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= Homing Configuration
[[cha:homing-configuration]] (((Homing Configuration)))
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== Overview
Homing seems simple enough - just move each joint to a known location,
and set LinuxCNC's internal variables accordingly. However, different
machines have different requirements, and homing is actually quite
complicated.
== Homing Sequence
There are four possible homing sequences, along with the associated
configuration parameters as shown in the following table. For a more
detailed description of what each configuration parameter does, see the
following section.
.Homing Sequences[[fig:motion-homing-sequence-diagram]]
image::images/emc2-motion-homing-diag.png[align="center"]
== Configuration
There are six pieces of information that determine exactly how the
home sequence behaves. They are defined in an [AXIS] section of the
inifile.
[width="80%", options="header", cols="4*^"]
|==========================================================
|Homing Type |SEARCH_VEL |LATCH_VEL |USE_INDEX
|Immediate |0 |0 |NO
|Index-only |0 |nonzero |YES
|Switch-only |nonzero |nonzero |NO
|Switch and Index |nonzero |nonzero |YES
|==========================================================
[NOTE]
Any other combinations may result in an error.
=== HOME_SEARCH_VEL (((HOME SEARCH VEL)))
The default value is zero. A value of zero causes LinuxCNC to assume that
there is no home switch; the search stage of homing is skipped.
If HOME_SEARCH_VEL is non-zero, then LinuxCNC assumes that there is a home
switch. It begins by checking whether the home switch is already
tripped. If tripped it backs off the switch at HOME_SEARCH_VEL. The
direction of the back-off is opposite the sign of HOME_SEARCH_VEL. Then
it searches for the home switch by moving in the direction specified by
the sign of HOME_SEARCH_VEL, at a speed determined by its absolute
value. When the home switch is detected, the joint will stop as fast as
possible, but there will always be some overshoot. The amount of
overshoot depends on the speed. If it is too high, the joint might
overshoot enough to hit a limit switch or crash into the end of travel.
On the other hand, if HOME_SEARCH_VEL is too low, homing can take a
long time.
=== HOME_LATCH_VEL (((HOME LATCH VEL)))
Specifies the speed and direction that LinuxCNC uses when it makes its
final accurate determination of the home switch (if present) and index
pulse location (if present). It will usually be slower than the search
velocity to maximize accuracy. If HOME_SEARCH_VEL and HOME_LATCH_VEL
have the same sign, then the latch phase is done while moving in the
same direction as the search phase. (In that case, LinuxCNC first backs off
the switch, before moving towards it again at the latch velocity.) If
HOME_SEARCH_VEL and HOME_LATCH_VEL have opposite signs, the latch phase
is done while moving in the opposite direction from the search phase.
That means LinuxCNC will latch the first pulse after it moves off the
switch. If HOME_SEARCH_VEL is zero (meaning there is no home switch),
and this parameter is nonzero, LinuxCNC goes ahead to the index pulse
search. If HOME_SEARCH_VEL is non-zero and this parameter is zero, it
is an error and the homing operation will fail. The default value is
zero.
=== HOME_FINAL_VEL
It specifies the speed that LinuxCNC uses when it makes its move from
HOME_OFFSET to the HOME position. If the HOME_FINAL_VEL is missing from
the ini file, then the maximum joint speed is used to make this move.
The value must be a positive number.
=== HOME_IGNORE_LIMITS (((HOME IGNORE LIMITS)))
Can hold the values YES / NO. This flag determines whether LinuxCNC will
ignore the limit switch input for this axis while homing. Some machine
configurations do not use a separate home switch, instead they route
one of the limit switch signals to the home switch input as well.
In this case, LinuxCNC needs to ignore that limit during homing. The default
value for this parameter is NO. To use only one input for all homing and
limits you will have to block the limits of the axis not homing with axis
that is homing in HAL.
=== HOME_USE_INDEX (((HOME USE INDEX)))
Specifies whether or not there is an index pulse. If the flag is true
(HOME_USE_INDEX = YES), LinuxCNC will latch on the rising edge of the index
pulse. If false, LinuxCNC will latch on either the rising or falling edge of
the home switch (depending on the signs of HOME_SEARCH_VEL and
HOME_LATCH_VEL). The default value is NO.
=== HOME_OFFSET (((HOME OFFSET)))
Contains the location of the home switch or index pulse, in joint
coordinates. It can also be treated as the distance between the point
where the switch or index pulse is latched and the zero point of the
joint. After detecting the index pulse, LinuxCNC sets the joint coordinate
of the current point to HOME_OFFSET. The default value is zero.
=== HOME (((HOME)))
The position that the joint will go to upon completion of the homing
sequence. After detecting the index pulse, and setting the coordinate
of that point to HOME_OFFSET, LinuxCNC makes a move to HOME as the final
step of the homing process. The default value is zero. Note that even
if this parameter is the same as HOME_OFFSET, the joint will slightly
overshoot the latched position as it stops. Therefore there will always
be a small move at this time (unless HOME_SEARCH_VEL is zero, and the
entire search/latch stage was skipped). This final move will be made at
the joint's maximum velocity. Since the joint is now homed, there should
be no risk of crashing the machine, and a rapid move is the quickest
way to finish the homing sequence. footnote:[The distinction between
'home_offset' and 'home' is that 'home_offset' first establishes the
scale location on the machine by applying the 'home_offset' value to
the location where home was found, and then 'home' says where
the joint should move to on that scale.]
=== HOME_IS_SHARED (((HOME IS SHARED)))
If there is not a separate home switch input for this axis, but a
number of momentary switches wired to the same pin, set this value to 1
to prevent homing from starting if one of the shared switches is
already closed. Set this value to 0 to permit homing even if the switch
is already closed.
=== HOME_SEQUENCE (((HOME SEQUENCE)))
Used to define a multi-axis homing sequence HOME ALL and enforce
homing order (e.g., Z may not be homed if X is not yet homed). An axis
may be homed after all axes with a lower HOME_SEQUENCE have already
been homed and are at the HOME_OFFSET. If two axes have the same
HOME_SEQUENCE, they may be homed at the same time. If HOME_SEQUENCE is
-1 or not specified then this joint will not be homed by the HOME ALL
sequence. HOME_SEQUENCE numbers start with 0 and there may be no unused
numbers.
=== VOLATILE_HOME (((VOLATILE HOME)))
If this setting is true, this axis becomes unhomed whenever the
machine transitions into the OFF state. This is appropriate for
any axis that does not maintain position when the axis drive is
off. Some stepper drives, especially microstep drives, may need
this.
=== LOCKING_INDEXER (((LOCKING INDEXER)))
If this axis is a locking rotary indexer, it will unlock before
homing, and lock afterward.
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