Page 120 - Advances In Productive, Safe, and Responsible Coal Mining

104 Advances in Productive, Safe, and Responsible Coal Mining

writing of this chapter, several of these systems were being developed for surface min-
ing applications.
There are a variety of ranging calculations that could be used to determine distance
between the reader and the tag. Commonly, ToF information, along with the propa-
gation speed of the signal, is used to assess the distance from a tag to a reader. Or,
similar to magnetic-field-based proximity detection systems, the use of multiple
readers could allow for the tag’s position to be determined through trilateration or
other means.
RFID can detect objects outside of line of sight and its performance is not affected
by dust in the air. This technology could provide localization that is similar in nature to
magnetic-field PDS. As such, some problems will be common between the two,
including electromagnetic interference (EMI), environmental effects, and general
performance.

7.3.4 Magnetic-field systems
Currently available PDS approved for use in underground coal mines are based on EM
technology. A typical system currently used in underground coal mines consists of
four generators positioned around the perimeter of the machine. Each generator is
a coil of insulated wire wrapped around a ferrite core. The magnetic-field strength
is proportional to the current running through the coil. Miners working on the section
have an MWC, which is a transceiver that measures the field strength emitted by the
generators and transmits a data packet containing the field strength reading over a
radio frequency (RF) link to the PDS controller mounted on the machine. The
magnetic-field generator’s pulses contain identification information so that the
MWC can determine which generator’s field strength it is reading. This type of system
utilizes the principle of magnetic flux density: the closer the MWC is to the field gen-
erators, the higher the field strength reading. Thus, a miner’s presence can be deter-
mined once he gets too close to the machine, because his MWC will measure a
magnetic flux density beyond a certain threshold. These thresholds are utilized on
a per-generator basis to “shape” the fields for both warning and stop zones around
a machine, as illustrated in Fig. 7.1.

7.3.5 Infrared-based systems

Infrared proximity sensors transmit an invisible infrared light beam and detect
reflections from nearby objects. Previously, infrared proximity sensors had limited
detection range and there were concerns with reliable operation in the mining envi-
ronment [5]. Improved systems with detection ranges of up to 9m (30ft) are now
available, and these technologies are gaining popularity in some construction and
industrial applications. It is not known how effective they would be in a mining envi-
ronment. Infrared video cameras (thermal imagers) detect the thermal signature
radiated from a person and provide an enhanced image, especially in low-light con-
ditions. Applications of these devices for avoiding collisions between vehicles and
people have been commercialized.