Position Measuring Systems
The following information on EC type examination of safe measuring systems refers to the detection of angles or position of drive shafts or linear axes. The instructions are to be applied if a safety-related application is intended for the described measuring system and its suitability is to be proven in a specific Performance Level and Safety Integrity Level.
Sine/Cosine measuring system
For a large number of safety functions, clear recognition of the direction of movement is required. Both the sine and the cosine signal are required for this purpose. If one of the signals is faulty, the correct detection of the direction of movement is no longer ensured; redundancy is therefore not given. Errors can be detected both within the measuring system and in the evaluation unit. To realize the ideal error detection, there must not be any error possibilities in the measuring system that cannot be detected.
If a loosened connection can lead to a dangerous failure of the measuring system, the reliability of a fault exclusion according to ISO 13849-2 must be proven. Fault exclusions can be achieved for mechanical connecting elements by over-sizing them as described in ISO 13849-2. Screw connections must be secured against self-loosening. The proof should be provided by means of suitable tests.
Systems which are operated exclusively by a PELV circuit only have to meet type requirements.
The protective conductor connection is not required for systems that are operated exclusively within a PELV power circuit. A protective conductor connection must be provided for protection class I. This also applies to ELV systems that do not meet the requirements for PELV.