STUMP CUTTER SAFETY involves many elements of machine design and safe operating practices. Manufacturers and dealerships provide extensive documentation on the proper operation and safe use of stump cutter machines. ANSI Z133.1-2006 requires that stump cutters be equipped with enclosures and guards to reduce the risk of injury. It also states that operators shall keep the enclosures and guards in place while the cutter is being operated (International Society of Arboriculture, 2006). However, in some cases, operators have not followed proper procedures and have been injured. American Rental Association (ARA) Insurance Services (2007) reports that since January 2000, six claims have involved people injured while using rented stump cutters. Incidents involving stump cutters can result in serious injuries with workers missing multiple days of work. An operator presence system may help prevent injuries that can occur when the operator approaches the cutter wheel with the clutch still engaged. Small machines equipped with handlebars have unique safety and operation characteristics and typically utilize traditional operator presence controls. These controls use a separate handle or switch that is activated and held in place by the operator while the machine is being used. These systems are typically found on walk-behind machines, such as lawn mowers, or in the seats of larger machines, such as tractors and forklifts. When the handle or switch is released, power to the cutter wheel disengages automatically. When utilized on small stump cutters, power to the cutter wheel is disengaged automatically. Larger stump cutters require hydraulic controls for the cutter wheel travel. These cutters have historically not been equipped with operator presence systems at the controls due, in part, to human factors concerns, such as hand/arm fatigue. Traditional operator presence strategies are not practical on these machines. To address this issue, NIOSH and Vermeer Corp. collaborated to investigate the application of capacitive-sensing technology to detect the operator at the controls of a hydraulic stump cutter.
Early NIOSH Research In early 2001, NIOSH became interested in developing a passive operator presence system that did not require the operator to engage a separate handle or switch. The system would automatically detect an operator around a machine and shut down the equipment if the operator entered a danger zone. At this same time, Quantum Research Group Ltd. in Great Britain was developing capacitive-sensing components that were inexpensive and easy to use. Capacitance is the ability of an object or surface to store an electrical charge. It is often discussed as an electrical characteristic between two objects. The objects are separated by some nonconductive material such as air. The substance between the two objects, the material of which the objects are made and the distance between the mall affect capacitance. The capacitance between two objects increases with an increase in the objects' surface areas and decreases with an increase in distance between them. The human body can readily accumulate a measurable charge, which makes it possible to detect using capacitance. Examples of capacitive sensing that uses the human body include touch lamps and faucets that turn on when a person's hands are placed on or near them.