
2.HOW TO USE SERVO MOTORS SPECIFICATIONS FOR THE αiS/αi SERIES B-65262EN/03
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2.3.5 Precautions for Using Linear Scale
In the case where the machine moves in a linear direction and
movement is directly detected by linear scale such as inductosyn,
magne-scale etc., special considerations are necessary in comparison
with the method where feedback is produced by detecting the motor
shaft rotation. This is because the machine movement now directly
influences the characteristics of the control system.
Machine system natural frequency
The following block diagram shows feedback produced using a linear
scale.
Motor
Pulse coder Linear scale
Command
Position
control
circuit
Velocity
control
circuit
The response of this control system is determined by the adjustment
value (position loop gain) of the position control circuit. In other
words, the position loop gain is determined by the specified response
time of the control system. In the diagram above, the section enclosed
by the broken line is called the velocity loop.
Unless the response time of this section where position signal is
detected is sufficiently shorter than the response time determined by
the position loop gain, the system does not operate properly. In other
words, when a command signal is put into point A, response time of
the machine where position signals are detected must be sufficiently
shorter than the response time defined by the position loop gain.
If the response of the sensor section is slow, the position loop gain
should be reduced to have the system operate normally, and as a result,
the response of the whole system becomes slow. The same problem is
caused when inertia is great.
The main causes for slow response are the mass of the machine and
the elastic deformation of the machine system. The larger the volume,
and the greater the elastic deformation, the slower the response
becomes.
As an index for estimating the response of this machine system, the
natural frequency of the machine is used, and this is briefly calculated
by the following equation.
L
J
m
K
m
W ×=
π
2
1
W
m
: Natural frequency
J
L
: Load inertia reflected to motor shaft
K
m
: Rigidity of machine system
(=Torque necessary to elastically deform 1[rad] at the
motor shaft when the machine table is clamped.)