Bryan L. Programmable controllers. Theory and implementation
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EXAMPLE 13: DELAYED DE-ENERGIZE DEVICE
This example (see Figure 11-84) illustrates the use of an OFF-delay timer to
de-energize a motor or another device after a delay period. Note that the
output of the OFF-delay timer before the Stop Motor 1 push button is pressed
is TRUE, thus keeping the TMR1 contact in line 1 closed. When the Stop
Motor 1 push button (wired as normally closed) is depressed while the motor
is running, it energizes the internal output, enabling the OFF-delay timer.
When the timer times out, the contacts open and the motor de-energizes.
Figure 11-83. Sequential motor-starting circuit.
Output M1
M1
Start
Stop *
Output M2
Output M3
Stop *
M2
Stop *
* Wired NC
Internal 1
Internal 1
Internal 1
PR: 30
TB: 0.1 sec.
PR: 30
TB: 0.1 sec.
TMR1
TMR1
TMR2
TMR2
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EXAMPLE 14: 24-HOUR CLOCK
A 24-hour clock has many applications, but it is typically used to display the
time of day or to determine the time a report is generated. Figure 11-85
shows the logic used to implement this type of clock. This logic consists of
three counters: one counts 60 seconds, another counts 60 minutes, and the
third counts 24 hours. The time is displayed by outputting the accumulated
register value of each counter to seven-segment BCD displays.
Figure 11-84. Delayed de-energize circuit.
EXAMPLE 15: ELIMINATION OF BIDIRECTIONAL
POWER FLOW
Sometimes, when converting relay logic to program logic, you will find
relay circuits that allow power to flow bidirectionally, as shown in Figure
11-86a. In this circuit, power can flow down through CR3 or up through
CR3 to make a complete path. PLCs do not allow bidirectional power flow,
so the circuit must be restructured to establish a circuit for each power flow
direction. The result, shown in Figure 11-86b, is two separate circuits that
allow unidirectional, left-to-right power flow.
Motor 1
OutputStart
Master *
Stop TMR1
TMR1
TOF
Internal 1
Stop
Motor 1*
M1
M1
* Wired NC
Internal 1
Internal 1
PR: 10
TB: 1 sec.
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Figure 11-85. 24-hour clock circuit.
Figure 11-86. (a) Hardwired circuit allowing bidirectional power flow at CR3 and
(b) reconfiguration to eliminate bidirectional power flow.
CR1 CR4
CR2 CR5
CR2 CR3
CR1 CR3
CR1 CR4
CR2 CR5
CR3
(b)
(a)
Seconds
Pulse
Enabled
Internal 1
Internal 1
CTR
Up
Reset
PR: 4000 = 60
AR: 4110
=
Internal 1
Enabled
Internal 2
Internal 2
CTR
Up
Reset
PR: 4000 = 60
AR: 4101
=
Internal 2
Enabled
Internal 3
Internal 3
CTR
Up
Reset
PR: 4001 = 24
AR: 4102
=
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Figure 11-87. Binary counting program.
EXAMPLE 16: EXCEEDING THE MAXIMUM COUNT
Some applications require the capability to count events that will exceed the
maximum allowable count number that a register can hold. The maximum
count in most controllers is either 9999 (BCD) or 32767 (binary). Counting
beyond BCD 9999 involves cascading two counters, where the output of the
first counter is used to input an up-count signal to the second counter.
This approach does not work for binary format however. If 32767 is the
maximum count, the first register would contain a value of 1 (after 32767 is
reached), and the second register would contain 00000. This result would
indicate a count of 100000 instead of the actual count of 32768. A solution
to this situation is to set the preset value of the first counter to 9999 and use
a second counter to register each time 10000 counts occur. The sequences in
Figure 11-87 illustrate this technique.
Event
Count
Enabled
Internal 1
Internal 1
Master Reset
CTR
Up
Reset
PR: 4000 = 9999
AR: 4100
=
Internal 1
Enabled
Master Reset
Internal 2
CTR
Up
Reset
PR: 4001 = 32767
AR: 4101
=
Preset Registers:
Maximum Count is 327679999
in Accumulated Registers 4100 and 4101
4000 = 9999
4001 = 32767
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control strategy
control task
flowcharting
program coding
Figure 11-88. (a) A push-to-start/push-to-stop circuit and (b) its timing diagram.
KEY
TERMS
EXAMPLE 17: PUSH-TO-START/PUSH-TO-STOP
CIRCUIT
Often, it is desirable for a single push button to perform both the start
(enable) and stop (disable) functions. In this example (see Figure 11-88a),
when the push button (PB1) is depressed for the first time, internal output 2
turns ON and remains ON. If the push button is depressed again, internal
output 2 turns OFF. The second logic rung detects the first time the push
button is pressed, while the first rung detects the second time the button is
depressed. The simplified timing diagram in Figure 11-88b shows the
operation of this circuit.
