Siemens SINUMERIK 840D sl Function Manual page 421
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Actual-value acquisition
A maximum of two measuring systems can be connected for each axis/spindle, e.g., a direct
measuring system for machining processes with high accuracy requirements and an indirect
measuring system for highspeed positioning tasks.
The number of encoders used is recorded in the machine data:
MD30200 $MA_NUM_ENCS (number of encoders)
In the case of two actual-value branches, the actual value is acquired for both branches.
The active measuring system is always used for position control, absolute value calculation
and display. If both measuring systems are activated at the same time by the PLC interface,
positioning measuring system 1 is chosen internally by the control.
Reference point approach is executed by the selected measuring system.
Each position measuring system must be referenced separately.
For an explanation of actual-value acquisition compensation functions, see:
References:
/FB2/ Function Manual, Extended Functions; Compensations (K3)
For an explanation of encoder monitoring, see:
References:
/FB1/Function Manual, Basic Functions; Axis Monitoring, Protection Zones (A3)
Switching between measuring systems
One can switch between the two measuring systems through the following NC/PLC interface
signals:
DB31, ... DBX1.5 (position measuring system 1)
DB31, ... DBX1.6 (position measuring system 2)
References
/FB1/ Function Manual, Basic Functions; Various Interface Signals (A2)
It is possible to switch over measuring systems at any time, the axes do not have to be
stationary to do this. Switchover only takes place if a permissible deviation between the
actual values and the two measuring systems has not been violated.
The associated tolerance is entered in the machine data:
MD36500 $MA_ENC_CHANGE_TOL (Max. tolerance on position actual value switchover)
On switchover, the current difference between position measuring system 1 and 2 is
traversed immediately.
The permissible deviation between the actual values of the two measuring systems is to be
entered in the machine data:
MD36510 $MA_ENC_DIFF_TOL
This tolerance setting must not be exceeded during cyclical comparison of the two
measuring systems or an error message will be generated.
If the axis is not referenced (at least in the current control measuring system) then the
related monitoring is not active if MD36510 = 0 or if neither of the two measuring systems in
the axis is active/available).
Basic logic functions: Velocities, Setpoint/Actual Value Systems, Closed-Loop Control (G2)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
Detailed description
2.3 Setpoint /actual-value system
33
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