Visual feedback system to reduce errors while operating roof bolting machines

Lisa J. Steiner a,⁎

⁎	 Corresponding author. 

E-mail address: steinercpe@hotmail.com (L.J. Steiner). 

Brianna Eiter a, William Porter a, Tim Matty a

a Center for Disease Control, National Institute for Occupational Safety and Health, Office of Mining Safety and Health Research, United States

, Robin Burgess-Limerick b,

b Minerals Industry Safety and Health Centre, Sustainable Minerals Institute, The University of Queensland, Australia

abstract

Problem: Operators of roof bolting machines in underground coal mines do so in confined spaces and in very

close proximity to the moving equipment. Errors in the operation of these machines can have serious consequences, and the design of the equipment interface has a critical role in reducing the probability of such errors. Methods: An experiment was conducted to explore coding and directional compatibility on actual roof

bolting equipment and to determine the feasibility of a visual feedback system to alert operators of critical

movements and to also alert other workers in close proximity to the equipment to the pending movement

of the machine. The quantitative results of the study confirmed the potential for both selection errors and direction errors to be made, particularly during training. Results: Subjective data confirmed a potential benefit

of providing visual feedback of the intended operations and movements of the equipment. Impact: This research may influence the design of these and other similar control systems to provide evidence for the use

of warning systems to improve operator situational awareness.

1. Introduction

Roof bolting stabilizes the roof of the mine after coal extraction,

reducing the risk of injury or fatality associated with a roof fall.

However, the task is performed in confined space with the operators

of roof bolting machines in close proximity to moving parts. Errors in

the operation of roof bolters have caused many fatalities and injuries.

Injuries caused by intentional control operation can be divided into

the following categories: the wrong control was operated; the correct

control was operated in the wrong direction; the intended control

was operated in the intended direction while the injured employee

(a roof bolter operator or another person) was in a position of danger

(Burgess-Limerick, Krupenia, Zupanc, Wallis, & Steiner, 2010;

Burgess-Limerick & Steiner, 2006, 2007, 2011).

According to the analyses of the U.S. Mine Safety and Health

Administration (MSHA) injury database analyses of the roof bolter

accidents from 1984 through 1994, 11 of the 16 fatalities involved

the inadvertent (by the operator, roof fall, etc.) activation of a control

(MSHA, 1994). Of the 16 fatalities, 14 involved the moving boom.

Including victims being crushed between the boom and the mine

roof, victims being crushed between the boom and the canopy,

victims being crushed between the boom and the machine frame,

and one victim being crushed between the boom and the automated

temporary roof support (ATRS). Two of the 16 fatalities involved a

drill mast head where the victims were crushed between the drill

head and the machine frame. Additional interviews with experienced

roof bolters mentioned the swing lever when controls were inadvertently operated.

There has been much discussion about the idea of standardizing

control design to help reduce the probability of such errors. Miller

and McLellan (1973) reported that there was a need to redesign

roof bolter machines. Helander et al. (1980) suggested that “poor

human factors principles in the design and placement of controls

and inappropriately designed workstations contribute to a large

percentage of injuries” (p. 18). A report by Klishis et al. (1993) confirmed that injuries due to incorrect operator control remain a

problem.

1.1. Selection errors

In response to several roof bolter operator fatalities, in 1994 MSHA

formed a committee called the “Coal Mine Safety and Health Roof

Bolting Machine Committee” to investigate and report causes for

these fatalities. The first author served as one of the U.S. Bureau of

Mines contributors to this investigation. The committee found that

one of the leading causes was the unintentional operation of controls.

A specific suggestion was a recommendation to provide the industry

with distinct and consistent knob shapes for the controls of the

machine. From this committee was proposed rule-making in 1997

titled “Safety Standards for the Use of Roof-Bolting Machines in

Underground Mines” which suggested that design criteria were

being developed, but no rule making was ever published (MSHA,

1997). Ten years later, the New South Wales Department of Primary

Industries published Mining Design Guideline 35.1, (NSW DPI,
2009), in which a standard set of knob shapes are recommended for

the primary bolting controls.

