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8. CALCULATION METHODS FOR DESIGNING
8.2 Position resolution and electronic gear setting
Position resolution (travel per pulse
) is determined by travel per servo motor revolution S and the
number of encoder feedback pulses Pt, and is represented by Equation 8.1. As the number of feedback
pulses depends on the servo motor series, refer to Section 5.1.
Pt
S
.............................................................................................................................................................(8.1)
: Travel per pulse [mm]
S : Travel per servo motor revolution [mm/rev]
P
t : Number of feedback pulses [pulse/rev]
Since
has the relationship represented by Equation 8.1, 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.
CMX
CDV
SM
P
t
8192pulse/rev
Command
pulse train f
0
Electronic gear
CMX
CDV
Deviation
counter
Encoder
As shown above, command pulses are multiplied by CMX/CDV set in the parameters to be position
control pulses. Travel per command pulse
is expressed by Equation 8.2:
0
P
t
S
CMX
CDV
CMX
CDV
.......................................................................................................................(8.2)
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 0 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 8.2:
CMX
CDV
0
P
t
S
0.01
8192
10
1024
125
<Relationship between position resolution and overall accuracy>
Positioning accuracy of machine is the sum of electrical errors and mechanical errors. Normally,
provisions should be made so that positioning accuracy are not affected by electrical system errors. As a
guideline, Equation 8.3 should be satisfied:
1
5
1
10
to
D .........................................................................................................................................(8.3)
where,
: Travel per feedback pulse [mm/pulse]
: Positioning accuracy [mm]