Siemens Oven P01-07 User Manual

Industry Automation and Drive Technologies - SCE

SEQUENTIAL FUNCTION CHART

OBJECTIVE

The students will be able to successfully implement sequential controls using step
sequences. The students understand the structure and effect of step sequences, and are
introduced to corresponding design methods. Knowledge about operating modes and
protective measures is expanded for sequential controls. The students understand the
interaction between the programs for basic automation and the sequential controls. They
know how to generate sequential controls in PCS7.

THEORY IN SHORT

Sequential controls allow for processing sequential and parallel operations in a mode that
is discrete with respect to time or events. They are used to coordinate different continuous
functions as well as controlling complex process sequences. Depending on defined states
or events, operating and mode changes are generated in the existing logic control systems
and as a result, the desired sequential performance is implemented. They are implemented
through one or several step sequences (in English: sequential function charts).

A step sequence is the alternating sequence of steps that trigger certain actions
respectively, and transitions that cause a step to change into another one as soon as the
corresponding step enabling condition is met. Each step sequence has exactly one start
step and one end step and in addition any number of intermediate steps that are
connected respectively through oriented edges by means of interposed transitions. The
diagrams may also generate feedback through loops within the step sequence. They also
can include parallel or alternative branches. However, in this case it has to be ensured
during the design that the sequence does not contain segments that are unsafe or
unavailable.

To design sequential controls, particularly the formal design methods of state diagrams or
Petri’s networks are available. State diagrams are easily learned, make automatic error
diagnosis possible and can be converted without a problem into many existing
programming languages for sequence controls. However, designing parallel structures is
not possible, since state diagrams have only exactly one state.

Petri’s networks are considerably more complex and more demanding mathematically. But
all structures that are permitted in sequential controls can be modeled and extensively
analyzed. Thus, required control properties can be proven formally. Likewise, Petri’s
networks allow for no-problem implementation in sequential controls.

Sequential controls parameterize and activate lower level logical control systems by setting
corresponding global control signals. These control signals can have a brief or a lasting, a
direct or a delayed effect. Sequential controls as well as logical control systems have to
support different operating modes. Particularly manual control of the transitions and
temporary or permanent interruptions of the process sequences has to be possible In
addition, process specific protective functions are implemented with sequence controls.

In PCS7, sequence controls are implemented with Sequential Function Charts
(SFC). SFCs provide for efficient operating mode management, high controllability through
several switching modes as well as extensive parameterizability through different sequence
options. The SFCs and CFCs interact and are linked in PCS7 by means of process
variables and control variables. The interactive behavior can also be controlled in detail.

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THEORY

CONTINUOUS AND SEQUENTIAL CONTROLS

Within the scope of basic automation, different logic control systems are developed that
implement a limited, clearly defined function. The functions continuously process input
signals and generate corresponding output signals. By means of different control signals,
the functions can also be activated and parameterized. To implement complex process
sequences -for example, manufacturing recipes for products- it is necessary to coordinate
the different functions and to activate them at the right time with the correct parameters.
This task can be handled using sequential controls.

Sequential controls make step by step, event-discrete processing of sequential and parallel
operations possible using step sequences. Depending on defined states or events, they
generate operating and mode changes in the existing logic control systems and thus
implement the desired sequential behavior. Step sequences are also referred to as

sequential function charts.

STRUCTURE OF STEP SEQUENCES

A step sequence is the alternating sequence of steps and transitions. The individual
steps activate certain actions. The transitions control the change from one step to the next.

The first step of a step sequence is referred to as the start step. It is the unique entry
point in the sequence and is always executed. The last step in a step sequence is
correspondingly referred to as the end step. It is the only step in the sequence that does
not have a sequence transition. After the end step is processed, the step sequence is
terminated, or processing starts anew. The latter case is also referred to as sequence loop.

Steps and transitions are connected to each other with oriented diagrams. It is possible to
connect a step with several sequential transitions; the reverse is possible also. A transition
is enabled if all series connected steps are active and the step enabling condition is met.
In this case, first the immediately preceding steps are deactivated and then the immediate
subsequent steps are activated.

The simplest form of a step sequence is the unbranched sequence. Each step is followed
by exactly one transition, and the transition in turn by exactly one subsequent step. This
implements a purely sequential process run. Figure 1 shows the corresponding graphic
basic elements.

S 1

t 1

S 2

Step 1
(Start s tep)

Transition 1

Step 2

Figure 1: Basic elements of sequential function charts

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Loops within the step sequence occur when by sequencing several steps a cyclical run
within a sequence is possible. The sequence loop represents a special case of a loop
where all steps are run cyclically.

