Control System Principles; Basic Positioning System Design - Omron CJ1W-C113 - REV 02-2008 Operation Manual

Control System Principles

1-7-2 Control System Principles
Open-Loop System
1-7-3

Basic Positioning System Design

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.
Position controller
(e.g., PCU)
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
: Stepping angle per pulse (degree/pulse)
s
N
Reduction gear
Stepping motor
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
= (P x θ
)/(360 x M)
s
Using the pulse rate, the number of pulses required to move the object a dis-
tance L is given by the following calculation:
= Travel distance ÷ Pulse rate
No. of pulses
= L ÷ Pulse rate
= L ÷ (P x θ
= (360 × M × L)/(P x θ
The pulse speed required to move the object with a feed speed of V mm/s is
given by the following calculation:
= Feed speed ÷ Pulse rate
Pulse speed
= V ÷ Pulse rate
= V ÷ (P x θ
= (360 × M × V)/(P x θ
The PCU performs positioning control by providing the number of pulses and
the pulse speed in the form of position instructions and speed instructions.
Stepping motor or
pulse-train input-type
servomotor
Pulse
train
Reduction gear
M
Object being
positioned
) x M))
s
)/(360 x M)
s
)
s
)/(360 x M)
s
)
s
Section 1-7
Turntable
Ball screw
V
Feed screw pitch
P
15