
4. SERVO FUNCTION DETAILS B-65150E/04
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4.17.1 Preload Function
By applying an offset to the torque controlled by position (velocity)
feedback, torques of opposite directions can be applied to the main-
(main motor) and sub-axes (sub-motor) to maintain tension at all times.
This function can reduce the backlash between the main- and sub-axes,
caused by the tandem connection of two motors through gears.
However, this function does not reduce the backlash between the ball
screw and table, which are a feature of the machine system.
For example, set preload +Pre for the main axis and preload -Pre for the
sub-axis. Then, torques are produced as shown below.
If a torque is required during acceleration/deceleration, a torque of the
same direction is produced with the two motors. (Load sharing
mode)
If no torque is required, for example, during stop state, preload torques
produce tension between the two axes. (Anti-backlash mode)
For an application which requires only anti-backlash mode, use the full
preload function, described in Subsec. 4.17.7.
Stopped
Table
Sub-axis Main axis
+Pre
−Pre
Sub-axis Main axis
Drive torque
Drive torque
Direction of
movement
Acceler-
ated
−Pre
+Pre
Sub-axis
Main axis
Drive torque
Drive torque
When friction torque < preload torque
At constant
speed
Direction of
movement
+Pre
−Pre
When friction torque > preload torque
Direction of
movement
Drive torque
Drive torque
Main axis
Sub-axis
Decelerated
Direction of
movement
−Pre
+Pre
Fig. 4.17.1 (a) Changes of torque during movement