Operating speed assessments of underground mining equipment

Numerous fatal/nonfatal accidents involving underground workers struck by powered machinery occur yearly, with continuous miners and roof bolters involved in the majority of these accidents. In an attempt to reduce these accidents, researchers at the the National Institute for Occupational Safety and Health (NIOSH) Pittsburgh Research Laboratory conducted studies of operator interactions with the motions of continuous mining machines and roof bolter boom arms. These operators generally perform their tasks in close proximity to the equipment in confined workspaces, often employing awkward postures. Since experiments with human subjects using the actual equipment were not feasible due to safety concerns, researchers opted to conduct controlled experiments in laboratory settings. Utilizing motion capture technologies and computer simulations, the studies collected data on equipment and operator movement in a range of seam heights and working postures. This article details the results of these studies to examine operating speeds based on usage and seam height. Introduction The U.S. Mine Safety and Health Administration (MSHA) reports that both fatal and nonfatal remotecontrol continuous miner accidents from 1999 to 2007 averaged 243 per year during routine mining activities, with the majority of accident victims working within the turning radius of moving continuous miners. The mining industry uses an educational aid called “red zones are no zones” (MSHA 2006) to help operators of continuous miners to understand which areas around the machine to avoid. However, fatalities and injuries continue to occur. During the same period, MSHA data shows an average of nearly 600 accidents per year related to roof bolting, accounting for nearly one-third of powered machinery accidents. There are currently no regulations or data on determining safe velocities for roof bolter boom arms operating in close proximity to workers in underground mines (MSHA, 1994 and Turin et al., 1995). Other industries, such as robotics, have conducted studies and implemented guidelines for safe machine velocities. The U.S. Department of Occupational Safety and Health Administration (OSHA, 1987) and the U.S. Department of Energy (DOE, 1998) recommend velocities be limited as low as 152 mm/sec (6 in./sec) for manufacturing robots during programming. Another study, (Etherton, 1987), recommends 254 mm/sec (10 in./ sec) as a safe velocity for robots operating in proximity to humans. To address these issues, the National Institute for Occupational Safety and Health (NIOSH) conducted studies to investigate safe operating speeds for these two machines. To minimize risk to the human subjects, the studies used motion capture technologies and computer simulations in a controlled laboratory setting. For the continuous mining machine study, the researchers attempted to determine how quickly subjects could escape from the perceived danger of a machine rotating towards them. Continuous mining machines move in a straight line relatively slowly, but their rotational speed can be appreciably faster when the tracks move in opposite directions. In that case, the machine pivots around the center point of its tracks. The front and back of the machine are far enough from this center to result in high velocities in the turning direction. Researchers conducted numerous motion capture sessions with human subjects in controlled laboratory environments. This data was used to create a virtual environment to analyze factors influencing struck-by accidents during the tramming of a continuous mining machine. The roof bolter boom arm motion studies had United Mine Workers of America (UMWA) volunteers operate a model of a Fletcher Ranger II left-hand roof bolter arm while recording the motions of the bolter boom arm and subjects. While this manufacturer has the majority of the underground roof bolter market, the use of this particular manufacturer’s design should not limit the application of the results of these studies because other manufacturers’ articulating boom arms use similar designs. NIOSH’s model is of the common 1,830 mm (72 in.) length arm and all testing was done with full sump extension. For the vertical boom speed research, a virtual environment with digital humans was created to run simulations. A level of randomness added to the motion of the digital operator during the drilling and bolting cycle enabled the simulation to realistically represent the operator’s motions while performing the bolting task. Volberg and Ambrose (2002) discuss in detail the development of these random motions. The simulator output produced data on the number of contacts between the operator and moving boom arm. This data was analyzed to show the effects of vertical speed on the risk of boom arm contact with the operator. For the horizontal boom speed study, it was only necessary to complete analysis of motion capture data from the human subjects testing to determine if an operator’s safety could be compromised by excessive speeds. Procedures The continuous mining machine investigation analyzed factors influencing struck-by accidents during tramming by using a digital human model (DHM) with simulations driven by captured human motion data with a variety of subjects, postures, facing orientations, environmental constraints and machine characteristics. The DHM used MSHA fatality information to validate the model parameters relating to operator position, which could pose a threat to operator safety. Some of these positions were in the MSHA red zone (MSHA, 2006). It should be noted that the results from this study make a case supporting or even expanding the red zone strategy. The human subjects recruited from local mines were asked to perform realistic movements in a laboratory setting (Fig. 1) that mimic escaping from the path of a moving machine. FIGURE 1 Motion capture for continuous miner speed study. � The motion data was obtained using various operator work postures and escape paths (directions) typical for tramming operations of continuous miners. Complete details of these tests, the development of the DHM and data analysis are contained in Bartels et al. (2008). The DHM, shown in Fig. 2, was developed using the motion capture data to provide the means to measure parameters that would be used to predict struck-by events when the operator tries to move out of the way of the moving machine. FIGURE 2 Digital human/continuous mining machine model. The digital human operator’s movements were constrained by using motion capture data of test subjects as discussed in (Bartels et al., 2007). To present a realistic operator response to the moving machine, researchers programmed the operator’s movement using a delayed start in accordance with reaction times reported by Drowatzky (1981). This delay ranged from 0.19 to 24 seconds. The DHM output parameters included: • The time when the machine first begins to move. • The time when the operator first begins to move. • The time when the operator is struck by an object. • Name of the object that struck the operator. • The operator’s distance from the start position when struck by an object. MSHA’s fatality reports (Dransite and Huntley, 2005) provided information to help validate the model regarding objects that struck the operator and the operator’s distance and location from the machine at the time of being struck.