Rib

Shotcrete wall

~
a

—— —— Approach line

0

KEY

1 2 = 3
Scale, ft

4

Rib

Figure 14.—Parallel approach to concave corner, path B, about 4 ft away.
ay


9. Walter, S. The Sonar Ring: Obstacle Detection for a Mobile
Robot. Paper in IEEE Robotics and Automation Conference Pro-
ceedings. IEEE CH2413-3/87/0000/1574, March 1987, pp. 1574-1579.

10. Denning Mobile Robotics (Wilmington, MA). Denning Ring
Operating Manual. 1987, 98 pp.

11. Doepken, W. G. The Henderson Mine, Ch. in Underground
Mining Methods Handbook. W. A. Hustrulid, New York, NY, AIME,
1982, pp. 990-997.

12, Everett, H. R. Survey of Collision Avoidance and Ranging Sen-
sors for Mobile Robots. Nav. Ocean Syst. Cent., Tech. Rep. No. 1194,
San Diego, CA, 1987, 179 pp.

13. Campbell, D. Ultrasonic Noncontact Dimensional Measurement.
Sensors, v. 3, July 1986, pp. 37-43.

44. Biber, C,, S. Ellin, E. Shenk, and J. Stempeck. The Polaroid
Ultrasonic Ranging System. Paper in Preprint of the Sixty-seventh Con-
vention of the Audio Engineering Society. Audio Eng. Soc., New York,
NY, 1980, 11 pp.

15. Pierce, A.D. Acoustics—An Introduction to its Physical Principles
and Applications. McGraw-Hill, New York, NY, 1981, 642 pp.

16. Chande, P. K,, and P. C. Sharma. A Fully Compensated Digital
Ultrasonic Sensor for Distance Measurement. IEEE Trans. Instrum.
and Meas., v. 3, No. 2, 1984, pp. 128-129,

17. Meyer, E., and E. G. Neumann. Physical and Applied Acoustics—
‘An Introduction. Academic, New York, NY, 1972, 412 pp.

18. Kinsler, L. E., A. R. Frey, A. B, Coppens, and J. V. Sanders.
Fundamentals of Acoustics. Wiley, New York, NY, 1982, 524 pp.

19. Brown, M. K. Feature Extraction Techniques for Recognizing
Solid Objects with an Ultrasonic Range Sensor. IEEE J. Robotics
and Autom, v. 1, No. 5, 1985, pp. 191-205.

20. Bass, H. E., L. C. Sutherland, J. Piercy, and L. Evans. Absorption
of Sound by the Atmosphere. Physical Acoustics, ed. by W. P. Mason
and R. N. Thurston, Academic, v. XVII, New York, NY, 1984, 326 pp.

21. Polaroid Corporation (Cambridge, MA). Ultrasonic Ranging Sys-
tem. Ultrasonic Ranging Marketing Div., 1982, 34 pp.

22. Cooper, W. E., D. D. Rapp, and J. B. Hudgins, Noise
Investigation—Cannon Mine. MSHA Rep. D-5903 P-588, Denver, CO,
1987, 20 pp.

23. Kue, R, and M. W. Siegel. Efficient Representation of Reflecting
Structures for a Sonar Navigation Model. Paper in IEEE Robotics and
Automation Conference Proceedings. IEEE CH2413-3/87/0000/1916,
March 1987, pp. 1916-1923.

24. Borenstein, J,, and Y. Koren, Obstacle Avoidance with Ultrasonic
Sensors. IEBE J. Roboties and Autom. v. 4, No. 2, April 1988, pp. 213-
218.

25. King, R, and A. Gordon. Intelligent Waveform Analysis for Ul-
trasonic Ranging in a Cluttered Environment. CO Inst. Artificial Intell.,
Univ. CO at Boulder, April 1990, 18 pp.

26, Milltronics (Arlington, TX). MultiRanger Plus Product Cata-
logue. 1989, 6 pp.



—20 ft

were conducted,

~=— Direction of travel

+20 ft

Figure 31.—Intersection as seen by sensor 3. Note: 9X and 94LD denote specific locations in the mine where this serles of tests

KEY
* Rib survey point

Let 1 41 4 4
0 2-4 6 8 10

Scale, ft

9x

~=— Direction of travel
+20 ft o4t

+10 ft

—20 ft

510 ft

pet lea eeeee see
: KEY
: + Rib survey point

Uo 4

0 2.4 6 8 10

Scale, ft

Figure 32.—Intersection as seen by sensor 4. Note: 9X and 94LD denote specific locations in the mine where this series of tests

were conducted.


CHARACTERIZATION OF ULTRASONIC RANGERS

Before an ultrasonic ranger can be used for machine
guidance, the operating characteristics of the specific
ranger must be tested within the environment in which it
will be used. This research started with laboratory tests,
and later moved underground.

LABORATORY TESTS

A series of laboratory experiments characterized the
sensors before underground tests began. The equipment
used for most of these experiments was an array of
24 Polaroid ultrasonic rangers mounted at 15° intervals
around a 27-in fiberglass ring, shown in figure 7. Denning
Mobile Robotics, Inc., of Wilmington, MA, developed the
ting and its control processor. Figure 7 also shows a
microcomputer interfaced to the ring through the RS-232
port to collect and store range data.

Interference between the rangers is prevented by puls-
ing each sequentially with a user-selected time delay of 18
to 250 ms between transmit bursts. The user can also
select the sensors to be fired, and the firing sequence. For
example, the controller can fire all sensors (0-23) in nu-
merical order, or only fire three rangers in the sequence
0,8, 16. The controller can repeat the previous sequence,
or change it to another, say 1, 9, 17, for the next sequence.
This allows the user to select only those sensors needed
for the experiment, and prevents the transmit pulse of one
sensor from interfering with the received signal of another
sensor.

Laboratory tests evaluated range resolution, intersen-
sor variation, and FOV by moving a 1.5- by 2.5-ft rectan-
gular cardboard target and a 3/4-in by 4-ft wooden dowel
to different locations within the FOV of sensor 0. When
measurements were completed on sensor 0, the ring was
rotated 15° and the experiment was repeated for sensor 1,
2, and so on, until eight sensors were tested, Researchers
stored range data sets from each of eight sensors at 2.5,
5, 7.5, 10, 12.5, 15, 20, and 25 ft.

Table 1 shows good resolution and little intersensor
variation up to 7.5 ft. From 10 to 20 ft the readings varied
from the measured distance by as much as 0.2 ft. Varid-
tion among the sensors, however, was less than 0.1 ft. The
target did not reflect enough sound energy to be detected
at longer ranges.

Researchers moved the targets at right angles to the
sensor axis and across the FOV (fig. 8) to define the ex-
tent of the FOV. The FOV was 17.6° (8.8° on either side
of the sensor axis) at a range of 5 ft. The sound energy is
highest close to the ranger and along the sensor axis. The
wave is weakest near the edges of the FOV; consequently,
the FOV appeared to decrease to 13.8° at 10 ft, and 10.6°

at 15 ft, because of the weak echo when the target was on
the edges of the 17.6° FOV.

