
B-65262EN/03 SPECIFICATIONS FOR THE αiS/αi SERIES 2.HOW TO USE SERVO MOTORS
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The above values can be obtained by calculating the elastic
deformation for each section of the driving system. The machine
should be designed so that the value of this natural frequency [Hz]
will be more than or equal to the value of the position loop gain [sec
-
1
]. For example, when setting 20 [sec
-1
] as the value of position loop
gain, natural frequency of machine system must be more than 20 [Hz].
In this case, the response of the control system becomes a problem for
extremely small amounts of movement. Consequently, the natural
frequency should be calculated from the rigidity at extremely small
displacement such as 10 [µm] or less.
Stick slip
If machine movement causes a stick slip, the control system does not
operate normally. That is, it does not stop where it is supposed to, but
a phenomenon occurs where it goes beyond and then back within an
extremely small range (hunting).
To avoid stick slip, the machine rigidity should be increased, or
friction characteristics of the sliding surface should be improved.
When the sliding surface friction characteristic is as in the figure
below, stick slip occurs easily.
Friction coefficient
Proper friction
characteristic
Friction characteristic which
causes stick slip
Speed
Value of machine overrun (Damping coefficient of machine system)
When the machine is floated by static pressure, etc., there are cases
where the machine keeps on moving within the range of backlash
although the motor shaft has stopped. If this amount is large, hunting
will also occur. To avoid this, backlash should be reduced (especially
the backlash of the last mass where position sensor is mounted) and
the appropriate damping should be considered.