Omron CJ1W-C113 - REV 02-2008, Operation Manual

Page: 42 / 406

15
Control System Principles Section 1-7
1-7-2 Control System Principles
Open-Loop System In an open-loop system, positioning is controlled according to the number of
input pulses that the motor receives, and no position feedback is provided.
The PCUs all employ pulse-output-type open-loop systems, and the most
commonly used motor for this type of control system is a stepping motor. The
angle of rotation of a stepping motor can be controlled through the number of
pulse signals supplied to the motor driver. The number of rotations of the step-
ping motor is proportional to the number of pulses supplied by the PCU, and
the rotational speed of the stepping motor is proportional to the frequency of
the pulse train.
1-7-3 Basic Positioning System Design
The following diagram and parameters illustrate a simplified positioning sys-
tem.
M: Reduction ratio
P: Feed screw pitch (mm/revolution)
V: Feed velocity of object being positioned (mm/s)
θ
s
: Stepping angle per pulse (degree/pulse)
The travel distance per pulse sent to the motor driver is called the “pulse rate”
and is calculated according to the following equation:
Pulse rate = P/(pulses per revolution x M)
= P/((360/
θ
s
) x M))
= (P x
θ
s
)/(360 x M)
Using the pulse rate, the number of pulses required to move the object a dis-
tance L is given by the following calculation:
No. of pulses
= Travel distance
÷
Pulse rate
= L
÷
Pulse rate
= L
÷
(P x
θ
s
)/(360 x M)
= (360
×
M
×
L)/(P x
θ
s
)
The pulse speed required to move the object with a feed speed of V mm/s is
given by the following calculation:
Pulse speed
= Feed speed
÷
Pulse rate
= V
÷
Pulse rate
= V
÷
(P x
θ
s
)/(360 x M)
= (360
×
M
×
V)/(P x
θ
s
)
The PCU performs positioning control by providing the number of pulses and
the pulse speed in the form of position instructions and speed instructions.
Position controller
(e.g., PCU)
Pulse
train
Stepping motor or
pulse-train input-type
servomotor
Reduction gear
Turntable
Ball screw
P
Stepping motor
Reduction gear
Object being
positioned
N
M
V
Feed screw pitch