Page 119 - Advances In Productive, Safe, and Responsible Coal Mining

Developing effective proximity detection systems for underground coal mines 103

obstacles are not good radar reflectors, such as plastics, dry wood, or objects with large
flat surfaces that can reflect signals away from the radar antenna. Possible obstacle
detection ranges for a radar-based proximity warning system vary from less than a
meter to 30m or more. Radar works well in dirty and dusty environments and does
not require any miner wearable component (MWC); however, it is only useful in
line-of-sight applications and cannot differentiate between objects and humans.

7.3.2 Sonar

Sonar or ultrasonic sensors operate on the principle of transmitting a high-frequency
sound wave at an object, and then measuring the reflected echo off of the target. The
sensors used in these systems are capable of converting an alternating current into
ultrasound and the reverse; converting ultrasound into an alternating current. Some
systems use separate sensors to transmit and receive; others combine both functions
into a single sensor. These systems determine distance based on ToF measurements
along with the propagation speed of the sonic wave in the propagation medium. The
technology cannot tell the difference between objects and humans. Also, no MWC is
required for an ultrasonic PDS. The frequency of the sound is above that of human
hearing (greater than 20KHz).
Sonar systems for vehicles have very limited range—typically less than 3m (10ft).
Thus, they will work well in near-range ranging applications but cannot detect through
barriers. These sensors are directional, and provide a narrow beam detection range.
Therefore, multiple sensors are needed to cover the width of a large vehicle. These
sensors can be sensitive to particles in the air (dust, snow, and rain) and must be kept
fairly clean to avoid any debris buildup on the face of the sensor. Improvements to
these types of sensors are possible, and new systems may become available that would
be suited for underground mining applications.

7.3.3 Radio frequency identification

Radio frequency identification (RFID) utilizes electromagnetic (EM) fields to identify
and track objects using a reader and tags. Tag-based proximity warning systems use
electronic tags that are worn by workers, attached to small vehicles, or attached to
stationary objects. Tag detectors or readers are installed on mobile equipment. There
are two types of RFID tags: active and passive. Active tags have their own power
source and operate at further distances than passive tags, which collect energy from
the RFID reader and must be near the reader. Active technology requires the tag to
transmit a marker signal that is detected by the tag reader. If the tag is within a certain
range, an alarm is generated in the cab of the equipment. Two-way communication
between the reader and the tag allows alarms to be generated at the tag also. The pas-
sive methodology is similar, but the reader transmits the marker signal. If a tag detects
this signal, an alarm condition is sent to the reader and an alarm is generated both in the
cab and at the tag. Several technologies have been used to generate the marker signals
that determine tag proximity: ultrasonic, magnetic, and radio frequency (RF). At the