1. Introduction
Programmable controllers are increasingly widely used in the field of industrial control due to their strong anti-interference ability, high stability, simple programming, and high cost-effectiveness. The stability issue of automatic control systems has been deeply studied, and methods have been proposed to improve the stability of system operation. Practice has proven that the adoption of these methods is effective in improving the stability of the system.
2. The main reasons for the reduced stability of the control system
Although industrial control machines and programmable controllers themselves have high stability, if there is an error in the input switch signal to the PLC, there is a significant deviation in the analog signal, and the executing mechanism controlled by the PLC output port does not act as required, these may cause errors in the control process and irreparable economic losses.
The main reasons for errors in on-site input signals to the PLC are:
1) Causing a short circuit or open circuit in the transmission signal line (due to mechanical pulling, the line itself aging, especially mouse damage), when the transmission signal line malfunctions, the on-site signal cannot be transmitted to the PLC, causing control errors;
2) Mechanical contact shaking, although the on-site contact is only closed once, the PLC believes it has been closed multiple times. Although the hardware has added a filtering circuit and the software has added differential instructions, due to the short scanning cycle of the PLC, errors may still occur in counting, accumulating, shifting and other instructions, resulting in incorrect control results;
3) The on-site transmitter and mechanical switch themselves have faults, such as poor contact of the contacts, and the transmitter reflects significant non electrical deviation or inability to work properly. These faults can also prevent the control system from working properly.
The main reasons for errors in the executing agency are:
1) The contact of the control load cannot operate stably, and the PLC issued an action command, but the executing mechanism did not act as required;
2) Control the start of the frequency converter, but due to the fault of the frequency converter itself, the motor carried by the frequency converter did not work as required;
3) Various electric valves and solenoid valves that should be opened and closed cannot be fully closed. Due to the actuator not being able to operate according to the control requirements of the PLC, the system cannot function properly and reduces system stability. To improve the stability of the entire control system, it is necessary to improve the stability of the input signal and the accuracy of the actuator action. Otherwise, the PLC should be able to detect problems in a timely manner, use sound and light alarm methods to prompt operators, eliminate faults as soon as possible, and ensure that the system operates safely, stably, and correctly.
Serial communication is used for data exchange between the central control unit and the lower computer PLC. Usually, 485 twisted pair communication is used for distances within 1000m, fiber optic communication can be used for more common distances, and wireless communication can also be used for longer distances. The lower computer is controlled by PLC. Depending on the number of control objects and the range of control objects, one or more PLCs can be selected for control. Data exchange between PLCs uses internal link registers to achieve data exchange and sharing. Due to the high stability of PLC for real-time monitoring on site, as well as its simple and flexible programming, it has been increasingly valued by people.
2. A well-designed three-level fault alarm system
In the design of the automatic control system, we have designed a three-level fault display alarm system. The idea is to add a timer after the on-site input of the contact, and the timing time is determined based on the contact jitter situation and the response speed required by the system, usually in the tens of milliseconds. This allows for stable closure of the contact before other responses can occur. Analog signal filtering can continuously sample on-site analog signals multiple times, and the sampling interval is determined by the conversion speed and the rate of change of the analog signal.
Level 1 is set on the control panel of each control cabinet on the control site, and indicator lights are used to indicate the normal operation and fault conditions of the equipment. To prevent the indicator light bulb from being damaged and not accurately reflecting the equipment's working condition, a fault reset/light test button has been specially set.
Press the button continuously at any time during system operation, and all indicator lights should be on. If there are any indicators that do not light up at this time, it indicates that the indicator light is broken and should be replaced immediately. After resetting the button, the indicator light will still display the equipment's working status according to its original working state.
Level 2 fault display is set on the large screen monitor in the central control room. When a device malfunctions, text displays the type of fault, the corresponding device on the process flowchart flashes, and the fault will be recorded in the historical event table.
Level 3 fault display is set in the signal box of the central control room. When the equipment malfunctions, the signal box will use sound and light alarms to alert the staff and promptly handle the fault. When dealing with faults, they are also classified. Some faults require the system to stop running, but some faults have little impact on the system's operation. The system can operate with faults, and faults can be eliminated during operation. This greatly reduces the downtime of the entire system and improves the stability and operation level of the system. Research on Input Signal Stability
To improve the stability of on-site input signals, it is necessary to first select transmitters and various switches with high stability to prevent various reasons from causing short circuits, open circuits, or poor contact in the transmission signal line. Secondly, add a digital filtering program during program design to increase the credibility of the input signal.
The sub sampling data is stored separately in the data register. After the end of the sub sampling, the data comparison, data exchange instruction, and data segment comparison instruction are used to remove the large and small values, and the intermediate value is retained as the result of this sampling and stored in the data register. This can improve the stability of the read in field signal and also utilize the characteristics of the control system to determine the credibility of the signal by using the relationship between signals.
If liquid level control is carried out, as the size of the storage tank is known, the opening and pressure of the inlet or outlet valves are known, and the approximate range of liquid change height in the tank within a certain period of time is known. If there is a significant difference between the data sent by the liquid level gauge to the PLC and the estimated liquid level height, it may be determined that the liquid level gauge is faulty. The fault alarm system notifies the operator to check the liquid level gauge.
In addition, if each storage tank has upper and lower liquid level * limit protection, a signal will be sent when the switch is activated to check whether this signal is true and stable. In the program design, we will compare this signal with the liquid level gauge signal of the tank. If the liquid level gauge reading is also at the * limit position, it indicates that the signal is true; If the reading of the liquid level gauge is not in the limit position, it may be a malfunction of the liquid level limit switch or transmission signal line. Similarly, the operator will be notified through the alarm system to handle the malfunction. Due to the use of the above methods in program design, the stability of the input signal has been greatly improved.