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A major issue in underground mining equipment design is the visibility from the operator's station. Each year, miners are killed and injured, equipment is damaged, cables are cut and roof supports are knocked down because the operators of mobile equipment cannot see. Their line of sight is blocked due to the design of the equipment or canopies. The term "visibility" refers to how well the human eye can see something. Because human judgment is involved, the measurement of visibility is always subjective. The table below lists the primary factors that determine the visibility of objects.

The first two items, Line of Sight and Contrast, are integral to the first principles of visibility for the purposes of underground mobile machine design:

Line Of Sight - Provide an unobstructed line of sight to those locations or objects that need to be seen to do a job safety and efficiently.

Contrast - Provide enough contrast between the luminance of the object or location of interest and the background against which the operator sees the object to ensure that it is visible.

Assessing Line of Sight

The following sections describe a suggested technique for assessing visibility line of sight from the operator station for underground mobile equipment. The technique consists of four main steps:

• Specify Information Requirements - Determine the information needs of the operator.
• Identify, Rate, and Locate Visual Features - Determine which visual features provide the information that the operator needs and rate the relative importance of each.
• Specify the Visibility Requirements - Indicate which areas around the machine that the operator needs to see.
• Assess the Fields of Visibility - Rate the visibility from the operator's stations.

We give an example of applying this technique for shuttle cars. The data for assessing shuttle car visibility were derived from a study of 12 underground coal mines comprising 28 working sections and approximately 100 different operators.

Note: This technique for assessing line of sight was developed under the Bureau of Mines contract J0387213, "Visual Attention Locations for Operating Continuous Miners, Shuttle Cars, and Scoops." The contractor was Canyon Research Group, Inc., and the final report was written in 1981 by Mark S. Sanders, Ph.D., and Gene R. Kelley. Much of the following discussion on assessing line of sight was taken directly from their final report. (1)

Step 1 - Specifying Information Requirements

Determining what needs to be seen to do a job safely and efficiently can be done best by developing an understanding of which tasks need to be accomplished to do the job. See the Task Analysis topic for references on the techniques required to do a thorough analysis to learn the information requirements for operating a given piece of equipment. Once you know the information requirements, specific visual features that serve as sources of that information can be identified. The table below lists the information requirements for shuttle car operators.

Step 2 - Identify, Rate and Locate Visual Features

Multiple sources of information are available to satisfy any given information requirement when operating mobile equipment underground. Sometimes redundant sources exist. In other cases, several individual sources must be combined to satisfy the requirement. Not all sources of information are visual; some are auditory and others are tactual. For assessing line of sight, the concern is with visual features that serve as sources of information.

Identifying Visual Features

Specifying information requirements is easy given a thorough knowledge of the individual tasks obtained through a task analysis. Unfortunately, specifying the visual features that provide the information can be very difficult. The principal reason is that there are multiple sources of information. Usually, operators are not aware of how they get the information, only that they become aware of it. To develop visual requirements, specific visual features must be identified that serve as the sources of the information required to operate the equipment safely and efficiently. (The table below lists the principal visual features that serve as sources for the information requirements of shuttle car operators.) As with specifying information requirements, the development of this list of visual features is based on an understanding of the tasks that the operators need to perform.

Rating Visual Features

The importance of visual features depends partly on their likelihood of occurrence, which varies from mine to mine. One example is roof hazards. In mines with an undulating roof and low hanging roof bolts, roof hazards can be important sources of information to avoid mishaps. On the other hand, when you have a good, flat, high roof, seeing it is less important.

Redundant visual sources are usually associated with a given information requirement. An example is the same-side leading corner of a machine and the opposite-side leading corner. Being able to see one corner generally gives the operator enough information to find the other. So how do you rate each corner? If you rate each without regard to the other, they might both get high ratings, implying that the operator must see both. If you rate each assuming the other can be seen, both might get low ratings, implying that neither must be seen.

Possible criteria that can be used to rate individual visual features include:

• How good is the cue for supplying the needed information?
• How often will it occur in a mine?
• How likely is it to be visible given the current configurations of underground equipment?
• If operators do not see the visual feature, what effect will it have on accidents and productivity?

Given the complexity of rating visual features, we need a simplifying approach. The suggested method first rates the importance of the information requirements and classifies them into importance categories. The importance categories comprise two three-point scales dealing with safety and productivity (see the table below).

Each of the information requirements should be rated by persons familiar with the operation of the equipment, preferably those who participated in the field observations and interviews that resulted in the development of the information requirements. A group decision-making technique (such as Delphi) can be used to resolve discrepancies in the ratings. The ratings of the two scales listed in the table above are then averaged and the averages added for a total importance rating. The table below lists the information requirements for shuttle cars showing the average scale rating and total importance rating for each.

