Way to improve the reliability of PLC automatic control system - Database & Sql Blog Articles

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The method to improve the reliability of the PLC automatic control system is as follows:

First, understanding the main reasons affecting the reliability of the control system.

Although industrial control units and programmable controllers themselves have high reliability, if the switching signal input to the PLC is incorrect, the analog signal has a large deviation, or the actuator controlled by the PLC output port fails to perform the required action, these issues can lead to errors in the control process, resulting in irreparable economic losses. The primary factors that affect the input of PLC signals from the field include: 1. Short-circuit or open circuit in the transmission signal lines (due to mechanical stress, aging, or loose connections). When such failures occur, the field signals cannot reach the PLC, causing control errors. 2. Mechanical contact bounce. Even though a field contact is only closed once, the PLC may interpret it as multiple closures. Although hardware filters and software differential instructions are used, if the PLC scan cycle is too short, errors like counting, accumulation, or shifting may still occur, leading to faulty control results. 3. Failures in field transmitters or mechanical switches, such as poor contact or incorrect readings, can also cause problems.

1. The control load's contacts may not operate reliably. Even if the PLC sends an action command, the actuator may not respond as expected.

2. The inverter may fail, preventing the motor from operating correctly.

3. Electric valves or solenoid valves may not open or close properly due to actuator failure, leading to improper system operation and reduced reliability. To enhance the overall system reliability, it's essential to ensure reliable input signals and accurate actuator responses. Otherwise, the PLC should detect issues promptly and alert the operator through sound and light alarms, allowing for quick fault resolution and ensuring safe and correct system operation.

Second, designing a comprehensive fault alarm system.

In the design of the automatic control system, we implemented a three-level fault display alarm system. The first level is on the control panel, where indicator lights show whether the equipment is operating normally or has a fault. A flashing light at 1 Hz indicates a problem. A dedicated fault reset/light test button ensures all indicators light up when pressed for 3 seconds, helping identify broken bulbs. After resetting, the indicators return to their original state. The second level is displayed on the central control room’s large-screen monitor, showing the fault type, flashing the relevant device on the process flow diagram, and recording the event. The third level is a signal box in the control room, which alerts staff with sound and light alarms. Faults are categorized—some require immediate shutdown, while others allow continued operation, minimizing downtime and improving system reliability.

Third, researching the reliability of input signals.

To enhance the reliability of signals entering the PLC, we selected high-reliability transmitters and switches, and ensured the transmission lines were protected against short circuits, open circuits, or poor contact. During programming, digital filtering was added to increase signal credibility. For digital signals, a timer was introduced after the field contact, with timing based on contact jitter and system response speed, typically tens of milliseconds. This helps ensure the contact closes reliably before further processing. For analog signals, three samples were taken, stored in registers, and the middle value was used after removing the max and min values. In practice, more samples improved accuracy. By analyzing signal relationships, such as comparing liquid level measurements with tank size and valve positions, we could detect faulty sensors and notify operators. For example, if a liquid level switch activates but the gauge doesn't reflect the extreme position, it might indicate a faulty switch or wiring, prompting maintenance.

Fourth, studying the reliability of the actuating mechanism.

Once the field signal is accurately input into the PLC, the program executes and controls the field devices via actuators. Ensuring the actuator works as required is critical. We implemented procedures to verify the reliability of contactors and valves. For contactors, we monitored the coil and auxiliary contacts, using a timer to check if the contactor operated correctly. If not, a fault bit was set and alerted. For valves, we set delays based on opening/closing times and checked if the position signal returned to the PLC. If not, a fault was flagged. Important outputs also used intermediate relays with feedback contacts to confirm actions, enhancing system reliability and fault detection.