Although these documents report the apparent need for shape

coding of controls to reduce inadvertent activation of the wrong control

(selection error), there is little scientific evidence that shape coding is

effective as a control. Many human factors textbooks refer to the need

to shape code, however the empirical evidence of the effectiveness

is not recorded or it could not be substantiated that the shape

coding was the main discriminating factor to improving performance

(Chapanis, 1999, p. 15–16; Roscoe, 1980; p. 274).

Weitz (1947) described experiments in which participants operated levers under varying shape coding conditions. No differences

were found in the number of selection errors between coded and

non-coded conditions in situations where the layout of the controls

remained constant during the experiment. In situations when the

controls were altered during the experiment, fewer selection errors

were made by the participants who were assigned to shape coded

conditions. Similarly, in a more recent series of experiments utilizing

a virtual reality analogy of bolting involving a bank of four levers

(Burgess-Limerick, Krupenia, Wallis, Pratim-Bannerjee and Steiner,

2010), shape coding was only found to reduce selection errors when

the spatial arrangement of levers was altered during the experiment.

1.2. Directional errors

As noted above, another cause of injury is directional compatibility

errors. Directional errors are those errors where the correct control is

operated but in the opposite direction than what was needed to produce the intended outcome. A contributing factor to this type of

error may be that the directional control-response relationship is

not compatible with the direction of movement or with the mental

model of the operator. The directional control response relationships

currently in use across mining equipment vary, even within manufacturers, and within similar functions, and sometimes change with

changes in vehicle direction (Zupanc, Burgess-Limerick, & Wallis,

2007). Helander et al. (1980) also found design deficiencies and

violation of control direction stereotypes associated with mining

equipment and suggested that these design flaws contributed to

increased injury risks.

Though it is agreed that it is important to ensure the compatibility

of directional control-response relationships, the design is not always

clear-cut. For example, it is relatively common on mining equipment

to find situations in which downward movement of a horizontal

control lever causes upward movement of the controlled element,

such as a boom, stabilizer jack or drill steel. While some authors

(e.g. Helander et al., 1980) have suggested that this is a violation of

compatible directional control-response relationships, Simpson and

Chan (1988) suggested that the response may be compatible if the

operators assume a ‘see-saw’ mental model of the situation, where moving the near end of the control downwards causes the far end (and the

controlled element) to move upwards. These issues have been examined

more recently using a virtual simulation (Burgess-Limerick, Krupenia,

Zupanc, et al., 2010).

1.3. Situational awareness

Improving the situational awareness of roof bolter operators is one

way to help reduce selection and directional compatibility errors.

Situational awareness can be defined as “the perception of the

elements in the environment within a volume of time and space, the

comprehension of their meaning and the projection of their status in

the near future” (Endsley, 1988). Expanding this definition Endsley

(1995) describes three levels of situational awareness as perception of

elements in the environment, comprehension of the current situation

and projection of future status. When an inadvertent activation of a

control occurs, the earlier this error is detected, the better. To improve

the operator's situational awareness, i.e. detecting errors then

correcting them, a feedback control to allow for errors to be caught

would be beneficial. The control feedback would enable the operator

to improve performance and reduce the probability of harm to either

himself/herself or to another individual nearby.

Currently, the operator must “notice” that the roof bolter arm

appendage is moving in the wrong direction before he/she can correct

the action. At that point, it may already be too late and the result can

be catastrophic. Operational errors become even more critical when

the error is performed out of the context of their routine job of placing roof bolts. For instance, the operator may get himself/herself

caught between the drill mast and ATRS (relating to the drill feed

control) or between the rib and the boom (relating to the swing

control). In that moment, there is no room for an incorrect activation

and yet, it may be highly probable for an error to be made when the

motion is not in the context of the normal routine of placing roof

bolts. Any feedback or warning information as to the correct directional movement would be invaluable.

A visual feedback system may serve to improve situational awareness by providing the operator with feedback prior to machine movement. A current technology being developed at the NIOSH Office of

Mine Safety and Health Research (OMSHR) gave promising results

of reducing time for subjects to detect and identify the direction of

movement of a continuous mining machine when operators were

standing in different operating positions during backing out and

tramming tasks (Sammarco, Gallagher, Mayton, & Srednicki, 2012).

