Reducing control selection errors associated with underground bolting equipment

Robin Burgess-Limerick a,*,

* Corresponding author. Tel.: þ61 7 3365 4718; fax: þ61 3 3365 6877.

E-mail address: robin@hms.uq.edu.au (R. Burgess-Limerick).

Veronica Krupenia a, Christine Zupanc a, Guy Wallis a,

a School of Human Movement Studies, The University of Queensland 4072, Australia

Lisa Steiner b

b Mining Injury Prevention Branch, National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, USA

abstract

Selecting the incorrect control during the operation of underground bolting and drilling equipment

causes serious injuries. Shape coding and the layout of dual control banks are two aspects of control

design which require further examination. The aims of this research were: (i) to determine whether

arbitrary shape coding was effective in reducing selection error rates in a virtual analogy of roof-bolting;

and (ii) to determine whether any advantages exist for mirror or place layouts for dual control situations

in this situation. Two experiments were conducted to address these questions. No benefits of arbitrary

shape coding were evident while control location remained constant. When control location was altered,

shape coding did provide a significant reduction in selection error rate. No differences between mirror or

place arrangements were detected and this question remains open.

1. Introduction

The introduction of roof and rib bolting to prevent rock falls in

underground coal mines was a major safety advance (Mark, 2002).

However, additional hazards were introduced in the process.

Analyses of narratives describing injuries occurring in both

Australia and the United States of America have highlighted the

potential of control errors during drilling and bolting to cause

serious injuries (Burgess-Limerick and Steiner, 2006; 2007).

Examples of typical bolting controls are illustrated in Fig. 1.

Standardising the controls on bolting machines has been suggested many times as a means of reducing the probability of control

errors. Miller and McLellan (1973) commented on the ‘‘obvious

need’’ to redesign roof-bolting machines, suggesting, for example,

that of 759 bolting machine related injuries, 72 involved operating

the wrong control, while Helander et al. (1983) determined that 5%

of bolting machine accidents were caused by control activation

errors. 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

the reported injuries’’ (p. 18).

A Society of Automotive Engineers (SAE) standard titled

‘‘Human factors design guidelines for mobile underground mining

equipment’’ which addressed these issues was defeated at a ballot

in 1984 (Gilbert, 1990). A subsequent report (Klishis et al., 1993)

again noted the lack of standardisation of bolting machine controls,

even among machines from the same manufacturer, and commented on the potential for injuries due to incorrect control

operation.

While there are a number of aspects of bolting control design

which require consideration, the experiments reported here focus

on two topics related to reducing the probability of operating the

wrong control, i.e. selection errors. The first issue is shape coding;

the second concerns the layout of controls in the situation where

multiple banks of controls are provided.

1.1. Shape coding

In a six-week period in 1994, three operators of roof-bolting

machines in the USA were killed. Two were crushed between

drill head and machine frame while rib bolting, the third

crushed between drill head and canopy. A ‘‘Coal Mine Safety and

Health Roof-Bolting-Machine Committee’’ was formed by the US

Mine Safety and Health Authority (MSHA) to investigate, and

a report released (MSHA, 1994) which determined the causes to

be the unintentional operation of controls. Amongst other

suggestions, a recommendation included in this report was:

‘‘Provide industry-wide accepted distinct and consistent knob

shapes’’.

The MSHA subsequently called for industry comment on an

advance notice of proposed rulemaking titled ‘‘Safety standards for

the use of roof-bolting machines in underground mines’’ (MSHA,

1997) which suggested that MSHA was developing design criteria

for underground bolting machines.

The New SouthWales Department of Primary Industries publishes

‘‘Mining Design Guidelines’’ to assist mining companies and
equipment manufacturers meet their obligations to provide ‘‘fit for

purpose’’ equipment. Mining Design Guideline 35.1 (MDG35)

‘‘Guideline for bolting and drilling plant in mines, Part 1: Bolting plant

for strata support in underground coal mines’’ (NSW DPI, 2009a) has

been published which stipulates a standard set of knob shapes for

the primary bolting controls (rotation, feed and timber jack).

Fig. 1. Examples of bolting rig controls. The controls illustrated in the top of the figure are on a stand-alone bolting machine; those on the bottom are for bolting rigs located on

a continuous mining machine (an integrated bolter-miner).

