Miners today use externally powered machines designed to be used by a single operator, the autonomous miner. Many companies offer computer-operated mining equipment that can be used for both high wall and underground applications. Programmable logic controls with ring laser gyroscopes and inclinometers are utilized to enable miners to follow prescribed paths in a coal seam with accuracy. Because of this technology, miners may be positioned in nonhazardous locations as they steer the mining equipment.
After a single tunnel has been dug through the coal seam, however, the operator needs to dig another tunnel parallel to the first while maintaining a certain wall thickness to support the mine. Although sophisticated electronic equipment aids miners, they may nonetheless creep edgewise. This can lead to the coal walls becoming too thin and ultimately collapsing.
An acoustic system was developed at the Army Research Laboratory (ARL) to improve the navigational abilities of autonomous mini systems by allowing the miner to sustain a prescribed sidewall thickness while excavating adjacent to other tunnels. Better control of the structural stability of the mine site can be maintained and more coal can be mined. The technology also allows coal to be distinguished from other elements such as rock, clay, and water so that the miner is more efficient.
ARL's technology is essentially a sensor that will enhance state-of-the-art mining equipment. It easily can be incorporated into existing systems. The technology will equip miners with a sound navigation and ranging, or sonar-like, system comprising one or more acoustic transducers for sending and receiving acoustic signals, and a processor that is used to determine the wall thickness as the miner digs coal. The sonar head, a ceramic transducer, and some associated electronics are placed on a retractable isolation arm so that they may be quickly positioned against the coal wall for operation. A short pulse of sound is projected into the coal sidewall in a narrow beam by placing a vibrating transducer solidly against the wall. The length of time it takes the sound to bounce back indicates the thickness of the coal wall. If the sound is returned too quickly, the miner is too close to the next tunnel.
If, for example, the interior wall of another tunnel is approximately five feet away, the sound will be strongly reflected because of the large impedance mismatch of sound propagation through air and coal. The sound waves will arrive back at the transducer in approximately 0.67 millisecond, and the transducer will now be reversed from the transit mode to the receiving mode. Feedback from ARL's sensor ties into the computer controls of existing mining equipment to help miners steer in the appropriate direction.