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6.6. Preset function
The preset function in the encoder is triggered as soon as the PLC sets the “preset bit” (see below) in the
output data. The encoder position is set to the given preset value.
For optimum alignment of mechanical position to preset we recommend to set the preset value during
encoder standstill. But for lower requirements it may also be executed while encoder rotates.
It is mandatory to parameterize the required resolution or code sequence (cw/ccw) prior to performing the
preset operation.
Upon performing a preset operation an internal offset (see subindex 8 of PNU 65001) is calculated and
stored non-volatile in the FLASH memory to make sure the encoder will be at the same position after cycling
power.
Although the FLASH provides typically 100000 write cycles, frequent software-triggered or event-triggered
preset operations may consume even this capacity what has to be considered in the PLC software
configuration.
Note:
Any alteration of the total measuring range, measuring units per revolution or of a gear factor parameter
during encoder re-parameterization will implicate clearing the internal preset offset value. This however has
no effect in practice since in these cases the position reference is lost anyway.
6.6.1. Preset in standard telegrams
Bit 31 in the 32 bit output data word is the preset bit. The lower bits (b
it 30…0) hold the right-aligned position
value which is adopted by the encoder in the preset operation.
The preset bit must be kept set for at least one full bus update cycle to be recognized by the encoder. A timer
control of e. g. 10 ms or more will do as well. The preset bit may stay active for an unlimited period of time
since only a change from „0“ to „1“ will be evaluated. We recommend however to clear the bit after completion
in order to provide the original status for a subsequent preset operation.
6.6.2. Preset in PROFIdrive telegrams 81 to 83
A preset operation in control word STW2 requires the PLC having bit CP (Control by PLC) set. The TAR bit
must not be set at the time of the preset operation.
The REL bit in control word G1_STW1 defines whether an absolute or relative preset operation will be
performed. In a relative preset operation the current position value will be shifted by the preset value.
The PR bit in G1_STW1 is the preset bit. The encoder executes the preset when the preset bit changes from
“0” to “1”. Once successfully completed bit PA (preset acknowledge) is set in status word G1_ZSW1. The PA
bit remains active as long as the preset bit remains set by PLC.
The preset value is retrieved from the value stored in parameter 65000 (see chapter 7.4.17 „Parameter
65000: Preset Value“).
Since firmware version 1.3.0 this has to be considered:
It is possible to set the preset value (PNU 65000) directly
in the “module parameters”. This means that the
PLC writes PNU 65000 during startup of the encoder.
To be able to use this functionality the following GSDML file has to be used:
“GSDML-V2.32-Baumer-EAx580_PN-20190715.xml”
