Programmable controllers, due to their strong anti-interference capabilities, high reliability, simple programming, and high performance-to-price ratio, are gaining increasingly widespread application in the industrial control field.
Central Control Unit and Subordinate MachinePLCs communicate via serial data exchange, typically using 485 twisted pair cables for distances up to 1,000 meters. For longer distances, fiber optic communication can be employed, or wireless communication for extended ranges. Subordinate machines are controlled by PLCs, and depending on the number and scope of control objects, a single or multiple PLCs can be used. Data exchange between PLCs is achieved through internal link registers, facilitating data sharing. Due to the high reliability of PLCs in real-time field monitoring, along with their simple and flexible programming, they are gaining increasing attention.
1. Primary Reasons for Decreased Control System Reliability
Although industrial control computers and programmable controllers are inherently highly reliable, if the inputs provided toAn error in the PLC's discrete signal and significant deviation in the analog signal may result in the actuator controlled by the PLC's output port not operating as required, which could lead to errors in the control process and incur irrecoverable economic losses.
Impact on live input toThe primary causes of PLC signal errors include:
1) Causing a short circuit or open circuit in the transmission signal lines (due to mechanical stress, inherent aging of the lines, especially rodent damage), when transmission signal lines fail, the field signals cannot be transmitted to the PLC, resulting in control errors.
2) Mechanical contact jitter causes the PLC to recognize multiple closures even though the现场contact only closes once. Despite the addition of a filter circuit in the hardware and differential instructions in the software, errors may still occur in counting, accumulation, and shifting commands due to the PLC's short scanning cycle, leading to incorrect control outcomes.
3) Field transmitters and mechanical switches may experience internal failures, such as poor contact between contacts. This can cause the transmitter to reflect significant non-electricity deviations from the field or fail to operate normally. These issues can also lead to the control system malfunctioning.
The primary causes of errors in executing agencies include:
The contact for load control failed to operate reliably; although the PLC issued the actuation command, the actuator did not respond as required.
2) The variable frequency controller failed to start, as the motor powered by the controller did not operate as required due to the controller's internal fault.
3) Various electric valves and solenoid valves failed to open or close properly, as the actuating mechanism did not respond to the PLC's control commands, leading to the system's inability to operate normally and reducing system reliability. To enhance the overall reliability of the control system, it is essential to improve the reliability of input signals and the accuracy of the actuating mechanism's movements. Otherwise, the PLC should promptly detect issues and alert the operators with sound and light alarms, allowing for the swift resolution of faults and ensuring the system operates safely, reliably, and accurately.
2. A well-designed fault alarm system
In the design of automatic control systems, we have developed...The 3rd-level fault display is an alarm system, with the 1st-level settings on the control panels of various control cabinets at the control site. Indicator lights signal the normal operation and fault conditions of the equipment; when the equipment is running normally, the corresponding indicator light is on, and when there is a fault, the indicator light blinks at a frequency of 1Hz. To prevent the bulb of the indicator light from being damaged and not accurately reflecting the equipment's working condition, a dedicated fault reset/light test button is provided. If the system is running, holding down this button continuously for 3 seconds should light up all indicator lights. If any lights remain unlit, it indicates that the light is faulty and should be replaced immediately. After resetting the button, the indicator lights will still display the equipment's working status as before. The 2nd-level fault display is set up on the large screen monitor in the central control room, displaying fault types with text and the corresponding equipment blinking on the process flow diagram, and recording the fault in the historical events table. The 3rd-level fault display is located inside the signal box in the central control room, alerting staff with audio and visual alarms when a fault occurs, prompting for timely fault handling. During fault resolution, the faults are categorized; some require the system to stop running, while others have minimal impact on system operation, allowing the system to run with faults, which can be resolved during operation. This significantly reduces the system's downtime and enhances its reliability and operational efficiency.
3. Input Signal Reliability Study
Please enhance the on-site input toThe reliability of PLC signals hinges on selecting high-reliability transmitters and various switches to prevent signal line short circuits, breaks, or poor contacts due to various causes. Secondly, enhance the credibility of input signals by incorporating digital filtering programs into the program design.
After inputting the touch point on-site, a timer is added. The timing duration is determined based on the jitters of the touch point and the required response speed of the system, usually in the tens.MS ensures that other responses are initiated only after the contact is indeed stable and closed. Analog signal filtering can be achieved using the programming method depicted in Figure 2b, where the analog signal is sampled continuously three times. The sampling interval is determined by the A/D conversion speed and the rate of change of the analog signal. The data from the three samples are stored in data registers DT10, DT11, and DT12, respectively. After the last sample is taken, data comparison and data exchange instructions are used to retain the intermediate value as the result of this sampling, which is then stored in data register DT0.
Enhance InputPLC field signal reliability can be enhanced by utilizing the inherent characteristics of the control system to assess the credibility of signals based on their interrelations. For instance, in level control, since the tank's dimensions are known, as well as the valve openings and pressures for incoming or outgoing fluids, the approximate range of liquid level change within a certain time frame is known. If the data from the level sensor to the PLC significantly differs from the estimated level height, it may indicate a sensor fault. In such cases, the fault alert system notifies the operators to inspect the level sensor.
For example, each storage tank is equipped with upper and lower level protection limits, which emit a signal when the switch operates.PLC: The authenticity and reliability of this signal are verified during programming by comparing it with the tank level sensor signal. If the level sensor reading is also at the extreme position, the signal is deemed genuine; if not, it may indicate a fault with the level limit switch or a transmission line fault. In such cases, the alarm system notifies the operator to address the issue. By employing this method in programming, the reliability of input signals has been significantly enhanced.
4. Actuator Reliability Study
When the signal on site is accurately input toAfter the PLC executes the program, it adjusts and controls the field equipment through the actuator. How to ensure that the actuator operates according to the control requirements, and how to identify a fault when the actuator does not function as required? We take the following measures: When the load is controlled by a contactor, switching from starting or stopping the load to controlling the contactor coil, whether the contactor reliably engages during startup and reliably releases during shutdown are our concerns.
X0 is the contactor action condition, Y0 is the control coil output, X1 is the auxiliary normally open contact of the contactor that returns to the PLC input, with the timer's set time greater than the contactor's action time. R0 is the set fault position; R0 ON indicates a fault, triggering an alarm; R0 OFF indicates no fault. Faults have a memory function and are cleared by the fault reset button.
When operating an electric valve, set a delay time based on the opening or closing duration. After the delay, check for signals indicating full opening or closing. If these signals fail to return accurately on time,PLC indicates a potential fault in the valve, and handles the valve fault alarm. The program design is shown in Figure 3b. X2 is the valve opening condition, Y1 is the control valve action output, the timer's timing exceeds the valve opening time, X3 is the valve in-position return signal, and R1 is the valve fault position.
