The primary objective of providing for safety in design is to prevent accidents, protect people from harm, and reduce overall risk. In some instances, hazards can be designed out of the machine, such as an adequately guarded fan drive. Where access to the hazard is needed for the machine to perform its intended function, a subsequent safeguarding method will be necessary. This may take the form of a jig or fixture (presence sensing safeguard) or a control procedure (two-hand control) to force the operator to keep clear of the hazard. When the hazard is an integral part of the process, for example, robotic operations or automatic palletizing, the safeguard may extend to controlling the behavior of the machine by separating the hazardous area from the operator. http://www.pilz.com
The above may be achieved by a simple control function such as electrically interlocking a gate to a guarded area. However, it is in the more complex and integrated control functions where the majority of safety-related control tasks are executed. The deviations from standard control functions and the more complex safety-related control functions can, however, add cost, increase downtime, or add complexity to maintenance. This, in turn, can lead to resistance to implementing safety-related control functions. A safety circuit solves this dilemma by providing a controlled means of initiating a safety-related control function in response to an unsafe event on a machine or an uncontrolled manner of processing in its intended function. A safety-related control function primarily consists of accepting an input from a sensor, a decision-making process, and an output to a final control element. Control is achieved by a safety relay. Generally, there are three control devices for mechanical motion or process. These are the emergency stop function, the interlock, and the guard safety switch. An emergency stop function is a method to immediately turn off power to the machine. Definition is taken from AS4024, Safety of Machinery Standards and General Principles of Design. http://www.pilz.de