Mitsubishi Electric MELSEC Q Series Programming Manual page 47

2 SAFETY OBSERVATION FUNCTION
POINTS
(1) Be sure to use A-phase/B-phase mode during standstill monitoring so that
rotation information is acquired.
(2) In the case of 1-phase mode, internal counter counts up regardless of rotating
direction. In the case of an operation pattern where the inverse rotation for
motor rotating direction is repeated in a short time, a speed monitoring error
may occur because an internal counter miscounts by input pulse chattering.
Use A-phase/B-phase mode to prevent the miscount.
(c) The safety speed depends on the number of external auxiliary pulse per
revolution and PLC CPU scan time. One pulse (ON to OFF, OFF to ON)
must be longer than one scan time. If there are two or more pulse edges
(OFF to ON, ON to OFF) in one scan, the count cannot be executed
properly.
Calculate the detectable maximum speed (pulse frequency) from the longer
time, the PLC CPU scan time or input response time.
Example)
Number of pulses per revolution of external pulse: 8 [pulse/rev]
PLC CPU scan time: 25 [ms] (In the case of maximum allowable scan
Input response time: 10 [ms]
• Maximum input frequency
25[ms] × 10
(Note-1): Two scans are required to detect the pulse input of OFF to ON.
• Detectable maximum speed
20[Hz] × 60[s/min]
8[pulse/rev]
Set "150 [r/min] or less" as the safety speed of the external auxiliary pulse
generation mechanism part.
1-phase mode
A-phase
A-phase/B-phase
mode
B-phase
time for speed monitoring function)
1
= 20 [Hz] (20 [pulse/s])
-3 (Note-1)
× 2
= 150[r/min]
25ms 25ms
or more or more
25ms(PLC CPU scan time)
25ms 25ms 25ms 25ms
or more or more or more or more
25ms(PLC CPU scan time)
2 - 10
Make the pulse width
(OFF to ON, ON to OFF)
of 25ms or more.
Make the pulse width
(OFF to ON (A-phase),
OFF to ON (B-phase))
of 25ms or more