Bryan L. Programmable controllers. Theory and implementation

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EXAMPLE 10-1

Implement the Boolean variable declaration (variable names and

variable types) for the input devices shown in Figure 10-2a for use in

a control program. Assume that the controller being used follows the

rack-slot-terminal address configuration (e.g., rack 0, slot 0, terminal

3 is address 003). Figure 10-2b shows the wiring to the input module.

Figure 10-2. (a) A traditional hardwired circuit and (b) its wiring diagram.

Rack 0

Slot 0

PB1

LS1

LS2

L1 L2

PB1 SOL

LS2

LS1

PLC Input Module Wiring

Hardwired Circuit

Description of Inputs

PB1: Used to manually start conveyor sequence

LS1: Detects parts in automatic start

LS2: Detects a no-jam condition

OLUTION

Figure 10-3 shows a sample variable declaration for this example.

All of the input devices are discrete; therefore, they are specified

as Boolean variables. PB1 is named MAN_START_PB, LS1 is named

AUTO_PART_Detect, and LS2 is named NO_JAM_Detect.

(a)

(b)

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Figure 10-3. Boolean variable declaration.

10-2 IEC 1131-3 PROGRAMMING LANGUAGES

While the IEC 1131-3 programming standard provides great new potential

for programmable controller users, it is actually based on the relay ladder

logic that has been inherent in PLCs since their inception. The IEC 1131-3 is

based on the ladder logic used in PLC ladder diagrams (including functional

blocks) because of its simplicity of use, representation, and to some extent,

programmability. The IEC 1131-3, however, reduces the need for complex

interlocking circuits within PLC ladder diagram circuits. It enhances the

languages previously used in programmable controllers and incorporates

them with a powerful framework—sequential function charts—making in-

terlocking, interpretation of the control program, and implementation of the

control system much easier for both the programmer and the final user of the

system. With this in mind, let’s briefly discuss the four languages that are

used with the IEC 1131-3 standard—ladder diagrams, function block dia-

grams, instruction list, and structured text—along with sequential function

charts. Note that, when programming in the IEC 1131-3, any of these

languages may be used either alone or as a group, with or without sequential

function charts. In Section 10-4, we will list all available IEC 1131-3

programming instructions.

LADDER DIAGRAMS (LD)

Ladder diagram language (LD) uses a standardized set of ladder program-

ming symbols to implement control functions. This type of programming

language is essentially the one that has always been available in PLCs (see

Figure 10-4). Users familiar with current PLC ladder diagrams can use the

same programming techniques and methods when using this language in an

IEC 1131-3 environment. However, as we will explain later, interlocking

ladder diagram programming is much easier to implement in the IEC 1131-

3 format due to the use of sequential function charts.

Note that these variable names, which can be chosen by the user,

describe the operational functions of the input devices.

MAN_START_PB

AUTO_PART_Detect

NO_JAM_Detect

Bool

Input Variable Name Variable Type Rack Slot Terminal

Address Location

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Figure 10-4. Ladder diagram representation of a PLC program.

FUNCTION BLOCK DIAGRAM (FBD)

Function block diagram (FBD) is a graphical language that allows the user

to program elements (e.g., PLC function blocks) in such a way that they

appear to be wired together like electrical circuits. Figure 10-5 illustrates this

type of function block diagram configuration. Some IEC 1131-3 systems use

logic symbols to represent the function blocks. Note that the output logic of

the block in Figure 10-5 does not incorporate an output coil because the

output is represented by the variable assigned to the output of the block. This

LS_Stop LS_OK

TMR

PR AR

Time_Value

Set/Reset

Dwell

AND

(&)

POS_RT

AT_TOP

SETDwell

Cont_Cycle

Reset_Sys Stop_Cycle

Move_Up

(

)

Section of a control program using a timer,

set/reset, AND, and OR function blocks

Output variable of timer (Dwell) becomes

the input to the set/reset block

Figure 10-5. Function block diagram language.

L1 L2

Start

Start Stop Done Drill_Motor

LS_Reach

LS_Reach LS_Top Done

LS_Top

Stop

L1 L2

Drill_Motor

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variable can be used throughout the program in other instructions and as a

control output through the address mapping performed during variable

declaration. The user may still choose to use an output coil representation if

desired; however, it will only be allowed in the last (right-most) block. The

FBD language uses both standard and vendor-specified function blocks. The

block functions typically used with the IEC 1131 standard include, for the

most part, the block functions discussed in Chapter 9.

