In the arrow diagram, there are some actual logical relations that cannot be expressed, so it is necessary to introduce the concept of virtual work into the arrow diagram.
Arrow diagram (ADM) is a plan that represents a project, so its logical relationship is clear and readable.
critical path method, cpm
It is very important that in addition to the correct logic of arrow diagram (ADM), good drawing habits are also necessary. Therefore, it is very important to abide by the above rules when drawing. In addition, straight lines and broken lines are generally used as much as possible when drawing, and diagonal lines can be used when it is inevitable, but attention should be paid to the clarity of logical direction.
When drawing an arrow diagram, there are mainly the following rules:
1. A loop cannot appear in the arrow diagram (ADM). As mentioned earlier, the loop is a logical error, which does not conform to the actual situation and will lead to infinite loop calculation, so this rule is a necessary requirement.
Arrow diagram (ADM) generally requires drawing from left to right. Although this is not necessary, it conforms to people's reading habits and can increase the readability of the arrow diagram (ADM).
13. Each node should be numbered. The numbering is not necessarily continuous, but it cannot be repeated, and it is incremental in order. This rule has many considerations. When drawing by hand, it can increase the readability and clarity of the graphics. In addition, it is also very important to run the arrow diagram (ADM) on the computer, because the time of each activity is generally determined by calculating the time of the nodes in the computer, so it is necessary not to repeat the node numbers.
4. General figures cannot be continuous, so a certain interval should be reserved. The main reason is that you may need to add activities to the completed arrow diagram (ADM). If the numbers are continuous, the newly added activities can't meet the requirements of numbers from small to large.
5. It is not necessary to have an arrow for the line indicating activity, but for the convenience of expression, it is generally recommended to use an arrow. The main purpose of this paper is to improve the readability of drawing arrow diagram (ADM).
Generally, it is required that the double code network diagram should start from one node and end at one node. This requirement can increase the readability of manual drawing and improve the efficiency of computer calculation and the clarity of results.
When drawing a network diagram, it is generally required that the connecting lines cannot intersect. When crossing is inevitable, you can avoid confusion by crossing the bridge or pointing. This requirement is mainly to increase the readability of graphics.
The calculation of arrow graph (ADM) generally includes forward transfer and reverse transfer. Forward transfer is used to calculate the earliest time of activities and nodes, and its algorithm is as follows:
1. Set the time of the first node in the arrow diagram (ADM), such as 1.
critical path method, cpm
2. Select the activity calculated from the first node.
3. Make the earliest start time of an activity equal to the earliest time of its start node.
4. Adding the duration to the earliest start time of the selected activity is the earliest end time.
5. Compare the earliest end time of the activity with the earliest end time of the activity node. If the end node has no set time, the earliest end time of the activity is the earliest time of the end node; If the end time of the activity is greater than the earliest end time of the node, the earliest end time of the activity is taken as the earliest time of the node; If the earliest end time of the activity is less than the earliest time of its end node, the node time will be taken as its earliest time.
6. Check whether there are other activities starting from this node, and if so, return to step 3 for calculation; If not, enter the calculation of the next node and return to step 3 until the last node.
The latest time of activities and nodes is backward.
Transfer) calculation, backward method (backward
Pass) is generally calculated from the last activity of the project until the time of the first node is calculated. In the calculation of the reverse method, the latest time of the last node is equal to the earliest time, and then the calculation begins. The specific calculation steps are as follows:
1. Set the latest time of the last node equal to the earliest time calculated by the forward method.
2. Select the activity that uses this node as the calculation end node.
3. Make the latest end time of the activity equal to the latest time of the node.
4. Subtract the duration from the latest end time of the activity to get its latest start time.
5. Compare the latest start time of this activity with the latest time of its start node. If the start node does not set the latest time, the latest start time of the activity is set as the latest time of this node. If the latest start time of the activity is earlier than the latest time of the node, the latest start time of the activity is set as the latest time of the node. If the latest start time of the activity is later than the latest time of the node, the time of the original node is kept as the latest time.
6. Check whether other activities end with this node. If yes, enter the second step of calculation; If not, go to the next node, and then go to the second step of calculation until the last node.
The latest time of the first node is the time when the project must start. Assuming that the latest time and the earliest time of the last node are equal, its value should be equal to 1.