This article conducts an in-depth study on the reliability issues of PLC automatic control systems and proposes methods to improve system reliability. Practice has proven that these methods are effective in improving system reliability.
1. Introduction
Programmable logic controllers (PLCs) are increasingly widely used in the field of industrial control due to their strong anti-interference ability, high reliability, simple programming, and high performance-price ratio.
The industrial PLC, serving as the central control unit, is equipped with configuration software and a large-screen real-time monitoring interface. This allows for dynamic display of control points, data modification, fault diagnosis, and automatic alarms. It can also display and query historical event records, cumulative operating time of major system components, process flow diagrams for each unit, and structural diagrams of each unit. Data exchange between the central control unit and the lower-level PLCs is conducted via serial communication. Typically, a 485 twisted-pair cable is used for distances within 1000m, fiber optic communication can be used for longer distances, and wireless communication can be used for even longer distances. The lower-level PLCs are controlled by PLCs; depending on the number and scope of the controlled objects, one or more PLCs can be used. Data exchange between PLCs is achieved through internal link registers. Due to the high reliability of PLCs for real-time monitoring and their simple and flexible programming, they are increasingly valued.
2. Main reasons for reduced reliability of the control system
Although industrial control computers and programmable logic controllers (PLCs) are inherently highly reliable, errors in the digital input signals to the PLC , significant deviations in the analog input signals, and failure of the actuators controlled by the PLC output ports to operate as required can all lead to control errors and cause irreparable economic losses.
The main reasons for errors in the signals input to the PLC from the field are:
1) Causes short circuits or open circuits in the transmission signal line (due to mechanical pulling, aging of the line itself, especially rodent damage). When the transmission signal line fails, the field signal cannot be transmitted to the PLC , causing control errors.
2) Mechanical contact jitter: Although the field contacts only close once, the PLC thinks they have closed multiple times. Although the hardware has added a filter circuit and the software has added a differential instruction, the PLC scan cycle is too short, and errors may still occur in the counting, accumulating, shifting and other instructions, resulting in incorrect control results.
3) Faults in the field transmitter or mechanical switch itself, such as poor contact, large deviations in the non-electrical quantities reflected by the transmitter, or failure to work properly, will also cause the control system to malfunction.
The main reasons for errors in the execution mechanism are:
1) The control load contacts cannot operate reliably. Although the PLC issues an action command, the actuator does not operate as required.
2) Controlling the start of the frequency converter: Due to a fault in the frequency converter itself, the motor driven by the frequency converter did not work as required;
3) Various electric and solenoid valves failed to open when they should have, or failed to close completely when they should have. Because the actuators did not operate according to the PLC's control requirements, the system could not function properly, reducing its reliability. To improve the overall reliability of the control system, the reliability of input signals and the accuracy of actuator actions must be improved. Otherwise, the PLC should be able to detect problems promptly and alert operators using audible and visual alarms to quickly troubleshoot and ensure the system operates safely, reliably, and correctly.
3. Design a comprehensive fault alarm system
In the design of the automatic control system, we designed a three-level fault display and alarm system. Level 1 is installed on the control cabinet panels at the control site, using indicator lights to indicate normal operation and fault status. When the equipment is operating normally, the corresponding indicator light illuminates; when a fault occurs, the indicator light flashes at a frequency of 1Hz. To prevent indicator light bulbs from malfunctioning and failing to accurately reflect the equipment's operating status, a fault reset/lamp test button is specifically provided. Pressing this button continuously for 3 seconds at any time during system operation should illuminate all indicator lights. If any indicator light remains off, it indicates that the indicator light is faulty and should be replaced immediately. After resetting the button, the indicator lights will continue to display the equipment's operating status as before. Level 2 fault display is located on the large-screen monitor in the central control room. When equipment malfunctions, the fault type is displayed in text, the corresponding equipment on the process flow diagram flashes, and the fault is recorded in the historical event table. Level 3 fault display is located in the signal box in the central control room. When equipment malfunctions, the signal box will alert personnel with audible and visual alarms, allowing for timely fault handling. When handling faults, they are further classified. Some faults require the system to stop operating, while others have little impact on the system's operation and can be operated with the faults. The faults can be eliminated during operation, which greatly reduces the downtime of the entire system and improves the reliability of the system.
4. Input signal reliability study
To improve the reliability of signals input to the PLC, firstly, select highly reliable transmitters and switches to prevent short circuits, open circuits, or poor contact in the signal transmission lines caused by various reasons. Secondly, add digital filtering programs during program design to increase the reliability of the input signals.
A timer is added after the field input contact. The timing interval is determined based on the contact bounce and the required system response speed, typically tens of milliseconds. This ensures that other responses only occur after the contact has stably closed. Analog signal filtering can be performed using the programming method shown in Figure 2b. The field analog signal is sampled three times consecutively, with the sampling interval determined by the A/D conversion speed and the rate of change of the analog signal. The three sampled data are stored in data registers DT10, DT11, and DT12, respectively. After the last sampling, data comparison, data exchange, and data segment comparison instructions are used to remove the maximum and minimum values, retaining the intermediate value as the sampling result, which is stored in data register DT0.
Improving the reliability of signals input to the PLC can also be achieved by leveraging the characteristics of the control system itself and using the relationships between signals to determine their reliability. For example, in liquid level control, since the tank dimensions, valve openings, and pressures are known, the approximate range of liquid level changes within the tank over a given time is also known. If the data sent to the PLC by the level gauge differs significantly from the estimated level, a potential level gauge malfunction can be identified, and an alarm system can be used to notify the operator to check the level gauge. Similarly, if each tank has upper and lower level limit protection, a signal is sent to the PLC when the switch activates. The reliability of this signal is assessed during program design by comparing it with the level gauge signal for that tank. If the level gauge reading is also at the limit position, the signal is considered valid; otherwise, a malfunction in the level limit switch or the signal transmission line is suspected, and an alarm system can be used to notify the operator to address the fault. By employing these methods in program design, the reliability of input signals is significantly improved.
5. Reliability Study of the Actuating Mechanism
Once the field signals are accurately input to the PLC, the PLC executes the program and uses the actuators to adjust and control the field devices. How do we ensure that the actuators operate according to control requirements, and how do we detect faults when they fail to operate as required? We take the following measures: When the load is controlled by a contactor, starting or stopping this type of load switches to contactor coil control. We are concerned with whether the contactor reliably engages during startup and reliably releases during shutdown.
X0 represents the contactor's operating condition, Y0 is the control coil output, and X1 is the normally open auxiliary contact of the contactor that returns to the PLC input. The timer's timing duration is longer than the contactor's operating time. R0 is the set fault position; R0 ON indicates a fault and triggers an alarm; R0 OFF indicates no fault. The fault has a memory function and can be cleared by the fault reset button.
When the electric valve is opened or closed, a delay time is set according to the different opening and closing times. After the delay, the open or closed position signal is detected. If these signals are not returned to the PLC in a timely and accurate manner, it indicates that the valve may be faulty, and a valve fault alarm is triggered. The program design is shown in Figure 3b. X2 is the valve opening condition, Y1 is the control valve action output, the timer timing period is greater than the valve open position time, X3 is the valve position return signal, and R1 is the valve fault position.
