Chapter 2 Selection of the RSF supermini Series
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19
1+NL
1+L
1+NL/R
2
1+L/R
2
NL
R
2
L
R
2
Chapter 2 Selection of the RSF supermini Series
2-1 Allowable load moment of inertia
To make full use of high precision and high performance of the RSF supermini series actuator, perform
temporary selection by considering the load moment of inertia and rotation speed.
As a guideline, the load moment of inertia should be 3 to 5 times the moment of inertia of the actuator.
For the moment of inertia of the actuator, refer to “1-4 Specifications of RSF supermini actuators.”
Refer to appendix 1 for the calculation of moment inertia.
The rotation speed cannot exceed the maximum rotation speed of the actuator. For the maximum
rotation speed, refer to “1-4 Specifications of RSF supermini actuators.”
2-2 Variable load moment of inertia
RSF supermini series actuators include Harmonic Drive™ gearing that has a high reduction ratio.
Because of this there are minimal effects of variable load moment of inertias to the servo drive system.
In comparison to direct servo systems this benefit will drive the load with a better servo response.
For example, assume that the load moment of inertia increases to N-times during its motion (for
example, robot arms). The effect of the variable load moment of inertia to the [total inertia converted
into motor shaft] is as follows:
The symbols in the formulas are:
J
S
:
total moment of inertia converted into
motor shaft
L:
Ratio of load moment of inertia to motor inertia
J
M
:
moment inertia of motor
N:
variation ratio of load moment of inertia
R:
reduction ratio of RSF supermini series
Direct drive
Before:
Js = J
M
(1+L)
After: Js’ = J
M
(1+NL)
Ratio: Js’/J
M =
RSF supermini actuator drive
Before: Js’ = J
M
(1+ )
After: Js’ = J
M
(1+ )
Ratio: Js’/J
M =
In the case of the RSF supermini actuator drive, as the reduction ratio is [R=30], [R=50], or [R=100] and
the square of the reduction ratio [R
2
=900], [R
2
=2500], or [R
2
=10000] the denominator and the
numerator of the ratio are almost [1]. Then the ratio is [F=1]. This means that drive systems are hardly
affected by the load moment of inertia variation. Therefore, it is not necessary to take the load moment
of inertia variation in consideration for selecting an RSF supermini actuator or for setting up the HA-680
driver.
2-3 Verifying loads
The RSF supermini series incorporates a precision 4-point contact ball bearing for direct support of
external load. To make full use of the performance of the RSF supermini series, check the maximum
load moment, life of the 4-point contact ball bearing, and static safety factor.
For detailed calculation methods for the maximum load moment, life of the 4-point contact ball bearing,
and static safety factor, refer to the “Harmonic Drive™ CSF Mini series” catalogue.
