Priorities
Problem:
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A car transports products from three independent conveyors to an evacuation conveyor:
The system has several sensors: ai – detects the presence of a part in line 1, with i=1,2,3; an – detects the presence of a part in the transportation car; aei – detects the presence of a workpiece at the beginning of the evacuation conveyor; aef – detects the presence of a piece on the end of the evacuation conveyor; pi – limit sensor, indicates the positioning of the transportation car in the end of line I, with i=1,2,3; pe – indicates the positioning of the car at the beginning of the evacuation conveyor. The actuators are the following: E – moves the car to the left; D – moves the car to the right; Ri – motor of conveyor i, with i = 1,2,3 (lines), n (car), e (evacuation); The transportation car capacity is one part and its initial position is near the evacuation conveyor. Logical controllers are implemented for the following cases with Grafcets:
Example 5.1:
- The proposed problem is solved by using partial grafcets
- In the transport car Grafcet the selection of sequences is defined by using priorities.
- It is defined a certain order to collect objects from the respective lines, in the case of more than one is line busy.
- It is possible for the user to change the priorities by using the menu “Priority”.
- The sequences priorities are defined from the left to the right. This can be observed by the changing of the transition conditions at the beginning of each sequence.
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.
Initially the car and all the conveyors are empty. When line 1 is operating, the motor it’s stopped when a part is detected by the respective position sensor, a1, remaining stopped until the transportation car arrives and the part is removed to be evacuated. The same applies to lines 2 and 3. By pressing button m, the car will transport a part. In the case of none part to be transported the cars waits. The priority to serve the lines is defined from the left to the right (initially, line 1 with the highest priority and line 3 with the lowest priority).
Example 5.2:
- The problem is solved with partial grafcets.
- The selection of sequences is defined by using priorities.
- The lines priorities is alternated, and in this way the lines do not hold the same priority always.
- The change performed in the functions associated with the receptivity’s, in the sequence selection, together with Grafcet controlling the priorities, allows to perform the change in priorities for the lines.
- Check how to write logical functions to the receptivity’s, with higher complexity, which allow to modify the model, to implement the controller in this case.
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.
For the same problem as the one proposed in 5.1, the aim is to implement priorities for the lines, but alternatively. A line has priority over the other two just once, then the priority is given to the following line. Initially the priority is given to line 1, followed by line 2 and finally line 3, returning to line 1, and so on.