Another option for structuring step sequences is jumps. When a jump mark is reached,
processing continues with the step to which the jump mark points. Jumps within the step
sequence can also result in loops. Since such a structure is difficult to follow, jumps should
be dispensed with if possible.

In many cases it is necessary from the process view to respond differently to different
events when the program is executed. This structure is referred to as alternative
branching. The step is linked with each possible subsequent step by means of its own
transition. To ensure that at any time at most one of these transitions is enabled (and the
branches are actually alternative), the transitions should be mutually locked or clearly
prioritized. Otherwise, in most control systems the transitions are evaluated from left to
right, and the first transitions whose step enabling condition is met is enabled.

Figure 2 shows, in principle, the structure of alternative branching with two branches. It is
represented by bordering horizontal single lines with protruding ends. As can be seen, the
alternative branches always start and end with transitions.

S 1

t 1

S 2 S 3

t 3

S 4

t 2

t 4

S 1

t 1

S 2 S 3

t 2

S 4

Alternative branch Parallel branch

Figure 2: Alternative and parallel branches

It is often required that after a step, several subsequent steps are to be processed
simultaneously. In this case, the initial step has one transition exactly that activates several
subsequent steps at the same time. We call this structure parallel branching. The
subsequent steps of the individual branches are processed independent of each other and
are then merged again. All branches end in a joint transition. Only after all branches are
processed completely and the step enabling condition for the subsequent transition is met
is it possible to activate the joint subsequent step.

Figure 2 also shows the sequence of a parallel branch with two branches. They are
represented with bordering horizontal double lines and protruding ends. As can be seen,
the parallel branches always start and end with actions.

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A particular control engineering problem is the possibility to generate -by unfavorably using
jumps and branches- faulty step sequences. We are distinguishing three possible cases.

– Uncertain sequence: A step sequence contains a structure whose availability is not

ensured through the defined sequential performance.

– Partially stuck: A step sequence contains an internal loop that is not exited.

Although the steps within this loop are executed, the steps outside the loop are not.
This makes parts of step sequence unavailable.

– Totally stuck: A step sequence contains a structure for which no permissible step

enabling condition exists. In this case, the step sequence remains permanently in one
state and all other steps are unavailable.

Such structures are not permitted in step sequences and have to be excluded with
corresponding design methods. Figure 3 shows examples of two step sequences with
impermissible structures.

In the left sequence we can’t ensure that Step S6 is available since the alternative branch
after Step S3 prevents -when transition t3 is enabled- that the parallel branch is merged
again in transition t4. For that reason, this sequence is uncertain. The right sequence, on
the other hand, is executed exactly once and then stops at Step S4. Since Step S2 is not
active in this state, the parallel branch can no longer be merged in transition t3. It is totally
stuck; Step S5 is unavailable.

S 1

t 1

S 2 S 3

S 6

t 6

t 2

S 4

t 3

t 5t 4

t 7

S 5

S 7

S 1

t 1

S 2 S 3

t 2

S 4

t 3

S 5

t 4

Uncertain structure

Figure 3: Uncertain and illegal structures

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Illegal structure

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DESIGN OF SEQUENCE CONTROLS

There are numerous formal design methods for sequence controls. In practice, however,
particularly the state diagrams and Petri’s network have proven themselves.

A state diagram is a connected, oriented diagram. States are represented as circles and
the state transitions as arrows that connect exactly two states. In a state diagram, always
exactly one state is active at a time. The states can be linked to certain actions. A certain
sequence performance can be assigned to these actions. They can be performed once
when entering the state or when leaving it, or cyclically as long as the state is active. State
transitions can be subject to transition conditions.

State diagrams can be arranged hierarchically, and linked to each other. State diagrams
are considered easy to learn and make automatic error diagnosis possible -for example,
through pair, time or state monitoring. They can be converted into many existing
programming languages for sequence controls, without a problem.

Petri’s networks are particularly suitable for modeling concurrent processes. Petri’s
network consists of locations and transitions that are linked to each other with oriented
edges. This generates an oriented diagram also. A location is represented as a circle, a
transition as a rectangle (often also reduced to a cross bar). Active locations are identified
with labels which are represented by a dot within the circle for the corresponding location.

In contrast to function diagrams, in Petri’s network the state is determined by the number of
active locations in the entire network. The dynamics of the system is modeled by the
movement of the labels within the network. The significance of the locations and transitions
for the modeled process (i.e., the semantics of Petri’s network) is not defined and has to
be specified depending on the application case. Petri’s networks whose semantics is
specified are referred to as interpreted Petri’s networks (IPN). For the control design,
control engineering interpreted Petri’s networks (CIPN) are used as a rule.