Table 1.Accuracy of range readings for sensors 0 to 7 used
in Denning ring segment, feet

(Temp, 28° C; humidity, 48%; elev, 5,775 ft;
pressure, 24.50 in Hg)

Measured

distance 0 1 2 3 4 5 6 7
to box

25. 25 26 25 25 25 25 25 25
5.0. 49 50 50 50 50 50 50 50
7.5. 75 75 75 75 75 75 75 75
10.0 99 100 99 99 99 99 100 99

125
15.0
20.0

124 124 124 124 124 124 124 12.4
148 149 148 148 148 149 149 148
198 199 198 198 198 198 199 198

To further investigate the effect of an object’s surface
characteristics on FOV, researchers moved a vertically
mounted 3/4-in wooden dowel, tall enough to intersect the
entire FOV horizontally, across the FOV. The use of a
nonplanar object reduces the effects of incidence angle on
the FOV. The dowel tests produced a 22° average FOV
at 5 ft, 15.3° at 10 ft, and 12.6° at 15 ft. The increase in
the FOV over the cardboard target was caused by the
dowel’s more diffuse scattering of the sound wave over a
wide angle. Although the echo was weak, at least some of
the energy reached the ranger. The cardboard echo was
not diffused and probably reflected along a line that
missed the ranger. In addition, cardboard absorbs more
sound energy than smooth wood.

UNDERGROUND MINE EXPERIMENTS

The Denning ring was tested at AMAX’s Henderson
Mine near Empire, CO, after the sensors had been char-
acterized in the lab. Three types of underground navi-
gation features were defined, consistent with Kuc and
Siegel’s corner-edge-wall navigation model: concave cor-
ner, convex corner, and linear rib (fig. 9) (23).

Linear Rib Experiment

The experiment began with choosing a section of
straight rib, surveying it with traditional techniques, and
producing the contour map of figure 10 showing the sur-
face characteristics. The Denning ring was then placed on
a platform 4.25 ft above the floor (fig. 11) and approxi-
mately 5 ft from a straight section of the rib, The sensor
axes aligned with the 18-in line shown on the vertical axis
of the figure 10 map.


Shotcrete wall

rom | /

va Rib

KEY

—— —— Approach line

Scale, ft

Figure 18.—Surveyed concave corner rib and range readings at 6-ft setup.


APPENDIX._PROGRAM FOR CONVERTING DATA FROM DENNING RING

The following program, Ultraran.pas, is written in Turbo Pascal. The program accepts the output file from the
Denning ring, and converts it to points in a .dxf file that can be interpreted by computer-aided software packages.

{$R+}
Program UltraRange (input, output, InputFile, OutputFile);

{This program accepts data from a file generated by a moving Denning ring and computes X,Y positions of the objects
that reflect the ultrasonic waves.}

var
StartX, StartY: real; {Denning ring starting coordinates}
Vel: real; {Denning ring velocity}
DeltaT: real; {Time interval between range readings}
Dist:real; {Distance that the Denning ring moved during the test}

Procedure EnterTestParameters (var StartX, StartY, Vel, DeltaT: real);

{This procedure enters the information that describes the Denning ring setup and calculates the total number of
readings.}

Begin
writeln (‘Enter the starting X coordinate of the Denning ring [0].’);
readin (StartX);
writeln (‘Enter the starting Y coordinate of the Denning ring [0].’);
readin (StartY);
writeln (‘Enter the velocity of the Denning ring in ft/sec [0.8].’);
readin (Vel);
writeln (‘Enter the time interval in sec. between range readings [0.075].’);
readin (DeltaT);
End; {EnterTestParameters}

Procedure ReadAndWriteThreeLines(reading:integer;
InputFileName, OutputFileName:string;

var ch:char;

var InputFile, OutputFile:text);

{This procedure reads the characters and range values in the 3 lines of the input file that contain data from all 24 sensors.
It writes the range readings to a file of 1 reading of 24 sensors per row.}

type RangeArray=array [1..200, 1..25] of integer;

var Range: RangeArray; {Range readings extracted from the Denning file and read to OutputFile}
sensor:integer; {Each column in the new range array is for a sensor; each row is a reading}
LineNumber:integer; {Represents the three lines of sensor data per reading}

First, Last:integer; {The numbers of the first and last sensors in the three lines}

Begin
{The first sensor number in the line}
{The last sensor number in the line}



RESSSSIBIRR

PRE

Sensor 1, 90° approach to convex corner . .
Sensor 2, 90° approach to convex corner
Data from 135° approach at 14 ft . .
. Data from 135° approach at 6 ft .
Intersection as seen by sensor 1 .
Intersection as seen by sensor 2 .
. Intersection as seen by sensor 3 .
Intersection as seen by sensor 4...
Fly by view of intersection, sensor 4
Enclosure for mounting sensors on production lo:
Protective faceplates for sensors

ILLUSTRATIONS—Continued

ad-haul-dump .

TABLES

Accuracy of range readings for sensors 0 to 7 used in Denning ring segment . . .

Typical Denning ring output
Range and field-of-view tests with protective grills

dB
deg
ft

ft/s

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

degree Celsius
decibel

degree

foot

foot per second
hour

inch

in Hg
K

kHz

inch of mercury (atmospheric pressure)
kelvin

kilohertz

millisecond

second

volt



Figures 29 through 32 show the intersecting ribs
mapped by traditional surveying techniques as dotted lines,
and the sonar rays of less than 25.5 ft as arrows. Range
readings that were greater than or equal to 25.5 ft (the
maximum for these rangers) were not plotted. The range
data defined the convex corners at the intersection very
well.

The next step in the intersection experiment was to
push the Denning ring through the intersection at a nearly
constant velocity without stopping every 2 ft to store data.
Sensor 7 was aligned along the track centerline, and sensor
1 was aligned perpendicular to the rib line. The sensors
pulsed constantly while the cart moved. Every complete
cycle of ranger pulses can be used to produce a map of
occupied and unoccupied regions. In this case, all the
readings from sensor 1 were combined to build a map of
the rib, Figure 33 shows the map compared to the actual
survey.

A program has been written in Pascal, Ultraran.pas
(see appendix), to automate the map building process,
The program takes the data file from the Denning ring
and produces an output file of data points for display in a
common computer-aided drafting software package.

Table 2 shows a sample data output from the Denning
ring. Ultraran.pas reads the Denning ring output file,
places the range reading from each of the 24 sensors in an
array, and eliminates the sensor identification characters.

Table 2.—Typical Denning ring output, 0.1 ft

Sensor AB C Dd E F G eH
Oto7. 66 65 «66 «670 76 (114 255 237
8 to 15 285 255 255 255 255 255 255 255
16to23... 255 255 255 255 255 255 255 255

NOTE.—A = sensors 0, 8, 16; B = sensors 1, 9, 17; C = sen-
sors 2, 10, 18; D = sensors 3, 11, 19; E = sensors 4, 12, 20; F =
sensors 5, 13, 21; G = sensors 6, 14, 22; H = sensors 7, 15, 23.