The results indicated that in a dark environment, such as found in a

mine, the visual feedback system on the continuous mining machine

in this study “vastly improves an individual's ability to quickly detect

machine motions, in many cases by well over one second”. The research suggests that such a system could be an important tool to

alert underground miners to impending or active machine motion

which could prevent struck-by or pinning accidents in underground

mining, such as the type of accidents involved with roof bolting machines. This same technology may be helpful for roof bolter operators

when used in the context of operating controls. With accurate and

improved situation awareness, roof bolter operators can react and respond more rapidly and with higher accuracy. An improvement in the

feedback mechanism may provide the additional benefits of situational awareness.

1.4. Objectives

The aims of this research were (i) to determine whether the

patterns of selection errors and direction errors observed while operating a real bolting machine in a laboratory are consistent with previous research using a virtual simulation analogous to bolting, and

(ii) to determine whether a visual feedback intervention could

improve the operators ability to recognize and correct a lever selection or direction error before adverse effects of the incorrect action

are realized.

2. Methods

2.1. Participants

Sixteen adult male participants (age range of 22 to 56 years,

mean=37, SD=11) who were practicing, experienced roof bolter operators (underground mining experience range of one month to 36 years,

mean=10.1 years, SD=11.4 years) were recruited using word of

mouth with the United Mine Workers of America and other mines. All

participants had experience working with a roof bolter machine (range

of 1 months to 20 years, mean=4.7 years, SD=5.5 years). Thirteen of

the 16 participants had experience on the Fletcher Roof Ranger II

machine, the machine used in these tests. An equal mix of union and

non-union roof bolter operators were recruited to ensure all types of
operations were included. All participants had normal, or corrected to

normal vision, and were right handed. All participants provided written

informed consent prior to the commencement oftesting and were free to

withdraw at any point in the experiments. Ethical approval to run all

studies was given by the National Institute for Occupational Safety and

Health (NIOSH) Human Subjects Review Board.

2.2. Apparatus

All experiments were conducted in the Human Performance

Research Mine (HPRM) at the NIOSH, OMSHR in Pittsburgh, PA. The

HPRM is a simulated underground mine environment with various

testing apparatuses, data acquisition and control systems, and

networked computers. The HPRM is large enough to house actual

working mining equipment.

Participants were tested on an actual Fletcher Roof Ranger II Roof

Bolter machine provided by Fletcher Manufacturing for a three-month

period (Fig. 1a and b).

Fig. 1. Operator on a RRII machine (a) and the joystick control on a RRII (b).

The roof bolter was modified by Fletcher and

NIOSH to provide additional safety precautions as indicated through

the HSRB procedures and enable comparison with previous experimental testing (not yet published) on a mock-up bolter arm. The original

machine had only 4 levers and a joystick. The joystick controls the

rotation and the drill feed. For this experiment, the joystick control

was decoupled and replaced with two levers to separate the controls

for rotation and drill feed. Therefore a total of six mechanical directional

hydraulic control valves were available. The directional hydraulic

control valves operate the canopy, stabilizer jack, boom extension,

boom swing, drill rotation and drill elevation. Additional changes

were made to the roof bolter machine on-site in order for participants

to encounter different lever assignments and directional operation

than they were expecting so that errors could be measured.

These changes mostly involved reversing and rearranging the

hydraulics to change the direction of movement of machine appendages. Fig. 2a and b show the experimental arm and one test subject

using the experiment set-up. Fig. 3a and b illustrate the original and

reconfigured testing layout of levers.

2.3. Instrumentation

For these experiments the roof bolting machine was equipped

with two LED indicator lights within view of the operator to designate

the operation of the boom swing out/away and the drill raise functions. The swing out/away used a red LED light that was placed just

above the control bank and was directed towards the outside of the

machine so that while the boom was swinging outward, its light

would illuminate the rib and could be seen by workers coming onto

the section; from this location the operator was able to easily see

this swing light peripherally. The drill feed used a blue LED light located on the drill feed mechanism pointing upwards toward the drill

hole. The operator can easily see this light while using the control

bank. When a small amount of pressure was applied to the lever,

the light was activated. When more pressure was applied to the

lever the machine appendage moved. Both LED light interventions

could be turned off or on to match a given testing condition. The

choice of providing an LED light to the drill feed and swing out/

away control directions were based partially on MSHA accident data

and partially on the authors’ discussions with several experienced

roof bolter operators (MSHA, 1994). These two LED lights comprised

the visual feedback system intervention. Additionally, because roof

bolts were not actually placed during the tests, incandescent lights

(green and yellow) were added to the drill head to represent the rotational directions of the roof drill. It was not possible to show the action of rotation on the video stimuli so the light colors substituted for

these actions. A green light was used for clockwise rotation and yellow for counterclockwise.

