Select interlock signal: off, on, 0, 1
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This post was last edited by zhoudingshengs on 2023-11-23 09:50. Selecting interlock signals, such as open, closed, 0, and 1, – instrument signal interlocks are an important measure for automatic monitoring and enabling automated operations. When certain process parameters in the equipment and pipelines exceed their limits or the operating conditions become abnormal, lights and sounds are used to alert the operators, who then manually or automatically adjust the operating conditions to keep the production process in a safe state. Choosing an interlock protection system appropriately can also improve the level of production automation to a certain extent and reduce the need for operators. During operation, improper functioning of the instrument signal interlock system often leads to unintended shutdowns of the production system, disrupting its normal operation and reducing product output ； Sometimes, the failure of the signal interlock system can cause damage to equipment, reduce product quality, and in severe cases, pose a threat to the safety of operators. Therefore, it is very important to ensure the stable operation of the instrument signal interlock system. (1) For system interlock signals, which is better: open, closed, 0, or 1? The system interlock signal: 0 represents disconnected, 1 represents connected ； 0 indicates that the solenoid valve is de-energized, while 1 indicates that it is energized. Generally, safety interlock signals are discussed; in that case, a \"0\" signal for interlocking is preferable. That is, the interlock signal is \"1\" in normal conditions and \"0\" in the event of a fault, meaning that it is the \"0\" signal that triggers the interlock. In fact, once the logical states are clearly defined, it becomes easy to switch all control logic variables between their positive and negative states during control design; thus, there is no real difference between using \"0\" or \"1\". The \"0\" logic can be considered a more conservative design approach, as well as one that is more reliable from a safety perspective, reflecting the principle of \"fail safe\". 1. The “0” interlock signal automatically incorporates faults such as open circuits, loose connections, poor terminal contacts, and air leakage into the interlock logic. This works on the same principle as whether to use normally open or normally closed contacts for an emergency stop button. 2. The state of the actuator in the event of an unexpected loss of power is determined by referring to the \"0\" interlock logic, which causes the system to automatically enter the \"interlock protection state\" in such cases; this is logically consistent. 3. System interlock signals, especially those related to safety interlocks, require that the interlock state be maintained once it is activated until a reset signal is received. By selecting the “0” interlock mode, it can be ensured that the system will not exit the interlock state even in the event of an accident while in that state. 4. General control systems are designed to have fail-safe (zero-voltage protection) features, meaning that the process will not restart automatically once the power is lost and then restored. Therefore, when the power is restored, all signals are in a de-activated state due to the pressure loss protection. For safety interlocks, if a signal value of “0” is taken as the condition for activating the interlock, the state of various variables at the moment the system returns corresponds exactly to the state required to activate the safety interlock, thus preventing any accidents from occurring. 5. Some controllers require the status of certain internal registers to be used as conditions for calculating interlock logic. These registers need to take into account the possibility of data loss due to prolonged shutdowns or power outages; in other words, the data in these registers will be \"resetted\" to \"0\". If a \"0\" signal is used as the condition for safety interlocks, then even if the loss of register data is not detected immediately upon restart, the system will automatically enter a safety interlock state, which is also a safe approach. Of course, the drawback of the “0” signal safety interlock is that it is prone to having its function interrupted due to equipment issues, and this can happen quite frequently, disrupting the normal progress of production. But with safety as the top priority, the “0” interlock signal has a natural logical advantage. (II) Considerations in practical engineering 1. Interlock signals can be divided into external signals to the control system and internal signals to the control system. 2. External signals of the control system usually include continuous analog signals (4–20 mA) and digital signals (open and closed). 3. Continuous analog signals and digital signals are typically involved in logic operations within control systems in a two-value format. GB/T 50770-2013 requires that the logical functions of application software utilize Boolean logic and the rules of Boolean algebra. 4. Boolean logic 1 and logic 0 have the same level of security; it doesn’t matter which one is more secure. In other words, when external signals from the control system enter the controller, the interlock state can correspond either to logic 1 or to logic 0 ； The normal state can correspond to either logic 0 or logic 1. 5. External signals of the control system are typically designed in a fail-safe manner; cable disconnection can usually be set to trigger corresponding interlocks. Specifically, the digital input signals and digital output signals can be set to: disconnect the corresponding interlock. For other considerations, you can refer to the relevant requirements specified in the supplementary reading. 1 and 0 are numerical representations of being powered on and powered off. For interlock signals, it is sufficient to achieve the required functions at both the input and output stages. The dominant terms are power on and power off, while the auxiliary terms are 1 and 0. If not the other way around, there will be no mistakes. Because the action itself is electricity, not digits; digits are used to represent electricity, with 1 indicating the presence of electricity and 0 indicating its absence. Of course, it’s also possible to do the opposite. Therefore, it’s not a 1/0 interlock, but rather an interlock based on the presence or absence of electricity. This is why digital descriptions are rarely used for motor start-stop control in electrical engineering.
