Omron SYSMAC CJ Operation Manual page 315

How-to's
5-1-5-3 Origin search against hardware parts blocking movement
5-1-5-4 Origin search using encoder reference pulse "Zero Mark"
This origin search procedure performs origin search against a physical object
and mechanically blocks the movement. There are no limit switches, no
absolute position switch and no reference pulses. The origin position is
detected by detecting a particular amount of torque against the blocking
objects. An adequate torque limit is required in order not to damage the
mechanics during the origin search process. The example for this homing
procedure is shown in the figure.
The program example that does this origin search sequence is given below.
BASE(0)
DRIVE_CONTROL=11 'Monitor torque with DRIVE_MONITOR
SERVO=ON
WDOG=ON
SPEED=CREEP
REVERSE
WA(1)
WAIT UNTIL DRIVE_MONITOR < -100
'Wait for particular amount of applied torque
CANCEL
DEFPOS(0)
MOVEABS(10) 'This is necessary, otherwise the position
'is kept pushing the hardware limit of the
'machine and the motor trips by overload
min. limit switch
moving part
moving part
ON OFF OFF ON
Section 5-1
encoder
max. limit switch
303