While shape coding is very commonly recommended within

Human Factors texts, empirical evidence of the effectiveness of

shape coding in reducing selection errors is scant. The most

frequently quoted example of the effectiveness of shape coding is

the instigation in World War II of the use of a wheel shaped knob to

correspond to the wheel raising/lowering in an aircraft to reduce

the probability of pilots retracting the wheels on landing, rather

than the flaps as intended. This evidence remains anecdotal, and as

the investigator concerned described the situation, shape coding

was not the only control employed viz:

‘‘I was asked to figure out why pilots and copilots frequently

retracted the landing gear instead of the landing flaps after

landing. What I found on inspecting the cockpits . was two

identical toggle switches side by side, one for the landing gear,

the other for the flaps. Given the stress of landing after a combat

mission, it is understandable how they could have been easily

confused. . The ad hoc remedies proposed at the time (separate

the controls and/or shape code them) were substantiated in the

human factors literature years later. Another remedy was a more

mechanical one – installing a sensor on the landing struts that

detected whether they were compressed by the weight of the

aircraft. If so, a circuit deactivated the landing gear control in the

cockpit’’ (Chapanis, 1999, p.15–16).

Despite the claim that shape coding was later substantiated,

only two published papers exist which address the issue. Both

were conceived with aviation applications in mind, and neither

provided unequivocal evidence for the benefits of shape coding.

This lack of evidence was commented on by Roscoe (1980), who

noted:

‘‘The discriminability of shape-coded switch knobs had been

studied vigorously following World War II. . However, further

application of shape coding was stalled because no investigator

had demonstrated a reliable improvement in any critical

switching operation attributable to the application of discriminably shaped switch knobs.’’ (p. 274).

Weitz (1947) reported two experiments in which participants

operated one of four levers in response to pairs of stimulus lights

under varying shape coding conditions. The task was to make as

many correct lever movements as possible in a one-minute trial. No

differences were found in the number of selection errors between

coded and non-coded conditions in situations in which the layout

of the controls was unaltered during the experiment. However,

fewer selection errors were made by participants assigned to

a shape coded condition when the layout (order) of the controls

was altered during the experiment.
Slocum et al. (1971) placed participants in a dual task paradigm

in which the secondary task was to rotate one of four knobs in

response to verbal commands. Three coding conditions were

employed: non-shape coded; arbitrary shape coding; and coding

which corresponded meaningfully to the commands given. A

reduction in selection errors was found for the meaningfully coded

knobs for the first block of 48 trials only. No difference was found

between arbitrary coded and non-coded knobs during this block,

nor were there any differences in error rate between any of the

conditions in the second block of trials.

In summary, there is limited evidence that meaningful coding

may reduce selection errors, however, arbitrary shape coding has

not been demonstrated to reduce selection errors when the location of controls remains constant. The only evidence for a benefit of

arbitrary shape coding suggests that a reduction in selection errors

may occur following a change in the layout of controls.

1.2. Mirror versus place layouts

Bolting machines, whether stand-alone, or integrated onto

continuous miners, typically have dual control stations. The operator will use different hands to operate the controls depending on

whether the control bank is to the left or right of the bolting rig. A

question then arises regarding the appropriate relationship

between the control layouts at the different stations. One alternative is for the control arrangement to be ‘‘mirrored’’, that is, for the

controls to be laid out such that controls to be operated by left and

right hands are in the same order relative to the drill head, that is

the control closest to the drill head on each side corresponds to the

same machine function. A non-mirrored arrangement, or ‘‘place’’

arrangement has the controls laid out in the same order left-toright, that is the left-most control on both sides controls the same

function, and so on (Fig. 2).

Fig. 2. ‘‘Mirror’’ and ‘‘Place’’ alternatives for situations where dual workstations are

provided on different sides of a machine (after Helander et al., 1980).

Helander et al. (1980) noted that the question was controversial,

and cited a Masters thesis by Pigg (1954) as the best available

evidence. This research involved 64 participants learning a four

choice reaction time task in which different colours of a light were

paired with four switches placed in a row facing the participant to

the left or right of a display containing the light. After six blocks of

eight trials, subjects changed hands and repeated the task with the

switches either presented in the same left-right sequence, or in

a mirrored sequence. The mean time taken to complete eight

correct responses (including the time taken for errors) was longer

for the first eight trials following the change of hand for those

participants in the left–right condition relative to the mirror

condition, but this difference was absent in the second and

subsequent eight trial series. Fewer errors were also made,

however no statistical analysis was undertaken of the error data. On

the basis of these results, Helander et al. (1980) recommended

a mirrored layout, and this recommendation was contained in the

proposed SAE standard (Gilbert, 1990).

