Principle Of Operation; Parameterization - Siemens Simatic Manual

Fail-safe function blocks for storage and retrieval machines
As example, if a spindle pitch of 10mm/revolution would be set at Fig. 11, a SLU_DEFINITION of
10mm/SLU would be entered at the block. Now, at each motor revolution, the block increases the
SAFE_POS value by 1 SLU.
If there is a real gearbox between the motor and load in the system, then this ratio must be additionally
taken into account.
Configuring the encoder at the block
To calculate a position actual value in SLU, the incremental relative position change of the modulo
actual value – as well as its overflows – are acquired by the block and summed. As a consequence, a
relative position is available to check the plausibility of the first channel POS_1 for CONFIG bit0 = 0.
The direction of rotation of the motor encoder is adapted in the SI section of the drive.
The 32-bit value of the direct encoder is scaled to SLU in the standard program section of the CPU.
The scaled value is interconnected at POS1; assuming a second direct encoder is being used, its
scaled value is interconnected at POS2 – and CONFIG bit0 is set to 1.
The plausibility of the calculated velocity from POS_SI is always checked using the encoder intercon-
nected at POS1.
For POS_CONFIG bit0 = 0, the plausibility of position POS1 is checked using the encoder intercon-
nected at POS_SI; for POS_CONFIG bit0 = 1, the plausibility of POS1 is checked with respect to
POS2.
Note
POS1 and POS2 expect opposing values!
Note
It should be ensured that the relative position change of the modulo actual value
between two calls of the F_SAFE_POS block does not have two overflows, as in
this case, only one overflow would be detected.
Safety notes and instructions
The signals interconnected at POS1 and POS2 must come from two independent
sources. If one signal source is connected at both inputs, then non-plausible val-
ues of this channel cannot be identified, for example. As a consequence, the
safety integrity of the block is no longer guaranteed.

3.2.4 Principle of operation

3.2.4.1 Parameterization

1. The position-defining encoder is interconnected at POS1, i.e. after the conversion to SLU (this
is realized in the standard program using block SCALE_DINT), the encoder raw value is avail-
able as a 16-bit value at output SAFE_POS.
2. A possibly available additional validity query for the position value (e.g. error bit of the module)
can be interconnected at input POS1_VALID. If information of this type is not available, then
the input must be permanently set to TRUE.
3. The "Position actual value referenced" information is interconnected at input POS1_REF.
4. The sampling time of the block is parameterized at input T_SAMPLE. For instance, the con-
figured call interval of the cyclic interrupt OB, from which the safety program is called.
5. The resolution of a position increment from POS_SI in µm is defined at
POS_SI_RESOLUTION; POS_SI_T_SAMPLE defines the sampling rate of SI in the drive,
POS_SI_COUNT should be interconnected with the counter value from the telegram 901.
6. The significance of SLU in mm is defined at SLU_DEFINITION; T_SAMPLE,
POS_SI_RESOLUTION and SLU_DEFINITION are relevant for calculations made inside the
block.
7. The modulo value of the motor encoder from telegram 901 is interconnected at input POS_SI
8. For applications involving a high degree of slip, it is possible to interconnect a second direct
encoder at POS2
S7-Distributed Safety/SIMATIC Safety - fail-safe function blocks for storage and retrieval machines
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