Instrumentation was also added to the Fletcher Roof Ranger II

Roof Bolter to record the operator's actions during the laboratory

tests. Two micro switches were used to monitor each of the six

mechanical hydraulic directional control valves to enable recording of

the following: canopy raise/lower, stabilizer jack raise/lower, boom

extend/retract, boom swing in/out, drill rotation CW/CCW, and drill

feed raise/lower. The operator was also provided with a momentary

push button to acknowledge he was ready for the start of each test.

The outputs from all the micro switches, operators acknowledge

push button, and LED lights statuses were input into a DATAFORTH

16 channel analog I/O backplane (SCMPB02) containing 15 isolated

signal input modules (SCM5B33-04D) which output 0 to 10 VDC. As

each control was activated by the subject an electrical signal was

sent to an analog-to-digital board (PCI-6033E National Instruments,

Austin, TX) and a control signal was sent to a hydraulic valve to

move the arm in the corresponding action. The A/D board was

controlled by Matlab's Data Acquisition toolbox (MATLAB®; The

MathWorks, Inc, Natick, MA) and data were collected at 1000 Hz. A

computer controlled Samsung 24” Syncmaster 245BW LCD display

(Microsoft PowerPoint 2004, Microsoft, Seattle, WA) was placed on

a cart in front of the bolter to produce a visual stimulus (described

in the stimulus section below). Subjects were asked to use the six

control levers to move the roof bolter arm to match those of the visual

stimulus. The levers were either identical, length coded, or shape

coded according to the recommendations of MDG35 and either in a

vertical or a horizontal orientation (Fig. 4a and b).

2.4. Stimuli

Video clips of all possible movements of the machine were recorded

without the levers in the video. These stimuli were used to show the

operator the movements of the machine that they would mimic using

the physical levers in front of them. The clips were all independent

movements of the machine and were uniform in time, distance and aspect ratio. They were taken from the perspective of where the operator
would stand. These stimuli were triggered when the administrator

clicked a computer mouse after the participant acknowledged, through

the pressing of the audible momentary button, that they were ready for

the next stimulus. A signal was recorded when the stimulus was initiated and the test started. The stimuli were shown to the participant via

the Samsung 24” LCD monitor located at eye level within 7 feet of the

participant's standing position.

Fig. 2. Experimental roof bolting controls (a) and Operator during experimental trials (b).

Fig. 3. Actual roof bolter set up (a) and changed set up for experiment (b).

2.5. Experimental design and procedure

Each participant was individually tested on the Fletcher Roof

Ranger II machine located in the HPRM. The participant's age, handedness, demographics and expected control-response relationship

for all machine movements were recorded. Additionally normal or

corrected-to-normal vision was confirmed. A randomized experimental condition was used for each subject (Table 1). Each subject

was asked to complete a total of 288 trials of 12 different actions

(six levers with two directions each). The tests were blocked into 6

separate sections around 6 minutes each with a two to three minute

break in between each section. Each section consisted of 48 trials with

four repetitions of each action presented in a randomized order.

