Electromagnetic (EM)-based sensors are often seen in the underground coal mining industry. While other methods may very well be possible, this toolkit discusses two commonly used methods in the Proximity Detection System (PDS) area: (a) the detection of objects based on the magnetic flux density; and (b) the use of Near-Field Electromagnetic Ranging (NFER).
Working Principles
There are two main elements that are typically implemented within an EM-based flux density Proximity Detection System (PDS): (a) a set of ferrite-cored generators (placed on the Local Object (LO)) that create a surrounding magnetic field; and (b) a set of magnetic probes or detectors (placed on the Remote Object (RO)) that allow detection of the generated fields (Li, Carr, Waynert, & Kovalchik, 2013; Li, Smith, Carr, & Whisner, 2019). These systems create a safe zone between the LO and RO (in most cases, this is a pedestrian) based on the detected magnetic flux density; therefore, it is important to note that the stability of the magnetic field is essential towards system accuracy (Li et al., 2019).
NFER, on the other hand, is a technology that relies on the near-field characteristics of the electric and magnetic components of an electric wave. In far-field EM propagation (such as RADAR), ranging is performed with Time of Flight (ToF)-based techniques. However, in near-field EM propagation, within approximately one-quarter wavelength of an electrically small transmitter, the electric and magnetic components are 90 degrees out of phase. When further away from the antenna, the electric and magnetic phases converge; therefore, by detecting, measuring and comparing these phases before they converge, the distance from a transmitter can be determined. It is important to note that the NFER principle requires the antennas (between a transmitter on one object and the receiver on another object) to be less than one wavelength from each other; this can potentially limit the detection range (H. Schantz, 2012; Hans G. Schantz, 2007; Hans Gregory Schantz, Weil, & Unden, 2011).
Application to Proximity Detection
In terms of both implementations, EM interference may be possible when considering EM-based sensors for use in PDS; however, due to the low frequency application of NFER, this may not be as prevalent, given that other technologies utilise much higher frequency bands. Another advantage of utilising lower frequencies is that the signal suffers much less signal loss and reflection errors from solid walls.
EM-based flux density technologies are able to propagate through various types of rock mass (including coal), with studies illustrating the induced interfering current of in situ coal mass caused insignificant changes in the magnitude of the EM generators: a notable advantage of the technology. However, it must be stated that temperature (both internal and ambient) affecting the generator current can cause location calculation errors; therefore, it is critical that developers consider this aspect into their development of EM-based flux density PDS (Li et al., 2013, 2019).
The overall functionality of EM-based technology require infrastructure on both the LO and RO. In terms of NFER, range is determined only by the receiving unit; therefore, additional communication infrastructure is required for range information to be communicated back to the transmitter units. Similarly, it is evident that both a generator (LO) and a detector unit (RO) must be implemented to create a complete magnetic flux based PDS, with additional communication required (i.e. detection alerts given to both the LO and the RO) depending on the system design and implementation.
Advantages
- Both methods do not require Line-of-Sight
- Applicable largely to underground/enclosed applications
- Suitable for both surface and underground operations
- Relatively robust to terrain and environmental effects
Limitations
- Both methods require infrastructure to be installed on both the LO and the RO
- Limited range between objects in comparison to other sensor modalities
- Regarding EM-based flux density PDS, temperature may cause increased location error between LO and RO