Internal 3PB1 Internal 2 Internal 1
Internal 3PB1
Internal 2
Internal 1 Internal 2
PB1 Internal 3
(a)
PB1
Internal 2
Set Reset
(b)
(a)
(b)
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PLC SYSTEM
DOCUMENTATION
CHAPTER
TWELVE
If you cannot—in the long run—tell everyone
what you have been doing, your doing has
been worthless.
—Erwin Schroedinger
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Documentation
While proper PLC programming is important for a well-run application
program, all of that work is lost without adequate system documentation.
Without documentation, system activities, such as changes, installation,
and maintenance, are difficult to accomplish. In this chapter, we will explain
how to document all aspects of a PLC—from system configuration to
register assignments. We will also explore some different documentation
methods. After that, you will be ready to learn how to implement a PLC
process control system.
Documentation is an orderly collection of recorded information about both
the operation of a machine or process and the hardware and software
components of its control system. These records are a valuable reference
during system design, installation, start-up, debugging, and maintenance.
To the system designer, documentation should be a working tool that is used
throughout the design phase. If the various documentation components are
created and kept current during system design, they will provide the following
benefits:
• They will provide an easy way to communicate accurate information
to all those involved with the system.
• They will serve as a reference to the designer during and after the
design phase.
• They will help the designer, or someone else, answer questions,
diagnose possible problems, and modify the program if require-
ments change.
• They will serve as training material both for the operators who will
interface with the system and for the maintenance personnel who will
maintain it.
• They will allow the system to be reproduced or altered to serve other
purposes.
Proper documentation comes from the compilation of hardware, as well as
software, information. The engineering or electrical group that designs the
system usually provides this information to the end user. Although docu-
mentation is often thought of as extraneous, it is actually a vital system
component and a good engineering practice. In this chapter, we will explain
the requirements of a good PLC documentation package, which will
facilitate the understanding of the control system. Table 12-1 lists the
components of a thorough documentation package.
CHAPTER
HIGHLIGHTS
12-1 INTRODUCTION TO DOCUMENTATION
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Table 12-1. Components of a good PLC documentation package.
12-2 STEPS FOR DOCUMENTATION
SYSTEM ABSTRACT
A good system design starts with a thorough understanding of the problem
and a good description of the process to be controlled. This assessment is
followed by a systematic approach that will lead to the implementation of
the control system. Once the system is finished, the personnel involved in the
design should provide a global description, or abstract, of the scheme and
procedure used to control the process.
A system abstract should provide the following:
• a clear statement of the control problem or task
• a description of the design strategy or philosophy used to implement
the solution to the problem, which defines the functions of the major
hardware and software components of the system, as well as why they
were selected
• a statement of the objectives to be achieved
As an example of an abstract, let’s examine a warehouse with a PLC that
controls some conveyors. The statement of the task would specify that the
PLC should control the conveyors so that the parts are sorted correctly (see
Figure 12-1). The design philosophy would indicate that a single CPU,
located in the warehouse’s central area, controls two product conveyor lines.
It would also indicate that remote subsystems, located in rooms 4 and 5,
control the sorters for those areas. Moreover, it would specify that the
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programmable controller will gather data about total production from both
lines and report it in printed form at the end of each shift. Finally, the statement
of the objectives (i.e., 90% sorting accuracy) would allow the user to measure
the success of the control implementation. This system abstract will convey
general design information to the end user or anyone else who needs to
understand the original control task.
Figure 12-1. System abstract.
SYSTEM CONFIGURATION
As the name implies, the system configuration is a system arrangement
diagram. In fact, it is a pictorial drawing of the hardware elements defined
in the system abstract. It shows the location, simplified connections, and
minimum details of the system’s major hardware components (i.e., CPU,
subsystems, peripherals, GUIs, etc.). Figure 12-2 illustrates a typical system
configuration diagram.
The system configuration not only indicates the physical location of sub-
systems, but also the designation of the I/O rack address assignments.
Referencing the rack address assignments allows for quick location of
ABC Warehouse Automation:
System Abstract
Task
:
• PLC must sort parts correctly. Type A parts belong in area
A; type B parts belong in area B.
Design Philosophy
:
• Control room CPU operates conveyor A and conveyor B.
Conveyor A is directed to sorter 4; conveyor B is directed to
sorter 5.
• Subsystem 00, located in room 4, controls sorter 4.
• Subsystem 01, located in room 5, controls sorter 5.
• PLC will report total production data (parts sorted correctly)
at the end of each shift. This data will be printed out on the
control room printer.
Objective:
• The PLC system should achieve a 90% average of correctly
sorted parts.