Prior to testing, each participant was shown the set-up and a set of

standard instructions was read to each participant. They were each

told that they would be operating the roof bolting machine and

would be required to face the computer screen, operating six different controls using their right hand only. They were asked which

way they believed the lever should be activated to make the appendage move in a particular direction. This was recorded as the perceived

stereotype of the directional relationship. The instructions about the

task were given with the emphasis on that it should be carried out

as fast and accurately as possible. Because of the relatively simple

nature of the task the participant was not given any practice time

prior to commencement of the experiment. The subjects were then

asked to stand by the bolter controls station with their right arm

depressing the button on the control arm and their left arm at their

side, the subjects were only allowed to use their right hand for testing. A testing session trial was then started provided the audible

button was depressed. For each trial a short pause of a still picture

of the video was given as a pre-cue and then a 2 to 3 second video

was shown on display. The subjects were instructed to release the

button once they made a decision as to what lever was needed to

duplicate the action. They then traveled to this lever to choose the
direction to operate the lever in order to duplicate the action shown

on the screen. The stimulus was only shown once and only one action

was shown for each trial (i.e. no sequence of actions). If an error was

made, the operator had time to recover the error. Only one correction

attempt was recorded. The subjects were asked to return the bolter

arm to the starting position and activate the audible button with

their right hand in preparation for the next trial. At the end of all

trials, baseline motor movement times were established by asking

participants to move a lever in a particular direction while recording

response time (i.e. no choices involved). For these trials the subjects

were instructed to release the red button and activate one of the six

levers in a particular direction once they hear the command to do

so. This was repeated three times for each lever/direction combination. Structured pre and post experiment questions were asked

regarding the operation of roof bolting machines in the mine and

the light interventions they experienced in the experiment.

Fig. 4. Typical lever shape (a) and suggested MDG35 shape-coded controls (b).

Table 1

Test Conditions.

Condition Control layout

1 Light On, Length Coded, Horizontal

2 Light On, Length Coded, Vertical

3 Light On, Shape Coded, Horizontal

4 Light On, Shape Coded, Vertical

5 Light On, Non-Shape Coded, Horizontal

6 Light On, Non-Shape Coded, Vertical

7 Light Off, Length Coded, Horizontal

8 Light Off, Length Coded, Vertical

9 Light Off, Shape Coded, Horizontal

10 Light Off, Shape Coded, Vertical

11 Light Off, Non-Shape Coded, Horizontal

12 Light Off, Non-Shape Coded, Vertical

2.6. Dependent measures

Control selection errors (SEs), and direction errors (DEs) were

recorded. A control SE was identified when the wrong lever was

selected for completion of the task. A control DE was identified

when the correct lever was operated, but in the wrong direction.

For this experiment participants were permitted to correct an error

if they made one on their first attempt.

3. Results

3.1. Error data

Fig. 5a presents selection error by block and coding. While there was

a significant effect of block [F(5,65)=11.6, pb0.0001], there was no

significant effect of coding [F(2,65)=1.055, p=0.376]. While the

median selection error rate for length and shape coded groups was

lower than the group with identical levers for blocks 2 and 3 (as

might be predicted if coding is a training aid), the interaction between

block and coding was not significant [F(10,65)=0.5806, p=0.8239].

Direction error rates as a function of lever, and lever orientation is

presented in Fig. 5b. A significant main effect of control lever was

evident [F(5,84)=3.551, p= 0.0058], however there was no significant effect of lever orientation [F(1,84)= 0.2818, p= 0.5969], nor

was there a significant interaction [F(5,84)= 0.2477, p= 0.0399].

Fig. 5c presents direction error rates as a function of lever, and of

whether the light intervention was enabled (for swing and drill functions). No significant effects of the light intervention were noted.

3.1.1. Subjective data

Participants were exposed to either the visual feedback system

(light on drill feed up and light on swing out/away condition) or

not. However, participants who did not have the visual feedback

system condition were shown the lights and how the system would

work on the machine after the end of their trials. The participants

were asked the following questions pre and post experiment:

Pre-test questions

1. Have you ever grabbed the wrong control when operating the

bolter?

o If so, was it while bolting or tramming?

o What control(s) did you grab incorrectly?

o Why do you think that happened?

2. Have you ever activated a control in the wrong direction?

o If so, was it while bolting or tramming?

o What control(s) did you operate in the wrong direction?

o Why do you think that happened?

All participants (100%) responded yes to #1 pre-test question ‘Have

you ever grabbed the wrong control when operating the bolter?’ Majority of the participants indicated doing this while bolting (14 of the 16

participants, or 87.5%). 12.5% (2 of 16) reported grabbing the wrong

control during both bolting and tramming. Participants erroneously

grabbed the swing (56%), the extend control (25%), the canopy and
drill feed (both 12%) and the rotation control (.06%). When asked ‘Why

do you think this happened?’ majority of participants report not looking

atthe controls while operating the roof bolter (12 of the 16 participants,

or 75%). 31% of participants reported grabbing the wrong control

because they were rushing or not paying attention to the task and 19%

said they either couldn't see the controls or made a mistake because

of the position of the controls.

