APPLICATION OF RADIO FREQUENCY IDENTIFICATION SYSTEMS TO

COLLISION AVOIDANCE IN METAL/NONMETAL MINES

TODD M. RUFF, P.E.

NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH

315 E. MONTGOMERY, 

SPOKANE, WA 99207 USA

DREW HESSION-KUNZ

ID INTERNATIONAL HOLDINGS, LLC

P.O. BOX 1125

CONCORD, MA 01742 USA

Abstract-Collisions between haulage equipment and

pedestrian workers or other vehicles claim an average of

five lives each year in surface and underground metal/

nonmetal mining operations. Many more workers are

severely injured. Although technologies exist that could

prevent these collisions, (radar, CCTV, and back-up

alarms), they do have limitations and are not being used

widely. New developments in radio frequency identification (RFID) systems show promise for reducing the number of collisions. This paper describes tests conducted on

off-the-shelf RFID systems and the subsequent development of a custom RFID system that could be used for both

surface and underground mining equipment.

 INTRODUCTION

Researchers at the Spokane Research Laboratory (SRL)

of the National Institute for Occupational Safety and Health

(NIOSH) have been studying methods to reduce accidents

associated with collisions between mobile mining equipment

and pedestrian workers or smaller vehicles. Preliminary research has focused on evaluating current technology used in

detecting pedestrians or vehicles in the blind spots of large

haulage equipment operated in both surface and underground

metal/nonmetal mines. The limitations of current methods

prompted researchers to develop a more reliable and robust

method of warning an equipment operator of an impending

collision. 

During the period from 1992 to 1996, 23 workers were

killed in surface and underground metal/nonmetal mining

operations when they were crushed by mobile mining equipment [1]. The number of near misses and nonfatal injuries is

much higher [1]. Although circumstances vary, most fatalities

could have been avoided if the equipment operator and/or

pedestrian had sufficient warning of the impending collision.

EVALUATION OF EXISTING TECHNOLOGY

Several technologies are available that provide equipment

operators with information on objects in blind spots. These

include Doppler radar units, video cameras, ultrasonic and

infrared sensors, and radio frequency transponders. Many of

these technologies were developed for the transportation and

automotive industries and a few are available for mining

equipment. The first step of the current work was to determine

why the available technologies are not being widely used in

the mining industry.

Radar

Several discriminating back-up alarms are available that

are based on Doppler radar technology. Two of the more

popular units were evaluated both in the lab and at field sites.

The first unit, originally designed for school bus safety, was

not robust enough for mining environments. The second unit

was specifically designed for construction equipment and

proved to be robust enough for surface mining applications.

However, tests on surface haulage trucks revealed severe limitations because of numerous false alarms. For example, if the

sensitivity was set high enough to detect a person at 20 to 30

ft from the vehicle, it would also detect buildings as far away

as 100 ft. These findings are similar to those discussed in [2].

Recent work in the transportation industry indicates that

forward-looking radar shows promise in the development of

vehicle-collision-avoidance technology, but further research is

needed.

Video

Video cameras that allow a vehicle operator to observe the

area in the vehicle’s blind spots have been used in several

industries. Cameras have also been used in surface mining,

but are not widely accepted because of high maintenance

require-ments. In underground mines, the same problems are

com-pounded by low light conditions. As the cost and size
of video cameras continue to decrease, this method of obstacle

detection is becoming more popular.

Automotive 

Back-up alarms based on ultrasonic and infrared systems

have been used in the automotive market with some success.

No systems were found that were packaged to endure the environmental conditions and maintenance requirements of mining.

Ultrasonic-based systems are difficult to package in a way that

makes the system robust enough to handle the vibration of

mining equipment and the harsh environment of a typical

mine. Slow response times also limit their usefulness [2].

Infrared systems are susceptible to generating false alarms

caused by direct sunlight, reflections, and hot equipment [2].

Radio Frequency Identification

The most promising technology seen to date involves the

use of radio frequency transmitting tags that are detected by a

tag reader on the equipment. This technology is known as

radio frequency identification (RFID) and is used in many industries for asset tracking and security applications. Personnel

and smaller vehicles are each outfitted with a tag that has a

unique identification (ID) code. Each piece of large equipment has a tag detector or reader that “listens” for the unique

transmitted frequency and ID of a tag in the equipment’s blind

spots. If any tag is detected, an alarm alerts the equipment

operator. 

