Siemens SINUMERIK 840D sl Function Manual page 337

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 high-speed 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 controller.
Reference point approach is executed by the selected measuring system.
Each position measuring system must be referenced separately.
For an explanation of encoder monitoring, see Section "A3: Axis Monitoring, Protection
Zones (Page 79)".
For an explanation of actual-value acquisition compensation functions, see:
References:
Function Manual, Extended Functions; Compensations (K3)
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)
For further information, see Section "A2: Various NC/PLC interface signals and functions
(Page 35)".
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.
Monitoring
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
For the cyclic comparison of the two measuring systems used, this difference must not be
exceeded, as otherwise Alarm 25105 "Measuring systems deviate" is generated.
Basic Functions
Function Manual, 01/2015, 6FC5397-0BP40-5BA2
G2: Velocities, setpoint / actual value systems, closed-loop control
7.4 Setpoint/actual-value system
337