The personnel who took part in the study of 12 underground coal mines mentioned previously generated these ratings. The table below is the frequency distribution of the importance ratings for shuttle cars. The ratings were divided into four priority categories for specifying visual requirements. The visual features take on the importance rating of the information requirement of which they are a part. Where two or more visual features serve as redundant sources of information, those features ahead and/or on the same side of the machine as the operator are given preference over features behind and/or on the opposite side of the machine. This preference takes into account the general layouts of equipment in which designing for same-side viewing will be easier than for opposite-side viewing.

Locating Visual Features

The location of a visual feature can be specified by its position in three planes: fore-aft, lateral (side-to-side), and vertical (up-down). To do this requires that reference points in each plane be identified. Ideally, the reference points, and therefore the location of the visual feature, should be determined so that they can be generalized across specific equipment types. For example, a continuous miner operator needs to see the cutting head of the machine. If the fore-aft location were specified with reference to the operator's head position, the location of the cutting head would vary depending on the length of the machine and the location of the operator's compartment. However, if the fore-aft position is specified with reference to a machine point, in this case the front edge, it does not matter what the length of the machine is or where the operator is positioned.

The philosophy underlying the specification of visual feature locations is that the location should represent the last point at which the information, if received, can be used by the operator. For example, consider a road obstruction, such as a pile of timber, and its location in the fore-aft plane. How far in front of the vehicle must the operator be able to see the obstruction? To see it 200 feet ahead would be nice, but it is not really necessary. The location where it must be seen is a necessary stopping distance ahead of the machine. This necessary stopping distance depends on the speed of the machine, the reaction time of the operator, and the inherent stopping capability of the machine.

Appendix A contains the complete list of visual features for shuttle cars, organized by information requirement, and their location in the three planes (fore-aft, side-to-side and up-down). There is also a list of abbreviations used to specify the locations.

Roadway Obstacles

Some explanation of roadway obstacles is necessary. (Reference the figure below for an illustration of the roadway hazard zone for moving equipment.) It is important that the operator see obstacles in the shaded zone of the illustration. Obstacles not in the shaded zone are either no threat, or even if the operator saw them, little can be done to avoid them. The shaded areas outside the widest machine points represent locations where if obstacles are seen, the operator will not attempt a change in direction (because there is no chance of stopping the machine in time to avoid hitting them). Note that as the machine moves or turns, these shaded areas also move and turn with the machine.

Rather than specify an area, specific points in the area that define the perimeter and midline are specified. These are marked as numbered circles in the illustration; the rationale is that if these points can be seen, effectively the area can be seen. This approach will later simplify the specification of visibility requirements.

Step 3 - Specifying Visibility Requirements

There are two generic approaches for specifying visibility requirements. The first is to specify "visual windows" of given size and location. The second approach is to specify specific points in space that must be visible. Note that neither approach deals directly with methods for assessing whether a particular piece of equipment meets the requirements.

Visual Windows

Visual windows are areas of unobstructed vision. Visual windows must be specified using a visual angle, rather than absolute size (unless the distance from the operator to the window is also specified). For example, specifying a window 20 in by 8 in (50.8 cm by 20.32 cm) in the forward direction is meaningless. Such a window, 10 in (25.4 cm) from the operator's face will provide much more visual access than if the same window were placed 10 ft (304.80 cm) in front of the operator (see the illustration). If the distance is specified, the visual angle can be computed. The farther away from the observer the window is placed, the larger it must be to maintain the same visual angle. Appendix B has more discussion of Visual Windows.

Visual Attention Locations (VAL's)

The second approach to specifying visual requirements eliminates the need to translate visual features into visual angles. The approach specifies the requirements using specific locations, or visual attention locations (VAL's), which must be visible from the operator's position. The key to the approach is to specify the locations with reference to machine points. In this way, the requirement will apply to all configurations of the equipment class. For example, operators may be required to see an object on the ground a necessary stopping distance (NSD) ahead of the machine. This point can be located in space as shown in the table below.

The NSD referenced in the table depends, in part, on the speed and braking capacity of the particular model of the machine. Once the NSD is determined (or estimated), the above requirement does not change if the length of the equipment changes, the operator's posture or position changes, or if the width or height of the machine changes. Thus, the requirements are generalizable to all equipment within a given class, i.e., all continuous miners, all shuttle cars, etc. This ability to generalize is one of the most compelling reasons for using this method to specify visual requirements. Appendix C has a discussion of the philosophy of specifying the NSD.)

When you use this method to specify visual requirements, you want to maximize the number of visual features accounted for by using a minimum number of VAL's. To accomplish this, you can combine all of the visual features for a particular class of equipment into a composite graph. This composite can be used to identify points of commonality among the visual features. You should also consider redundancy among visual features associated with the same information requirement.

The operator has three alternatives for movement to be able to see a VAL:

• Without head or torso flexion
• With head flexion, but no torso flexion
• With head and torso flexion.

The alternative selected depends on the importance priority assigned to the visual features at the specific location. The highest priority prevails where several features are found at the same place. For example, if at one location there are four visual features with priority ratings 1, 2, 2, and 3, the overall priority rating of that location would be 1.

