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PLC User's Manual - Positioning Control Edition
Built-in Positioning Functions
13 Troubleshooting
13.4 If Operation Is Stopped at a Wrong Position
B - 178
13.4 If Operation Is Stopped at a Wrong Position
If operation is stopped at a wrong position, check the following items.
1) Check whether the electronic gear of the servo amplifier (drive unit) is set properly.
→ For the electronic gear setting method of the MELSERVO Series, refer to Subsection 4.6.2.
2) Check whether the origin is set properly.
• Properly set the DOG so that the near-point signal (DOG) can be kept ON until the speed is reduced to
the creep speed. The zero return instruction will start speed reduction at the front end of the DOG, and
will stop the operation at the rear end of the DOG or at detection of the first zero-phase signal after
passing the rear end of the DOG. After that, the current value register will be cleared (reset to "0"). If
the speed is not reduced to the creep speed before detecting the rear end of the DOG, the operation
may not be stopped at the specified position.
• The creep speed should be slow enough. The zero return instruction will not reduce the speed before
stopping. For this reason, if the creep speed is not slow enough, the operation may not be stopped at
the specified position due to inertia.
• Devices for the near-point signal (DOG)
- DSZR instruction
If an input (X000 to X017)
*1
of the main unit is specified for the near-point signal (DOG), the rear end
of the near-point signal (DOG) will be monitored (detected) at 1-ms intervals (interruption). Under the
following condition, however, monitoring (detection) of the near-point signal (DOG) rear end may be
affected by the input time constant or the scan time of the sequence program, and the operation may
not be stopped at the specified position.
a) An input number of X020 or below (or other device (auxiliary relay, etc.)) is specified.
*1.When using an FX
3U
-16M
, FX
3UC
-16M
, specify an input in the range of X000 to X007.
X000 to X007 for FX
3G
PLC (main unit).
- ZRN instruction
If an input (X000 to X007) of the main unit is specified for the near-point input signal, the PLC
interruption function will be used to stop the operation. Under the following condition, however,
operation may be affected by the scan time of the sequence program, and the operation may not be
stopped at the specified position.
a) An input number of X010 or below (or other device (auxiliary relay, etc.)) is specified. If an input
relay X010 or below is specified for the near-point signal, the input filter (10 ms) will apply.
• If the DSZR instruction is used:
Since the zero-phase signal of the servo motor is used, adjust the relation between the rear end of the
DOG and the zero-phase signal as shown in the following figure. If fine adjustment of the origin
position is needed, adjust the position of the near-point signal (DOG).
3) If reciprocating operation (operation in the forward rotation direction and then reverse rotation direction) is
not stopped at the specified position: The built-in positioning function cannot correct the mechanical
backlash (clearance, play) during positioning operation. If it is necessary to correct the backlash,
preliminarily set the number of output pulses considering the backlash that may be caused when
changing the transfer direction.
DOG
Rear end
Zero-phase signal
Table
Feed screw
Backlash
(clearance, play)