set the red lamp to flashing repeatedly to warn an operator. If
loading were to continue, the equipment would be protected
through automatic mechanical hydraulic pressure reliefs.

The slow and fast cycle times are 2 and 8 sec, respectively,
and can be easily changed by the user if desired. To set the
trigger valucs for the red and ycllow lamps, one must convert
loading rates in kilopascals per second to bits per cycle. Table
1 presents an example of input values for lamp triggers.

In the example, the red lamp is lit when the loading rate
equals or exceeds 168 kPa/sec for the fast cycle and 84 kPa/sec
for slow cycle. The yellow lamp is lit when the loading rate
equals or exceeds 84 kPa/sec for the fast cycle and 42 kPa/sec
for the slow cycle. At startup and low loading rates, the green
lamp indicates that the device is functioning properly.

B. Static Load Program

As previously mentioned, the load rate monitoring system was
modified to add a static load monitoring option. The coding for
both the static load and load rate programs are very similar.
Only one hardware change was required, that is, the addition of
a toggle switch to provide the operator with a means for
sclecting the load rate or static load program. The original load
rate program was modified to monitor the position of the switch
to determine which opcrating program to usc. It then loads the
light trigger values appropriate for that program. The program’s
toggle switch would be set in an area away from the active min-
ing face. The MRS will have to be shut down and de-energized
whenever the switch position or operating parameters (light
trigger values) of the programs are changed. This safety re-
quirement must he met before removing the access panel to gain
access to the program selector switch or to the embedded
processor.

IIL. CONCLUSIONS AND FUTURE WORK

The load rate monitoring system is designed to be MSHA
permissible. With the cooperation of a major MRS manufac-
turer, an experimental permit was obtained from MSHA to
allow testing of the MRS load rate monitoring system in
underground mining operations. The system was first tested on
an MRS in the manufacturer's shop facility. These and other

studies identified inaccuracies in hydraulic cylinder pressure
measurements of support loading when the bottom stage of a
multistage cylinder was fully extended [2]. Load changes would
not occur until the roof load acting on the hydraulic cylinder
exceeded the force applied to the mechanical stop of the bottom
stage.

To maximize the performance of the load rate monitoring
system, the MRS should be set against the mine roof with the
bottom stage partially extended. Upcoming ficld studies will
focus on evaluating the performance of the load rate system,
developing a protocol on how to use the system as a tool for
alerting miners to dangerous ground conditions during pillar
extraction, and determining critical load rate settings for the
system under different geological conditions. An integrated
ground monitoring system will be tested in which the simplicity
of roof-floor convergence measurements are combined with
load rate monitoring on MRS leg cylinders. Measurements from
four mines with various geological and support conditions have
shown that monitoring roof-floor convergence rates enables
miners to detect unstable roof conditions within the whole area
of interest at the mining face [3]. Monitoring MRS loading
rates will provide warnings about conditions that could result in
premature roof caving. It is anticipated that adding this type of
device to MRS’s will significantly improve the safety of room-
and-pillar retreat operations utilizing these machines.

ACKNOWLEDGMENTS

The support of J.H. Fletcher & Co. is gratefully acknowl-
edged. Installation and testing of the load rate monitoring
system could not have been accomplished without its assistance.
Fletcher not only provided MRS’s and multistage hydraulic
cylinders for testing the load rate monitoring system, but assist-
ed in installing the system and locating mine siles for the up-
coming field tests. The company’s diligent work was invaluable
in obtaining an experimental permit from MSHA to test the
system on MRS'’s in active retreat mining sections.

REFERENCES

(1) J. Shepherd and T. Lewandowski, “Modified Wongawilli extraction with
mobile breaker line supports: preliminary geomechanics,” Australian Coal
Journal, no. 38, pp. 41-47, 1997.

TABLE |
EXAMPLE OF LOAD RATE PROGRAMMING
Lamp Fast cycle Slow cycle
Load rate, KPwsec ——2-see cycle, bits Load rate, Load rate, kPa/sec 8-see cycle, bits Load rate, bits/sec
bits/sec
Yellow... 84 20 10 42 40 5
Red ........ 168 40 20 84 80 10

NOTE: | bit = 8.4 kPa


the user, The green light, for example, could easily be program-
med to indicate hydraulic pressures between 0 and 13.8 MPa,
the yellow light indicates pressures between 13.8 and 20.7 MPa,
and the red light indicates pressures over 20.7 MPa. The red
light is programmed to flash continuously when the hydraulic
cylinder is within 5% of yield pressure.

A. Load Rate Program

The code was developed in Assembly and C. A compiler
package from Franklin Software was used. The program was
compiled using a small memory library. It has a custom startup
code that must be linked. In addition, special linking options
are used to ensure that the code does not interfere with resident
on-board programs (the BASIC Interpreter and Monitor-51).

