Safety of servomotor presses

Outline of reasons and objectives
This study relates to a generation of innovative machinery, namely servomotor presses, distribution of which is going to increase because the features they offer are of interest to users. For example, the slide displacement and force characteristics, which are variable in real time, offer the possibility of performing complex work cycles. There are not yet any national or international safety guidelines that take account of the specificities of this type of press. This machinery, which is potentially dangerous and implements new technologies, should achieve a level of safety equivalent to that of conventional presses.
It was thus essential to study the techniques specific to these new presses, and to analyse their impact on the safety functions and on the validity of the protective means usually implemented for presses.
Approach
This study was conducted in three main phases:
Phase 1: Inventory of the techniques specific to servomotor presses;
Phase 2: Detailed study of those specific techniques;
Phase 3: Validity of the conventional protective means on servomotor presses.
Main results
Unlike conventional presses, servomotor presses use a power drive system that does not merely supply the energy necessary for moving the slide. It becomes the core of a control system making new operating modes possible, while also offering a large number of safety functions that are necessary for protecting operators. Precautions need to be taken at the variable speed drive or the mechanical transmission so as to provide said safety functions, even in the event of failure. Similarly, a mechanical braking system needs to be provided, mainly for mitigating failures of the electronic control system. Despite these measures, the response time for the stop functions can be affected, which needs to be taken into account.
The protective means used previously remain effective for servo-motor presses. The use of guards with interlock systems would make it possible to overcome certain technical constraints such as controlling the stops (braking capacity and stop time), related to taking account of failures of the drive system.
Discussion
The servomotors make it possible to manage keeping the machinery stopped by energising. In such a situation, failure can generate untimely displacement of the slide, and the operator can then be in danger. It will remain to define, during standardisation work, whether the principle of tolerating an untimely displacement stroke is acceptable on the presses and within what limits it would not give rise to any risk.
The contributions from this study will serve directly for the standardisation work in progress on the presses, and will be usable as a reference for other machinery using servomotors.