Visual Attention Locations for Operation of Shuttle Cars

Appendix D has the coordinates (fore-aft, lateral, and vertical) for each of 54 recommended VAL's associated with operation of shuttle cars. For shuttle cars, these 54 requirements apply to both directions of travel.

A total of 86 visual features were identified in shuttle car operations. The 54 recommended VAL's account for 80 (93%) of all of the visual features identified. A more detailed breakdown by visual feature importance priority is shown in the table below. Considering only the visual features with priorities 1, 2, and 3, the 54 requirements account for 95% of all of the visual features.

Step 4 - Assessing Fields of Visibility

Steps 1, 2, and 3 resulted in a list of VAL's that contain visual features important for the safe and efficient operation of the equipment. They describe what needs to be seen. This section discusses a methodology for determining whether a particular equipment and operator compartment configuration permits the operator to see those VAL's.

Line-of-Sight Techniques

There have been numerous approaches suggested in the literature for assessing fields of visibility of mobile equipment. (2, 3) These approaches differ in terms of complexity, type of information generated, and utility for the intended application. (Appendix E has a summary of two such techniques - panorama photograph and shadow graph.) Our discussion will focus on the Line-of-Sight techniques.

Line-of-Sight techniques are predicated on the knowledge of what the operator needs to see to operate the equipment. Thus, it is particularly well suited to the present application. There are two variations on the technique. The first is an inside-out technique in which a camera (or observer) is positioned in the operator's compartment. A target is positioned at the VAL and a picture is taken from the cab, or the observer in the cab indicates if he or she can see the target. The main disadvantage of this inside-out procedure is that separate sets of photographs must be made for the 5th- and 95th-percentile operators. Other than that, the procedure is simple to do, requires a minimum of equipment, produces a permanent record of the obstructions, and requires no data reduction - either the target can or cannot be seen.

The second variation of the line of sight technique is the outside-in method. Here a manikin like device is positioned in the operator compartment. The device is designed to show both the 5th- and 95th-percentile eye positions and can be configured to show eye positions associated with neck and torso flexion. The camera is then positioned at each VAL and a picture is taken looking back to the cab. If the "eyes" of the device can be seen, then an operator could see the VAL. The setup is simple, the photographs are readily interpretable, machine obstructions are easily identified, and the consequences of changing the machine's configuration are readily predictable. The added advantage of this system is that a single set of photographs is all that needed is to evaluate visibility with a 5th- or 95th-percentile operator, with or without neck and torso flexion. Also, this technique is readily adaptable to computerized methods. It is for these reasons that the outside-in line of sight method is the suggested technique for visibility assessments.

HERMI and the Outside-In Technique

The outside-in line of sight procedure is centered around a human eye reference measurement instrument (HERMI) and specific VAL's. HERMI is a device that represents the eye positions of the 5th-percentile female and 95th-percentile male performing reasonable neck and trunk flexion. The HERMI illustrated here has an eye arc that represents the eye positions for the 95th-percentile male.

HERMI is placed in the operator's cab, simulating the position of the operator. At each VAL, the evaluator takes a picture of HERMI in the operator's cab. Examination of the photograph allows direct determination of whether the 5th- and/or 95th-percentile operator could see that location using a reasonable amount of the neck and/or trunk flexion. An alternative to photography is to position a sight tube at the VAL and have an observer report what parts of HERMI are visible. The disadvantage of this, of course, is that no permanent photographic record is made.

Appendix F describes the procedure for assessing visibility using a HERMI type instrument. At least two people are required to complete the procedure efficiently.

How Good is Acceptable?

Based on the limited sample of shuttle cars, continuous miners, and scoops evaluated to develop this assessment technique, seeing some VAL's for underground mobile equipment will be virtually impossible. The critical question is whether all the VAL's must be seen in order for a machine to have an adequate field of view. The answer, undoubtedly, is "no." This is the logical consequence of the fact that visual features at one VAL can be redundant sources of information to visual features at another VAL.

You can approach the question of how good is acceptable visibility by envisioning a scale of "goodness" ranging from high to low. The question then transforms itself into assigning a cutoff point along the scale that separates acceptable from unacceptable levels of goodness. The first step toward establishing this cutoff is to attempt to quantify the evaluation of visibility so that specific equipment can be placed on the scale of goodness. To do this, we can list some factors that should influence the level of goodness associated with a particular machine:

• The more VAL's that can be seen, the better it is (i.e., the higher the level of goodness).
• The less body movement (i.e., neck and torso flexion) required to see the VAL, the better the visibility from the equipment.
• The greater the percentage of operators that can see a VAL, the better the visibility from the machine. If the 95th-percentile operator could see the location but the 5th-percentile operator could not, this would be less acceptable than if both the 95th- and 5th-percentile operators could see the location.
• The higher the priority (i.e., 1, 2, 3, 4) of the VAL that can be seen, the better is the visibility from the machine. If an operator can see a priority 1 VAL, that should get more "weight" than if a priority 3 VAL can be seen.

