Jingcheng Li

Measurements of medium frequency propagation characteristics of a transmission line in an underground coal mine

In underground coal mines, medium frequency (MF) communication systems couple their signals to metal infrastructures such as ac power cables and wire-based telephone lines, which guide the signals to propagate for a long distance. To better understand the propagation characteristics of MF signals, an easy-to-use measurement method was recently developed at the National Institute for Occupational Safety and Health. The method will be introduced along with an equivalent transmission line model for a long metallic infrastructure in underground coal mines. The model serves as the fundamental driver for the method development. Propagation measurements on a twisted pair of telephone lines in an underground mine were made using this method. The measurements confirmed the low MF signal attenuation rate and the dependence of the propagation characteristics of the line on the electrical properties of surrounding coal and rock as theoretical studies predicted.

Measurements of Medium-Frequency Propagation Characteristics of a Transmission Line in an Underground Coal Mine

In underground coal mines, medium frequency (MF) communication systems couple their signals to metal infrastructures such as AC power cables and wire-based telephone lines, which guide the signals to propagate for a long distance. To better understand the propagation characteristics of MF signals, an easy-to-use measurement method was recently developed at the National Institute for Occupational Safety and Health. The method will be introduced along with an equivalent transmission line model for a long metallic infrastructure in underground coal mines. The model serves as the fundamental driver for the method development. Propagation measurements on a twisted pair of telephone lines in an underground mine were made using this method. The measurements confirmed the low MF signal attenuation rate and the dependence of the propagation characteristics of the line on the electrical properties of surrounding coal and rock as theoretical studies predicted.

Development of a method to determine operator location using electromagnetic proximity detection

Researchers at the National Institute for Occupational Safety and Health (NIOSH) are advancing the emerging technology of electromagnetic proximity detection, which provides a promising means of protecting workers around any machinery that presents striking, pinning or entanglement hazards. This technology is particularly applicable to mobile underground mining equipment such as remote-control continuous mining machines, which offer perhaps the most difficult safety challenges in the mining industry. Other industries have effectively implemented proximity detection technology, with successful test cases at surface and underground mines. However, applying this technology to remote-control continuous mining machines presents uniquely difficult challenges. These machines typically weigh close to 100,000 pounds and have heavy, articulated parts. Due to visibility and space limitations, machine operators often work in very close proximity to the machine despite the clear hazards that this proximity creates. To protect miners without preventing them from doing their jobs or causing nuisance alarms, intelligent electromagnetic proximity detection technology is now being developed at the NIOSH research facility in Pittsburgh. At the heart of this technology are a number of electromagnetic field generators mounted on a mining machine and magnetic flux density sensors built into a Personal Alarm Device (PAD) worn by the operator. In this paper, the authors present a novel algorithm created to calculate an accurate position based on PAD readings from multiple field generators coupled with a previously developed model of the generated magnetic field. The use of this algorithm allows for the calculation of an accurate PAD location relative to the mining machine. A prototype of this intelligent proximity detection system has been successfully implemented and demonstrated on a Joy 14CM continuous mining machine at the NIOSH research facility in Pittsburgh. This technology has the potential to significantly affect the mining industry by greatly advancing the current state-of-the-art in proximity detection technology, leading to increased operator safety and preventing serious injuries and fatalities.

Comparison of medium frequency propagation characteristics of a transmission line measured from both ends in a coal mine entry

An underground coal mine medium frequency (MF) communication system generally couples its electromagnetic signals to a long conductor in a tunnel (entry). The entry plus the conductor acts as a transmission line (TL). Transceivers exchange signals along the line. The propagation characteristics of the TL, which is generally the longest signal path, play a major role in determining the system performance. In this paper, we present MF measurements of TL characteristics as measured from both ends of the TL in a coal mine entry. The characteristic impedances from both ends have some significant differences, but the other propagation parameters are very similar.

Comparison of Magnetic Field Distribution Models for a Magnetic Proximity Detection System

Magnetic proximity detection technology is rapidly advancing as a promising method of protecting underground mine workers from striking and pinning hazards associated with mobile mining machines. A magnetic proximity detection system requires a magnetic distribution model to estimate the proximity of the sensor to the generators. This paper presents a comparative analysis of magnetic flux density distribution models in three different field distribution design patterns. The accuracy of these models is determined with a laboratory magnetic proximity detection system. These field distribution design patterns are spherical, ellipsoidal, and sphere-cosine, respectively. The analyses show that the sphere-cosine model is the most accurate model for the proximity system followed by the ellipsoidal and spherical models.

