Servomotors and power drive systems - Key elements for the failsafe design of a servomotor press

Servomotor presses are innovative machines and their sales are increasing. Development of servomotors and the recent arrival on the market of power drive systems, featuring so-called “pre-defined” safety functions, have contributed to their emergence. There is still no specific European or international safety standard for these machines, although such a reference frame is currently being drafted by the ISO's “Metal forming machine tools" standardisation group. Against this background, INRS has conducted a study to assess the impact of new servomotor press design principles. This paper describes the three main stages fulfilled in this study and its different results. These stages involved making an inventory of techniques specific to servomotor presses, conducting a detailed study of these techniques and reviewing the validity of conventional protective devices for use on servomotor presses. More specifically, the present study indicates that a safety-related power drive system (PDS/SR) reacts to a failure by initiating a failsafe position and this may be different from the intended function. This can result in safety function degradation in the case of a servomotor press. To conclude, the paper describes the conditions, under which the protective devices included in design standards for "conventional" machines, can remain valid. For example, stopping time control is decisive in calculating the minimum safety distance required by protective devices.