Only one RFID collision avoidance system, manufactured

by a foreign company, is known to be available for mining

equipment at this time. The cost of this existing system is

much higher than the cost of other collision avoidance sensors,

but the potential of the technology justifies continued work to

make this type of system more affordable for any size of mining operation. It is the goal of NIOSH researchers to develop

a low-cost system that is applicable to both underground and

surface mining operations.

RFID systems solve many of the problems of the earlier

technologies. With a properly designed system, there is mimimal chance of a false alarm. Either a tag is present in the read

range of the tag reader or it is not. If every pedestrian worker

is wearing a tag, an equipment operator will know if someone

is in the blind spot of the equipment. Packaging of the reader

and tags can be robust with no decrease in performance. An

RFID system would involve very little maintenance. And

finally, with new developments in radio frequency components, costs and size requirements can easily be met.

 

EVALUATING OFF-THE-SHELF RFID SYSTEMS

Researchers needed to determine if RFID was in fact

suitable for use on mining equipment and whether costs could

be kept low. The first step was to evaluate existing off-theshelf systems for other applications, such as vehicle and asset

tracking and security. The following specifications for system

performance were set by NIOSH researchers and were based

on typical distances of blind spots behind and to the sides of

equipment, equipment characteristics, and cost requirements.

The approach was to determine if an off-the-shelf system

could be modified to meet the specifications and if not, to

develop a custom system either in-house or under contract.

System Specifications

Tags: Tags must be small and unobtrusive. The tag could

be worn on a belt or eventually mounted on a cap lamp battery

for underground operations. The tag could be passive if the required read range of 50 ft is achieved. Otherwise, the tag

should be powered by rechargeable batteries that last for at

least 12 hours. The cost of each tag would depend on the other

uses of the tag system, e.g., personnel tracking, but should be

under $40 a unit when produced in large quantities. The tags

must be able to remain operational when exposed to shocks,

vibration, and the wet, dirty mining environment. Supplying

an audible alarm at the tag may be an option if power requirements and delays can be kept to a minimum.

Dimensions: As small as possible, but no larger than 3 in

square by 1 in thick.

Temperature: -20 to 70 °C.

Humidity: 100%, rain, snow.

Range: 50 ft maximum.

Delay: 0.5 sec maximum.

Voltage: Supplied by battery or 4-V dc if attached to

cap lamp.

Current: Low enough for 12 hours of operation.

Transmit Power: Must meet limits of human exposure as per

47 CFR 2.1093.

Cost: Under $40 per unit.

Reader System: The functions of the reader system can be

simplified to detect only the presence of a tag. It is not required that a unique ID of the tag be read, although this might

be useful in other applications. The maximum working range

of the system would need to be around 50 ft to allow the

equipment operator time to stop if a tag suddenly entered the

path of the machine. Each piece of mobile equipment would

have one or more readers mounted on it. The number of

readers would depend on their ability to sense a tag from any

blind spot around the machine. The reader must have some

method of ignoring the tag of the equipment operator while he

is operating the machine. The system would have to be robust

and able to handle jarring, vibration, and equipment washdowns. The frequency of the system should not interfere with

other radio systems or radio-remote-controlled equipment. 

Dimensions: As small as possible but no larger than 1 ft

square by 6 in thick.

Temperature: 20 to 70 °C.
Humidity: 100%, rain, snow.

Range: 50 ft maximum.

Delay: 0.5 sec maximum.

Voltage: 12- or 24-V dc, depending on equipment.

Transmit Power: Transmitting power must be kept very low

to avoid inducing current in blasting cap

wires and must also to meet the limits of

human exposure as per 47 CFR 2.1091.

Cost: Under $1,200 per unit, including the

operator warning system.

Operator Warning System: Output from the reader would

need to supply the operator with an indication that a tag is

within the sensing range. Both an audible and visual alarm are

preferable. These devices must be robust, industrial-type

indicators that are mounted in the operator’s cab.

Physical: Small indicator lamp and buzzer to indicate

the presence of a tag. Buzzer noise must be

heard above equipment noise.