However, while not citing any evidence, a contrary recommendation was made by Muldoon et al. (1980) ‘‘Once an operator

learns that rotate is to the left of feed, he should not have to relearn

that rotate is to the right of feed on the right boom.’’ . ‘‘Since

mirror image controls confuse the operator and do not increase

efficiency, they should not be used’’ (p. 41). The proposed ‘‘Human

Factors Guidelines for Mobile Underground Equipment’’ provided

by Essex Corporation in 1984 (Gilbert, 1990) also recommended

against mirror image control configurations for drill stations.

Machine Design Guideline 35 (NSW DPI, 2009a) similarly stipulates

that controls should be arranged ‘‘so that the controls are handed

identically, irrespective of the location on the bolting plant e.g., the

right hand lever should operate the same function on both sides of

the bolting plant’’.

1.3. Objectives

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

arbitrary shape coding as proposed within MDG35 is effective

in reducing selection error rates in a virtual analogy of operating bolting controls; and (ii) to determine whether any

advantages exist for mirror or place layouts for dual control

situations. Two experiments were conducted to address these

questions.

2. Methods

2.1. Apparatus

A computer generated simulation of a generic device (Fig. 3)

capable of slewing left and right (rotation about a vertical axis of

rotation), elevation and depression (rotation about a horizontal

axis of rotation), extension and retraction (lengthening or shortening of the virtual device), and changing colour were created by

a Silicon Graphics Onyx 3000 ® equipped with InfiniteReality II

graphics.

Fig. 3. The computer controlled stimulus image (left) prompted participants to

manipulate levers to cause the same change in the controlled image (right).

The image was projected onto a reflective screen using

a BARCO 808S analogue projector with a 24 Hz frame rate and

screen resolution of 1280 x 1024 pixels. Participants used four

levers to cause the movements of the virtual device on the right to

match those of the computer controlled stimulus image on the

left. The levers were either identical, or shape coded according to

the recommendations of MDG35. These shapes, and the effect of

each lever, are described in Fig. 4. A schematic layout of the

situation is provided in Fig. 5.
Fig. 4. Levers manipulated by participants in either non-shape coded (left) or shape coded (right) conditions. The shape coded levers utilise the shape provided in MDG35.1 (NSW

DPI, 2009a,b).

Fig. 5. Schematic of experiment layout (not to scale).

2.2. Procedure

Each trial required each of the four levers to be operated in

varying orders. The required lever and direction of operation was

indicated to the participant by the computer controlled image

moving (or changing colour). The participant’s task was to move

the correct lever in the correct direction to cause a matching change

in the virtual device controlled by the participant. If the correct

control was operated in the correct direction, a C Major chord was

played, and the next required movement presented. If an error was

made, a series of descending tones were played, the participant

controlled virtual device returned to its prior position, and the

movement repeated until the correct movement was achieved. The

nature of the initial error (selection or direction) was recorded. For

correct responses, the time taken until the lever was operated was

recorded (choice reaction time). The order and direction of lever

movements varied pseudo-randomly in that each of 16 different

combinations of required movements was presented in random

order during each block of trials.

In Experiment one, 48 participants were assigned to shape

coded or non-coded horizontal controls, with the controls initially

placed on the left or right side of the participant. The participants

completed five blocks of 16 trials (each trial involving moving each

of four levers, prompted via a visual stimuli of a virtual device).

Following the initial five trials, the participants changed sides, and

performed a second set of five blocks of trials with the levers on the

opposite side. For 24 of the participants the order of the levers

remained constant from left to right (‘‘place’’ transfer condition),

while for the remaining 24 participants the lever order was

reversed (‘‘mirror’’ transfer condition).

In Experiment two, 36 participants completed 10 blocks of 16

trials, with the location of the levers alternating between left and

right with each block. The participants were randomly assigned to

(i) the initial side (left or right) and (ii) mirror, place, or random

arrangement of levers. The levers were non-shape coded and

direction compatibility was constant.

2.3. Analysis

Percent error was calculated for selection error and direction

error measures. Error data are bounded by zero, and the distributions are skewed as a consequence. Median and interquartile

ranges for these data are presented graphically, and inferential

statistical analysis were undertaken on log transformed accuracy

(100 - %error) data. Selection error rates and reaction time were

examined as a function of coding and block.

