
B-65150E/04 4. SERVO FUNCTION DETAILS
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(4) Descriptions of servo parameters for adjustment
The load inertia ratio to be specified for axes subjected to tandem
control differs from that for ordinary axes.
1875
−
Load inertia ratio (LDINT)
2021
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[Standard setting] (Load inertia/motor inertia) × 256
(NOTE) In typical tandem control, the total load inertia of the machine is borne
by two motors. So, calculate the load inertia for the above formula as
follows:
(Load inertia) = (Total load inertia of machine)/2
When the full preload function is used, the motor on the driving side is
required to bear the total load inertia of the machine and the motor
inertia of the other motor. So, calculate the load inertia for the above
formula as follows:
(Load inertia) = (Total load inertia of machine) + (Motor inertia)
Example of setting The example shown in Fig. 4.17 (a) is used. Assume that the inertia of
each section applied to the motor shaft as follows:
• Inertias of the reducers of the main- and sub-axes: J
1m, J1s
• Inertias of the pinions of the main- and sub-axes: J2m, J2s
• Inertia of the rack: J3
(Total load inertia of the machine) = J1m + J2m + J3 + J1s + J2s
When the total load inertia of the machine is double that of the motor
inertia, for example, set the following:
When typical tandem control is used:
(Load inertia ratio) = (2/2) × 256 = 256
When the full preload function is used:
(Load inertia ratio) = (2 + 1) × 256 = 768
The result obtained from the above formula may cause oscillation due
to the mechanical structure. In such a case, set a smaller value.
Notes on stable tandem control operation
To ensure stable tandem control operation, the machine must be
capable of performing back-feed.
Back-feed is the moving of the sub-motor from the main motor, or vice
versa, through the connected transmission feature. When the back-feed
capability is disabled, unstable operation results. In this case, machine
adjustment becomes necessary.
The user can check whether the back-feed capability is enabled. To
make this check in the case of the example shown in Figs. 4.17 (a) and
(b), turn the main motor with the power line for the sub-motor
disconnected, and check that the main motor can be turned with one-
third or less of the rated torque of the motor (See (2) in Subsec. 4.17.9).