07.97 12 Functional Descriptions
12.11.3 Range extension with ENDAT absolute encoder (as from SW 6)
Each time the absolute encoder is evaluated (Power On or deselection of parking axis), both
MD 396*, absolute offset, and the rough encoder position are used to determine the actual
position. The absolute offset must have been declared active (MD 1808*, bit 3=1). MD 396* is
not changed internally and is used as a storage medium to make it possible to load old NC-MD
files.
12.11.3.2 Storing absolute information
Until now all absolute information was stored in the encoder. The absolute encoder only had to
be restarted (axis measured) after the encoder was replaced. Restart is now also necessary
on data loss in the SRAM of the NC-CPU when the range extension of the absolute encoder is
used. However, axis measurement is not necessary if an NC-MD file was stored immediately
before data loss in the SRAM. Possible applications are:
• SW update
• Replacement of the NC-CPU
• Replacement of the CSB board
Motion limitations when the axis is switched off
When switched off, the axis must only turned through half the definite traversing range of the
absolute encoder (e.g. coasting after a power failure). A violation of this condition is not
detected by the control and results in an incorrect actual position!
Example:
Rotary axis with position control resolution 0.5 · 10
-3
=> 1 revolution=720000 units[MS];
Gearing 1/59;
Encoder EQN1325 with a definite traversing range of 4096 rev
Calculation of half of the definite traversing range S
G
:
S
G
=4096 rev/2·1/59·720000 Units[MS]/rev=24992542.4 units [MS]
S
G
is the maximum permissible movement of the axis when it is switched off (=34.7 load
revolutions).
© Siemens AG 1992 All Rights Reserved 6FC5197- AA50
12–91
SINUMERIK 840C (IA)
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