Abstract of SD-No.26

National Institute of Occupational Safety and Health, Japan

A Study on a Procedure for Determining the Protective Stopping Time of Mechanical Servo Presses

Tsuyoshi SAITO

:"Servo press" is the name given to such an innovative power press machine that the motion of its slide can be changed flexibly and easily by the control of programmable servo drive system. A kind of the servo press which is so constructed that the power of a servo motor is transmitted to the slide by means of a mechanical rotating mechanism or a direct drive mechanism (e.g. crank, ball screw, etc.) is termed "mechanical servo press".
    In order for a safe use of the servo press to be ensured, the time interval between the actuation request for a protective stop function and the completion of a slide stoppage (i.e., the protective stopping time) is an important parameter to determine an appropriate distance to a hazard zone from the detection zone of a sensitive protective equipment or a two-hand control device. JIS B 6410 which provides safety requirements of servo presses, therefore, requires such a treatment of servo press manufacturers that the protective stopping time has to be determined by considering a single fault condition which gives the longest stopping time (i.e., the worst case). However, the extent of components and failures which may impact on the stopping time, the relationships with the required performance level, the applicability of fault exclusion and the forth are not mentioned in this standard, consequently, a common interpretation of the requirement for the protective stopping time still has not been shared.
    Concrete examples of procedures to identify the worst case and to evaluate the protective stopping time for several types of the mechanical servo press are described in this document. For this purpose, at the first, the meaning of the requirement for the protective stopping time is clarified from the results of considerations about the concepts and technical requirements of the ISO/IEC international standards on functional safety and the actual approach on the Japanese construction code for power press. In the second, concrete evaluation methods for the respective single fault condition, the extent of failures to be considered and constraints for fault exclusion is discussed. System architectures related to the protective stop function shown in the examples are modeled by reference to commercially available servo presses, which is one of results of a collaborative research project by JNIOSH and six domestic servo press manufacturers.

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