Ultraran.pas uses the range array to compute coor-
dinates of points that produced an echo detected by the
Denning ring. The first step calculates the world coordi-
nates of the ring each time it receives an echo. Then, the
code computes rib point coordinates from the recorded
ranges. The ring changed position for each row of data
in the range array. The equations for updating the ring
equations are

RingX[row] = §P - cos(RingAzimuth)
+ RingX[row -1],
and RingY[row] = 6P - sin(RingAzimuth)

+ RingY[row -1],

where:  RingX[row] = current X coordinate (ft) of the
Denning ring,
RingY[row] = current Y coordinate (ft) of the
Denning ring,
RingAzimuth = vehicle (cart) heading (degrees
from true North),
RingX[row -1] = previous Denning ring X co-
ordinate (ft),
RingY[row -1] = previous Denning ring Y co-
ordinate (ft),
and 6P = distance traveled (ft) from the
last reading.
6P = Vel- 6T
where §T = Denning ring interfire delay (s).
and Vel = vehicle (cart) velocity.

Ultraran.pas computes the rib point X and Y coor-
dinates from the ring positions with the following
equations:

X: = [Range/10 - cos(SensorRayAngle)] + RingX,
and Y: = [Range/10 - sin(SensorRayAngle)] + RingY,
where X_ = rib point X coordinate (ft),

Y = rib point Y coordinate (ft),

Range = range reading (ft) from the
range array,

and — SensorRayAngle = azimuth (deg) of the sensor
axis.

SensorRayAngle = [15 - (KeyRanger - Transducer)

+ n/180] + RingAzimuth,

where KeyRanger = number of the transducer
pointing along the drift
centerline,

and Transducer = number of the transducer

producing the range reading,


\ Shotcrete wall

Rib\ | Rib

Db
w
a
i=)

KEY
—— — Approach line

ne a a

Scale, ft

4

Figure 15.—Parallel approach to concave corner, path C, about 8 ft away.



45° 90° 135°

Ui
012345
Scale, ft

Figure 21.—Approach lines to convex corner.


Shotcrete wall

Frame /

Rib

KEY
—— —— Approach line

Litt | | | j

Scale, ft

Figure 17._Surveyed concave corner rib and range readings at 10-ft setup.


9x

¢
~=— Direction of travel
- 94 LD
20 ft _ +20 ft 4
|| |
. oooee *
N : KEY
. : +Rib survey point
: im 4
. Oo 2 4 6 8 10
. Scale, ft

Figure 29.—Intersection as seen by sensor 1. Note: 9X and 94LD denote specific locations in the mine where this series of tests

were conducted,

g
~=— Direction of travel
+20 ft

—20 ft

*Rib survey point

Oa 46870

Scale, ft

Figure 30.—Intersection as seen by sensor 2. Note: 9X and 94LD denote specific locations in the mine where this series of tests

were conducted.


eoveeere®

Denning ring

KEY
@ Rib survey point

i?) 1

Scale, ft

2

Figure 12,—Son:

phenomena may be explained by the sides of the sound
energy radiation pattern echoing from rib protrusions.

Convex Corner

Next, researchers surveyed and mapped a convex corner
(fig. 20) and then moved the cart-mounted sonar ring
along the approach lines shown in figure 21, again stop-
ping every 2 ft to collect data. The sensor 0 centerline
aligned with the 0°, 45°, and 90° headings, and the sensor
4 centerline aligned along the 135° heading.

Figures 22 through 26 show the results of the 0°, 45°,
and 90° tests. The data did not describe the corner with
perfect precision, but the data from most approach angles
should be adequate for computer-aided machine guidance,
because it closely describes the shape and location of the
corner. \

The ranger FOV and incidence angle are key factors
in defining a corner like this. When an ultrasonic ranger
approaches a convex corner, the reflecting walls are ef-
fectively turning away from the sensor axis, But the edges
of the sensor FOV are still identifying an occupied region
and suggesting a wall of some type. If more precise data
are necessary for high-speed machine guidance, data cor-
rections can be attempted by recognizing patterns in the
echo waveform. King and Gordon (25) recognized objects

ray representation of rib versus surveyed rib.

in ultrasonic transducer echoes with pattern recognition
algorithms. King believes the FOV edge reflection shape
will be significantly different from an echo reflected from
an object located more centrally in the FOV.

Figures 27 and 28 show the data from the 135°
approach at 6 and 14 ft. The range readings from the
sensor ring do not define the corner very well at this ap-
proach angle. As the rib turned away from the ranger, the
edges of the ranger’s FOV echoed from protrusions or
angled surfaces on the actual mine rib.

Even though sensor 4 moved toward the corner in
precisely 2-ft increments, the tabular data show different
results. The narrowing of corner (the intersection of the
two ribs), the fact that the corner was not perpendicular
to the central ray of the sensor, and the fact that the area
viewed by the sensor decreased as it moved toward the
corner all contributed to this unexpected result.

Intersection

The ability of the rangers to represent a drift inter-
section was first determined by placing the ring on a track-
mounted push cart, moving through the intersection, and
stopping to collect data in 2-ft increments. The sensor 7
axis aligned with the direction of travel; consequently, the
sensor 1 axis was perpendicular to the rib.


Ranger axis

Ranger



KEY
@ Rib survey point

ee a ee
(0) 1 2 3 4
XQ Scale, ft

Figure 27.—Data from 135° approach at 14 ft.


Rib

\\

KEY
e@ Rib survey point

Oo 1 2 3 4
Scale, ft

Figure 24,—Sensor 6, 90° approach to convex corner.

J@ADAL JO UOHOBIIq


INTRODUCTION

STATEMENT OF PROBLEM

Computer-aided vehicle guidance requires sensor data
to relate the vehicle path to the desired trajectory and
to identify obstacles that the vehicle must avoid. In an
underground mine, the trajectory is projected based on
targets or a digital map, unless the machine is excavating
new territory. Consequently, sensor data are necessary to
relate the vehicle’s path and position to digital maps of
mine features. An appropriate set of features to identify
a vehicle’s location are those present in the ribs: convex
corners, concave corners, intersections, and roughly linear
planes. Obstacles to be identified might include equip-
ment, rocks from roof and rib falls, or personnel. The
problem is to find a reliable and suitable sensor or array
of sensors that locate the machine in relation to a wide
range of objects using range data or other information.

Several imaging sensors, such as scanning laser rang-
ers, stereo vision, and swept focus video, can accomplish
this task; however, their purchase cost, maintainability,
reliability, and computing power requirements make them
difficult to apply in the mining environment, Some sensor
systems, such as lasers and microwaves, are not readily
accepted by work forces because of suspected health
hazards.

