
B–65142E/04
4. SELECTING A MOTORDESCRIPTIONS FOR THE α series
41
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.
Command
Position
control
circuit
Servo
amplifier
Pulse coder Motor Linear scale
This method is shown in the figure above by block diagram. 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.
When the response of the detector section is slow, the position loop gain
must be reduced to have the system operate normally, and as a result, the
response of the whole system is slow. The same problem is caused when
inertia is great (see Subsec. 4.1.1)).
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.
Wm +
1
2p
Km
JL
Ǹ
Wm: Natural frequency
JL : Load inertia reflected to motor shaft
Km : Rigidity of machine system
(=Torque necessary to elastically deform 1 [rad] at the motor
shaft when the machine table is clamped.)
4.2
PRECAUTIONS FOR
USING LINEAR
SCALE
Machine system natural
frequency