flow control

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A sequence control (English: sequential control ) or step sequence is a control which gradually runs out. This sequence takes place inevitably, the switching from step A to step B being carried out by switching conditions (transitions), e.g. B. a cylinder extends, transports a workpiece, this is then clamped.

Sequential controls can be designed in accordance with DIN EN 60848 with GRAFCET , and implementation in accordance with IEC 61131-3 with Sequential Function Chart (SFC). If a sequence control is implemented by a PLC , then the sequence language can be used for programming .

Examples of simple controls and sequence controls

A simple control is, for example, the leakage of water after the tap has been opened. This state does not change until another simple control (closing the tap) has been carried out.

A one-step process control exists when the closing of the water tap due to a filling message (feedback) is part of the control program.

As a rule, sequential controls are several successive steps that are carried out by a control program. In a washing machine, the water supply and water pumping out, detergent addition, heating and drum drive are started and stopped by a controller by processing information about the water level, temperature and time according to a selected program in such a way that cleaned and spun or dried laundry is produced. Sensors report back to the control that the selected values ​​for water level, temperature and time have been reached, after which the next process step is triggered.

Definition of the sequence control

Definition of the sequence controls according to DIN IEC 60050-351

The following definition exists for the term sequence control in the standard DIN IEC 60050-351:

A sequence control is a control with a step-by-step sequence in which the transition from one step to the next takes place in accordance with the specified transition condition.

From this definition, however, the actual operational processes of sequence control do not emerge. It is not possible to see what the control variables of this type of control are and how they are influenced. It is also not possible to understand from this how the “specified” transition conditions come about.

Novel approach to defining sequence controls

Zander introduces the term “event-discrete process” for the processes taking place in sequence controls as a more precise definition of the previously used term “discontinuous process”, whereby he assumes that the control variables are predominantly analog variables, e.g. B. Pressures, temperatures, levels, distances, angles, speeds.

An essential feature of this new approach is that during the course of a discrete event process, the binary control signals output by the control device act as step functions on the analog control variables and that their functional values ​​change as a result of step responses according to the respective time behavior. So z. B. the change in the level when filling a container on an integral behavior (I behavior).

Corresponding threshold values must be specified for the control variables . If a control variable reaches a threshold value provided for it, the binary control signal that caused the change in the control variable is set to the value zero by the control device. According to the control algorithm implemented in the control device, the next actuating signal is then output and the next step in the process is triggered.

Reaching the threshold value of the control variable is referred to as an "event". This explains the name “Discrete Event Process”.

An event is also present when an operator action is carried out or a predetermined period of time has expired in a timer. When an event occurs, a change of operation is initiated in the process by definition. For this purpose, the events are reported to the control device by so-called event signals. Event signals are binary measurement signals, binary operating signals and binary output signals from timing elements.

On this basis, sequence controls, i.e. control of discrete event processes, are defined according to Zander as follows:

A sequence control is a process in which an event signal arriving in the control device generates a binary control signal in accordance with the implemented control algorithm and thereby a step function is exercised on an analog control variable, so that this control variable carries out a step response and thus an operation runs until again an event signal related to it arrives, which ends the current step response and activates further step functions of the control device, etc.
Sequence controls are characterized by a closed operating sequence through feedback and predominantly analog control variables .

In comparison with regulations , the terms Zander chose for the sequence controls , which consist of several steps, refer to a closed process and feedback to other, simpler factual relationships. The feedback (feedback, closed operational sequence ) only sets the control variable to zero and that for the following sub-process to one. There is no negative feedback between an analog output variable (actual value of the controlled variable ) and an analog input variable ( setpoint ).

In contrast to sequence controls, logic controls do not change the values ​​of analog control variables, but rather discrete-value (e.g. binary) control variables as outputs of the control path. For this purpose, binary control signals are generated in the control device by logically combining the binary input signals, which cause the control variables to be switched. There is no feedback on a switching operation carried out from the outputs of the control path to the inputs of the control device with logic controls. The structure of the information flow is therefore an open chain .

Examples of logic controls are toggle and cross circuits for switching lights or units on and off using switches at different locations.

Descriptive means and technical languages ​​for sequence controls

Petri nets interpreted in terms of control technology, as worked out by König and Quäck, can be used as descriptive means for sequence controls . In France, the description language GRAFCET was created on the basis of Petri nets , which today is standardized as DIN EN 60848.

The essential display elements of Petri nets interpreted in terms of control and GRAFCET are steps (also called places or places) and transitions . The steps can be regarded as "states" are actions (eg., Valve open), and the transitions are transition conditions associated through which the transition will be described from one step to the next step.

On the basis of Petri nets and GRAFCET, sequence diagrams were developed for programming programmable logic controllers (PLC), the prototype of which is standardized in DIN EN 61131-3 as SFC (Sequential Function Chart). With the STEP7 control system from Siemens , the sequence chart is called "S7-GRAPH".

Example of a sequencer

The initialization step is an excellent wait state. This is exited when the start transition for the sequencer occurs and the automatic sequence begins. In the example, the processing of step 1 is started after switching the start transition.

In addition to the linear sequence of steps, alternative branches, parallel branches and loops can be modeled.

Alternative branch

Step 1 is active until transition 1a or 1b switches. If 1a is true, then step 1 is ended and step 2a is processed next; Transition 1b leads to the processing of step 2b. If both are true, a precedence rule comes into play, e.g. B. that step 2a is processed.

Parallel branch

If transition 3 switches, step 3 is ended and steps 4a and 4b are carried out simultaneously. If transition 4 occurs, both are terminated at the same time and the execution of step 5 begins.


Step 6 is processed until 6a or 6b occurs. If transition 6a is true, the execution of the sequencer is completely ended in this example. If transition 6b occurs (and 6a is not true), step 5 is processed again.

See also


  • Ch. Duhr: Grafcet. Work book. EUROPA-Lehrmittel Bildungsverlag, 2015, ISBN 978-3-8085-3763-3 .
  • R. König, L. Quäck: Petri networks in control technology. Verlag Technik, Berlin 1988, ISBN 3-341-00525-0 . (also: Petri networks in control and digital technology. Oldenbourg Verlag, Munich 1988)
  • H.-J. Zander : Control of discrete event processes. Novel methods for describing processes and designing control algorithms. Springer Vieweg Verlag, Wiesbaden 2015, ISBN 978-3-658-01381-3 .

Web links

Individual evidence

  1. H.-J. Zander: Control of discrete event processes. Novel methods for describing processes and designing control algorithms. Springer Vieweg Verlag, Wiesbaden 2015, ISBN 978-3-658-01381-3 , pp. 38–43 and pp. 185–192.
  2. R. König, L. Quäck: Petri networks in control technology. Verlag Technik, Berlin 1988, ISBN 3-341-00525-0 .