Bryan L. Programmable controllers. Theory and implementation

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An action in a sequential function chart is executed when its corresponding

step is active. When the step becomes active, the software control instructions

contained in the action will be executed and scanned until the token is

transitioned to the next step in the chart. A step action can take several forms,

depending on the desired operation and result. These types of actions are:

• Boolean actions

• pulse actions

• normal actions

• SFC actions

10-4 TYPES OF STEP ACTIONS

Y19

X20

Bool_Var_1

Bool_Var_2

Bool_Var_3

Bool_Var_4

Y20

20 (Boolean_Action)

Bool_Var_1;

/Bool_Var_2;

Bool_Var_3(S);

Bool_Var_4(R);

Figure 10-64. Example of a Boolean action.

BOOLEAN ACTIONS

A Boolean action assigns a Boolean value (i.e., TRUE/FALSE) to a variable

during the step’s action. A Boolean variable may be a real output or an

internal output. The instruction simply reflects the state (ON/OFF) of the

corresponding variable with respect to the state of its action. Let’s take, for

example, the action shown in Figure 10-64. Once step 20 is active (X20 is

ON), the variable Bool_Var_1 will be turned ON as long as the step is

active. The variable /Bool_Var_2—i.e., NOT Bool_Var_2 (/ = NOT)—is the

NOT value of the active step X20 and, accordingly, of the variable

Bool_Var_2. The variables Bool_Var_3 and Bool_Var_4, followed by (S)

and (R) respectively, indicate set and reset instructions to the variable. The set

(S) parameter becomes active when the step becomes active, setting the

variable to TRUE. The set variable stays active until it is reset in the same step

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EXAMPLE 10-6

Using SFC Boolean actions, implement a chart that will turn ON and

OFF two pilot lights according to the timing diagram shown in Figure

10-66. In the timing diagram, PLight_1 is ON for one second while

PLight_2 is OFF, then PLight_1 is OFF for one second while PLight_2

is ON. Assume that a normally open push button labeled as Start

initiates the pilot light sequence and that a normally open push button

labeled as Reset resets the whole operation, turning both pilot lights

OFF. Include a light enable (Light_EN) pilot light indicator that is ON

at the start of the operation and OFF when the operation is reset.

or in another step; however, it keeps the variable as TRUE, even when the step

is deactivated. Conversely, the reset (R) parameter resets the variable to

FALSE when the step activity is TRUE. The reset action remains FALSE

until the variable is set. Figure 10-65 shows a similar example with different

variables. Note that the Solenoid_2(R) instruction resets the variable Sole-

noid_2, which was set to ON in a previous action.

Y19

X20

Motor_1

Motor_2

Solenoid_1

Solenoid_2

Y20

20 (Boolean_Action)

Motor_1;

/Motor_2;

Solenoid_1(S);

Solenoid_2(R);

Figure 10-65. Example of a Boolean action controlling a motor and a solenoid.

Figure 10-66. Timing diagrams for two pilot lights.

OFF

1 sec

Start

Reset PLight_2

PLight_1

1 sec

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SOLUTION

Figure 10-67 illustrates the desired timing diagram of the two inputs

(Start and Reset) and the three pilot lights (PLight_1, PLight_2, and

Light_EN). Figure 10-68 depicts the SFC implementation of this

timing diagram, where the initial step sets both PLight_1 and PLight_2

to an OFF (FALSE) state. Once the Start push button is pushed, the

token passes to step 2, which has no action, and continues to the

Figure 10-67. Timing diagram for the SFC implementation in Example 10-6.

3 True

10 TMR/X10/1 sec

2 4Reset Not_Reset

(ON1_OFF2)

PLight_1;

/PLight_2;

Light_EN(S);

11 TMR/X11/1 sec

(OFF1_ON2)

/PLight_1;

PLight_2;

(Initialize)

PLight_1:=False

PLight_2:=False

(Reset)

Light_En(R);

Start1

Figure 10-68. SFC implementation of the two pilot lights in Figure 10-66.

Start

Reset

Light_EN

PLight_1

PLight_2

1 sec

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OR divergence. At the OR divergence, control goes to step 10

(ON1_OFF2) if the Reset push button is not pressed (Not_Reset),

thereby turning ON PLight_1, keeping PLight_2 OFF (opposite

state of the step activity), and turning ON Light_EN using a set

parameter. The timer transition Y10 is triggered one second after

step X10 is activated, passing control to step X11, which reverses

the state of the pilot lights using Boolean actions. Like the Y10

transition, the Y11 transition also allows one second of activation

before it turns OFF the step and passes the token to step 2, where

the sequence is repeated.

