Page 123 - Advances In Productive, Safe, and Responsible Coal Mining

Developing effective proximity detection systems for underground coal mines 107

to thresholds that define the field strengths representing zones of safety, caution, and
danger. Each threshold can be adjusted and affects the size of the danger and caution
zones for the entire machine. The size of the protection zone for a particular or specific
location is changed by adjusting the current to the appropriate generator.
Several manufacturers adopted the concepts developed with HASARD and further
refined the technology. Several systems are now commercially available to the mining
industry; however, the complex and nonspherical shape of the magnetic fields still
makes accurately determining distance difficult. Both HASARD and the systems cur-
rently available on the market simply trigger alarms or machine shutdown based on
predetermined threshold values for the magnetic flux density. This results in a some-
what ambiguous protection zone around the machine, which is difficult to shape, and
does not provide for situational or intelligent response to hazards. While the proximity
detection manufacturers have spent a great deal of effort trying to shape the protection
zones, the currently available systems sometimes interfere with the operator’s free-
dom to efficiently perform his job.
Due to visibility and space limitations, miners must routinely work in very close
proximity to the CMM, and it is common for an operator to be located within 1m
of the machine in order to see the visual cues needed to operate it [7]. To be acceptable
to miners and to avoid false alarms, a PDS must provide the necessary protection while
still allowing normal operation of the machine. This is difficult to achieve without an
intelligent system that can make decisions based on situation-specific conditions.
Accurate knowledge of worker position and posture enables the implementation of
intelligent protection capable of issuing alarms that are more meaningful or disabling
only specific machine functions, depending on the case at hand.

7.4.2 Magnetic-field modeling

A magnetic proximity detection system relies on magnetic flux density measurement
(B) to determine the position of a worker relative to a mobile mining machine. It is
desirable for the magnetic flux density distribution to be automatically adjustable
to conform to the protection requirements for the different types of machines and
working environments. In support of the development of an automatic field distribu-
tion adjustment process, NIOSH researchers developed a transferrable magnetic flux
density distribution model [8], which can also be used to control and stabilize the field
against field drift to enhance system performance.
Previous NIOSH research [9] showed the B field distribution from a ferrite-cored
generator was described in terms of magnetic shells that are surfaces of revolution
around the axis of the generator. Each shell (Fig. 7.3) represents a surface of constant
B field magnitude. A shell function is an analytical expression for the magnetic sur-
face. Shells vary in shape and size depending on the distance to the generator.
The general properties and parameters of the shell-based magnetic flux density dis-
tribution model for a generator are as follows. Eq. (7.1) shows the model covering the
three-dimensional (3D) space around a magnetic generator. The model defines a mag-
netic shell with a given B value. The coordinate system and the symbols used in (1) are
defined as shown in Fig. 7.4, in which a generator of length L lies along the x-axis and