
B-63783EN-1/01 1.AXIS CONTROL
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- Preload function
By applying an offset to a torque controlled by position (velocity)
feedback, tension can be maintained at all times with opposing torques
applied to the master axis and slave axis. With this function, the effect
of backlash that occurs between the master axis and slave axis due to
the tandem connection of two motors through a mechanism such as a
gear can be reduced. This function, however, does not reduce the
backlash inherent to a mechanical system, such as that between a ball
screw and table.
For example, by setting X for the master axis and -X for the slave axis
as preload values, tension is maintained by the preload torques even
during a stop.
Master axis Slave axis
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CAUTION
1 Set as small a preload value as possible. Never set a
value that exceeds the rated torque. Otherwise, an
overload alarm is issued because a torque is output
even during a stop. Set a value that is slightly larger
than the friction value. As a guideline, set a torque
that is about one-third of the rated torque.
2 If the direction of one motor is opposite to that of the
other motor (if a value with the opposite sign is set in
parameter No. 1879), set a preload value with the
same sign.
- Velocity feedback averaging function
As can be seen from the tandem control block diagram, the motor of the
slave axis is not subject to velocity control. For this reason, the
operation of a machine with a large backlash may become unstable
because the slave axis vibrates, for example, due to gear backlash. In
this case, machine operation can be made stable by exercising velocity
control on the slave axis as well. This function is referred to as the
velocity feedback averaging function. This function is enabled by
setting bit 2 (VFB) of parameter No. 1952.
- Improvement in closed loop system stability
For a closed loop system employing a linear scale, for example, two
functions are available which can improve the system reliability and
which allow a higher position gain.
- Dual position feedback function
- Machine velocity feedback function
For details of these functions, refer to the "FANUC AC SERVO
MOTOR αi series Parameter Manual (B-65270EN)."