The manual emergency stop Was recognized as an im- portant aspect of machine control very early during the history of powered mechanical systems. The purpose, of course, was to allow machinery to be quickly stepped during emergencies to minimize equipment damage and/or personal injury. Today, however, the emphasis of control is being shifted more and more toward totally automatic systems. Often, the importance of manual emergency stopping and override of these systems is lost in the zeal to perfect automatic controls. The problem that evolves is how and when to provide an efficient human-machine interface to allow a manual emergency stop.

Here, some common emergency stop devices are studied in light of ergonomic principles that apply during manual emergency response. Information on legal and consensus standards applicable to emergency stopping devices are included with a summary of useful recommendations regarding design and placement of devices for efficient human use.

The focus of this study is an ergonomic experiment concerning different arrangements of emergency stop pushbuttons commonly found on machinery. The results indicate that an unguarded pushbutton is superior to a guarded button in terms of the average human response time to activate and a reduction in the frequency of miss response (unsuccessful attempts at activation). Further results indicate that a large, 3 inch mushroom style pushbutton is superior to a 7/8 inch unguarded pushbutton and a significant reduction in frequency of miss response under adverse conditions is possible by redesigning the surface of the standard mushroom head.