Characterization of medium frequency propagation on a twin-lead transmission line with earth return

This work presents research in the development of medium frequency communications systems that the National Institute of Occupational Safety and Health (NIOSH) is performing in support of the 2006 MINER Act. In this work, a 280 m twin-lead transmission line (TL) is used to measure complex input impedance and line currents resulting from open and shorted terminations at medium frequencies (MF). The line is placed in free space and positioned close to a lossy earth surface such that two line configurations can be chosen in order to control the extent to which the grounds presence affects MF propagation. In this way, the effects of the earth return may be characterized and modeled. Further, full-wave computational models using moment methods are used to compare with the experimental measurements as a way to determine the material properties of the ground medium. In the future, these experiments will be conducted in underground mines in an effort to develop and improve MF communication systems for mining operations and emergency response.

Simulation and measurement of medium frequency signals coupling from a line to a loop antenna

The underground-mining environment can affect radio-signal propagation in various ways. Understanding these effects is especially critical in evaluating communications systems used during normal mining operations and during mine emergencies. One of these types of communications systems relies on medium-frequency (MF) radio frequencies. This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating MF coupling between a transmission line (TL) and a loop antenna in an underground coal mine. Two different types of measurements were completed: 1) line-current distribution and 2) line-to-antenna coupling. Measurements were taken underground in an experimental coal mine and on a specially designed surface test area. The results of these tests are characterized by current along a TL and voltage induced in the loop from a line. This paper concludes with a discussion of issues for MF TLs. These include electromagnetic fields at the ends of the TL, connection of the ends of the TL, the effect of other conductors underground, and the proximity of coal or earth. These results could help operators by providing examples of these challenges that may be experienced underground and a method by which to measure voltage induced by a line.

A Hybrid Approach for Efficient Modeling of Medium-Frequency Propagation in Coal Mines

An efficient procedure for modeling medium frequency (MF) communications in coal mines is introduced. In particular, a hybrid approach is formulated and demonstrated utilizing ideal transmission line equations to model MF propagation in combination with full-wave sections used for accurate simulation of local antenna-line coupling and other near-field effects. This work confirms that the hybrid method accurately models signal propagation from a source to a load for various system geometries and material compositions, while significantly reducing computation time. With such dramatic improvement to solution times, it becomes feasible to perform large-scale optimizations with the primary motivation of improving communications in coal mines both for daily operations and emergency response. Furthermore, it is demonstrated that the hybrid approach is suitable for modeling and optimizing large communication networks in coal mines that may otherwise be intractable to simulate using traditional full-wave techniques such as moment methods or finite-element analysis.

Environmental impact on the magnetic field distribution of a magnetic proximity detection system in an underground coal mine

A magnetic proximity detection system mounted on an underground mobile mining machine detects whether a worker is hazardously close to the machine. The system generates magnetic fields covering the extended spaces around the machine. A magnetic detector worn by the worker measures the magnetic field flux density and determines the distance from it to the machine. The system is frequently in close proximity to coal as the machine moves, causing the magnetic field flux, in part, to enter massive in situ coal. This has the potential to have an adverse effect on the accuracy of the system and on the safety of the worker if the coal were to significantly alter the magnetic flux density distribution. Two experiments were conducted to study the impact of in situ coal on these magnetic fields. Measurements in one mine show that coal mass has no significant impact on the magnetic field flux distribution.

Propagation parameters for medium frequency signals in a transmission line at different positions within a mine entry

A medium frequency (MF) communication system used in underground coal mines generally couples its electromagnetic signals to existing conductors in a mine entry (tunnel), and exchanges signals with transceivers near the conductors. The conductors act as a transmission line (TL). The propagation characteristics of the TL, which play a major role in determining the performance of an MF communication system, can be affected by a mine environment. This paper compares the measured propagation parameters of a TL in three different positions within a mine entry. This comparison shows that the differences between the electrical properties of local coal and rock close to the TL can influence the propagation characteristics of the TL when in a different position.