Based on these factors and the notion of a scale of goodness, developing a system by which a visibility score from a particular machine can be computed is possible using the HERMI technique. Such a scoring system is described below. The system can be used to explore alternative equipment designs, such as the impact of canopy height on visibility for operation of underground equipment.

The procedure assigns points to each VAL based on body movement required to see the location and the priority of the location. These points are determined separately for the 95th-percentile male operator and the 5th-percentile female operator and added. The total is then weighted by the priority of the visual attention area. This composite is then summed over all visual attention areas. This can be expressed by the following formula:

V = Visibility score Pi = Priority weight for Visual Attention Location i Ui = 95th-percentile (upper eye arc) points for Visual Attention Location i Li = 5th-percentile (lower eye arc) points for Visual Attention Location i

The upper and lower eye arc points (i.e., Ui and Li), corresponding to the 95th-percentile male and 5th-percentile female operator, are determined using the table below.

This scheme is applied to both the upper and lower eye arcs of HERMI. In each case, the highest number of points attained are assigned. For example, if at a priority 1 visual attention area, the photograph shows that the entire upper eye arc is visible, but only a part of the torso-flexion portion of the lower eye arc is visible, the following points would be assigned:

Thus, a priority 1 has a priority weight of three, a priority 2 has a priority weight of two, and a priority 3 has a priority weight of one. (There are no visual attention areas with priority values of 4.)

This scoring technique is only one of several that could be employed. For example, a machine might be considered acceptable if, and only if, all priority 1 VAL's are visible. Such a system, however, must take into account redundancy between visual features at different VAL's. Further, such a system would not be sensitive to design changes that reduce or enhance visibility above the acceptable level. The particular scoring technique chosen depends on the purpose for which it is intended to serve. For evaluating design changes, the visibility index scoring system described above would be useful. For specifying minimum requirements, perhaps the if-and-only-if priority 1 system would be more appropriate.

The visibility index system described above has one drawback. It is an additive system (i.e., totaling up points across Visual Attention Locations) and as such, it is a compensatory system. It is possible that a machine design could obstruct vision to a few Priority 1 VAL's and yet obtain a relatively "high" visibility index by affording visibility to all Priority 2 and 3 locations. In like manner, a machine could afford visibility to all Priority 1 VAL's and yet score relatively "low" because vision is obstructed to all Priority 2 and 3 locations.

llumination Levels at VAL's

It does little good to provide an unobstructed line of sight to important visual features if equipment operators cannot see them due to a lack of illumination. In this section, basic underground coal mine illumination standards are discussed, and a simple method of determining the illumination levels at VAL's is presented.

Basic Illumination Terminology

Note: An excellent summary document on mine lighting entitled "Underground Coal Mine Lighting Handbook" was published in 1986 by William H. Lewis. Part One (USBM IC 9073) covers the basics of lighting. (4) Part Two (USBM IC 9074) covers the application of lighting in underground coal mining. (5)

Unfortunately, describing the concepts related to the measurement of light is made more difficult due to the two systems of measurements used: the United States system and the International System of Units. Because MSHA regulations related to lighting underground equipment still use the U.S. system terminology, the following definitions will be related in those terms.

Illumination (also called illuminance) is a measure of the areal density of light reaching an object or surface. The common U.S. unit of measurement is the footcandle (fc). A footcandle is the density of light falling on the inner surface of a sphere of 1 foot radius when a point source of light with an intensity of 1 candela is placed at the sphere's center. The illumination diminishes according to the cosine of the angle between the surface and the direction of incoming light. The illumination also diminishes with distance according to the inverse square law as illustrated below.

Luminance (L) is the amount of light per unit area reflected from or emitted by a surface. For most purposes, this is the important measurement, because a portion of this light is what usually enters the eye. Although this measure is frequently called brightness, strictly speaking, brightness is the resulting subjective sensation and is influenced by contrast, adaptation, and other factors besides the physical energy in the stimulus. Luminance is commonly expressed by a variety of units. The one with the most relevance for underground mine lighting is the footlambert, which is a unit of luminance equal to that of a perfectly diffusing and reflecting surface illuminated by 1 footcandle.

With these definitions, it is possible to define contrast. Contrast is a measure of luminance difference, usually between that of the luminance of the object of interest and the luminance of the background against which the object is seen. It is computed by following formula:

Contrast can vary from 100% (positive) to zero for objects darker than their backgrounds, and from zero to infinity (negative) for targets brighter than their backgrounds.

See the figure below for an illustration of the concept of contrast. As the shading of the block on the right becomes lighter and the contrast between the blocks becomes greater, the visibility of the vertical border between them increases. (Note: You need at least a 256 color display to see this example properly.)

How contrast effects visibility.