Majority of participants (69%) also responded yes to pre-test

question #2, ‘Have you ever activated a control in the wrong direction?’ Participants indicate doing this primarily while bolting (73%)

and erroneously grabbing the swing (64%). When asked ‘Why do

you think this happened?’ Participants reported activating a control

in the wrong direction because they were either not paying attention

(45%) or in a hurry doing the job (27%).

Post-test questions

1. Did you find that the light feedback helped you to choose the correct, direction or to help you correct your error if you made one?

2. Would a light feedback system be useful for you on the job? When

would it be most useful?

For post-test question #1, ‘Did you find that the light feedback

helped you to choose the correct direction or to help you correct

your error if you made one?’ 43% (3 of 7) of the participants who

were NOT exposed to the light feedback said they would find the

light helpful while 78% (7 of 9) of the participants who were exposed

to the light feedback found it useful.

Prior to seeing the light feedback, one participant indicated that

the light “...would be distracting”. He thinks they would cover it up

if it were on the machine. He also did not think it would be helpful.

After the experiment was complete, he was shown the visual feedback lights and thought “it very well could be helpful for training.”

For post-test question #2, ‘Would a light feedback system be useful

for you on the job? When would it be most useful?’, the majority of

participants in both conditions indicated that having a light feedback

system would be useful for them on the job. 100% of those participants

exposed to the light feedback found it useful while 71% of participants

not given light feedback found it useful (5 of the 7 participants

responded yes in this condition while one participant refrained from

responding).

Participants who thought the light feedback system would be

useful for their job indicated the system would be most useful during

training (6 of 16 participants – 38%), as a warning system (4 of 16

participants – 25%) and as a signal that the roof bolting arm was

swinging (3 of 16 – 23%; one participant chose not to respond to

this question).

Fig. 5. a. Selection error by block and coding (length, shape or non-coded). b. Directional error by control lever and orientation (horizontal or vertical). c. Direction

error by control lever and visual feedback light intervention.

4. Discussion

The selection error data were consistent with those previously

gathered from an examination of shape coding in a virtual simulation

(Burgess-Limerick, Krupenia, Wallis, et al., 2010), and with the previous literature. Selection errors are likely to be made, particularly in

the early stages of learning a new lever layout, even by experienced

operators. Shape coding may be beneficial if it is consistently applied

to the same machine functions across different machines.

The direction error data similarly confirm that directional error

rates are relatively high, but that the error rates vary considerably

across functions. Where directional control response compatibility is

maintained (e.g. drill feed) the direction error rate was very low.

Sump (extension) movements of the bolter arm required movements

of the control levers which were perpendicular to the direction of the

response, and this lack of compatibility resulted in higher direction

error rates. Directional error probability can be reduced by, as far as

possible, ensuring directional control-response compatibility. This

finding is consistent with the previous literature and the examination

of a virtual analogy of bolting (Burgess-Limerick, Krupenia, Zupanc, et

al., 2010).

Although the direction error rate was not significantly different

between groups with and without the visual feedback system, the

qualitative data indicated that the workers generally found the lights

helpful. The control that most operators said they selected incorrectly

was the swing control. Most operators report they do not look at the

controls when operating their equipment. Other explanations include
being in a hurry or selecting the wrong control because of the positioning of the controls. Since the controls all look and feel the same

as each other, it is difficult to distinguish between controls. Operators

said they “count” or “feel” for the right control in terms of its position

or order and sometimes make a mistake. They also reported that

when they operate the controls in the wrong direction it is while

they are bolting and mostly with the swing control. This further

establishes that the swing is a control that gives the operators the

most difficulty and may not match their mental models of directional

compatibility. The swing (left and right motion) moves in a different

plane than the action direction of its control lever (up and down). The

drill feed, as evidenced by the MSHA injury and fatality data, and the

swing as evidenced by discussions with roof bolter operators and

answers to these experimental questions, are the two controls

which can be confused and may result in catastrophic events when

operated at the wrong time or in the wrong direction.