Petri’s networks can be extensively examined analytically. They also permit the conversion
into existing programming languages for sequence controls without a problem. There are
numerous expansions for Petri’s networks that are optimized for certain application cases
respectively, or permit a more detailed modeling of the process. For that reason, Petri’s
networks can become quite complicated which makes them rather demanding as design
models. Because of their structural similarity to step sequences and the option of modeling
parallel sequences, Petri’s networks offer clear advantages, however.

Which design method is used depends ultimately on the requirements of the design task as
well as on the developer’s preference. For additional information, we refer to the pertinent
technical literature.

INTERACTION OF SEQUENCE CONTROLS AND LOGIC CONTROL SYSTEMS

As described above, certain actions can be assigned to each step in step sequence.
Generally, these actions consist of the parameter assignment and the activation of logic
control systems. To this end, corresponding control signals are set.

Process and control signals used by step sequences have to be declared globally so that
they are
logic controls. Usually, the signals are contained in a symbol table.

Control signals always are effective as long as the corresponding step is active. To
implement more complex function sequences, it is possible to vary the processing of a
control signal itself (latching or non-latching, delayed or limited).

Usually, process specific functions are implemented with sequence controls, while logic
controls implement all device specific functions.

<<available equally? something missing in original>> to the programs of the sequence and

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PROTECTION FUNCTIONS AND OPERATING MODES IN SEQUENCE CONTROLS

Just as for the individual drive functions, adequate protection functions and operating
modes have to be implemented for sequence controls. It has to be possible to operate
sequence controls manually if there is a fault. To this end, corresponding operating modes
have to be provided for in the control.

– Automatic mode: The action of the step sequence is executed if the preceding

transition is enabled.

– Manual mode: The operator triggers the action of step sequence, even if the

preceding transition is not enabled.

– Mixed mode: The action of the step sequence is executed if the preceding transition

is enabled, or the operator triggered it. As an alternative, operator activation as well as
enabling the preceding transition may be required.

The manual mode prevents that the sequence control may be permanently blocked
because of a fault. The mixed mode allows for the manual interruption of the sequence for
testing or commissioning. The step enabling conditions of all transitions of the sequence
control have to be expanded accordingly.

Step sequences have to be able to react to faults in the controlled devices. To this end,
continuous fault monitoring is required. It recognizes and signals faults in the controlled
devices. It makes automated safety of the plant possible by stopping the step sequence
automatically if there is a fault. In addition, it has to be possible for the operator to stop and
cancel the step sequence if there is a fault.

In both cases corresponding protection functions have to be activated to take the plant to a
safe state. If a sequence is stopped, it has to be ensured that it can be continued safely
and in a way that is permissible regarding process engineering, even if the interruption was
of a longer duration. In the sequence controls, process specific protection functions are
implemented, such as sequential locking of several devices if there is a fault in the process.

SEQUENCE CONTROLS IN PCS7

In PCS7, sequence controls are implemented with Sequential Function Charts
(SFC). They contain the step sequences and define their sequence topology, the

conditions for the transitions and the actions of the steps. It is possible to define and
prioritize the start conditions and the sequence characteristics separately for each step
sequence. In addition, the preprocessing and post-processing steps can be defined that
are executed once before or after processing the step sequence.

Operating Modes and Switching Modes

The performance of a sequence control in PCS7 depends on the following: the selected
operating mode, the specified switching mode, its current operating mode, and the
sequence options. Two different operating modes can be selected for sequence controls:

– Auto: The program controls the sequence.

– Manual: The operator controls the sequence through commands, or by changing the

sequence options.

In the manual mode, the following commands are available to the operator: Start, Stop, Halt, Exit,
Cancel, Continue, Restart Reset and Error, to operate the sequence control manually. Depending on
the selected operating mode, the behavior of a step sequence can be controlled through different
switching modes when further switching active steps to the subsequent steps.

– Switching Mode T: The sequence control is running process controlled; i.e.,

automatically. If a transition is enabled, the preceding steps are deactivated and the
subsequent steps are activated. (T = transactions)

– Switching Mode O: The sequence control is running operator controlled; i.e.,

manually. The transition is enabled by an operator command. To this end, each

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subsequent transition of an active step automatically sets an operator prompt (O =
Operator).

– Switching Mode T or O: The sequence control is running process controlled or

operator controlled. The transition is enabled either through an operator command or a
step enabling condition that was met.

– Switching Mode T and O: The sequence control is running process controlled and

operator controlled. The transition is enabled only based on an operator command and
if the step enabling condition is met.