Temperature: -20 to 70 °C.

Humidity: 100%, rain, snow.

Power: Depends on output of reader.

Cost: Included in reader system.

Evaluation of Off-the-Shelf Systems

Two RFID systems were purchased for evaluation.

System 1: The first system had been successfully applied

to vehicle and asset tracking in other industries. It used active

tags and a sophisticated reader that incorporated many functions that were not needed for the collision avoidance application. The system transmitted in the UHF spectrum and had a

read range of over 200 ft. Modifications to the system allowed

the read range to be decreased and incorporated an audible

alarm if a tag was within range. 

The system was tested on a 2-1⁄2-yd front-end loader at

SRL’s Hanson site. Fig. 1 shows the configuration of the tests.

The reader was mounted on the rear grill of the machine, and

the tag was worn on the belt of the pedestrian. Several

problems were encountered. Because of the sophistication of

the reader’s functions, a delay of at least 6 sec was required

between read times. This, of course, is unacceptable with

mobile equipment moving at any speed. Researchers continued with tests, however, to define what type of detection

zone could be achieved with the system. 

 As seen in fig. 2, two detection zones were defined. The

first zone (in dark gray) shows the area in which the reader

detected the tag and alarmed nearly 100% of the time. Tag

detection was not dependent on tag orientation. The second

zone (in light gray) shows the area in which tag detection was

sporadic. Tags within this zone were not detected when the

pedestrian’s body was directly between the tag and reader. 

 Figure 1. Test configuration of UHF RFID system.

The detection zones were adjustable out to 70 ft; however,

because of the sporadic detection region, only a small reliable

zone of around 20-ft radius from the front-end loader was

useful. If the reliable detection zone was extended beyond 20

ft, false alarms from tags at distances of 100 ft from the frontend loader were prevalent.

The cost of this system was also a limiting factor. At

$5,000 for the reader and $100 for each tag, the cost would not

be acceptable for widespread use. The manufacturer was contacted to see if it was interested in making modifications to the

system to meet our specifications. It declined. 

System 2: The second system tested was primarily used

in security applications. Both the tags and the reader transmitted in the LF range. The tags were passive and were detected by the reader using a loop-type antenna mounted on the

rear of the front-end loader. Fig. 3 shows the configuration of

the tests. This system met cost requirements, but the detection

range was only about 7 ft, too small to be used with mining

equipment. While body shielding was not a problem at these

frequencies, the directionality of the tags was too severe for

reliable operation. Extensive modifications to the antennae

and packaging of the system would be required.

Conclusions

We determined that, rather than try to modify an existing

system extensively, it would be more cost effective to develop

our own system to meet the specifications. Preliminary efforts

to develop a system in-house revealed the need for increased

expertise and decreased development time. Development of

the system is being done under contract with ID International

Holdings (IDI), Concord, MA. 
 Figure 2. Test results of UHF RFID system. 

 

Figure 3. Test configuration of LF RFID system.
SYSTEM DEVELOPMENT

Approach

Development of a customized collision avoidance system

focused on using commercially available off-the-shelf, lowcost radio-frequency components. While the specifications for

the system were relaxed somewhat for the prototype, these

specifications will have to be met in a final product. Thus, an

iterative approach is being taken in the development of the

system to make sure that each phase is progressing toward the

specifications. This is a work in progress and the following

sections describe the prototype system in its first phase.

System Description

The system, developed by IDI, consists of tags mounted

on pedestrian workers’ helmets, a reader-antenna unit on the

vehicle, and an alarm unit mounted in the operator's cab and

connected to the reader via a cable (fig. 4). Each pedestrian

worker and each vehicle in a work area or job site are outfitted

with their appropriate parts of the system. A pedestrian’s tag

detected within the range of the reader activates the alarm,

which warns the operator of the vehicle.

The prototype tags are housed in high-impact, sealed

plastic cases approximately 2.4 by 2.7 by 1 in. Components

include a microprocessor, a 315-MHZ transmitter, and an onboard battery. Production tags are projected to be less than

one-quarter that size, plus antennae. The tags in the prototype

system are fixed to the side of a hard hat with hook and loop

fasteners. In production, permanent mounts will be used that

will allow the tag to be removed or replaced.

