Roof bolting machines in underground coal mines are operated in restricted space and in close proximity to the people. Unintended movement of the machines is a high risk event. Analysis of injury narratives has revealed that some unintended movements occur when the wrong control is operated (a selection error) or when a control is operated in the wrong direction (a direction error).
The objectives of this research were to address the following questions: (1) what principles of directional control-response compatibility are appropriate for use in the design of an underground coal bolting machine? (2) Is shape coding as proposed by MDG35.1 justified as a control measure to reduce selection errors? (3) Is length coding justified as a control measure to reduce selection errors? and (4) Does a light feedback mechanism have potential to reduce the probability of injuries resulting from unintended bolting machine movements?
Four experiments were conducted. The first three experiments were undertaken using a physical simulation of a roof bolting machine, in part to provide a more ecologically valid comparison with related experiments previously conducted in a virtual environment. The final experiment utilized a modified roof bolting machine operated by experienced operators.
In Experiment 1, 48 participants completed 6 blocks of 40 trials in which a physical model of a roof bolting machine arm was manipulated using a bank of control levers. The experiment examined the effect of different direction control-response relationships on direction errors, and the effects of shape and length coding on selection error rates. The independent variables for this experiment were coding (shape-coded, length-coded, or non-coded control levers), lever orientation (vertical or horizontal), and directional controlresponse relationships (CRR1 and CRR2). The dependent variables were selection errors, direction errors, and reaction time.
Experiments 2 and 3 examined the effect of altering the layout of controls on selection error rates in different coding conditions. In Experiment 2, 42 of the 48 participants in Experiment 1 repeated the same conditions as Experiment 1 for the first three blocks of 40 trials. The arrangement of controls was then altered and participants completed a further four blocks of 40 trials. For those groups where coding was employed, the relationship between coding and lever function remained constant. In Experiment 3, sixteen new participants performed three blocks of 40 trials in the shape coding condition utilized in Experiment 1 and 2. Then, both the arrangement of controls and the relationship between shape and control function was altered, before a further three blocks of trials were completed.
A modified Fetcher Roof Ranger II Roof Bolting Machine was utilized for the Experiment 4. Modifications included the ability for a light to be illuminated prior to boom swing out or drill feed up movements of the machine. Sixteen experienced roof bolting operators operated six levers to cause different responses of the roof bolting machine. Each participant performed six blocks of 48 trials in one of 12 combinations of feedback light (on, off), coding (shape code, length code, none), and lever orientation (horizontal, vertical). The directional control-response compatibility remained constant; however, the relationships were chosen to provide relationships predicted to be both compatible and incompatible. The operators were interviewed at the conclusion of the experiment to gather their opinions regarding the light feedback control measure.
Directional control-response compatibility
Directional error rates are increased when the control and response are in opposite directions, or if the direction of the control and response are perpendicular. The pattern of direction error rates was consistent with experiments obtained from a generic device in a virtual environment.
In the situation when operators are not under time constraints, are without divided attention, and are able to view the controls they are operating, the benefits of arbitrary shape-coding and length-coding for reducing selection type errors are restricted to certain situations. Mainly, the benefits may only be realized in situations in which the arrangement of the controls is altered after the coding has been assigned for one function and then the coding is switched to another function. If shape-coding is used, measures should be taken to ensure that the relationship between the shape and control function is not altered.
Length-coding may be at least equally effective as arbitrary shape-coding in reducing selection errors rates in situations when the arrangements of levers is altered, however further investigation is justified.
The light feedback control measure shows a potential benefit of increasing situation awareness of future movements of the equipment.
Ensuring that the design of equipment controls maintains compatible directional control-response relationships will reduce the probability of direction errors. Arbitrary shape-coding and/or length coding may reduce the probability of selection errors in some situations, though further research is warranted. The implementation of a visual feedback system has potential to mitigate the consequences of unintended bolter movements. Future research will involve field trials of selected equipment modifications.