Ultrasonic rangers are an attractive alternative because
they are simple, durable, relatively inexpensive, and ac-
cepted by the people working near them. An initial dem-
onstration using ultrasonic rangers to guide Carnegie
Mellon University’s Terregator mobile robot through an
underground mine showed rangers have promise for use
in mines (1)? However, ultrasonic rangers have not been
characterized carefully for guidance of underground min-
ing mobile equipment. These sensors have, however, been
used widely at mines for bin level monitoring, and under-
water sonar use has been well studied (2-3). Ultrasonic
rangers have also provided data for laboratory mobile
robot guidance (4-9). This study was part of the US.
Bureau of Mines computer-assisted mining program to
improve miner safety.

SCOPE

The research reported here attempts to determine
whether ultrasonic range data can meet some of the

Italic numbers in parentheses refer to items in the list of references
preceding the appendix at the end of this report.

navigation requirements of a computer-aided mining ve-
hicle by clearly identifying mine features such as linear
ribs, convex corners, and concave corners. The equipment
used for these experiments was an array of 24 Polaroid‘
ultrasonic rangers mounted at 15° intervals around a 27-in
fiberglass ring (made by Denning Mobile Robotics) inter-
faced with a microcomputer for data collection and storage
(10).

Researchers designed the experimental procedures to
test the ability of ultrasonic rangers to identify and define
the common navigation features found in an underground
mine. Laboratory tests established operating character-
istics such as field-of-view (FOV) and ranging accuracy
before underground tests began. The underground tests
were designed to evaluate the ability of the sensors to
delineate normal mine features such as corners, inter-
sections, and ribs. Experimental data were collected at
AMAX’s Henderson Mine near Empire, CO (11). The rib
feature was first surveyed using standard techniques, then
the ultrasonic range data were collected. The range data
were analyzed by comparing them to the rib outline as sur-
veyed. Researchers mounted sensors on actual under-
ground production load-haul-dump (LHD) units. This
lead to a series of laboratory experiments on sensor covers
or faceplates to improve durability.

RESULTS

This study showed ultrasonic ranging is a valid method
for machine navigation. The rangers outlined convex and
concave corners, ribs, and intersections. The rock and
shotcrete surfaces in the underground mine normally cause
a diffuse echo, so at least some of the sound wave returns
to the sensor even at high angles of incidence. Accurate
range readings were obtained up to a 60° angle of
incidence.

A thorough knowledge of the task requirements, as well
as the ranger’s individual characteristics, is important.
When used incorrectly, the rangers do not provide reliable
data. For example, the ranger’s FOV should be consid-
ered when analyzing range data.

‘Reference to specific products does not imply endorsement by the
US. Bureau of Mines.


CHARACTERISTICS OF ULTRASONIC RANGING SENSORS
IN AN UNDERGROUND ENVIRONMENT

By W. H. Strickland’ and R. H. King?

ABSTRACT

Ultrasonic ranging sensors are inexpensive, have no moving parts, have no lenses to clean, are nor-
mally small and unobtrusive, and can measure distances through moderate amounts of dust, smoke, and
humidity, so they are well suited to underground mines. In the work reported here, conducted by the
US. Bureau of Mines, researchers tested ultrasonic ranging sensors for their ability to define rib line
features for computer-aided navigation of underground mine mobile equipment. The investigation began
with laboratory tests of field of view, angle of incidence, intersensor variation, and ranging accuracy of
the individual rangers in a ring array produced by Denning Robotics of Wilmington, MA. The results
showed that the sensors have good accuracy and low variability.

Additional experiments at AMAX’s Henderson Mine showed the sensors could accurately and reliably
measure the distance to mine features, including convex and concave corners and rib intersections, The
results showed that when used properly, the ranger data are accurate enough for reliable mine vehicle
navigation.

When used incorrectly, ultrasonic rangers do not provide the anticipated data. Therefore, this report
explains the principles of ultrasonic range measurement, describes the ranger’s strengths and weaknesses,
and explains proper ranger use and data analysis.

'Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA (now manager, Experimental Mine, Colorado School of Mines,
Golden, CO).

?Mining engineer, Pittsburgh Research Center; professor of mining engineering, Colorado School of Mines, Golden, CO.


Shotcrete wall

LL:

Rib Rib

KEY
——  —— Approach line

LLiip oj
0 1° 2 3° 4
Scale, ft

a i
Figure 13.—Parallel approach to concave corner, path B, about 8 ft away.


FIELD OF VIEW

An ultrasonic ranger can be treated as a plane, circular
piston set in an infinite baffle. The radiated energy is
conically symmetric about an axis perpendicular to the
vibrating surface and passing through its center. That is,
the ultrasonic ranging transducer consists of a plane sur-
face that, at rest, lies flush with an acoustically hard wall.
In operation it vibrates perpendicular to its surface and
with constant velocity over its entire surface (17).

The acoustic field close to the ranger is very com-
plicated (8). However, in the far field [distances greater
than D?/(2d)], the spread of the acoustic beam depends
on the ranger’s diameter and wavelength as shown in the
following equation (19):

@ = 1.22 (A/D),

where § = acoustic beam angle in radians,
d = wavelength,
and D = ultrasonic ranger diameter.

The acoustic beam angle can be described as a contour
drawn by connecting the points in space at which the
energy by the ranger has dropped an equal level, for
example, 3 or 6 dB (18). Researchers tested a 14-in-
diameter, piezoelectric ultrasonic ranger operating at

330°

3002

270°

o°

13 KHz, made by Milltronics, that measured ranges up to
200 ft with 5.5° acoustic beam angle.

When measuring distance to an object, the beam
geometry is important, but the surface characteristics of
the object and the sensitivity of the ranger are equally
important. The experiments reported herein measure the
combination effect of these factors and consequently use
the term "field of view" (FOV) instead of beam angle.
The FOV is the beam angle at which an object with cer-
tain surface characteristics reflects enough sound to be
detected by the ranger.

The sound energy radiation pattern from a ranger has
many side lobes when the ranger’s radius is large in com-
parison to the transmitted wavelength (18). The Polaroid
ranger, with a 0.75-in radius and a wavelength of 0.27 in,
produces a three-lobe pattern. In this research, the aver-
age FOV of the major, on-axis lobe is represented with the
conical envelope shown in figure 1.

An acoustic sonar ray (vector OE in figure 2) is used
to compare ranger and physical survey data in this report.
The sonar ray is the center of the sound energy transmis-
sion. The angle of incidence (I) is the angle between a
perpendicular-to-the-target surface and the sonar ray as
shown in figure 3.

ATTENUATION

Because the transmitted ultrasonic energy wave spreads
over an increasingly large area the farther it travels from

30°

0-10 -30 -50 -70 O

0°
O -70 -50 -30 -100 8

Figure 1.—Polarold sound energy radiation pattern at 50 kHz (after Polaroid, 1982 (27)). Note: Decibels normalized to

on-axis response.


Shotcrete wall

ee ee

Frame

KEY
—— —— Approach line

LIL | | { |
ie} 1 2 3 4 5
Scale, ft

Figure 19.—Surveyed concave corner rib and range readings at 2-ft setup,

Rib


sooo

: REPORT OF INVESTIGATIONS/1993 Ee

Characteristics of Ultrasonic Ranging Sensors
in an Underground Environment

By W. H. Strickland and R. H. King

__ UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES.


sensor and the reflection cannot be distinguished from the
background noise.

