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

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, or the analog signal has a large deviation, or the actuator controlled by the PLC output port fails to perform the required action, these issues may lead to errors in the control process, resulting in significant economic losses. The primary factors that affect the input signals to the PLC from the field include: 1. Short circuits or open circuits in the transmission signal lines due to mechanical stress, aging, or loose connections. When these failures occur, the field signals cannot reach the PLC, leading to control errors. 2. Mechanical contact bounce. Even if the field contact closes only once, the PLC might interpret it as multiple closures. Although hardware filters and software differential instructions are used, if the PLC scan cycle is too short, it could still result in counting, accumulating, or shifting errors, causing faulty control results. 3. Failures in the field transmitters or mechanical switches, such as poor contact, inaccurate readings, or failure to function properly, also contribute to system instability.

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

2. Inverter control issues can cause motors to fail to operate correctly due to internal faults in the inverter.

3. Electric valves or solenoid valves may not open or close properly, as their actuators may not follow the PLC’s commands, leading to improper system operation and reduced reliability. To enhance the overall system reliability, it's essential to ensure both the accuracy of input signals and the correct performance of actuators. If any issue arises, the PLC should detect it promptly and alert the operator through visual and auditory alarms, allowing for quick fault resolution and ensuring safe, reliable, and accurate system operation.

Second, designing a comprehensive fault alarm system.

In the design of the automatic control system, we implemented a three-level fault display and alarm system. The first level includes indicator lights on each control panel, showing whether the equipment is operating normally or experiencing a fault. When the equipment is normal, the corresponding light is on. If there is a fault, the light flashes at 1 Hz. To prevent bulb damage from prolonged use, a dedicated reset/light test button is provided. Pressing this button for 3 seconds at any time will illuminate all indicators. If any indicator is damaged, it should be replaced immediately. After resetting, the indicators return to their original operational status. The second level of fault display is on the large-screen monitor in the central control room. When a device fails, the system displays the fault type, highlights the relevant equipment on the process flow diagram, and records the event in the historical log. The third level of fault display is located in the central control room signal box, which uses sound and light alarms to notify staff of the fault. Faults are categorized—some require immediate system shutdown, while others allow continued operation with minimal impact. This approach significantly reduces downtime and enhances system reliability.

Third, researching the reliability of input signals.

To improve the reliability of field inputs to the PLC, it's important to select high-reliability transmitters and switches, and avoid issues like short circuits, open circuits, or poor contact in the signal lines. During programming, digital filtering techniques can be added to increase the credibility of the input signals. For digital signals, a timer can be added after the field input contact, with the timing set based on contact jitter and system response speed, typically in the range of tens of milliseconds. This ensures the contact is fully closed before further processing occurs.

Analog signal filtering can be achieved by continuously sampling the signal three times. The sampling interval depends on the A/D conversion speed and the rate of change of the analog signal. The three sampled values are stored in data registers DT10, DT11, and DT12. Once the last sample is completed, comparison and sorting instructions are used to remove the maximum and minimum values, retaining the middle value as the current reading in data register DT0. In practice, increasing the number of samples can yield better results. Improving signal reliability can also involve using the relationships between signals within the control system. For example, in liquid level control, if the measured level differs significantly from the expected value based on known tank size and valve settings, it may indicate a faulty sensor, prompting an alarm for inspection.

Another example involves upper and lower liquid level limit protection for each tank. When the switch activates, it sends a signal to the PLC. During programming, this signal is compared with the level gauge reading. If the gauge shows the extreme position, the signal is considered valid. If not, the system may flag the switch or wiring as faulty, triggering an alarm for maintenance. These methods greatly enhance the reliability of input signals.

Fourth, studying the reliability of the actuating components.

Once the field signals are accurately input into the PLC, the system executes the program and controls the field devices through the actuator. Ensuring that the actuator performs according to the control requirements is crucial. When the actuator fails to operate as intended, identifying the fault becomes essential. One measure is to monitor the contactor coil when controlling a load. We designed a program to check if the contactor is reliably energized or de-energized. X0 represents the contactor's operating condition, Y0 is the coil output, and X1 is the auxiliary normally open contact feeding back to the PLC. The timer is set for a duration longer than the contactor’s action time. R0 is the fault bit. If R0 is ON, a fault is detected, and an alarm is triggered. If OFF, no fault is present. The fault memory is cleared via a reset button.

When controlling electric valves, a delay is set based on the valve's opening/closing time. After the delay, if the position feedback signal isn't received, the valve may be faulty, and an alarm is generated. The programming involves X2 (valve open state), Y1 (valve output), a timer exceeding the valve's opening time, X3 (position feedback), and R1 (valve fault). Additionally, important control outputs often use intermediate relays, with their auxiliary contacts feeding back the action status to the PLC.