Page 140 - Programmable Logic Controllers, Fifth Edition

1 - Filling operation 2 - Capping operation

Figure 6-59 Sequential control process.
Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

6.11 Converting Relay Schematics PLC
into PLC Ladder Programs

The best approach to developing a PLC program from a
relay schematic is to understand first the operation of each
relay ladder rung. As each relay ladder rung is understood, Furnace
an equivalent PLC rung can be generated. This process Feedback
will require access to the relay schematic, documentation
of the various input and output devices used, and possibly Controller
a process flow diagram of the operation.
Most control processes require the completion of several
operations to produce the required output. Manufacturing,
machining, assembling, packaging, finishing, or transport-
ing of products requires the precise coordination of tasks. Figure 6-61 Automatic control process.
A sequential control process is required for processes
that demand that certain operations be performed in a spe- Automatic control involves maintaining a desired set
cific order. Figure 6-59 illustrates part of a bottle filling point at an output. One example is maintaining a certain
process. In the filling and capping operations, the tasks set-point temperature in a furnace as illustrated in Fig-
are (1) fill bottle and (2) press on cap. These tasks must ure 6-61. If there is deviation from that set point, an error
be performed in the proper order. Obviously we could not is determined by comparing the output against the set point
fill the bottle after the cap is pressed on. This process, and using this error to make a correction. This requires
therefore, requires sequential control. feedback from the output to the control for the input.
Combination controls require that certain operations The converting of a simple sequential process can be
be performed without regard to the order in which they examined with reference to the process flow diagram illus-
are performed. Figure 6-60 illustrates another part of the trated in Figure 6-62. The sequential task is as follows:
same bottle filling process. Here, the tasks are (1) place 1. Start button is pressed.
label 1 on bottle and (2) place label 2 on bottle. The order 2. Table motor is started.
in which the tasks are performed does not really matter.
In fact, however, many industrial processes that are not in- Pilot lights
herently sequential in nature are performed in a sequential
manner for the most efficient order of operations. R

Label solenoid 1 G
Package Motor
Label
solenoid 2
Start Limit
Stop switch
Figure 6-60 Combination control process. Figure 6-62 Sequential process flow diagram.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 121

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