The human eyes sensitivity to contrast is affected by the level of illumination. If the illustration is viewed in a dimly lit room, the point where it is possible to distinguish the border would be farther down the figure. Stated another way, the less light there is, the higher the contrast necessary between objects to distinguish them.

Other factors that influence the ability to perceive the border include adaptation level, observer age, object size, time, and movement.

U.S. Mine Lighting Standards

The MSHA standards for underground coal mine lighting are only intended to specify minimum requirements for meeting the primary visual needs at the mine face. Current lighting knowledge and the variability of mining circumstances make specification of more extensive standards inappropriate. However, to truly optimize visibility, exceeding these minimum requirements may occasionally be necessary, such as when trying to illuminate VAL's.

The heart of the underground coal mine lighting standard is expressed in Section 75.1719-1(d), Title 30 of the CFR: "The luminous intensity (surface brightness) of surfaces that are in a miner's normal field of vision of areas in working places that are required to be lighted shall be not less than 0.06 footlamberts."

The basis for the illumination regulations is research that shows that the 0.06-fL level is adequate for performance of most mining tasks where fine perception of details is not required. In other words, this illumination level provides enough contrast between the objects normally needed to be seen during mining and the background provided by the mine environment (usually the coal seam). This result was based on a study of visual tasks in mining that took into account such physical components as distance, duration of observation, target identity, target brightness, background type, and the age of miners. The table below summaries some of the recommended luminance levels for tasks associated with shuttle car operation based on this study.

Where greater levels of illumination are desirable for close-in work, it was assumed that the miner's cap lamp would act as a supplement to the general lighting. Also, the 0.06-fL general level of luminance is low enough that adaptation problems in going from the illuminated area to darker areas of the mine are not severe.

The requirement is expressed in terms of luminance since only the physical measure of luminance correlates to what the eye sees. Unfortunately, luminance, which is a measurement of reflected light, is more difficult to determine than illumination, which is a measure of incident light. However, since the nominal reflectivity value for coal is 3%, approximately 2 footcandles of illumination is needed to produce the required luminance of 0.06 fL.

The wording of the lighting standard is important. The statement that 0.06 fL applies to "surfaces that are in a miner's normal field of vision" implies that mine lighting should accommodate peripheral vision. Peripheral vision is important in mining for early recognition of conditions that might give pre warning of potential safety hazards in the peripheral field and performance of tasks that require knowledge of the relative spatial relationships among objects separated by significant distance. Lighting all surfaces in a miner's normal field of vision eliminates the "tunnel vision" effect of the narrow cap lamp beam and overcomes shadowing by machine or roof support structures.

The following is an example of the interpretation of this lighting standard by MSHA for underground coal mine shuttle cars (Section 75.1719-1(e)(6), Title 30, Code of Federal Regulations).

Unfortunately, the VAL concept was not used as a basis for the lighting standards; therefore, fulfilling the regulatory requirements is possible while having peripheral VAL's receive inadequate levels of illumination to provide 0.06 fL of luminance.

The figure below illustrates the illumination pattern measured around a typical shuttle car. Illumination was measured within the areas bordered by the black lines to the front, rear, and side of the machine. The locations of shuttle car VAL's are indicated by the letters (A' to T' and A to T).

Illumination levels for inby (A'...T') and outby (A...T) directions of travel

 

Determining Illumination at VAL's

As stated previously, if the nominal reflectivity value for coal is 3%, then approximately 2 footcandles of illumination is needed to produce the required luminance (surface brightness) of 0.06 fL. This fact makes possible a simple method to determine the illumination level at VAL's.

The technique uses a photometer and a specially designed light measurement tube (see the cut away sketch of the tube below). The tube is constructed to eliminate all stray light from reaching the photometer. Only the illumination emitted from the luminaire, or stray light reflected off the face of the luminaire, is measured. The use of this measurement tube permits one to measure the illumination level of luminaires in daylight conditions.

To make a measurement, the measurement tube is positioned at a VAL using a tripod and aimed at a luminaire. The machine light is draped (shaded) with black cloth to reduce the stray light reaching the face of the luminaire. After adjusting the front iris of the tube so that an area slightly larger than the luminaire is visible, the photometer is attached to the rear of the tube and the light reaching the photometer is measured. The luminaire is then turned off and the measurement repeated. This second measure is used to correct the lights-on reading for stray light reflected off the face of the luminaire. The same procedure, lights-on, lights-off, is repeated aiming the light tube at all of the luminaires.

The illumination values taken are conservative, inasmuch as they do not include light reflected from the floor, roof, or ribs. However, because the reflectivity of coal is very low, not much additional light would be added.

An analysis of illumination levels at the VAL's for six shuttle cars revealed that, as a whole, illumination levels meet and often exceed the MSHA shuttle car illumination requirements. Inadequate illumination levels were recorded for VAL's on the side of the machine opposite the operator. It is unlikely that cap lamp illumination would rectify the problem. In contrast, although inadequate levels of illuminance were recorded for some VAL's on the same side of the machine as the operator, it is likely that cap lamp illumination would solve the problem.

