Mitsubishi Electric MELSERVO-JN Series Instruction Manual page 22

(2) How to find the cause of position shift
Controller
(a)Output pulse
counter
Q
Cause A
Servo-on (SON),
Stroke end
(LSP/LSN) input
When a position shift occurs, check (a) output pulse counter Q, (b) cumulative command pulses P, (c)
cumulative feedback pulses C, and (d) machine stop position M in the above diagram.
Cause A, Cause B and Cause C indicate position shift causes. For example, Cause A indicates that noise
entered the wiring between the controller and servo amplifier, causing the command input pulse to be miss-
counted.
In a normal status without position shift, there are the following relationships.
1) Q P (output pulse counter
2) When using the electronic gear
CMX (parameter No. PA06)
P 
CDV (parameter No. PA07) 
C (cumulative command pulses
Note. When "0" is set to the FBP (parameter No. PA05), the FBP becomes the servo motor encoder resolution.
3) C  Δ M (cumulative feedback pulses
Check for a position shift in the following sequence.
1) When Q ≠ P
Noise entered in the pulse train signal wiring between the controller and servo amplifier, causing
command input pulses to be miss-counted. (Cause A)
Make the following check or take the following measures.
Check the shielding.
Run wiring away from the power circuit.
Install a data line filter. (Refer to section 11.9 (2) (a).)
Servo amplifier
Electronic gear (parameters No. PA06, PA07)
P FBP
FBP conversion
(b)Cumulative command pulses
(c) Cumulative feedback pulses
Cause C
cumulative command pulses)
Servo motor encoder resolution
FBP (parameter No. PA05) (Note)
electronic gear
POINT
The noise tolerance can be enhanced by setting parameter No. PA13 to "1
when the command pulse frequency is 500kpps or less or "2
200kpps or less.
Servo motor
CMX
CDV
C
cumulative feedback pulses)
travel per pulse machine position)
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Introduction
Machine
L
M
(d) Machine stop position M
Cause B
Encoder
" when
"