Position Resolution And Electronic Gear Setting - Mitsubishi Electric MELSERVO MR-J2-A Product Specifications And Installation Manual

11.SELECTION

11-2 Position resolution and electronic gear setting

Position resolution (travel per pulse∆R) is determined by travel per servo motor revolution ∆S
and the number of encoder feedback pulses Pt, and is represented by Equation 11-1:
∆S
∆R=
............................................................................................................................................ (11-1)
P
t
∆R : Travel per pulse
∆S : Travel per servo motor revolution
Pt : Number of feedback pulses
Note: As these values depend on the servo motor series, confirm them in the specifications.
Since ∆R has the relationship represented by Equation 11-1, its value is fixed in the control
system after the drive system and encoder have been determined. However, travel per
command pulse can be set as desired using the parameters.
Command
pulse train f0
As shown above, command pulses are multiplied by CMX/CDV set in the parameters to be
position control pulses. Travel per command pulse ∆R is expressed by Equation 11-2:
CMX
P
t
∆Ro= ∆R •
•
∆S
CDV
CMX: Electronic gear (Command pulse multiplication numerator)
CDV: Electronic gear (Command pulse multiplication denominator)
Using the above relationship, travel per command pulse can be set to a value without fraction.
[Setting example]
Find a parameter value for ∆Ro=0.01 [mm] in a drive system where ball screw lead PB = 10 [mm]
and reduction ratio 1/n = 1.
The encoder feedback pulses Pt of the HC-MF = 8192 [pulses/rev].
Since ∆s = 10 [mm/rev], the following is obtained according to Equation 11-2:
CMX P
t
= ∆Ro •
= 0.01 •
∆S
CDV
<Relationship between position resolution ∆R and overall accuracy>
Overall accuracy (positioning accuracy of machine) is the sum of electrical errors and mechanical
errors. Normally, provisions should be made so that overall errors are not affected by electrical
system errors. As a guideline, Equation 11-3 should be satisfied:
1 1
∆R< • ∆ε
to
qqqqqqqqqqqqqqqqqqq
5 10
where, ∆R: Travel per feedback pulse [mm/pulse]
∆ε:Positioning accuracy [mm]
+
CMX
CDV
-
Electronic gear
(Parameters No. 3, 4)
CMX
................................................................................................... (11-2)
CDV
8192 1024
=
10
125
..................................................................................................................... (11-3)
11– 3
[mm]
[mm/rev]
[pulse/rev]
Deviation
SM
counter
P 8192pulse/rev
t =
Encoder
11