Appendices

Appendix A - Visual Features for Shuttle Cars
Appendix B - Visual Windows
Appendix C - Philosophy of Specifying the Necessary Stopping Distance (NSD)
Appendix D - Recommended VALs for Shuttle Cars
Appendix E - Field of Visibility Assessment Techniques
Appendix F - Using a HERMI

Appendix A - Visual Features for Shuttle Cars

The first table describes the abbreviations used to specify the locations of visual features. The second table is the complete list of visual features for shuttle cars, organized by information requirement, and their location in the three planes (fore-aft, side-to-side and up-down).

Some assumptions that were made when locating these visual features for shuttle cars are:

• Roadway width:20 ft (609.6 cm).
• Shuttle Car width:9.5 ft (289.6 cm).
• Distance from the Widest Machine Point on the same side as the operator to the rib: 3 ft (91.4 cm).
• Tram Speed: 5 mph (8.0 km), resulting in a Necessary Stopping Distance (NSD) of 20 ft (609.6 cm).

CH	Canopy Height	OCL	Operator Center Line
FE	Front Edge	OEH	Operator Eye Height
HMP	Highest Machine Point	OH	Operator's Head
MBH	Maximum Boom Height	OS	Opposite Side from Operator
MBS	Maximum Boom Swing	RE	Rear End
MCL	Machine Center Line	SH	Seam Height
MCH	Maximum Cutter Height	SS	Same Side as Operator
MMH	Median Machine Height	SST	Safe Stopping Time
NSD	Necessary Stopping Distance	WMP	Widest Machine Point
		WE<	Whichever is Less

Appendix B - Visual Windows

The logic behind specifying requirements using windows is that by defining a window, one can see some specific visual feature through it. Visual windows are, in essence, second-order approximations to the primary requirement of seeing a particular visual feature.

A major problem is encountered in translating the primary requirement of seeing a particular visual feature into a visual window specification. The problem is that differently sized and positioned visual windows would be required to see the same visual feature from differently configured equipment. For example, the illustration below shows the horizontal visual angles required to see the same visual feature from the two major designs of shuttle cars (i.e., a point 5 ft in front of the machine and 3 ft from the widest machine point on the same side as the operator). Here, the larger angle (14 degrees for shuttle car B) is more than 50% greater than the smaller angle (9 degrees for shuttle car A). If the 9-degree angle were used as a standard, the visual feature would not be visible from shuttle car B. Thus, if visual windows were used to specify requirements, a different set of windows would have to be specified for each configuration of equipment. If the machine height, length, width, cab position, or seat back angle changes, the visual angles (horizontal and vertical) to a fixed point outside the cab will also change.

Given the locations of the visual features and the relative position of the operator's head (resulting from a particular equipment configuration), it is, of course, possible to compute the various visual angles involved, but the process is laborious and would only apply to the specific equipment configuration upon which it was based.

Appendix C - Philosophy of Specifying the NSD

The philosophy in specifying the NSD is to assume reasonable worst-case situations. To come up with reasonable NSD's, operator reaction time was taken to be 2 seconds. This corresponds well with reaction times reported for automobile braking. For example, researchers have found that 90% of the reaction times measured under actual driving conditions were less than 1.5 seconds. (6) The 2-second value chosen here takes into account the adverse environment encountered underground and larger movement times that might occur due to the less-than-optimum control positions found on many mining machines.

The distance required for the vehicle to stop once the brakes are applied can be determined empirically as outlined in SAE Subcommittee 29, Working Group 8, Recommended Brake Standards for Underground Machines. (7) Maximum tram speed is supposed to be used in the determination. However, since adequate data do not exist on braking distances for specific models of machines, an alternative procedure is used here. The SAE Subcommittee 29 Brake Standard contains minimum performance criteria for brake systems for rubber-tired, self-propelled underground mining machines. These criteria are used as the worst-case braking distances.

The table below presents the Necessary Stopping Distances for various maximum tram speeds using the assumptions discussed above. For very slow-moving equipment (such as continuous miners), the computed necessary stopping distance is less than 10 ft (304.8 cm). Interviews with continuous miner operators revealed a desire to see at least 10 ft (304.8 cm) ahead of the machine during tramming. Therefore, a minimum 10 ft (304.8 cm) necessary stopping distance was assigned. Any vehicle with a maximum tram speed of 2.5 mph (4.02 km) or less is automatically assigned a 10 ft (304.8 cm) NSD.

One point of confirmation for this procedure is that during the shuttle car operator interviews, respondents consistently said that they needed to see at least 20 ft (609.6 cm) ahead of them. Most shuttle cars have a maximum tram speed of approximately 5 mph. From the table below, this translates to a necessary stopping distance of 20 ft (609.6 cm).