3. Results

3.1. Experiment one

Fig. 6 illustrates the median selection error rates and reaction

time for each group for the 10 blocks of trials. Median error rates

were very low following the first block of trials and, apart from

a main effect of block (F [9396] 1⁄4 24.7, p < 0.001), there were no

significant effects of group, nor were there any significant two-way

or three-way interactions. Similarly, apart from a main effect of

block on reaction time (F [9396] 1⁄4 77.9, p < 0.001), there were no

significant effects of group, nor were there any significant interactions for reaction time.

Following five blocks of trials, the side on which the levers were

placed changed from right to left, or vice versa. The error and

reaction time data for the block of trials immediately following the

change of sides (block six) was extracted for further analysis. A

Two-way ANOVA on the log transformed data revealed a significant

effect of coding (F [1,44] 1⁄4 7.54, p 1⁄4 0.009), but no significant effect

of lever layout (mirror or place) (F [1,44] 1⁄4 0.005, p 1⁄4 0.947), nor

any interaction (F [1,44] 1⁄4 0.19, p 1⁄4 0.665). No significant differences were found for reaction time during this block of trials.

(Effect of coding F [1,44] 1⁄4 2.422, p 1⁄4 0.127; Effect of group F

[1,44] 1⁄4 0.033, p 1⁄4 0.858; Interaction F [1,44] 1⁄4 0.128, p 1⁄4 0.723).

3.2. Experiment two

Fig. 7 provides median selection error rates and mean reaction

time for the three groups of participants across ten blocks of trials.

A Two-way ANOVA performed using log transformed accuracy data

revealed a main effect of block (F [9297] 1⁄4 6.37, p < 0.001) but no

significant effect of group (F [2297] 1⁄4 0.63, p 1⁄4 0.537), nor any

interaction (F [18,297] 1⁄4 0.74, p 1⁄4 0.773).

A Two-way ANOVA conducted on the reaction time for correct

responses revealed that while the main effect of group was not

significant (F [2297] 1⁄4 0.64, p 1⁄4 0.534); there was a significant effect

of block on reaction time (F [9297] 1⁄4 63, p < 0.001) and a significant

interaction between block and group (F [18,297] 1⁄4 2.014, p 1⁄4 0.0091)

indicating that longer time was taken to operate the levers by

participants in the random lever arrangement during some blocks
(two to five). No significant differences between place and mirror

arrangements were present.

Fig. 6. Median (interquartile range) selection errors and mean (95% confidence

interval) reaction time as a function of group and block for Experiment 1.

Fig. 7. Median (interquartile range) selection errors and mean (95% confidence

interval) reaction time as a function of group and block for Experiment 2.

4. Discussion

The results of Experiment one do not provide any evidence that

arbitrary shape coding was effective in reducing the probability of

selection errors during the performance of a task requiring the

manipulation of one of four levers, when the layout of the controls

remained constant. However, for the block of trials immediately

following the change of side, a significantly lower selection error

rate was found for participants assigned to the shape coded

conditions.

The frequency of selection errors was very low after the initial

block of 16 trials (64 correct lever movements) suggesting that

although the duration of the experiment was relatively brief, the

task was quickly learned, and increasing the duration of the

experiment would be unlikely to lead to differing conclusions.

However, the task characteristics, particularly the relatively small

number of levers, may have created a situation in which selection

errors were unlikely to occur once the task was learned. It may be

that a larger number of levers may have lead to an observable

advantage of shape coding whilst control location remained

constant.

The virtual environment paradigm suffers from a lack of

ecological validity, particularly in absence of significant cognitive

demands or competing attentional demands. A more realistic

scenario in which the operator’s attention was divided, may have

yielded a greater number of selection errors, and hence greater

opportunity for the potential benefits of shape coding to be realised. The experimental participants consisted of a convenience

sample drawn from university staff and students, and none had

experience operating underground bolting machines. It is possible

that experienced operators may have utilised the shape coding cues

more effectively (although few operators have experience of shape

coded controls). Additional investigations are underway at NIOSH

Pittsburgh Research Laboratory to explore these issues further

using physical models of bolting machines.

