Interpreted Paralelism
Problem:
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Consider the following material transportation system to two locations B and C:
There are limit position sensors in points A, B and C represented respectively by: a,b and c. Initially the car is in position A (resting position). The car is controlled by using buttons p1 and p2. If button p1 is pressed the car performs the trip ABA. If button p2 is pressed, then the car performs the trip ACA. The cars motors move the car to the right and left controlled by D and E, respectively. Actuator V controls the direction of the trip in the following way: V=0 – selects the direction C. V=1 – selects the direction B. Grafcets representing logical controllers for the following situations are presented here:
Example 2.1:
- In this Grafcet, the receptivity’s associated to the sequence selection are implemented with mutual exclusion.
- Without the mutual exclusion, in the case in which buttons p1 and p2 are pressed simultaneously the Grafcet will evolve to an unstable situation. By using mutual exclusion this type of simultaneous order is ignored and the system stability is maintained.
- In order to the Grafcet evolves properly, just one of the two orders must be followed (p1 or p2)
- The button p1.p2 allows to simulate the situation in which the buttons p1 and p2 are simultaneously pressed. In this case, given the mutual exclusion in the sequence selection, these orders are ignored.
- The direction choice is performed by actuator V. The order is transmitted and memorized in the activation of step 1 (path ABA) or step 3 (path ACA)- action in activation.
The actions are continuous and associated with the respective steps.
Note: There are several buttons, bellow the button/switch, m, which allows to control the pace of the animation. Thus, for a better understanding of the Grafcet evolution it is possible to pause
the animation and go through the evolution frame by frame
. At any moment the normal pace can be resumed
. Button
allows to re-start the animation.
By pressing button p1 the Grafcet evolves through a sequence (trip ABA); By pressing button p2 the Grafcet evolves through a different sequence (trip ACA). In both cases, the car returns to the initial position and waits for a new command. In the case when both buttons p1 and p2 are pressed simultaneously the order is ignored and the car remains in position A.
Example 2.2:
- In this case, relatively to the previous one, it is presented another solution which passes through the use of priorities. In the situation in which buttons p1 and p2 are pressed simultaneously it is execute one sequence – priority given to p1.
Note: There are several buttons, bellow the button/switch, m, which allows to control the pace of the animation. Thus, for a better understanding of the Grafcet evolution it is possible to pause the animation and go through the evolution frame by frame
. At any moment the normal pace can be resumed
. Button
allows to re-start the animation.
This example differs from the previous one in the sequence selection. If both buttons p1 and p2 are pressed simultaneously the car will perform trip ABA (priority is given to button p1).
