Question:
How can the actual position value be saved fail-safe and initialised accordingly after power on?
Answer:
Normally, for the 8400 HighLine, an HTL incremental encoder (DI1/DI2 or DI6/DI7) is used for the feedback for the position control. For positioning applications (application 'Table positioning'), the resulting actual position value is usually 'adjusted' once by means of homing. Normally, homing must be repeated after mains switching. For certain positioning applications, the actual position value shall be saved during power off to initialise it again with the saved value after power on. In this way, homing, which would otherwise be required, can be done without.
The 'Fail-safe saving of the actual position value' function is
not part of the standard functions of the 8400 HighLine. The function can, however, be reached applicatively for the 8400.
Application example for saving the actual position during power off and initialising it during power on:
The below figure shows the configuration of two parallel function blocks (FB) Sample & Hold
(L_SampleHold_x). Via the two function blocks the current position information is detected during operation of the axis and saved automatically during power off (power off incl. the 24 V supply voltage).
Since the
L_SampleHold_x function blocks can only process 16-bit values, the 32-bit position value to be saved must be converted into two 16-bit values (LOW and HIGH word) and converted back again by means of the conversion FBs
L_ConvDIntToWords_x and
L_ConvWordsToDInt_x.
After power on, the 'Set home position' function (
LA_TabPos.bHomeSetPositon) can be used to initialise the actual position value with the previously saved value. At the same time, the 'Home position known' status will be set so that homing is no longer required.
Please note:
- Due to the basic principle, it is not possible to detect and correct position changes while the controller is switched off. Therefore, it must be ensured that the drive and the motor shaft will not move in this phase, e. g. by applying a holding brake.
- Depending on the precision requirements of the machine, it may be necessary to carry out a homing process after a larger number of power on/off cycles.