Conversely, if the Reset push button is pressed (Reset), the program

activates step 3, which resets the light enable output and transitions

the sequence to step 1, where the program will wait until the Start push

button is pressed. Note that this SFC program requires the operator to

depress the Reset push button input at transition 2 for at least two

seconds in order to reset the lights to OFF. The reason for this is that

the program may be at the opposite OR divergence (transition 4),

which will last for two seconds before the reset signal can be scanned

at transition 2.

Start

TMR/X2/1 sec

TMR/X3/1 sec

(Initialize)

If Reset Then F/Chart_1;X1

PLight_1:=False

PLight_2:=False

(ON1_OFF2)

PLight_1;

(OFF1_ON2)

PLight_2;

Chart 1

Stand–Alone Action

Figure 10-69. Implementation of the process in Example 10-6 using a stand-alone action.

The implementation of the previous example could have been done many

different ways using Boolean actions. For instance, instead of using the

/PLight_2 and /PLight_1 instructions in steps 10 and 11, the program could

have specified only the ON conditions of PLight_1 and PLight_2 in steps

10 and 11, respectively, letting the transition trigger turn OFF the variables.

A stand-alone action could also have been programmed to detect the reset

function and send the program back to step 1 in the main chart. Figure 10-69

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shows this stand-alone configuration, along with an alternative set of

Boolean actions for this program. Although a stand-alone action is not

linked to the program, it will direct a transition move to a specified step if its

logical conditions are satisfied. A stand-alone action basically acts as an

interrupt jump to instruction, specifying the chart program and the step to go

to. Note that a stand-alone action is active at all times, ready to force the

program to the specified step. If the Reset push button in Figure 10-69 is

pressed, the stand-alone action will force the program to go to step 1 of the

Chart_1 program, regardless of where it is in the execution of the Chart_1

program. Also, in this configuration, the Reset push button may be pushed

momentarily, so it does not require a two-second push like it did before.

(Pulse_Action_Ex)

Action (P):

•

Instructions

•

End_Action;

Figure 10-70. Syntax of a pulse action.

The notation (P) indicates a pulse action. A pulse action may be represented

in a timing diagram as shown in Figure 10-71, where its execution is shown

at the start of the step activity. Figure 10-72 illustrates a typical SFC with a

Figure 10-71. Execution of a pulse action.

PULSE ACTIONS

Pulse actions allow the execution of one or more instructions in a step’s

action only once after the activation of the step. That is, once the step is

activated, a pulse action will occur only once before the step is deactivated.

Depending on the IEC 1131 software system, the instructions in the action

may be in one or more of the available languages. The typical syntax of an

SFC pulse action looks like the block in Figure 10-70.

Step Activity

Pulse Action

Execution

Single execution (pulse) at the beginning of the step

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Figure 10-73. Execution of a normal action.

Figure 10-72. A count-up instruction implemented as a pulse action.

CMD

NOT_CMD

CMD

(Initialize)

Action (P):

Count:=0

End_Action;

(Counting)

Action (P):

Count:=Count+1;

End_Action;

Note: Step 3 is included as a dummy step to wait for

the CMD (command signal to count) to go from

OFF to ON to count again.

Step Activity

Normal Action

Execution

Multiple execution of the normal/nonstored action during the active step

pulse action implementing a count-up (add by one) instruction using ST

instructions. Pulse actions are well suited for applications that require one-

time execution of an action, for instance, the initialization of variables in a

process. Pulse action instructions are similar in operation to the one-shot

functions discussed in Chapter 9.

NORMAL ACTIONS

Normal actions, also called nonstored actions, incorporate IEC 1131-3

language instructions that are executed continuously during the activity of a

step. In other words, the instructions within a normal action will be executed

and scanned over and over until the step is deactivated (see Figure 10-73). The

basic syntax for a normal instruction is shown in Figure 10-74, where (N)

indicates normal. Normal actions may also omit the (N) parameter in the

instruction syntax.