– Switching Mode T/T and O: In this switching mode we can specify for each step

individually whether the sequence is controlled by the process or the operator. In the
test mode, this allows for defining stop points in the sequence control (T/T = Test
Transactions)

In the operating mode Auto, only the switching modes T and T/T and O can be
selected. The operating mode of the sequence control indicates the current state in the
sequence and the resulting performance. A corresponding operating mode logic defines
the possible modes, the permissible transitions between the modes as well as the
transitional conditions for a mode change. PCS7 defines a separate operating mode logic
for sequence controls and for step sequences respectively. It is possible to run step
sequences depending on the mode of the sequence control.

Sequence Options

By using sequence options, it is possible to control the execution time performance of
sequence controls. For example, we can specify whether a sequence control is processed
once or cyclically (option cyclical mode) or whether the actions of the active step are
actually performed (option command output). In addition, time monitoring for the
individual steps of a step sequence can be activated which signals a step error if there is a
timeout (option time monitoring).

Interaction Performance

In the PCS7, CFCs and SFCs interact by means of process values and control values.
These values are linked by means of the desired signals either from the global symbol
table or by entering the absolute signal address. Controlling the processing of the control
signals is possible by means of the SFC characteristics. In the SFC Library, the PCS7
makes available preassembled step sequences for different standard scenarios. These
templates can be used and adapted to current projects.

LITERATURE

[1] Seitz, M. (2008): Speicherprogrammierbare Steuerungen. Hanser Fachbuchverlag
(Programmable Logic Controllers. Hanser Technical Publications)

[2] Wellenreuther, G. and Zastrow, D. (2002): Automatisieren mit SPS: Theorie und

[3] Uhlig, R. (2005): SPS - Modellbasierter Steuerungsentwurf für die Praxis:

Praxis. Vieweg+Teubner (Automating with PLC. Theory and Practice. Vieweg+Teubner Publishers)

Modellierungsmethoden aus der Informatik in der Automatisierungstechnik.
Oldenbourg Industrieverlag (Model Based Control Design in Practice: Modeling Methods from
Computer Science in Automation Engineering. Oldenbourg Industrial Publishers)

[4] Siemens (2009): Process Control System PCS 7: SFC for SIMATIC S7.

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STEP BY STEP INSTRUCTIONS

TASK

Based on the recipe in the chapter 'Process Description’ we are setting up and
programming an SFC step sequence.

For this chapter, we reduced the recipe to the following sequence:

1. First, 350ml are to be drained from educt tank =SCE.A1.T1-B003 to reactor
=SCE.A1.T2-R001.

2. When reactor =SCE.A1.T2-R001 is filled, the liquid it contains is to be heated to 25°C
and the stirrer is to be switched on.

3. When the temperature of the liquid in reactor =SCE.A1.T2-R001 has reached 25°C,
this liquid in this reactor =SCE.A1.T2-R001 is to be stirred another 10 seconds at
25°C.

4. Now, the liquid in reactor =SCE.A1.T2-R001 is to be heated to 28°C with the stirrer
being switched on.

5. When the temperature of the liquid in reactor =SCE.A1.T2-R001 has reached 28°C,
this liquid is then to be drained into product tank =SCE.A1.T3-B001.

OBJECTIVE

In this chapter, the student learns the following:

– Setting up and editing SFC step sequences

– Establishing logic operations between SFC step sequences and CFCs

– Establishing logic operations between SFC step sequences and the operands in the

– Testing sequence step programs

symbol table

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PROGRAMMING

1. To start, we are setting up a new SFC in the plant view in the folder ’A1_multipurpose_
plant’.

( A1_multipurpose_pant  Insert New Object  SFC)

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2. Next, we are selecting the SFC properties.

( SFC(1)  Object Properties)

3. Under General, we change the name to ’SFC_Produkt01’.

( General  SFC_Produkt01)

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4. We are keeping the operating parameters; they can be changed later in the online
mode ( AS Operating parameters)

5. Regarding the tab OS it is important that the checkmark is set so that the SFC will be
available later in visualization.

( OS  Transfer chart to the OS for visualization)

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6. Under the tab Version, we accept all parameters with OK.

( Version  OK)

7. Now, with a double click, we open the step sequence ’SFC_Produkt01’ in the
SIMATIC Manager. ( SFC_product01)

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8. In the SFC editor, it is now possible to set up the sequence control with the following
symbols from the tool bar.

Button Switch on Select

Button Insert Step and Transition

Button Insert parallel branch

Button Insert alternative branch

Button Insert loop

Button Insert jump

Button Insert text field

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