating temperature sensors were installed at four underground locations, including one by an underground coal storage/refilling site, one by a wind-returning gate and one at a coal production front. The sensor distance was between 5 and 6 km from the monitoring center. The data, which was remotely interrogated, was stored in a database and accessed via the Internet.
To evaluate their accuracy, the methane sensors were regularly filled with 1.0 percent standard methane gas; the apparent error of approximately –0.05 percent was partly the result of on-site calibration conditions. The underground blasters raise the concentration of ambient methane gas. This phenomenon can be observed from the recorded changes of methane gas concentrations. Underground temperatures recorded over a period of six weeks varied from 22 to 23 °C, comparing well with standard mercury thermometers. The mining pressure also was found to be stable during this period.
Fiber Bragg grating sensor systems offer the potential for a comprehensive, reliable solution and lower first-cost installation, compared with existing safety technologies, and a significant reduction in ongoing maintenance. However, fiber Bragg grating sensor systems do not yet have the resolution or precision of metal foil strain gauges. To avoid errors when designing systems, temperature changes also must be given careful consideration. Thermal residual stress can deform optical fiber, causing birefringence and widening the spectrum of the fiber Bragg grating sensor, requiring special arrangements or readout of two fiber Bragg gratings. Current work continues on helping to educate and train mining employees in monitoring and analyzing reported data.
Meet the authors
Tongyu Liu is general manager and founder of Shandong Micro-Sensor Ltd. of Jinan, China, a joint Sino-British venture; Eric Bergles is vice president of sales and marketing, and William Yang is president/CEO and founder of BaySpec Inc. in Fremont, Calif.; e-mail: [email protected].