Appendix D - Recommended VALs for Shuttle Cars

The table below contains the coordinates (fore-aft, lateral and vertical) for each of 54 recommended VAL's associated with operation of shuttle cars. The table is grouped into 20 fore-aft/lateral positions, at which from one to four vertical heights are shown. The abbreviations used to specify locations are listed at the bottom of the table.

The figure following the table presents a schematic illustrating these 20 positions. At each position are numbers that correspond to those in column 1 of the table. For shuttle cars, these 54 requirements apply to both directions of travel.

Visual Attention Locations for shuttle car operation. The numbers refer to the preceding table

Appendix E - Field of Visibility Assessment Techniques

Panorama Photograph Technique

This technique consists of positioning a camera at the operator's eye position inside the cab and taking a series of overlapping pictures as the camera is rotated 360 degrees. The pictures are then pasted together to produce a panoramic composite. Two variations of this technique are used differing only in how reference angles are established. One technique requires that concentric rings be painted on the ground at fixed distances from the camera. The other technique places a ranging pole (a vertical pole marked off in fixed vertical increments) at a fixed distance from the camera. Either technique permits the resulting pictures to be calibrated for visual angles.

Panoramic photography is simple in principle, yet becomes complex and often unwieldy in practice. First, the camera must be positioned perfectly level, and the point of rotation must be at the node point of the camera-lens combination. If either of these conditions is violated, the resulting composite picture is distorted. Second, the viewing angle of the lens usually does not match that of the human eye so that obstructions that would be visible to the operator (in the extremes of the vertical dimension) are often not recorded in the picture. Third, the panorama composite is at only one eye position, so that additional panoramas must be photographed to represent the 5th-percentile operator, the 95th-percentile operator, the operator with the torso flexed left, with the torso flexed right, etc. The fourth, and perhaps most significant, problem with panoramic photography is analyzing the resulting composite photographs. Visual angles must be determined for the unobstructed spaces. This is time-consuming and requires very precise measurement of the photographs. The resulting data are also not readily compatible with the VAL approach for specifying visibility requirements. Due to differences in machine configuration, each VAL must be converted to visual angle coordinates and then located on the composite pan photograph.

Shadow Graph Technique

This technique consists of positioning a point source of light at the operator's eye position in the cab. At a fixed distance from the machine around all sides, large screens covered with paper or cloth are built. The technique is performed in a darkened room. The shadow pattern produced by machine obstructions is cast on the paper screens. These shadows are then traced, or photographed (in which case the screen is gridded).

There are several problems with this technique, some of which are similar to those encountered with the panorama technique. First, the apparatus is bulky and difficult to transport. The screens are difficult to build, and data reduction is laborious and time-consuming. The same problem of translating the shadow graphs to VAL's remains. Separate graphs must be produced for 5th- and 95th-percentile operators. Although identifying the machine part that is creating the obstruction is possible, this is not easy to do when only the graphs are available. It is often difficult to visualize the effect of altering the machine design on the resulting shadow pattern.

Although the shadow graph technique is recommended by SAE for assessing visibility from mobile equipment (SAE XJ1091), no attempts have been made to define VAL's for each class of equipment. Thus, the technique gives an overview of the field of visibility without assessing whether what can be seen is important in terms of the safe, efficient operation of the equipment.

Appendix F - Using a HERMI

Step 1. Prepare the Machine

Place the machine in an open area. The machine must be placed on a flat surface: to facilitate access, it is recommended that a clear area be provided, approximately 25 ft (762.0 cm) in front, 15 ft (457.2 cm) to each side, and 25 ft (762.0 cm) to the rear. Examples for configuring the equipment follow. (If desired, visibility can be assessed with other configurations in addition to those described below. For example, canopies, booms, and buckets can be raised, or loads can be simulated in shuttle cars and scoops.)

Step 2. Position HERMI in the Cab

HERMI is constructed for use on non-compressible seats, such as is typically found in underground mobile equipment. If a compressible, padded seat is used, the seat must be compressed to compensate for HERMI's light weight. A 100-lb (45.4-kg) weight or cord tied tightly around the seat, where HERMI's stabilizer joint rests on the pad, will adequately compress the seat.

HERMI must be positioned in the cab to simulate the actual operator's posture. Use the following procedure to position HERMI:

• Position HERMI facing in the inby or outby direction.
• Position the 95th-percentile HERMI on the seat with the lower stabilizer legs angled flat on the seat. Duct tape may be used to position HERMI securely.
• Tilt the eye arcs into a position that is facing forward, perpendicular to the ground, and at right angles to the fore-aft machine center line. The center clip spring can be touching the mining machine canopy, but must not be bent. Tighten the neck joint screw to prevent the joint from moving.
• Adjust the eye arc for any side obstructions by moving the arcs inward. The end clip springs can touch, but must not be bent by any side obstructions.

