Sequential Function Chart

Transcription

1 Production Systems Control Automation Engineering Sequential Function Chart Prof. Luca Ferrarini Eng. Giancarlo Mantovani Politecnico di Milano Dipartimento di Elettronica e Informazione Index Introduzione Elementi Base Operatori Base Regole di Evoluzione LD ed SFC Basic elements Basic operators Evolution rules 2

2 History: SFC is a programming language defined in IEC61131 standard SFC was born from Grafcet, which is a representation language defined by AFCET (Association française pour la cybernétique économique et technique) SFC is its implementation as a Programming language for PLCs 3 History In 1975 in France, si a commitee was formed to define a new formalism Result: GRAFCET, from GRaph + AFCET (Association française de cybernétique économique et technique) In 1988 standard IEC 848 (International Electrotechnical Comission) is defined: «Preparation of function charts for control systems», with name «Sequential Function Chart», based on grafcet In 1993 SFC is included in IEC standard on PLC programming 4

3 SFC was born for the following reasons: Use formal models to represent control logics function for industrial automation Standardize representation formalisms Enrich automata with parallelism Simplify Petri nets in order to adapt them to industrial applications Be easily interpretable by control devices 5 Basic elements Few and simple ideas Step: sequence execution state Transition: passage between two steps during program execution Action: every step is associated with one or more actions to be performed Logic conditions: enables transition trigger, so execution evolution (true = transition triggered, false = transition stopped) Oriented arc: identifies passages between steps through a transition which is enabled by a logic condition 6

4 Basic elements - Step Step: ID (usually an integer sequential number) Can be active or inactive Graphical representation: square (with double border if starting step) Has associated actions to be performed when the step is active serie di azioni. To every action a qualifier is associated, specifying execution time or duration (N, S, P, etc.) Identifier 2 Active step is marked X Turn Motor on Qualifier: P Activation N Deactivation S Set R Reset Step Action 7 Basic elements - qualifiers Introduzione Elementi Basic Base elements Operatori Basic operators Base Regole Evolution di Evoluzione rules LD LD ed and SFC SFC Qualifiers: N Non-Store Action terminates when the step becomes inactive. S Set (Stored) Actions continues even if the step becomes inactive and is terminated when the action is reset. R Reset Terminates an action activated with qualifiers S, SD, SL or DS. L D P SD DS SL Time Limited Time Delayed Pulse Stored and time Delayed Delayed and Stored Stored and time Limited Action is started when step becomes active and continues until the step becomes inactive or a certain time interval has passed. A timer is set when the step becomes active; if the step is still active and time is elapsed, the action is started and terminates when step goes inactive. The action is performed when the step goes active and is executed only once. The action is started with a time delay even if the step has gone inactive in the meanwhile and is performed until it is reset. A timer begins counting when step becomes active. If the step is still active after timer completion, the action is started and carries on until is reset. The action starts when the step becomes acrive and is carried on until it is not reset or a certain time is elapsed. 8

5 Basic elements Transitions Transitions Identifier Enabled or disabled Can be triggered Associated to a logic condition (or receptivity ) Graphical representation: horizontal segment C1 AND C2 Transizione Logic condition (receptivity) 9 Basic elements Oriented arc Connection rules: A step can be directly connected only with transitions, not with steps; similarly, a transition can be directly connected with steps, not with other transitions. Graphical conventions: Arc with right angles Arc directed downwards: without arrow Arc directed upwards: with arrow Start Activate Pump P1 Tank target level reached Switch-off Pump P1 Pump is off 10

6 Basic operators In SFC it is possible to represent two basic features for discrete events systems: Choice (OR operators) Parallelism (AND operators) Graphical operators characterized by a start (divergence) and an end (convergence) correspond to these two features 11 Basic operators Or divergence Choice start Two transitions after a single step 3 C1 NOT(C1) 4 5 NOTE: mutual exclusion is necessary in order to have a deterministic behavior of the program 12

7 Basic operators Or convergence Choice end A step with two transitions before 9 10 C1 C Basic operators AND divergence Parallelism start Simultaneous activation of more steps

8 Basic operators AND convergence Parallelism end Simultaneous deactivation of two or more steps (syncronization) Evolution rules A transition is enabled (by the associated logic condition) if all the steps before it are active A transition can be triggered if is enabled and its receptivity has logical value true (the logic condition is verified) Enabled 22 16

9 Evolution rules Rule 1 if a transition can be triggered, it is actually triggered: all steps before the transition are deactivated, while all steps after it are activated Rule 2 all triggerable transitions in a certain time instant are triggered at the same time 17 Evolution rules AND divergence Transition triggering 15 Transition enabled

10 Evolution rules AND divergence Introduzione Elementi Basic Base elements Operatori Basic operators Base Evolution rules LD LD ed and SFC SFC Transition triggering 15 Transition passed Evolution rules AND convergence Transition triggering Transition enabled 22 20

11 Evolution rules AND convergence Transition triggering Transition passed Evolution rules OR divergence Transition triggering C1 true, C2 false 3 C1 C

12 Evolution rules OR divergence Transition triggering C1 true, C2 false 3 C1 C Evolution rules OR divergence Transition triggering C1 true, C2 true 3 C1 C

13 Evolution rules OR divergence Transition triggering C1 true, C2 true 3 C1 C Evolution rules OR divergence Transition triggering C1 true, C2 true Ambiguous situtation which can be solved with mutual exclusion or transition priority (interpretation depends on the PLC..) 3 C1 C2 AND NOT(C1)

