
B–63173EN–1/01
1. AXIS CONTROL
82
By adding an offset to the torque controlled by the position (velocity)
feedback device, the function can apply opposite torques to the master and
slave axes so that equal and opposite movements are performed for both
axes. This function can reduce the effect of backlash on the master and
slave axes caused by the tandem connection of the two motors via a gear.
This function, however, cannot reduce backlash between the ball screw
and table or other backlash inherent to the machine.
If a preload of x is set for the master axis and –x for the slave axis, the
opposing preload torques are continuously applied to the two axes, even
at rest, as shown below:
Master axis Slave axis
CAUTION
1 Specify as low a preload as possible. Avoid specifying a
preload higher than the rated torque. Too high a preload will
trigger an overload alarm because the specified torques
continue to be applied, even at rest. A preload that is very
slightly higher than the frictional force is recommended.
Thus, the recommended preload may be about one–third of
the rated torque.
2 If the motors rotate in opposite directions (different signs are
specified in parameter 2022), specify the preload values
with the same sign.
As shown in the block diagram of tandem control, the motor of the slave
axis is not subject to velocity control. A machine with a large amount of
backlash may become unstable if the motor of the slave axis vibrates as
a result of backlash in the gear. This can be overcome by applying
velocity control to the slave axis also. This velocity feedback average
function is enabled when bit 2 of parameter 2008 is set to 1.
The following two functions can increase the stability and position gain
of a closed–loop system having a linear scale:
Dual position feedback function
Machine velocity feedback function
For details of these functions, refer to FANUC AC SERVO MOTOR
series PARAMETER MANUAL (B–65150E).
An important factor affecting stability in tandem control is the capability
of back feed. Back feed is to cause movement along either the master or
slave axis from the other axis, via the transmission mechanism connecting
the two axes. A machine without this capability may be inclined to
become unstable and require adjustments.
D Preload function
D Velocity feedback
average function
D Improved stability of a
closed–loop system
D Notes on stability of
tandem control