In addition to standard and vendor-specified functions, the IEC 1131-3

allows users to “build” their own function blocks according to control

program requirements. This is referred to as encapsulating a block function.

The advantage of creating user-defined blocks is that they can be built using

other function blocks, instruction list, or structured text programming with

or without ladder diagram instructions. This allows great flexibility in

function block programming. Encapsulation also lets the user store a newly

created block in a library and use it as many times as needed in the program,

just like any other function block. Example 10-2 illustrates how ladder

diagrams can be used to create a custom function block.

L1 L2

Start

PL1

Stop

Figure 10-6. Start/stop circuit.

EXAMPLE 10-2

Illustrate how the hardwired start/stop circuit shown in Figure 10-6 can

be implemented using ladder diagrams in a custom-built function

block to turn ON motor M1 and pilot light PL1.

SOLUTION

Figure 10-7 illustrates the ladder diagram equivalent of the

hardwired start/stop circuit. Note that there are two rungs for the two

outputs and that both the input and output variables are specified

with the same names that they had in the hardwired circuit.

To implement this simple ladder diagram as a function block, it must

be programmed or stored in an encapsulated block (see Figure 10–

8a). The final function block will look like the diagram shown in Figure

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Stop M1Start

PL1

PLC Program

Figure 10-7. Ladder diagram equivalent of the circuit in Figure 10-6.

Motor

Stop MotorStart

Motor

Pilot_Light

START

STOP

MOTOR

PILOT_LIGHT

START

STOP

MOTOR

PILOT_LIGHT

Start/Stop Block

Figure 10-8. (a) Encapsulated ladder diagram and (b) start/stop block function.

10-8b. Note that the inputs to the start/stop block will act according

to the logic used to program the block. If the driving logic to the start

input is ON, then the motor and light will turn ON. If the stop input

is ON, then both the motor and light outputs will be OFF. The two

input variables (the START and STOP commands), as well as the

two output variables (the MOTOR and PILOT_LIGHT signals), are

Boolean variables.

(a)

(b)

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The flexibility of custom block creation is enhanced by the fact that the user

can build custom blocks using ladder diagrams or any of the other IEC

1131-3 languages (IL and ST). Also, custom blocks can be used in conjunc-

tion with other standard or vendor-specified function blocks. This allows the

programmer to create very powerful function blocks that can be integrated

into any ladder diagram or function block diagram. Figure 10-9 shows a

custom block instruction that was created in a B&R Industrial Automation

PLC using their instruction list language.

Figure 10-9. Custom function block from B & R Industrial Automation.

Courtesy of B & R Industrial Automation, Roswell, GA

Instruction list (IL) is a low-level language similar to the machine or

assembly language used with microprocessors (see Figure 10-10). This type

of language is useful for small applications, as well as applications that

require speed optimization of the program or a specific routine in the

program. As mentioned earlier, IL can be used to create custom function

blocks. A typical application of IL might involve the initialization to zero

(i.e., reset) of the accumulated value registers for all the timers in a control

program. As shown in Figure 10-11, a programmer could use IL to create a

function block that would load the contents of all the timers’ accumulated

registers (AR) with a value of zero.

INSTRUCTION LIST (IL)

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Figure 10-10. Example of the machine/assembly language used in microprocessors.

Figure 10-11. Instruction list custom function block that resets timer values to zero.

AND

ANDN

(*current result:=TRUE*)

(*current result:=b1 AND b2*)

(*current result:=b1 AND b2 AND NOT b3*)

(*b0:=current result*)

Instructions Comments

Note: The current result is held in a result register.

The last instruction stores the result register as

the variable b0.

ZERO:=Ø

ZERO

TIMER_AR_1

TIMER_AR_2

TIMER_AR_3

TIMER_AR_4

Start: LD RESET_TIMER

JMPNC Prog_End

(*Load reset condition*)

(*If not TRUE, jump to end of program*)

(*Go back to start*)Prog_End: JMP Start

(*If TRUE, continue and reset all AR in timers*)

(*Timer_AR_1 is the address of the first

timer’s accumulated register*)

Reset_Timer

Address of TMR1 AR

Address of TMR2 AR

Address of TMR3 AR

Address of TMR4 AR

TMR_RESET_FBD

Reset_Timer

Timer_AR_1

Timer_AR_2

Timer_AR_3

Timer_AR_4

TMR1 AR is the address of the first timer’s accumulated register. In the

FBD, this address is known as Timer_AR_1 so that the IL program can

interpret it. The result of the IL program will be that the values in the

specified accumulated registers will be reset to 0. The variable

Reset_Timer will trigger the block and start the IL instruction. The IL

routine will cycle back to start while the block is enabled by the

Reset_Timer variable being ON. There are also ways to “pulse” just

once through the program so that the instruction is executed only one

time, if enabled.

