
4. SELECTING A MOTOR
B–65142E/04
DESCRIPTIONS FOR THE α series
42
The above values can be obtained by calculating the elastic deformation
for each section of the driving system. If the value of this natural
frequency (Hz) is more than the value of position loop gain (sec
–1
), it
operates normally in most cases. That is to say, 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, attention must be paid to the fact
that response 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 less than 10 µm.
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
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 detector is mounted) and the appropriate
damping should be considered.
Stick slip
Value of machine
overrun
(Damping coefficient of
machine system)