2-2 Change in moment of inertia of load
Selection guidelines
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2-2
Change in moment of inertia of load
FHA-C mini series actuators include HarmonicDrive
®
gearing that has a high reduction ratio. Because of this, the
effects of change in load inertia moment on the servo performance are minimal. In comparison to direct servo
drive mechanisms, therefore, this benefit allows the load to be driven with a better servo response.
For example, assume that the load inertia moment increases to N-times. The total inertia moment converted to
motor shaft which has an effect on servo response is as follows:
The symbols in the formulas are:
J
S
: Total inertia moment converted to motor shaft
L: Ratio of load inertia moment to inertia moment of motor
J
M
: Motor inertia moment
N: Rate of change in load inertia moment
R: Reduction ratio of FHA-C mini series actuators
Direct drive
Before:
After:
Ratio:
Driven by FHA-C mini series
Before:
After:
Ratio:
With the FHA-C mini series, the value of R increases to 30, 50 or 100, which means that the value of
R
2
increases substantially to 900, 2500 or 10000. Then the ratio is Js'/Js 1. This means that FHA-C
mini series are hardly affected by the load variation. Therefore, it is not necessary to take change in
load inertia moment into consideration when selecting a FHA-C mini series actuator.
+
2
M
R
NL
J
Js'
1
＝
(
)
NL
J
Js'
M
+
1
＝
(
)
L
J
Js
M
+
1
＝
+
2
M
R
L
J
Js
1
＝
L
1
NL
1
/Js
Js'
+
+
＝
2
2
R
/
L
1
R
/
NL
1
/Js
Js'
+
+
＝
