Things to note before writing a PLC program - Database & Sql Blog Articles

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Before writing the plc program, first analyze the characteristics and operation process of the system. In a typical industrial production process, each device in the system is initially in an initial state. The initial state includes:

a. The power supply for the equipment is normal.

b. The device is selected in the automatic mode, ie the PLC control mode.

c. The device's protection, control and signals have been reset.

After determining that each device meets the initial state, the operator issues a start command, and the entire system enters the starting process from the initial state. The start operation cannot be performed by any device in the self-test that does not meet the initial conditions for starting. During the starting process, the status of each device changes continuously, and each single device starts to run according to the sequence of the process flow, advances to a stable running state, and finally enters a stable running state. The time for stable operation is determined by the production situation. When a production work is completed, the operator will operate or the parking condition will automatically issue a stop command, and the system will enter the stop process. After the last device is stopped, the entire system returns to the initial state and waits for the next cycle.

1) In the initial stage, the faults occurred in the self-test of each device of the system a. The power supply or equipment is not working properly. b. Whether the device control status selects the automatic mode. c. The fault has not been eliminated.

2) Start fault The common start fault is the start timeout fault, that is, the PLC drive output relay operates, and the motor fails to start correspondingly during normal time.

3) Operation failure During the operation of the system, sudden events or malfunctions such as motor overload trip, automatic change of the automatic mode, and emergency stop switch action to protect the human body and equipment may occur.

If any of the above faults and signals appear, the PLC program should be immediately transferred to the execution stop command phase, and the production process should be stopped according to the program setting. For such a fault that needs to immediately suspend the production process, it is called a type of fault. There is another fault in actual production that does not require immediate stopping of the production process, such as a dust collector. This type of equipment is an accessory device throughout the production process. If it is not in operation, it will not affect the normal operation of the production. The PLC system can first stop the device and send an audible and visual alarm signal to the operator. The general dispatching room assigns maintenance personnel to carry out equipment maintenance and the PLC system can continue to execute the main production process. This type of fault can be called a second type of fault. .