• DOK-DIAX02-DDS02.1*ANA-ANW1-EN-E1,44 • 12.96
76
7. Commissioning the functions of the digital AC servo drive
7.9. Actual-position
sensing by indirect abso-
lute
position measuring system
An NC control requires position measuring systems to sense the axis
position on machine axes. These position measuring systems can either
be mounted directly on the machine’s mechanical construction (direct
position measuring system) or on the motor (indirect position measuring
system).
For indirect sensing of the axis position INDRAMAT supplies an interface
as standard on digital drives with ANALOG interface. This interface
comes in two versions:
– indirect relative position sensing (refer to Section 7.8).
– indirect absolute position sensing.
Benefits of absolute position sensing:
– no new homing cycle necessary when position information is lost by a
shutdown of the NC control (saves time).
– no risk of damaging the machine or workpiece during homing cycles
after losing instantaneous position information.
– elimination of reference cams along with switches and wiring.
The motor feedback senses the position of the rotor in the servo motor,
the process being cyclically absolute. The signal thus obtained under-
goes high resolution conditioning in the drive controller.
Motors whose feedback circuitry includes an additional absolute encoder option
can achieve absolute resolution of up to 4096 motor revolutions.
The precondition for this is:
The axis has been equipped according to system configurations RA 02
or DA 02 (see Section 2.3).
The output connector of the ANALOG interface with absolute encoder
emulator (DAA1.1) is provided with absolute encoder signals in Gray
code. These are transferred to an external control system by an SSI
interface (synchronous serial interface).
When a DDS with absolute encoder motor (multiturn) is shut down, the
instantaneous actual position is stored. When the unit is started up again,
the position calculated by the absolute encoder evaluation is compared
with the stored value. If the deviation is greater than the amount given in
the absolute encoder position monitoring window in the "Operating
modes, scaling" menu, the error message "76" "absolute encoder error"
will be generated.
Remedy:
Danger!
The axis has been moved while it was shut down and is now
outside the limits parametrized for the "absolute encoder
monitoring window".
Check whether a new start command will cause damage.
– Reset the error using the reset key (S1) (see Fig. 15).
– If error 76 cannot be reset, there is a fault in the feedback circuit. In this
case, replace the motor.
Indirect absolute
position sensing
Absolute encoder
monitoring
