Learning phase 9: Parallel Sequences

A parallel sequence is used when several partial sequences are to be started simultaneously after a transition. The first steps in the partial sequences are activated simultaneously by a transition here. The partial sequences are thus started synchronously, which is the reason why the symbol for a parallel branch is also referred to as a synchronisation symbol. The partial sequences then remain independent from one another, i.e. they run in parallel. At the end, the partial sequences flow back to a synchronisation symbol and a subsequent transition. This transition is only released once all the partial sequences have been processed in full. Therefore, the partial sequences are also synchronised at the end.

Learning steps:

  • Introduction to parallel sequences
  • Application of parallel sequences

Fig. 3.137 Parallel sequence

The adjacent figure shows a parallel sequence. The double line above steps 2 and 3 is the aforementioned synchronisation symbol. A transition with the condition S1Start is located above the symbol. This transition activates both steps 2 and 3. Afterwards, both partial sequences are autonomous. You can see from the different time designations that the partial sequence with steps 3 and 6 is processed faster than the partial sequence with steps 2, 4 and 5. Because steps 5 and 6 are again linked over a synchronisation symbol, the transition with the condition S2Continue is not released until both step 5 and step 6 are active. Only then can the transition to step 7 take place over S2Continue = True. This also results in step 5 and 6 being deactivated.

PLC-Lab Filename GRAFCET-Studio Filename
FluessigkeitenMischen.plclab FluessigkeitenMischen.grafcet

Two liquids are to be mixed in a blending tank. After the start, 20 units of Liquid 1 and 30 units of Liquid 2 are to be pumped into the container. A lamp (H1) indicates that the quantities for both liquids have been completely pumped into the tank. This is then confirmed by pressing a button. Emptying the blending tank takes place manually. You can only re-start the procedure once the completed operation has been confirmed and the blending tank has been emptied completely (S4 = True).

Fig. 3.138 Diagram of the technology for the mixing tank

Definition of the operands:

Symbol Description
S1Start "Start" button, value = True if pressed
S2CountLiquid1 "Counting impulse for pumping in Liquid 1" sensor, pos. edge corresponds to one unit
S3CountLiquid2 "Counting impulse for pumping in Liquid 2" sensor, pos. edge corresponds to one unit
S4MixTankLowerLevel "Blending tank lower level" sensor, value = True if lower level is reached
S5MixTankUpperLevel "Blending tank upper level" sensor, value = True if upper level is reached
S6Confirm "Confirm" button, value = True if pressed
M1PumpLiquid1 Pump for Liquid 1
M2PumpLiquid2 Pump for Liquid 2
P1ProcessCompleted Lamp H1: "Process Completed"
CounterLiquid1 Integral operand for storing the counting pulses for Liquid 1
CounterLiquid2 Integral operand for storing the counting pulses for Liquid 2

Fig. 3.139 Solution for the blending tank

The enclosing steps and the enclosures are observable in together with the GRAFCET chart. You can see the following situation in the figure: Liquid 1 has already been completely pumped into the mixing tank. You can see this from the fact that enclosing step 2 is no longer active. In contrast, enclosing step 3 is still active. Liquid 2 is therefore still pumped into the blending tank and the enclosure of step 3 captures the corresponding counting pulses.

Since only step 5 is active and not step 7, the transition after the synchronisation symbol has not yet been released. This means that 'waiting' takes place until the second partial sequence with the steps 3 and 7 has also been processed in full.

Fig. 3.140 Liquid 1 has already been completely pumped into the blending tank.

  • In a parallel branch, the first steps in the individual partial sequences are linked by the synchronisation symbol. This symbol is preceded by a transition which activates these steps simultaneously.

  • The final steps in the individual partial sequences of a parallel branch are also linked by the synchronisation symbol. A transition follows this. This transition is not released until all final steps in the partial sequences are active. In other words: All partial sequences need to have been executed in full; only then is the final transition released.

PLC-Lab Filename GRAFCET-Studio Filename
FluessigkeitenMischen.plclab FluessigkeitenMischenMakro.grafcet

The GRAFCET for the blending tank is to be modified so that the enclosing step 3 used there is replaced by a macro step.

In macro step 3 is contained in the partial sequence for the parallel branch. It has replaced enclosing step 3. The expansion of the macro step is situated in border 3.

You can see the following situation in the figure: Liquid 2 has already been completely pumped into the blending tank. This can be seen by the fact that in the partial sequence step 7 is active and thus the pump for liquid 2 has already been switched off. Liquid 1 is still being pumped into the tank since enclosing step 2 is still active. The transition after the synchronisation symbol is only released when this partial sequence has been completely processed, thus making step 5 active as well.

Fig. 3.141 Solution for the training "Parallel sequence with a macro step"

  • Which GRAFCET element does a parallel sequence begin and end with?
  • When is the transition that follows the final synchronisation symbol released?