The bounced sound wave may reflect from several sur-
faces before returning to the sensor, thus traveling over a
longer path and causing an apparent return from an object
at a more distant range (fig. 4). The surfaces encountered
in these tests were rough and diffused the sound reflec-
tions, so this was not a problem.

Highly absorptive surfaces, such as plastic foam, may
reflect so little sound at certain wavelengths that the return
is too weak. Extremely smooth surfaces might produce
reflections that interfere with the transmitted sound wave.
The interference effect is wavelength dependent, so an
echo that is weak at one frequency may be much stronger
at another frequency. Consequently, Polaroid produces
a ranger with a transmit burst composed of 60-, 57-, 53-,
and 49.7-kHz waves (15).

RANGING ACCURACY

Ranging accuracy is greatly improved if the actual
ambient speed of sound is used for all calculations.
Inaccuracies can be introduced to the range readings by
the length of the transmit burst; the surface character-
istics of the target, as described previously; the detection
threshold of the ranger; and the beam width.

The ultrasonic ranger reports the range to the very first
object that produces an echo above the threshold point.
There may be more than one object in the spreading
acoustic beam. Therefore, the range reported may not be
the range to the object being used to safely and accurately
guide the machine.

The ranging circuitry normally starts timing for an echo
the instant that it transmits a wave. If the transmit pulse
is long and the target surface irregular, the last piece of
the transmit may produce the only echo strong enough to
trigger the ranger’s threshold. It is difficult to determine
which part of the transmit wave created the echo; there-
fore, the transmit burst should be kept as short as
possible.

Other errors result from the method of echo amplifi-
cation. The Polaroid ranger has a “step” amplification
gain; the gain of the amplifier increases in discrete steps
with time (14). If the increase in gain occurs in the midst
of an echo, a false range reading may result.

THE POLAROID RANGER

The Polaroid ultrasonic ranger is an electrostatic trans-
ponder that transmits a pulse and then turns into a re-
ceiver (or microphone) (27). Figure 5 shows a typical
transmit and receive pulse from the Polaroid ranger in the
Denning ring array. The ranger precision is 0.1 ft over a
0.9- to 35.5-ft range, but rangers can be modified to obtain
greater resolution over shorter ranges. The rangers used
in this research measure from 0.9 to 25.5 ft.

In transmit mode, the sensor’s components, a thin,
sensitive foil composed of gold-coated polyester film
stretched over an aluminum backplate (fig. 6), act as a
capacitor. The foil vibrates at the impressed ac voltage
frequency to emit sound cnergy.

Any echo received by the ranger also causes the foil to
vibrate. The ranger now functions as a microphone and
produces an ac that ranges from volts to microvolts
depending on the echo signal strength. The farther an
echo source is from the ranger and the more absorptive
the echo object surface, the lower the echo’s energy and
the lower the amplitude of the voltage produced by the
ranger. Therefore, amplification is required to distinguish
low-strength echoes.

Researchers studied sources and amplitudes of inter-
nally and externally generated noise in the ranger circuit.
Numerous waveforms captured at high-frequency sampling
with a digital storage oscilloscope showed that the internal
noise in the Polaroid ranger’s circuitry is minimal. The
frequency of 49.7 kHz, used in the Polaroid sensors tested,
was above that measured on diesel-powered mining equip-
ment, so there were no interference problems from ex-
ternal sources (22). However, additional testing is neces-
sary before using lower frequency sonic rangers.

The Polaroid ranger differentiates between a legiti-
mate reflection and noise by assuming the reflection am-
plitude will exceed a constant threshold. The first echo
that exceeds the threshold is used to determine a range
reading. The object that produced this echo may or may
not be the object of interest. For example, the objective
of this research was to measure distance to rib features.
Sometimes, the sensor reported distance to small objects
such as wires or protruding bolts that reflected enough
sound to trigger the ranger. Some manufacturers, such as
Milltronics, incorporate programmable processors with the
ranging system to solve this problem.


the sensor, signal power decreases as 1/d?, where d is the
distance traveled (14), The reflected signal strength is
again reduced by a factor of 1/d2. In addition, there are
viscous losses and heat conduction losses (classical absorp-
tion), and losses associated with molecular exchange of
energy.

Viscous losses are similar to friction losses but occur
during the expansion and compression of the air as the
sound wave advances (18). The sound wave causes tem-
perature variations that produce heat conduction losses.
Heat transfers from the regions of higher temperature (the
sound wave) to those of lower temperature (the atmos-
phere). Molecular losses occur during the collision of gas
molecules as the sound wave travels. Energy transfers
from the sound wave to the internal structure of the
molecule (20).

Thus, the power of a sound wave transmitted in the
atmosphere dissipates rapidly with distance according to
the following equation (14):

P, = [(6*)/d4} Py,

Ranger

Figure 4.—Multiple reflections causing

where P, = power of received signal,
P, = power of transmitted signal,
a = absorption coefficient (atmospheric ab-
sorption per unit distance),
and d_ = distance from the source.

SURFACE CHARACTERISTICS

The characteristics of the reflecting surface have a
significant effect on the reflected signal. Some very
smooth surfaces, such as polished metal, water, and some-
times a rock surface with water on it, reflect the sound
without any significant scattering. This reflectivity causes
problems if the angle of incidence is large, that is, if angle
Tin figure 3 is not close to 0°. At large angles the sound
wave bounces away from the ranger (fig. 4). Consequent-
ly, only a small amount of sound reflects back to the

Ise range reading.


writeln (OutputFile);
reading: =reading+1
end; {while not eof}
close (InputFile);
close (OutputFile);
end; {ExtractRangeData}
Begin
StartX:=0.0; StartY:=0.0; Vel:=0.8; DeltaT: =0.075;
EnterTestParameters (StartX, StartY, Vel, DeltaT);
ExtractRangeData;
End.


60°

e
Rib
‘KEY
e@ Rib survey point
Lily
o ?t 2 3 4
Scale, ft

Figure 26.—Sensor 2, 90° approach to convex corner.

JAADAL $O UOlLIB1IG


e Rib survey point

Scale, ft

Figure 25.—Sensor 1, 90° approach to convex corner.

JBADAL JO UOlLOE4IG


ACKNOWLEDGMENTS

The authors would like to express their appreciation
to the many persons, hourly and management, at AMAX’s
Henderson Mine, especially William Hinken, Robert
Wishart, John Timmons, Kenny Schrader, and Doug
Crosby, for their assistance and guidance on this project.

The authors also greatly appreciate the contribution of
George Schnakenberg, Jr., research physicist, Pittsburgh
Research Center, U.S. Bureau of Mines, for the experi-
mental plan.

PRINCIPLES OF ULTRASONIC RANGE MEASUREMENT

Ultrasound frequencies are above the range of human
hearing. The use of sound for ranging was developed in
Europe just after World War I and was further refined
during World War II (12-13). Underwater sonar ranging
generally uses sound in the audio frequency range, the
frequencies in the normal hearing range, but atmospheric
ranging has usually been done with ultrasonic frequencies.