As shown in the software flow diagram (Fig. 6), the software
operates in several modes. On power-up, the system executes
the program stored in battery-protected random access memory
(RAM). This program cannot be changed by a user. The param-
eter storage area is also in battery-protected RAM, but it can be
modified by the user. As the program starts, it accesses the
parameter storage area to determine triggering values on the
lights as set by the user. These triggering values are stored as
conjugates, one positive value and an equivalent negative value.
If any error or improper user input has corrupted these values,
the program will use default values stored with the program and
not accessible by the user. Validation is important even though
parameter corruption is virtually impossible as long as the back-
up battery is functioning. Next the program initializes the data
acquisition system. The multiplexer (MUX) is set for the first
reading and the analogue-to-digital converter (ADC) is reset. In
this way, the transform parameters are set. The system is imme-
diately set in its normal (safc) condition with the green load-rate
lamp lit.

The program now waits for user input via the optional RS232
link from the laptop computer. If no inputs are present, the fore-
ground operation begins. If any user input is detected, fore-
ground operation is disabled while the user sets operating
parameters {rom a laptop computer. Background processing
occurs in response to detected user inputs. If the optional RS-
232 serial cable is not connected, these operations are not per-
formed, and the system operates solely in the foreground mode.
If the user is changing parameters via the RS-232 cable from a
laptop, all foreground operations are disabled until restarted by
user command.

Once the user input has been processed, the foreground pro-
gram is enabled. A “Start Pad” is sct to inhibit warnings or
alerts until the program has collected enough data to make valid
decisions. In other words, changing the lamp status is inhibited
until enough time has passed to establish a baseline. Once in the
foreground processing modc, as in most real-time operating
systems, both foreground and background tasks are supported.
While performing the foreground task, the system monitors for
background commands. The foreground task operates the sys-
tem in real time, monitors pressures, calculates load rates, and

PROGRAM POWER-UP

Retrieve standard warning and alarm limits from
battery-protected RAM.

If stored values are valid, then use the given values.

Otherwise, set-up values are corrupt.

Retrieve backup values from ROM.

Initialize data acquisition system.

Set green lamp to “on” to begin sequence.

BACKGROUND OPERATION

Monitor for user input from RS-232 line.

Start monitoring.

If user input is received during or before operation
begins, halt monitoring and accept inputs as follows:

Set fast yellow setting
Set slow yellow setting
Set fast red setting
Set slow red setting
Set flashing level

FOREGROUND OPERATION

Begin measuring pressures. <——--

Disable warning or danger for several cycles to avoid
false transient errors.

Collect enough data for fast-cycle measurement.

Calculate effective fast delta-p - fast rate change.

If enough fast cycles have passed to constitute
completion of one slow cycle, calculate effective
slow delta-p - slow rate.

Check fast and/or slow rate against set trigger values.

If rates exceed warning or danger limits, turn on
yellow or red lamp.

If absolute pressure exceeds safe level, flash red
lamp.

Fig, 6.—Flow diagram of load rate program

sets the annunciator lamps.

Basically, the program takes a series of measurements that
constitute a fast cycle of pressure measurements. At the end of
this fast cycle, it calculates the rate of change over the cycle.
After accumulating the specified number of the fast cycle rates,
it computes a slow-cycle rate. The slow- cycle rate is calculated
from the load change over a specified number of fast cycles.
The intent is that the user can specify two-rate trigger levels.
Generally the user will choose to make the slow-cycle triggering
rate smaller than the fast-cycle triggering rate. This process
allows for warnings to be generated over a large range of both
rapid changes and long-term rate changes. No matter what the
current load rate range is, when the absolute pressure level of
the monitored system approaches that of the pre-set level of
automatic system overload (yield) protection, the program wil!



(2) T. Barezak and D. Gearhart, "Performance and safety considerations of
hydraulic systems,” in Proceedings. 17th International Conference on
Ground Control in Mining, 8. 8. Peng, ed. Morgantown, WV: Dept. of
Mining Engineering, WV Univ., pp. 176-186, 1998.

[3] H. Malcki, “Detecting stability problems by monitoring rate of roof
movement.” Coal, vol. 25, no.12, pp. 34-38, 1988.


Mobile Roof Support Load Rate Monitoring System

Wayne L. Howie and John K. Owens
National Institute for Occupational Safety and Health
Spokane Research Laboratory
315 E, Montgomery Ave.

Spokane, WA 99207

Abstract—Mobile roof support (MRS) machines are used as roof
support during pillar recovery in retreat operations in lieu of
wooden posts, cribs, or hydraulic props. A Mine Safety and Health
Administration-permissible load rate monitoring system was
developed for the MRS by researchers at the Spokane Research
Laboratory of the National Institute for Occupational Safety and
Health. The system uses a dedicated embedded processor to
monitor changes in pressure inside the hydraulic cylinders of the
MRS through multiplexed data acquisition channels and converts
these pressure changes to load rates that sequentially activate three
load-rate-indicator lights mounted on the machine. Each light
represents a different loading rate to alert mine personnel to
dangerous conditions during pillar extraction.