14 Evolution rules Note By applying evolution rules it can happen that a step (31) should be activated and deactivated at the same time Conventionally, this step stays active (if it has an associated action which is being performed, this action is not stopped and started again) But this situation should be used with caution C10 and C11 both true C10 C11 27 Evolution rules Note By applying evolution rules it can happen that a step (31) should be activated and deactivated at the same time Conventionally, this step stays active (if it has an associated action which is being performed, this action is not stopped and started again) But this situation should be used with caution C10 and C11 both true C10 C11 28

15 LD vs SFC SFC PROs CONs PROs CONs Easier control logics design Allows functional design of project specifications The project has even a documentation function Easier maintainance of already developed functions Even if it was born more than 25 years ago and is present in IEC industrial standard, is not supported by all PLCs Supported by all PLCs LD Really low level language Difficult to read and so to maintain Not suitable for complex projects 29 LD vs SFC SFC PROs CONs PROs CONs Easier control logics design Allows functional design of project specifications The project has even a documentation function Easier maintainance of already developed functions Even if it was born more than 25 years ago and is present in IEC industrial standard, is not supported by all PLCs Supported by all PLCs LD Really low level language Difficult to read and so to maintain Not suitable for complex projects 30

16 Boolean equivalent equations Consider the generic step [4] of an SFC T1 Cond1 4 T2 Cond2 5 6 T3 Cond3 T4 Cond4 31 Boolean equivalent equations State activation equation: X4 = X1 Cond1 + X2 X3 Cond2 State permanence equation : X4 = X4 NOT (Cond3 + X5 X6 Cond4) Output equation (depends on qualifier): N: Action4 = X4... Problem: they are not very readable and maintainable and iterpretation rules are missing 32

17 SFC interpretation The formal representation of evolution (or interpretation) rules of an SFC program is called evolution algorithm (or interpretation algorithm) In literature there are many alternatives for evolution algorithms, because the same program can be interpreted in different ways 33 SFC interpretation 1 Let s make an example: What if A and B are both verified? A 2 B 3 34

18 SFC interpretation 1 A Let s make an example: What if A and B are both verified? Step 2 would be activated and 2 B 3 35 SFC interpretation 1 A Let s make an example: What if A and B are both verified? Step 2 would be activated and... immediately after it, step 3 2 B But immediately cannot be obtained with real devices and in any case needs to be interpreted

19 SFC evolution algorithms Evolution algorithms without stability search Algorithmic representation of evolution rules such as in presence of transitions sequences with logic condition equal to true, they are triggered in different cycles, not in the same one All the outputs associated to intermediate steps are assigned (even if activation token stays in the step for an arbitrarily small time) Four steps for each cycle: 1. Input scanning 2. Triggerable transitions definition 3. New state definition (due to transition trigger) 4. Output writing 37 SFC evolution algorithms Evolution algorithm without stability search Input scanning Steps initialization Output writing Input scanning Triggerable transitions definition New state definition Output writing 38

20 SFC evolution algorithms Evolution algorithm with stability search Input scanning Steps initialization Impulsive actions execution Input scanning Triggerable transitions definition Impulsive actions execution New state definition Varied condition Output writing 39 SFC to LD translation without stability search Ladder Diagram implementation of an SFC schema without stability search A memory bit is associated to each step (representing if the step is active or not) A memory bit is associated to each transition (meaning that a transition is triggerable in a certain state) Program divided in 4 sections: 1. Initialization 2. Actions executions section 3. Transitions evaluation secton 4. Steps update section 40

21 SFC to LD translation [1] Initialization Executed once: initializes to 1 initial steps: W1 LBL JMP W1 L Initial_step_1 L Initial_step_k L LBL 41 SFC to LD translation [2] Actions For every action we need to perform these translation steps For every action with N qualifier, it is necessary to include a rung with inside all the steps performing that action, put under an OR condition. If the action is under other conditions (example: TIMER), it is necessary to include that condition in the rung (AND condition). For actions with set or reset qualifiers (S, R), Latch or Unlatch coils are needed. For impulsive actions it is necessary to use positive/negative edge detectors 42

22 SFC to LD translation [2] Actions Step i Step j Tk Action A Perform Action A (N qualifier) if you are in step i, k or in step j and Tk=1 (timer elapsed) (D/L qualifiers) Step k Step m Step n Action B L Start performing Action B (S qualifier) when step m or n become active (until Action B is reset) 43 SFC to LD translation - [3] Transitions The bit associated to the transition is high if the transition is triggerable A rung is defined for each transition i j A Receptivity = Step i Step j Receptivity Transition A 44

23 SFC to LD translation [4] Steps update In correspondence of a triggerable transition: 1 is written for steps after the transition (steps going active) 0 is written for steps before the transition (steps going inactive) Transition A Step i L Step j L Step k U Step m U 45 SFC to LD translation Timing variables Timing variables are made with non retentive timers which start counting when steps become active (D/L qualifiers) They can be put in a separated section Step i Tk

24 SFC to LD translation Stability search algorithm T_tr bit means that at least one transition has been triggered in this cycle during conditions evaluations T_tr is activated during conditions evaluation if a transition is triggered (so the overall state has changed) IF T_tr==1 THEN JMP to Input scanning Transition A T_tr U Step i L Step j L Step k U Step m U T_tr L T_tr is unlatched at the beginning T_tr Input_Scanning JMP 47