Ultrasonic rangers are transponders, that is, they act
as transmitters by converting electrical energy into sound
energy, then change roles to act as detectors (micro-
phones) and convert sound energy into electrical energy.
‘Two types of ultrasonic ranging transducers are commonly
used for ranging in air: electrostatic and piezoelectric. A
piezoelectric transducer impresses an ac voltage on a
crystal, which vibrates and transmits a sonic pulse. While
piezoelectric transducers couple very well to solids and
liquids, they transmit only a small percentage of their
energy to air.

Electrostatic transducers operate by causing a metallic-
coated plastic film to vibrate at the same frequency as an
applied voltage. They couple well to air because they have
a large displacement amplitude (they vibrate with a great
deal of intensity), which is needed to couple efficiently to
low-density media that have high compressibility (media
such as the atmosphere) (73).

Ultrasonic rangers transmit short bursts of high-
frequency sound energy, commonly in the 13- to 60-kHz
frequency range. The transmitted sound waves travel to
an object, are reflected, and travel back to the sensor.
Consequently, the distance can be calculated from the
round trip time (time of flight) and the velocity of sound
as shown in the following equation,

R = (V,)(t/2),
where R= range (ft),
V, = velocity of sound (ft/s),

and t = round trip time (s).

The ultrasonic ranging transducer vibrates after the
transmit wave has been sent out. This vibration can cause
false range readings. To overcome this, most manufac-
turers have introduced a "blanking range." During this
interval, the ranger is turned off and will not read any
return signals. This blanking range is 0.0 to 0.9 ft for the
rangers used in these experiments.

In most applications, the transducer is pulsed many
times and the resulting ranges are averaged. The pulse
rate of a ranger should be based on the range. The longer
the range, the longer the ranger must remain in the re-
ceive mode, waiting for an echo, and the longer the re-
quired delay between transmit pulses.

VELOCITY OF SOUND

The velocity of sound at 20° C is 1,125 ft/s. Biber,
Ellin, Shenk, and Stempeck (14) report that it changes
only 0.35% over the maximum humidity change at 20° C
and is virtually independent of pressure and elevation
changes. However, temperature variation causes a 7%
deviation from 0° to 40° C, because the velocity of sound
changes with temperature of the air as described in the
following equation:

V, = VoVT/273
where V, = velocity of sound in air (ft/s),

Vv,

‘9 = velocity of sound at 0° C (1,087.6 ft/s),

and T

temperature (K).

Ambient temperature sensor data can be fused with
ranger data to correct the error, or the actual speed of
sound can be measured over a known distance, and used
to adjust the calculated range (16).


Shotcrete wall

=

Rib Rib

Db
w
io)
oO

KEY
—— — Approach line
Lit
o 1 2 3 4
Scale, ft _

Figure 16.—Parallel approach to concave corner, path C, about 4 ft away.
.


cocerchceceaee6

jane ak ee Soe gr PES arene ceed

: x :
- : : -

Eccmenotes ices

. ‘8 * ‘

- : io ag

- - Sa A Eee -

eo ees

oe

23

Peeceaare epee eee

Figure 35.—Protective faceplates for sensors (upper left = 1/4-in expanded metal, upper right = 1/2-in wire
mesh, middle = Polaroid design).


Some ultrasonic rangers, such as the Milltronics ST-25
and ST-100 rangers, are designed for severe environments
(26). Milltronics coats these piezoelectric transducers
with polyurethane to protect them from dust, water, and
abrasion. They are designed to withstand shock, and be-
cause they are piezoelectric, there is no polyester film foil
to rip. They have been field proven in applications with

dust, steam, and acids, and have been used to reliably
detect live ore storage levels under the main gyratory
crusher at the Henderson Mine. The Milltronics ST-25
ranger has a rated beam width of 12°, and the ST-100
ranger has a beam width of 7° (28). They appear to be
more rugged and have better beam angles; they will be
tested in the future.

CONCLUSIONS

This study showed that ultrasonic rangers can identify
mine rib navigation features and can provide adequate
data for computer-aided mining vehicle guidance. ‘The
rangers defined convex corners, concave corners, inter-
sections, and roughly linear planes when the incidence
angle was appropriate. Ultrasonic rangers will probably
not give all the data needed for navigation of a computer-
aided vehicle. For instance, in their present form, ultra-
sonic rangers cannot fully define all the features of a muck
pile. However, they give simple range data that can iden-
tify common mine features with a good deal of clarity. To
give all the information a vehicle needs for navigation, a
suite of sensors will be required.

Large angles of incidence (greater than 60°), combined
with the FOV, caused improper definition of the rib line.
The rangers did not completely define the rib intersection
point of concave corners in all cases. The rangers did,
however, measure the distance to this wide range of ob-
jects with adequate precision to define the mine’s features
for a computer-guided vehicle.

Underground mine features appear to be excellent
sound reflectors. Because they are rough, they diffuse the
sound energy so multiple strong reflections and deflec-
tions are not present, and because they are hard and solid,
they absorb very little sound energy. Because mine ribs
are so rough, they provide enough surface area that is
nearly perpendicular to the sensor axis for the rangers to
work well with relatively large angles of incidence. The
only time the rock roughness created problems was when

the transmit burst hit at an extremely high angle of inci-
dence. When this event occurred, high data variability
resulted, because the roughness intermittently reflected
enough energy from the transmit wave’s edges to trigger
the ranger amplitude threshold. This problem may be ap-
proached by recognizing patterns in the echo waveform.

The rangers were sensitive to small objects protruding
from the ribs, since 1/2-in rock bolts reflected enough
energy to trigger the amplitude threshold. Consequently,
the bolts formed part of the rib outline map produced
from ranger data. This might be considered an error in
object recognition, but the rib outline that resulted would
help prevent collision between computer-guided vehicles
and the rib, protruding bolts, or any other obstacle.

The rangers that were mounted on a production LHD
withstood everything in the mine environment except di-
rect hits from sharp objects like rocks. The rangers are
durable enough for mine operating conditions that include
extremes in vibration, dust, and humidity. Experiments
showed that faceplate protection can be added to the
mounting without creating serious sound wave attenuation
problems.

Certain types of piezoelectric ultrasonic rangers, such
as the Milltronics unit described, can probably survive the
mine environment even better than the electrostatic sen-
sors tested in these experiments.

A thorough knowledge of the task the ultrasonic range
sensor is to perform and the ranger’s individual charac-
teristics is important. When used incorrectly, the rangers
do not provide reliable data.

REFERENCES

1. Oppenheim, I. J., and W. L. Whittaker. Demonstration of Robotic
Mapping of Mine Spaces. Civ. Eng. Robotics Lab. Rep. Carnegie
Mellon Univ, Pittsburgh, PA, 1985, 11 pp.