 Reader components include receiver, signal conditioning

electronics, and microprocessor. The readers are housed in 4-

by 4- by 6-in NEMA-type enclosures, plus a partial-coil whip

antenna. These sealed, grounded steel boxes are mounted with

bolts and brackets to the host vehicle, powered by the

 Figure 4. IDI’s prototype collision avoidance system

vehicle’s 12-V dc system, and connected to the alarm unit in

the operator's cab. The sealed alarm unit is 2.25 by 1.75 by

1.25 in, and has a light and a 105-dB alarm.

For testing and development purposes, the readers have

two external switch-controlled adjustments: range and sensitivity. There are three selectable distance ranges (near, middle,

far) and a continuously adjustable sensitivity setting, which

controls the software's signal discernment characteristics.

Each tag transmits an "I'm here" signal three to four times

per second. The reader receives the signal and converts it into

a digital format. A noise rejection algorithm is applied to the

signal by the software, and the signal is then checked for validity. If a valid signal is detected within the selected range, an

alert condition is sent to the alarm unit, which flashes the light

and buzzes the alarm, which is near the operator of the vehicle.

If two tags are detected in the read range, a discernable

“double buzz” alerts the driver to the presence of two or more

tags. Up to four tags within the range of the reader can be discerned in this way.

Currently, the tags are powered by lithium coin cells,

which have a projected working life of 6 months to a year

(verification tests to follow). Design features that contribute

to extended battery life include a minimum parts count

coupled with careful design of the transmission duty cycle.

Developers are reviewing the addition of battery checking

capability or a “low battery” alarm to ensure that the battery

doesn't fail mid-shift, creating a hazardous condition.

Test Results

Tests were conducted on the prototype system using the

same platform as previous tests on commercial systems. The

reader was mounted on the rear of the front-end loader on the

engine cover at a height of approximately 7 ft. Tags were

placed on the side of the pedestrian’s hard hat. Fig. 5 shows

the various detection zones of the system at a medium range

setting. The dark gray area indicates the region in which a tag

was detected nearly 100% of the time. Detection was not

dependent on the physical orientation of the pedestrian. The

medium gray zone indicates the region in which tag detection

was reliable if the pedestrian was standing upright so that the

tag and its antenna were in line-of-sight of the reader. If the

pedestrian bent over in this region, the reader failed to detect

the tag. The light gray zone indicates the region in which tag

detection was unreliable and sporadic.

 

Needed Improvements

Having completed the proof-of-concept phase, researchers

will now focus on improvements to the effectiveness of the

system. There are two important issues with the system as

built. First is the potential for missing a tag as a result of

changes in the physical orientation of the tag, i.e., if a worker

bends over. Second is the potential for missing a tag because
 

Figure 5. Test results of IDI system.

of nulls in the field within the range of the system. The directionality of the tags will be reduced through less directional

antennae, both on the tags and the reader, or perhaps through

orthogonal pairs of antennae, to improve uniformity of field

strength in all directions. The nulls in the field will be

addressed through analysis of the interaction of the tag's signal

with the vehicle and changes in the mounting location and

antennae of the readers.

Additionally, IDI is studying software modifications that

would allow the system to discern between a signal in the

excluded far field and one coming from a null within the

expected range of the reader. This would also shorten the

sporadic region of detection and minimize false alarms from

tags that are a safe distance from the reader.

A further objective of the current phase is to test the radio

frequency output of the tags to ensure that they fall within both

the blasting cap safety limits and health exposure safety limits

currently in force. These tests will determine if the tags can be

safely worn during standard work shifts in all environments.

CONCLUSIONS

Avoiding collisions between pedestrian workers and

vehicles in surface and underground mines is an important

safety goal. While RFID systems hold promise for solving this

problem, existing systems do not meet the functionality or cost

needs of the industry. NIOSH is sponsoring the development

of a low-cost, high-reliability system through a contract with

IDI. Once commercialized and properly deployed, this system

could be important in reducing accidents and saving lives.

REFERENCES

[1] MSHA mining accident database.

[2] Johnson, G.A., R.E. Griffin, and L.W. Laage. Improved

Backup Alarm Technology for Mobile Mining Equipment. U.S. Bureau of Mines Information Circular 9079,

1986, 18 pp.