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STRUCTURED TEXT (ST)

Structured text (ST) is a high-level language that allows structured pro-

gramming, meaning that many complex tasks can be broken down into

smaller ones. ST resembles a BASIC- or PASCAL-type computer language

(see Figure 10-12), which uses subroutines to perform different parts of the

control function and passes parameters and values between the different

sections of the program. Like LD, FBD, and IL, the structured text language

utilizes variable definitions to identify input and output field devices and any

other internally created variables that are used in the program. ST also

supports iterations, such as WHILE...DO and REPEAT...UNTIL, as well as

other conditional executions, such as IF...THEN...ELSE. Moreover, struc-

tured text language supports Boolean operations (AND, OR, etc.) and a

variety of specific data, such as time of day information.

IF Manual AND NOT Alarm THEN

Level:=Manual_Level;

Mixer:=Start AND NOT Reset

ELSE_IF Other_Mode THEN

Level:=Max_Level;

ELSE Level:=(Level_Indic × 100)/Scale;

END_IF;

Figure 10-12. Example of a BASIC-like computer program.

The structured text language is extremely useful for executing routines like

report generation, where English-like instructions explain what is being done.

Remember that ST can be used to encapsulate, or create, a function block that

will perform a certain task when triggered by the control logic (see Figure 10-

13). This function block routine can be used repeatedly throughout the

control program.

Some PLC manufacturers enhance the standard features of ST by using it to

integrate real-time force I/O and monitoring I/O (analog and digital) data in

the same manner as a standard PLC would using ladder diagrams. For

example, an ST instruction such as FORCE Variable_One would force

Variable_One to be ON regardless of any other conditions, as long as

Variable_One is Boolean. If the variable was analog, the instruction may be

FORCE Variable_One = 5000; in which case, the value of the analog variable

would be set to 5000 during the forcing.

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Finished

Conditions

to trigger report

Custom Report

Function Block

Temp_Var

Press_Var

Variable_1

Report

Variable_2

Values

passed from

variables

IF REPORT THEN

Message “Target Temperature:”:=Variable_1

Message “Actual Temperature:”:=Variable_2

END_IF

Figure 10-13. Report generation function block created using structured text.

SEQUENTIAL FUNCTION CHARTS (SFC)

Structured text programming is particularly suited to applications involving

data handling, computational sorting, and intensive mathematical applications

utilizing floating-point values. ST is also the best language for implementing

artificial intelligence (AI) computations, fuzzy logic, and decision making.

Sequential functional chart, or SFC, is a graphical “language” that provides

a diagrammatic representation of control sequences in a program. Basically,

sequential function chart is a flowchart-like framework that can organize the

subprograms or subroutines (programmed in LD, FBD, IL, and/or ST) that

form the control program. SFC is particularly useful for sequential control

operations, where a program flows from one step to another once a condition

has been satisfied (TRUE or FALSE).

The SFC programming framework contains three main elements that orga-

nize the control program:

• steps

• transitions

• actions

A step is a stage in the control process. For example, the mixing application

shown in Figure 10-14 has three steps—the initial step, the mixing step, and

the emptying step. When the control program receives an input, it will execute

each of these steps starting with step 1. Each step may or may not have an

action associated with it. An action is a set of control instructions prompting

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LS_Reach

IF Temp_1≥100

PB_Return

Action_2

Action_3

Figure 10-15. Transitions in a sequential function chart.

the PLC to execute a certain control function during that step. An action

may be programmed using any one of the four IEC 1131-3 languages. After

the PLC executes a step/action, it must receive a transition before it will

proceed to the next step. A transition can take the form of a variable input,

a result of a previous action, or a conditional IF statement (e.g., IF

Temp_1≥100). So, for the application shown in Figure 10-15, the PLC will

execute action 2 only after step 1 receives a valid input and transition 1

Trans_1

Trans_2

Trans_3

Mix_Batch (Ladder Diagram)

Empty_Batch (FBD)

STEP ( means initial step)

TRANSITION

ACTION

Figure 10-14. Sequential function chart of a mixing process.