23
Under normal wiring conditions a position control loop is created when power
is turned ON, and a servolock occurs. For example, if the motor rotates in the
CW direction during servolock, the encoder will detect this. Consequently,
feedback signals notify the error counter in the Position Control Unit about
the direction and magnitude of this movement. Since the error counter count
is ordinarily zero unless otherwise designated, even if the motor moves in the
CW direction and the feedback signals transfer this direction and movement
as a count to the error counter, it will zero this count figure by rotating the
motor in the CCW direction with the appropriate control voltage. The control
voltage is output to the servomotor driver, and the motor rotates in the CCW
direction. Again, when the motor rotates in this CCW direction the encoder
will detect the movement and notify the error counter in the Position Control
Unit with feedback signals. This loop subtracts the count figure in the error
counter to zero it. Hence, the creation of a position control loop and valid ser-
volock will ordinarily allow the servomotor to constantly correct and maintain
its stopped position.
If feedback input lines Phase A and Phase B are wired in reverse (broken
lines at (1) in the figure), the servolock is ineffective and the motor runs out of
control. Suppose the motor rotates in the CW direction and the encoder de-
tects this. Because the feedback input wiring is reversed at the Position Con-
trol Unit, the error counter receives the information as a magnitude in the
CCW direction. Then the error counter attempts to zero the count figure by a
control voltage output in the CW direction. The reversed wiring thus causes
further CW rotation when the servomotor driver receives this control voltage.
The error counter continues to total a count in the CCW direction, and the
motor runs out of control in the CW direction as the cycle repeats. This can
occur not only from reversed wiring of Phases A and B of the feedback in-
puts, but also from reversed wiring of the speed voltage and the ground (zero
volt) lines. Run-away motors are quite dangerous because they become ap-
parent only after switching power on.
The C500-NC222-E Position Control Unit counters this danger with the fol-
lowing faulty wiring diagnostics in order to prevent run-away motors.
•
When the power is turned ON the error counter capacity has a limit so that
a control voltage beyond
±
0.3 V is not output.
•
In the event of faulty wiring, this limit ensures that the motor will not run too
fast to be stopped externally. Immediately check the wiring and parameters
after the motor stops.
•
This faulty wiring diagnostic function clears when the first axis operation
command arrives such as SRT, JOG, PLS, ORG, etc. Thus during ordinary
operation this limit does not interfere.
•
If a motor does run away and the error counter overflows because of an
error, stop the motor by a 0-V control voltage and clear the servolock func-
tion. Then immediately turn OFF the servomotor driver power supply since
the motor may continue to move slowly under a 0-V drift.
Other reasons why the servolock does not engage and causes a run-away
motor include 1) reversed connections of A and A or B and B, 2) faulty wiring
between the servoamp and servomotor, or 3) incorrect rotation direction set-
tings for parameters (addresses 402/802). Check for these if you suspect
wiring problems.
Faulty Wiring Diagnostic Function
Section 2-3