2. Urick, R.J. Principles of Underwater Sound, McGraw-Hill, New
York, NY, 1975, 384 pp.

3. Sutton, J. L. Underwater Acoustic Imaging. Modern Acoustical
Imaging, ed. by H. Lee and G. Wade. IEEE, New York, NY, 1986,
pp. 229-241.

4. Crowley, J. L. Dynamic World Modeling for an Intelligent Mo-
bile Robot Using a Rotating Ultra-Sonic Ranging Device. Carnegie
Mellon Univ. Robotics Inst. Rep. CMU-RI-TR-84-27, Pittsburgh, PA,
1984, 17 pp.

5. Drumheller, M. Mobile Robot Localization Using Sonar. IEEE
Trans. Pattern Anal. and Mach. Intell, v. 9, No. 2, March 1987, pp. 325-
332,

6. Elfes, A. A Sonar-Based Mapping and Navigation System. Paper
in IEEE Robotics and Automation Conference Proceedings. IEEE
hi2252-2/86 0000/1151, 1986, pp. 1151-1156.

. Sonar-Based Real-World Mapping and Navigation. IEEE
iL ova ‘and Autom. v. 3, No. 3, June 1987, pp. 249-265.

8. Moravec, H. P., and A. Elfes. High Resolution Maps From Wide
Angle Sonar. Paper in IEEE Robotics and Automation Conference
Proceedings. IEEE CH2152-7/85/0000/0116, 1985, pp. 116-121.


KEY
e@ Rib survey point
, .
ce) 1 2 3.4

Scale, ft

Figure 22.—Sensor 6, 0° approach to convex corner.

|BADA, JO UOlLOs4IG


CONTENTS

Page

Pt ¢.(~ ec is bee eeeeeee 1
Introduction 2
Statement of problem 2
Scope 2
Results 2
Acknowledgments . 3
Principles of ultrasonic range measurement 3
Velocity of sound ... 3
Field of view 4
Attenuation 4
Surface characteristics .. 6
Ranging accuracy 1
The Polaroid ranger 7
Characterization of ultrasonic rangers . 9
Laboratory tests . + . 9
Underground mine experiments. 9
Linear rib experiment . 9
Concave corner .. 2
Convex corner 13
Intersection . . 2B
Sensor durability 33
Conclusions 35
References . anne eee neenees . 35
Appendix—Program for converting data from Denning ring .........+..ss.scceceeeee veeeeeees 37

ILLUSTRATIONS

1. Polaroid sound energy radiation pattern at 50 KHz .

2. Ultrasonic ranger’s ideal field of view
3. Angle of incidence .
4, Multiple reflections causing false range reading ......

5. Sample of transmit and receive pulse from Polaroid ranger as used in Denning ring
6. .

7

8.

Polaroid Instrument Grade Ranger
Denning ring and microcomputer . .
. Field of view test ..

9. Mine navigation features
10. Contour map of rib surface
11. Side view of Denning ring in linear rib experiment .
12, Sonar ray representation of rib versus surveyed rib .......
13. Parallel approach to concave corner, path B, about 8 ft away
14. Parallel approach to concave corner, path B, about 4 ft away
15. Parallel approach to concave corner, path C, about 8 ft away ..
16. Parallel approach to concave corner, path C, about 4 ft away ..
17. Surveyed concave corner rib and range readings at 10-ft setup . .
18. Surveyed concave corner rib and range readings at 6-ft setup ..
19, Surveyed concave corner rib and range readings at 2-ft setup
20. Convex corner at Henderson Mine . .
21. Approach lines to convex corner .
22. Sensor 6, 0° approach to convex corner . .
23. Sensor 6, 45° approach to convex corner .
24. Sensor 6, 90° approach to convex corner... .


Figure 7.Denning ring and microcomputer.

Ranger
Ranger axis

KEY

a
@ Target detection point

duit yp a
01234 5
Scale, ft

Figure 8.—Field of view test.


Back

Cable tray
i
9.3" Denning
ring
12.5’
r a 172
4.25"
| Floor
[ 0

Figure 11.—Side view of Denning ring in It

Researchers rotated the ring through 105°, stopping at
each 5° increment to gather range data. Figure 12 shows
the rib as surveyed along with the position of the ring, and
sonar rays to the range reading. The mode of 10 range
readings was used to produce the drawings shown. The
range readings from the sensors produced a very good,
though not exact, delineation of the rib.

The modes, in feet, of 10 range readings at 0° angle of
incidence, listing ranging accuracy to mine rib at a distance
of 5 ft, follow: sensor 0, 4.8; sensor 1, 4.8; sensor 2, 4.8;
sensor 3, 4.9; sensor 4, 4.9; sensor 5, 4.9; sensor 6, 4.9;
sensor 7, 5.0, Temperature was 28° C, humidity 54%, ele-
vation 7,500 ft, and pressure 22.71 in Hg. The resolution
compares favorably with that found in the laboratory ex-
periments. Intersensor variation was quite a bit higher,
however, with range readings varying by as much as 0.2 ft
at a measured range of 5 ft. Misalignment of the sensors
in their ring mountings contributed to this variance. In ad-
dition, each sensor’s FOV enveloped different rib features,
causing variations in range readings.

Angle of incidence to a rough mine rib is an important
measurement. If reliable data cannot be obtained at high
enough incidence angles, sensors on the front of a
computer-aided machine will not be able to define the rib
lines far enough ahead for reliable trajectory generation.
At 60° incidence, range readings defined the rib with
adequate precision (a maximum error in this experiment
of 9 in). Sixty degrees is probably adequate for mine
vehicle guidance.

Borenstein and Koren (24) determined 25° was the
maximum angle of incidence for a "smooth" surface. The
rangers detected the mine rib at angles greater than 60°
(0° being perpendicular to the rib), but the data varied
too much, The variability results from rib protrusions

ar rib experiment.

intersecting the FOV, which produces weak reflections that
intermittently exceed the ranger’s amplitude threshold.

As the rangers were oriented parallel to the drift
centerline, they did not measure the expected maximum
distance of 25.5 ft. Instead, they sometimes received an
echo from the floor, the back, or the overhead cable tray
because the transponders were misaligned in the ring.
This problem was not a fault of the rangers, but was
probably due to their mounting.

Concave Corner

Next, researchers determined the ability of the rangers
to define concave corners by using traditional surveying
techniques to develop a rib baseline, setting the ring and
computer on a cart as shown in figure 7, and then map-
ping the ribs with the ultrasonic rangers for comparison.
The ribs were rough surfaces with numerous large, angular
features, similar to the linear rib, created by drill-blast-
muck operations. The wall between the ribs was shoterete
sprayed on plywood (fig. 7).

The cart was moved along a set of four approach lines
parallel to the ribs, shown in figures 13 through 16, and
along the diagonal approach path shown in figures 17
through 19, stopping every 2 ft for data storage. The
sensor 7 axis paralleled the approach paths, and the diag-
onal approach line was midway between sensors 3 and 4,
The figures show the sonar ray to the most frequently
occurring (mode) range reading, superimposed on the sur-
veyed rib edges.