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(Normal_Action_Ex)

Action (N):

•

Instructions

•

End_Action;

Figure 10-74. Syntax for a normal action.

Start_Counting

Stop_Counting

(Initialize)

Action (P):

R_Count:=0;

End_Action;

(Count_Step)

Action (N):

If Cmd_Cnt AND NOT (Last_Cmd) Then

R_Count:=R_Count+1;

End_If;

Last_Cmd:=Cmd_Cnt;

End_Action;

Figure 10-75. Example of a counting program using a normal action.

Figure 10-75 shows an example of a counting program using a normal action

in step 2. Note that step 1 uses a pulse action to set the value of the variable

R_Count to zero. As the next example illustrates, the normal action in step

2 (programmed using ST language) performs a counting procedure on the

rising edge of the signal Cmd (command) and stores the total count value as

variable R_Count. This counting procedure is executed for as long as step 2

is active.

EXAMPLE 10-7

Referring to Figure 10-75, explain the operation of step 2. Also, draw

a timing diagram of the signals indicating when the counter variable

R_Count begins and ends during each count.

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Start_ Counting

Stop_Counting

Cmd_Cnt

Last_Cmd

R_Count

Beginning of 1st Count

End of 1st Count

R_Count = 1

Beginning of 2nd Count

End of 2nd Count

R_Count = 2

Figure 10-76. Timing diagram of step 2 in Figure 10-75.

SOLUTION

Figure 10-76 illustrates the timing diagram of step 2. The variable

R_Count increases its value by one every time the signal Cmd_Cnt

goes from OFF to ON. The IF condition in step 2 of Figure 10-75

ensures that the signal is tested to make sure that it has gone OFF

after the OFF-to-ON transition. The Last_Cmd:=Cmd_Cnt instruction

traps the last value of Cmd_Cnt, so that the count does not get

executed again without Cmd_Cnt going OFF first. When the action is

deactivated by the Stop_Counting transition variable, the status of

Cmd_Cnt and Last_Cmd are reset to OFF (not stored). Note that the

R_Count value is reset to zero at step 1. However, the value of R_Count

will be stored as a normal integer value in the program until it is

changed, as in this example, in step 1.

SFC ACTIONS

An SFC action is a child-type SFC sequence program that can be activated

(started) or deactivated (killed) when the step is active. Remember that a

child program belongs to a father, or main, program. SFC actions may

have normal, set, or reset parameters that influence the operation of the

SFC action (see Table 10-3). Figure 10-77 illustrates a batching process SFC

that uses SFC actions. The main SFC program has two child programs,

Batch_Mix and Batch_Pump, which are activated by the main (father)

program. The main SFC program uses normal, set, and reset operands.

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Figure 10-77. Batching process implemented using SFC actions.

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Table 10-3. Syntax for SFC action parameters.

Once Start is triggered, the SFC activates both of the child programs. The

Batch_Mix program has a normal (nonstored) parameter, while the

Batch_Pump program has set and reset parameters. The Batch_Pump pro-

gram becomes active as soon as step 20 is activated. It remains active until

the signal Level_Full is turned ON, activating step 30 and resetting, or

killing, the Batch_Pump program.

1 Start

Level_Full

2 Continue

Batch_Mix (N); Batch_Pump (S);

Batch_Pump (R);

Main Chart

Child Charts

Batch_Mix

Batch_Pump

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SFC actions may be started or killed using any of the programming

languages, depending on the IEC 1131-3 software system manufacturer. The

syntax differs slightly from one system to another and may take the form

shown in Table 10-4. The start and kill instructions have the same effects as

the set (S) and reset (R) parameters, respectively. Figure 10-78 illustrates an

SFC action using structured text. The starting and killing of the child

program can be either nonstored or pulse actions, but in this example, the

Table 10-4. Alternative syntax for SFC action parameters.

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1 Start

2 10

2 Level_Full

3 Continue

Main Chart

Child Chart

Batch_Pump

(First_Action_Status)

Action (N):

If Status (Batch_Pump)=0

Then

Message:=“Batch Stopped”;

Else

Message:=“Batch Running”;

End_If;

End_Action;

(First_Action_Start)

Action (P):

Start (Batch_Pump);

End_Action;

(First_Action_Kill)

Action (P):

Kill (Batch_Pump);

End_Action;

Figure 10-78. An SFC action programmed using ST and alternative SFC action syntax.