Fail-Safe Function Block F_Safe_Pos; Introduction - Siemens Simatic Manual

Fail-safe function blocks for storage and retrieval machines

3.2 Fail-safe function block F_SAFE_POS

3.2.1

Introduction

The fail-safe function block F_SAFE_POS generates a safety-related position actual value
based on the discrepancy comparison from two encoders. A velocity is calculated from the
motor encoder value and is verified by a position discrepancy comparison using an absolute
encoder. Within a time that can be parameterized, the positions between the motor encoder
and the second encoder used to check the plausibility may not differ from one another by
more than a slip tolerance so that the velocity value can be considered to be safety-related.
The safety-related position and velocity form the basis for the other blocks described in this
document.
Redundant position sensing is always required if the position cannot be uniquely sensed using
the Safety Integrated (SI) motor encoder in the drive. This is the case if the encoder cannot be
mounted in a safety-related fashion, the mechanical system has slip or is subject to elongation
(e.g. travel gear with wheel-rail system or hoisting gear with cable winch). Then, the position-
related safety functions in the SI of the drive cannot be used. A direct measuring system must
be employed to monitor the position. This is realized in the F-CPU using this particular block.
The motor measuring system can then only be used to check the plausibility of the direct posi-
tion actual value.
For applications where considerable slip is to be expected, and therefore the motor encoder
cannot be used regarding the plausibility of the position, this block offers the option of deriving
the safety-related position by deriving the discrepancy between two direct measuring systems.
S7-Distributed Safety/SIMATIC Safety - fail-safe function blocks for storage and retrieval machines
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