Examples and tips
PROGRAMMING MANUAL
253
Revi
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on 3.0
6.1.5
Origin search
The origin search or homing functionality is often seen as a particular
sequence of movements of an axis at the start-up phase of the machine.
This sequence is done automatically in most cases, without the input from
the operator of the machine. In general, an origin search procedure couples
a position to a specific axis. It depends on the encoders used (absolute or
relative), on the system used (linear or circular), and on the mechanical
construction of the machine. Absolute encoders do not need a movement
during the origin search procedure, because the exact positions are
transferred directly to the system. For other encoder types, a movement is
necessary, since there is no knowledge of the exact position within the
system. Basically, this movement is at low speed in some direction until a
certain measuring point is reached. Such a measuring point can be scanned
from both directions to increase the precision.
At startup, the current positions of the axes using incremental encoders are
0. Because these positions do not match with the mechanical 0 of the
machine, it is necessary to execute the homing sequence. If an absolute
encoder is used, the absolute position is read at startup from the encoder
and homing is not necessary. In this case, a startup sequence must be
executed one time during the machine commissioning.
In practice there are several different origin search sequences. They are
different in these areas:
•
The means used to detect limit positions of the moving part (sensors,
switches, etc.)
•
Origin (home) position or reference.
•
Possible positions of the moving part related to limit positions and origin
position.
Trajexia includes some pre-defined basic homing sequences:
•
DATUM(0)
This is not really an origin search. This command sets
DPOS=MPOS
and cancels the axis errors.
•
DATUM(1)
This does an origin search in forward direction using the Z mark of an
encoder as homing switch.
•
DATUM(2)
Does an origin search in reverse direction using the Z mark of an
encoder as homing switch.
•
DATUM(3)
Does an origin search in forward direction using the input selected in
DATUM_IN
as homing switch.
•
DATUM(4)
Does an origin search in reverse direction using the input selected in
DATUM_IN
as homing switch.
•
DATUM(5)
Does an origin search in forward direction using the input selected in
DATUM_IN
as homing switch and searches the next Z mark of an
encoder.
•
DATUM(6)
Does an origin search in reverse direction using the input selected in
DATUM_IN
as homing switch and searches the next Z mark of an
encoder.
For more details on these pre-defined homing sequences, see section
3.2.68.
In some situations, more complex homing sequences are required:
•
Absolute switch origin search plus limit switches.
•
Origin search against limit switches.
•
Origin search against hardware parts blocking movement.
•
Origin search using encoder reference pulse "Zero Mark".
•
Static origin search, forcing a position from a user reference.
•
Static origin search, forcing a position from an absolute encoder.
I52E-EN-03.book Seite 253 Freitag, 29. Juni 2007 11:55 11
