Yaskawa Large Capacity Sigma II Series - Manual (Page 211)

5 Servo Adjustment
5.4.2 Basic Rules of Gain Adjustment
5-28
Generally speaking, the responsiveness of the position loop cannot be higher than that of the
speed loop. Therefore, to increase the position loop gain, you must first increase the speed
loop gain. If only the position loop gain is increased, oscillation will result in the speed ref-
erence and positioning time will increase, not decrease.
Position loop gain can be increased only to the point where oscillation begins in the mechan-
ical system.
• If the position loop response is faster than the speed loop response, speed reference out-
put from the position loop cannot follow the position loop response due to the slow
speed loop response. Therefore, smooth linear acceleration or deceleration will not be
possible and the position loop will keep accumulating errors, thus increasing the amount
of speed reference output.
As a result, the motor speed will be excessive and the position loop will try decreasing
the amount of speed reference output. The speed loop responsiveness will be poor, how-
ever, and the motor will not be able to catch up with the speed reference. As a result, the
speed reference will oscillate as shown in the following graph.
If this happens, reduce the position loop gain or increase the speed loop gain to prevent
the speed reference from oscillating.
Fig. 5.2 Speed Reference with Position Loop Gain and Speed Loop Responsiveness
Not Well-balanced
• The position loop gain must not exceed the natural frequency of the mechanical system.
For example, if the mechanical system is an articulated robot, the rigidity of the machin-
ery mechanism is very low because the mechanism incorporates a wave reduction gear
and the natural frequency of the mechanical system is 10 to 20 Hz. In this case, the posi-
tion loop gain can be set to 10 to 20 (1/s).
If the mechanical system is a chip mounting machine, IC bonding machine, or high-pre-
cision machining tool, the natural frequency of the system is 70 Hz or more. Therefore,
the position loop gain can be set to 70 (1/s) or higher.
• When high responsiveness is required, it is not only important to ensure the responsive-
ness of the servo system that is employed (the controller, SERVOPACK, motor, and
encoder), but it is also necessary to ensure that the mechanical system have high rigidity.
Speed
reference
Time
Actual speed reference output from controller
Speed reference as a result of calculation in controller