When the canopy of the mining machine is set at the lowest canopy position, it is likely that the back "standoff" of the 95th percentile HERMI will be at a substantial angle to the seat backrest in order to position the eye arcs correctly. Moreover, when HERMI is placed in a low-seam mining machine (30 in (76.2 cm) machine height or less), it is likely that the 95th- and 5th-percentile eye arcs will be at the same heights. In this case, only the 95th-percentile HERMI is needed.

Step 3. Measure the Machine

Several machine dimensions must be measured to determine the positions of the Visual Attention Locations. Measure and record the following heights:

• Median Machine Height (MMH) - The vertical distance from the floor to the overall height of the main machine frame.
• Highest Machine Point (HMP) - The vertical distance from the floor to the highest vertical point on the machine.
• Operator Eye Height (OEH) - The vertical distance from the floor to a point midway between the 95th- and 5th-percentile eye arcs on HERMI.
• Canopy Height (CH) - The vertical distance from the floor to the highest point on the canopy.
• Seam Height (SH) - The anticipated minimum vertical distance from the floor to the roof in which it is reasonably likely the machine will be operated.
• Maximum Boom Height (MBH) - Continuous miners only. The vertical distance from the floor to the highest point on the boom, with the boom positioned at its maximum height.
• Maximum Cutter Head Height (MCH) - Continuous miners only. The vertical distance from the floor to the highest point on the cutter head with the head positioned at its maximum height.

In addition to heights, the following reference points must be identified and marked on the floor:

• Widest Machine Point(s) (WMP) - Both sides. The widest machine point is that part of the machine that extends farthest from the centerline of the machine.
• Front Edge (FE) - The front edge of the machine is that part of the machine that extends farthest forward.
• Rear Edge (RE) - The rear edge of the machine is that part of the machine that extends farthest rearward.
• Operator's Head Position (OH) - The operator's head position is the location that the operator's head would occupy if a person of the 95th-percentile body height sat in the cab seat. Once the 95th-percentile HERMI is completely assembled and correctly placed in the cab seat, the top center of the eye arc plate may be used as the reference point.
• Machine Center Line (MCL) - The machine center line is a hypothetical line that bisects the machine lengthwise into two equal halves.
• Operator Center Line (OCL) - The operator center line is a hypothetical line that bisects the cab seat (and thus an operator) into two equal halves and is parallel to the machine center line.
• Necessary Stopping Distance (NSD) - The necessary stopping distance is the distance in which the mining machine can safely brake to a stop and is based on operator reaction time and the distance required to stop the machine traveling at maximum tram speed.

Step 4. Mark Location of VAL's

Mark the pattern of VAL's for the machine being investigated on the floor using a tape measure and chalk stick. It is only necessary to place a mark on the floor corresponding to every unique set of fore-aft and side-to-side locations for the VAL's.

Shuttle cars tram in two directions, which requires that the HERMI procedure be repeated for both the inby and outby ends of the car. Shuttle cars, however, often have the operator's cabs located on the side at the center of the car. When the configuration of the car is approximately the same fore and aft, the procedure may be performed for only the outby end of the car. The outby end is the most important to investigate, because the car is loaded when it trams away from the face.

Step 5. Measure Visibility

Visibility is measured by examining HERMI from each of the Visual Attention Locations. The out-side-in technique is based on the rationale that if any part of the HERMI eye arcs can be seen from a VAL, then an operator using a reasonable amount of neck and trunk flexion could see the VAL. Determining whether or not any part of HERMI can be seen may be accomplished several ways. Experience has shown that using a sight tube or camera provides accurate data. We recommend that a 35-mm camera be used to take black-and-white photographs at each VAL.

References

1. Sanders, M. S., and G. R. Kelley. Visual Attention Locations for Operating Continuous Miners, Shuttle Cars, and Scoops - Volume 1 (contract J0387213, Canyon Research Group, Inc.). USBM OFR 29(1)-82, 1981, 142 pp.; NTIS PB 82-187964.
    
2. Measurement and Evaluation of Visibilty - SAE XJ1091. Society of Automotive Engineers, Warrendale, PA, 1980.
    
3. Mason, S., R. Rhodes, and C. Best. Summary of Sight Line Techniques for Use With Mining Machinery. ON79/58, National Coal Board, England, 1979.
    
4. Lewis, W. H. Underground Coal Mine Lighting Handbook. (In Two Parts.) 1. Background. USBM IC 9073, 1986, 42 pp.
    
5. Lewis, W. H. Underground Coal Mine Lighting Handbook. (In Two Parts.) 2. Application. USBM IC 9074, 1986, 89 pp.
    
6. Johansson, G., and K. Rumar. Driver's Brake Reaction Times. Human Factors, 1971, v. 13, No. 1, 6 pp.
    
7. SAE Subcommittee 29, Working Group 8. Recommended Minimum Performance Criteria for Braking Systems for Rubber-Tired, Self Propelled Underground Mining Machines. Society of Automotive Engineers, Warrendale, PA, 1979.