The lack of significance in the quantitative data may be due to the

simple nature of task and the lack of other distractions during the

experiment. Since there are limited action choices as a roof bolter

operator, the simplicity of the task itself only generates a small number of errors even with the changes to the machine with which they

had experience. The number of errors may have been reduced further

depending on whether operators prioritized accuracy over speed.

The lower positive response rate to QUESTION #1 by the miners

exposed and NOT exposed to the light feedback may be related to

these same issues. The error rate of both these subject groups was

low to begin with; therefore, it is less likely that they would feel the

need for additional aid.

The anecdotal comment from one of the subjects NOT exposed to

the light feedback adds credence to this thought; after exposure to

the system, he responded more favorably. However, both groups

responded positively to QUESTION #2 indicating that the light feedback would be helpful even if it did not help them. They reported

that it would have been very helpful when they were training to

operate the roof bolter machine. This cue should likely be incorporated into the design of the machine to address the possibility of operators relying on that feedback while performing bolting tasks. There is

evidence in the literature that cueing decreases error rates and that

withholding a cue that a participant has been trained to use increases

error rates (Ament, Lai, & Cox, 2011). Because error rates for both

groups of participants was low to begin with, it can be speculated

that participants did not need the additional aid because majority of

the participants could be considered an expert at the task (median

time working as a roof bolter=4.67 years).

The positive response to the use of the visual feedback system is

encouraging because based on human factors principles the light

feedback should improve attention (Lynas & Horberry, 2011 and

Stanton, Chambers, & Piggott, 2001). The light feedback increases

task attention spatially and temporally as well as providing feedback

redundancy in conjunction with the bolter motion itself (Wogalter &

Mayhorn, 2005). The additional lighting on the drill feed up motion

will also provide additional lighting when placing the roof bolt, a

request that several roof bolters have mentioned during underground

discussions of roof bolting tasks.

5. Limitations and future direction

A limitation to this study was the small number of roof bolter operators that were available for this study. It is costly and difficult at

this time of mine worker shortages to have workers tested. Other

types of interventions could have been compared and tested along

with placing the visual feedback system on controls other than the

drill feed up and swing out/away. It may have been helpful to test

selection errors and directional compatibility errors with a larger

number of controls. This may help identify when shape-coding,

length-coding and location coding is most beneficial. Also, it may be

possible to compare error rates and reaction times of new bolter operator trainees with and without this feedback system as well as determine how such an intervention can change the way they do their

jobs.

6. Summary and impact on Industry

The data illustrate the importance of multiple control barriers to

both reduce the probability of errors (shape coding and compatible

directional control-response relationships) and control measures

aimed at making systems error tolerant, and allowing operators to

detect and recover from errors before negative consequences are

realized. The experiment confirmed the need for additional attention

to the swing and drill feed. The visual warning system may be an

acceptable and appropriate intervention while the standardization

of controls is further examined. It is possible that this type of intervention is effective for times when there is little time for decisions

and an error in direction of a lever could result in injury or even

death. The situation where the operator is using the controls out of

the context of their normal routine or when the operator is training

for the first months of their jobs, this visual feedback can maximize

safety and minimize learning time. It may be necessary to carry-on

this feedback once the operator has used it for training. This

cost-effective solution should be studied in the environment to

ensure its intended use. Applications beyond mining can extend to

any control design where feedback is critical to the health and safety

of the operator and there is a need to improve situational awareness.

Disclaimers

Mention of any company or product does not constitute endorsement by the National Institute for Occupational Safety and Health.

The findings and conclusions in this article are those of the authors

and do not necessarily represent the views of the National Institute

for Occupational Safety and Health.

Acknowledgements

The studies described in this article were conducted with assistance from A. Cook and M.E. Nelson. D. Beshero provided the realistic

representations of the bolting controls in this article graphics. Also,

we want to thank all of the mines that realized the importance of

this research and allowed their bolter operators to participate. Finally,

a special thanks to Fletcher Mining Company for providing expertise

and the roof bolting equipment to conduct this experiment.

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