The results show that in the diagonal approach ex-
periment, the sensor array clearly defined the concave
corner, However, the rangers could not see all the way
into the corner in the parallel approach experiments. This


Library of Congress Cataloging in Publication Data:

Strickland, W. H. (William H.)
Characteristics of ultrasonic ranging sensors in an underground environment / by
W.H. Strickland and RH. King.
Pp. cm. — (Report of investigations; 9452)
Includes bibliographical references (p. 35).
1, Mining machinery—Automatic control. 2. Sonar. I. King, Robert H. II. Title.
Ill. Series: Report of investigations (United States. Bureau of Mines); 9452.

TN345.S745 1992 622—dc20 92-26918 CIP



Figure 33.—Fly by view of intersection, sensor 4.

The occupied points (in this case, rib points) are written
to a file with a “dxf extension that can be read by the
computer-aided drafting package for display and com-
parison. Figure 33 shows good correlation between the
range readings and the surveyed rib line.

SENSOR DURABILITY

The ability of ultrasonic rangers to survive the mine
environment is extremely important if they are to be
used to navigate underground vehicles. The researchers
mounted three Polaroid rangers and the circuitry for one
ranger in a steel box (fig. 34) and bolted it to a production
LHD. The LHD operated normally, exposing the sensor
faces to the environment, and all components in the box
to shock and vibration.

The rangers were removed and tested after 68 h of
production exposure. Two of the rangers gave reliable
range readings, even though they were covered with dust.
The gold foil on the third was ripped, probably by a sharp
object such as a rock, causing sensor failure. The remain-
ing two rangers on the LHD were replaced, along with a
replacement for the failed ranger, for another 149 h of
operation. Again, one of these rangers failed after being
struck in the faceplate. The impact broke the plastic
frame of the ranger, and forced the faceplate into the foil.
The other rangers continued to give accurate range

readings.

— Rib as surveyed
* Sensor 4(45) data

Loy
0246 810
Scale, ft

The final durability test lasted 287 additional operating
hours. This time, all three rangers survived in good con-
dition and produced accurate range readings when tested.
There were no failures in the analog or digital ranging
circuit boards stored in the enclosure during the tests.

Several different enclosures were tested to protect the
frame and faceplate of the Polaroid rangers in the labora-
tory. The first unit tested was Polaroid’s Environmental
Enclosure, marketed for use in industrial environments.
The grill is made of Valox resin (fig. 35) and is specially
tuned to reduce interference with the sonar beam. How-
ever, this plastic covering cannot withstand direct impacts
by rocks.

Protective enclosures were fabricated from plastic-
coated 1/2-in wire mesh and 1/4-in expanded metal
(fig. 35), and were mounted 1/8 in from the face of the
sensor, within blanking zone.

The results of range and FOV tests (table 3) show that
none of the grills placed at this position severely degraded
tanger operation. However, the grills are seen as an
object in the FOV when they are placed more than 3/4 in
from the ranger face. Protective covers such as these
should improve the Polaroid ranger’s resistance to impact.
It may also be possible to mount the ranger in such a way
that the sound wave is reflected in the desired direction
by bouncing it off a metal or plastic plate, while the
ranger itself remains protected within the housing. This
arrangement was not tried, but it would probably reduce
echo strength.

Table 3.—Range and field-of-view tests with protective grills

Measured distance Polaroid test enclosure

1/4in expanded metal 1/2.in wire mesh

to dowel, ft Range, ft FOV,deg Range, ft__—*~FOV, deg _—Range, ft___-FOV, deg
25 0 25 0 25 0
5.0 19.9 5.0 23.4 5.0 22.4
78 0 78 0 75 0
99 16.5 9.9 77 10.0 18.2
124 0 12.4 ) 12.5 °
14.9 14.6 14.8 15.0 14.9 16.6



Figure 20.—Convex corner at Henderson Mine.


Report of Investigations 9452

Characteristics of Ultrasonic Ranging Sensors
in an Underground Environment

By W. H. Strickland and R. H. King

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt, Secretary

BUREAU OF MINES


KEY

Rib @ Rib survey point

vist | | J
1 2 3 4

Scale, ft

7
ja Approach line
l
y 10)

~~

Figure 28.—Data from 135° approach at 6 ft.


LineNumber: =1;
while (LineNumber< =3) and not (eof(InputFile) or (ch='>’))
do begin
repeat
read (InputFile, ch);
write (ch);
until (ch="]’) or (ch=’>’) or eof(InputFile);

{'T' is the last character in the string that is to be omitted from the range array.}

LineNumber: =LineNumber+1;

Sensor: =First;
while (Sensor <= Last) and not eof(InputFile) and not (ch=">’)
do begin {Read range values into OutputFile}

read (InputFile, Range [reading, sensor]);
write (OutputFile, Range [reading, sensor]:4);
writeIn(Range[reading,sensor],’,’,
reading,’,’,sensor);
sensor: =sensor+1;
end;{while sensor <= Last}
First: =First+8;
Last:=Last+8
end; {while LineNumber <= 3}

End; {ReadAndWriteThreeLines}

Procedure ExtractRangeData;
{The Denning ring writes range data via an RS232 link to a PC file. This procedure removes initial characters from the

file, for example, [0-7], that identify the sensors, and writes the integer range values to the output file for further
processing}

var ch:char; {Character variable for reading the sensor identifiers that are being removed
from the range data}

InputFile:text; {The Denning ring file of raw data}

OutputFile:text; {The file of range values produced by this procedure}

InputFileName:string[45];
OutputFileName:string[45];
reading:integer; {Each line in the new range array is a reading}

Begin
writeln (‘Please enter the name of the file produced by the’);
write (‘Denning Ring that will be input to this program. > >');
readin (InputFileName);
assign (InputFile, InputFileName);
reset (InputFile);
write (‘What do you want to name the output file? > >’);
readin (OutputFileName);
assign (OutputFile, OutputFileName);
rewrite (OutputFile);
reading: =1;
ch:="*;

while (not eof (InputFile)) and (not (ch=’>’)) do begin
ReadAndWriteThreeLines(reading, InputFileName, OutputFileName,

ch, InputFile, OutputFile);


KEY
e Rib survey point

LLti} of
ie) 1 2 3 #4
Scale, ft

Figure 23.—Sensor 6, 45° approach to convex corner.

J@ADAL JO UOlLOBJIG


150

100F
50F
> FE
ul
[=)
2D
E
5 OF
a e&
= &E
< E
—50k
—100F
—150
0.0 o.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

TIME, ms

1513 1.690
diam = diam

— |-o.128

TOP VIEW SIDE VIEW END VIEW

Figure 6.—Polarold Instrument Grade Ranger (after Polarold, 1982 (27)). Dimensions are In Inches.


N

Concave corner Convex corner

BRC RA

Wall

Figure 9.—Mine navigation features.

VERTICAL HEIGHT, in

Figure 10.—Contour map of rib surface (contour interval = 2 in). Sensor axes were located at y = 18.