I. INTRODUCTION

Mobile roof support (MRS) machines are used during pillar
recovery in retreat coal mining operations in lieu of wooden
posts, cribs, or hydraulic props (Fig.1). These machines consist
of a roof canopy, four hydraulic cylinders, a caving shicld
canopy, and associated electromechanical systems mounted on
crawler tracks. Because they are very mobile and allow higher
rates of resource recovery, they are currently being used in
about 36 U.S. coal mines, as well as in a number of Australian
mines [1].

One of the drawbacks with the MRS is its lack of a warning
system that would alert miners to dangerous ground conditions
during pillar removal. In the past, cracking and crying sounds
emitted by wooden posts and visual observations of
displacement and failure warned miners of dangerous loading
conditions. Hydraulic supports such as used in an MRS provide
little or no discernible audible or visual indications of
impending roof caving. Therefore, miners have come to rely on
the hydraulic dial gauges on the MRS, as well as indications of
ground movement, to determine when to cease operations and
remove themselves and equipment from the active mining face
before a dangerous roof fall occurs. These gauges monitor
pressure in the hydraulic cylinders of the MRS and often show
a rapid increase in pressure when a roof failure is imminent.

However, these gauges are difficult to read and require that
miners approach the MRS to see the gauges (Fig. 2), which
brings them close to the active mining face. Not only is this
area susceptible to roof falls, but mobile equipment poses
additional safety risks. For these reasons, and because miners
are often busy performing other work, the gauges are only
checked periodically.

Fig. 2.—Pillar extraction using mobile roof supports


Front Rear

Microchip
computer

Analog
to digital

Red - danger

(ADC)

Analog
pressure
transducers

(2)

Hydraulic cylinder
system (2)
Analog
multiplexer converter
(MUX)
a

Yellow - warning

=)
Ss

User input and control —#}
via RS232 serial connection
to laptop computer

Green - normal

Annunciator
lamps (3)

Fig. 3.—Diagram of load rate monitoring system

A load rate monitoring system was developed by personnel of
the Spokane Research Laboratory (SRL) of the National In-
stitute for Occupational Safety and Health (NIOSH) to track and
display loading rates on an MRS in real time. Such a system
installed on an MRS could warn of major events, such as fail-
ures of fenders and pillars that generally trigger roof falls. The
system was designed to be scen casily by all miners in the
vicinily of an MRS.

Tl. LOAD RATE MONITORING SYSTEM

Fig. 3 is a block diagram of the load rate monitoring system.
The system uses a dedicated embedded Micro-485 program-
mable controller to monitor pressure inside two hydraulic
systems using standard analog pressure transducers (0 to 34.5
MPa). Pressure is translated by the transducer (o an analog
voltage of 0 to 5 V, transformed by the analog-to-digital
converter to a 12-bit (0 to 4096 level) digital value, and sent to
the embedded processor via its integral interface for processing.
The controller is based on a highly integrated version of the
world standard 8051 microcontroller family. It calculates
loading rates using Assembly and C languages and controls
output signals to the three Joad-rale indicator lamps. A socketed
80C51FA central processing unit (CPU) is ideally suited to the
control and data acquisition requirements of the system. Fig. 4
shows the nonintrinsic components of the system installed in an
electrical starter box located at the front of the MRS.

Loading on a hydraulic cylinder is proportional to internal
pressure and surface area of the piston head and is determined
by the formula

F=AxP, 0)

where F = force (N), A = area (m’), and P = pressure (Pa).

The embedded processor reads changes in cylinder pressure
through two multiplexed data acquisition channels of the
controller and converts these pressure changes to load rates.
Green, yellow, and red lights globe lights are activated as the
load rate increases. Green indicates that there is minimal
change in load rate on the MRS. Yellow indicates that the load
rate is increasing and that additional caution is recommended.
Red indicates a rapid load rate increase and that a roof fall may
occur soon. A continuously flashing red light indicates that the
hydraulic cylinder load is approaching the yield of the MRS and
the unit may soon collapse. As shown in Fig. 5, these lights can
be installed on the MRS canopy near the hydraulic pressure
gauges. These lights can casily be seen at all viewing angles at
a safe distance from the active mining area.

Other devices could be used to meet specific warning
requirements for a mine operator or MRS manufacturer,
including multicolor probes, LED's, or audible alarms. The
system will operate as an integral part of an MRS. Operating
parameters can be set prior to or during installation of the
system or periodically as mine conditions change, but need not
be done by operating personnel at the mine. Operating
parameters are set by connecting the system to a laptop
compuler via an RS-232 null modem cable with the
communication terminal emulator acting as the laptop client
program. This allows a trained user to change parameters casily
to trigger the various load rate indicator devices to suit
geotechnical conditions at a mine.

At the request of a MRS manufacturer and mines using
MRS’, a static load indicator feature was added to the main
load rate monitoring system. The system is transformed into a
static hydraulic pressure monitoring system by simply flipping
a toggle switch to activate the static load program on the
embedded processor. When the machine is operating in the
static load mode, cach light indicates a range of pressure set by



Intrinsi@7
barrie
and

signal
conditioners

Fig, 4.—Components of load rate monitoring device installed in electrical box on mobile roof support

Fig. 5—Load rate indicators installed on mobile roof support