Page 27 - Programmable Logic Controllers, Fifth Edition Mobile 2

Figure 1-15 Typical PC software used to create a ladder logic program.
Source: Image Courtesy of Rockwell Automation, Inc.

1.3 Principles of Operation operation of the motor is provided by means of a sepa-
rate pushbutton station. The process is monitored with
To get an idea of how a PLC operates, consider the sim- temperature and pressure sensor switches that close
ple process control problem illustrated in Figure 1-16. their respective contacts when conditions reach their
Here a mixer motor is to be used to automatically stir preset values.
the liquid in a vat when the temperature and pres- This control problem can be solved using the relay
sure reach preset values. In addition, direct manual method for motor control shown in the relay ladder
diagram of Figure 1-17. The motor starter coil (M)
is energized when both the pressure and temperature
switches are closed or when the manual pushbutton is
pressed.
Motor

L1 L2
Pressure 120 VAC
sensor
switch Pressure Temperature
switch switch OL
M
Temperature
sensor switch Motor
starter
coil

Manual
Manual pushbutton station pushbutton
Figure 1-16 Mixer process control problem. Figure 1-17 Process control relay ladder diagram.

8 Chapter 1 Programmable Logic Controllers (PLCs)

pet73842_ch01_001-016.indd 8 03/11/15 7:09 PM