The previous literature examining shape coding provides

consistent evidence, however. Slocum et al. (1971) utilised a dual

task paradigm, and similarly, found no evidence of a benefit of

arbitrary shape coding. The second experiment reported by Weitz

(1947) required participants to select between seven levers, also

leading to consistent results. This experiment is worthy of closer

examination. Fig. 8 is redrawn from Weitz (1947) and presents the

average number of selection errors for four groups of 25 participants who performed 16 one-minute trials in which four of seven

controls were manipulated in response to four pairs of stimulus

lights. The levers were either shape-coded (groups I and II) or

identical (groups III and IV), and for two groups (I and III) the lever

locations were changed after eight trials; while the lever locations

remained constant for the remaining groups (II and IV).

The average error data for groups II and IV demonstrates no

evidence of an advantage of shape coding when the location of the

levers remained constant. However, a comparison of the data from
groups I and III demonstrates a reduction in error rates achieved by

shape coding in the situation in which the location of levers is

altered. These data, in conjunction with the data provided by

Experiment one here, justify a recommendation that shape coding

should be employed for bolting control levers. However, for

maximum benefit, and perhaps for any benefit, to be realised, the

shapes must have a consistent relationship to lever function

between different bolting equipment, as well as within the same

piece of equipment. Consequently, the shapes proposed for bolting

equipment in MDG35 should be employed universally for all

underground drilling and bolting equipment.

Further, the provision of shape coding creates an additional risk

that selection errors may be provoked if the shapes were inadvertently swapped during equipment maintenance. This risk was

highlighted by the investigation of a serious injury (NSW DPI,

2009b) in which it was noted that:

‘‘The bolter rotation and bolter raise control handles are located

beside each other at the same height but are differing shaped

handles. The handle shape was not consistently applied to all

control stations on the machine’’.

Consequently, it is recommended that standards which specify

shape coding should also include a provision stipulating that the

design of the controls must incorporate a means of ensuring that

the shaped handles cannot be inadvertently fitted to the incorrect

lever. This recommendation is consistent with clause 5.4.1.4.4.e of

MIL-STD-1472f (DOD, 1999) which states ‘‘Shape-coded knobs and

handles shall be positively and non-reversibly attached to their

shafts to preclude incorrect attachment when replacement is

required’’.

Neither experiment was able to demonstrate a difference

between mirror or place layouts for situations in which dual control

workstations are provided. Limited evidence was available to

suggest that both mirror and place layouts had an advantage over

a random arrangement, at least in the absence of shape coding. It

may be that either place or mirror arrangements are satisfactory,

although there may be advantages in standardising on one or the

other. The only previous examination of the topic (Pigg, 1954)

provided evidence only of a very short-lived temporal advantage of

a mirror arrangement (although in this case the orientation of the

controls was constant, with the only change being the hand used).

Again, the limited ecological validity of the paradigm means that

the results must be interpreted with caution. However, the question of whether control layouts for dual control situations should

be mirrored or place remains open and should be the subject of

further investigation, ideally using a paradigm which more closely

resembles the real task.

Fig. 8. Average selection errors for four groups of participants manipulating four of seven levers in response to pairs of lights. For groups I and III the order of the levers was changed

after eight trials, while the order remained constant for groups II and IV. The levers were shape coded as illustrated (labelled ‘‘Condition B’’) for groups I and II, and non-coded for

groups III and IV. (Figure redrawn from Weitz, 1947).

5. Conclusion

Evidence exists to suggest that beneficial consequences of

arbitrary shape coding exist in situations in which the relationship

between shape and function is constant, but the location of the

controls is altered: either by changing to a different workstation; or

a different machine. Consequently it is important to ensure that the

relationship between shape and function is standardised, and that

a means is provided to prevent shaped handles being placed on the

incorrect lever. Whether arbitrary shape coding has benefits when

control layout remains constant is unknown. Further research is

required to determine whether mirror or place arrangements

should be used where dual controls banks are provided for operation by left and right hands. To overcome the limited ecological

validity of the current paradigm, future research should be undertaken using a paradigm which more closely resembles the real task.

Acknowledgements

Initial work on this research was undertaken whilst the first

author held a National Academy of Sciences Senior Research

Associateship at NIOSH Pittsburgh Research Laboratory. Excellent

assistance was provided by the NIOSH PRL Librarian, Kathleen

Stabryla. The project was funded by the Australian Coal Association

Research Program (Project C16013). Dave Mellows (Xstrata Coal

NSW) and John Hempenstall (Centennial) acted as Industry Monitors for the project.

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