When selecting system interlock signals, the key considerations are safety and reliability. In most cases, for safety interlock signals, the “0” state (which can also be understood as an off or de-energized state) is usually a more reliable and safe choice. This is because, with this design, any unexpected power outage or failure will automatically cause the system to enter safe mode. This design embodies the Fail-Safe principle. Specifically, the advantage of using “0” as the safety interlock signal is that: 1. It automatically takes into account potential faults such as broken wires, loose connections, poor terminal contacts, and air leaks, thereby helping to ensure that the system switches to a safe state when these issues occur. 2. In the event of an unexpected loss of power, the default state of the actuator generally corresponds to the \"0\" logic, thereby ensuring that the system enters a protected state in case of a sudden power outage. 3. If the system is already in interlock mode, using “0” as the signal ensures that even in case of an accident while in interlock mode, the system will not exit that mode. 4. After the power is restored, since loss of voltage protection causes all signals to be in a de-activated state, using a “0” signal as the interlock start condition prevents the system from restarting automatically without manual inspection, thereby avoiding potential accidents. 5. Some controllers may lose the data in their internal registers after a long power outage, and the default value of these registers is often “0”. If “0” is used as the interlock condition, the system will automatically enter a safe interlock state upon power restoration. Of course, choosing “0” as the safety interlock signal also has its drawbacks; for example, it may cause the interlock to activate frequently due to equipment issues, thereby disrupting the normal progress of production. However, from a safety-first perspective, choosing “0” as the interlock status has logical advantages. In engineering practice, the following aspects need to be considered when dealing with interlock signals: 1. Consider both external and internal signals of the control system. 2. Consider continuous analog signals and digital signals. 3. Boolean logic and Boolean algebra operations should be applied to handle control logic. 4. Whether it is logic 1 or logic 0, their safety levels should be the same; the key is how to configure these logics to meet the interlock conditions. 5. For external signals, fail-safe design principles are generally followed, such as setting a broken cable as a condition that triggers interlocks. Finally, the decision regarding whether to use an open or closed state, and 0 or 1 as the safety interlock signal, should be made based on specific engineering and safety requirements, in accordance with relevant standards and specifications. .
This post was last edited by zhoudingshengs on 2023-11-23 at 10:27. \"Fault safety\" refers to the ability to reach a predetermined safe state in the event of a failure. A fail-safe design ensures that the system can maintain the production process in a predetermined safe state in the event of a failure in any component, device, link, or power source within the safety instrumented system. The greatest advantage of a fail-safe design is that it ensures all components within a safety instrumented system are constantly monitored, thereby guaranteeing the system’s reliability. It can also best meet the requirements for safety integrity. In the non-excitation shutdown design, special attention should be paid to addressing system failures caused by sticking of relay or digital instrument contacts. Where the system performs important security functions, a voting system should be considered to meet the requirements for safety integrity.
I remember that sil seems to be negative logic
This post was last edited by zhoudingshengs on 2023-11-23 at 12:24. SIL (Safety Integrity Level) refers to a discrete scale used to specify the integrity requirements for the safety instrument functions assigned to an SIS. It serves as a measure of the importance of safety functions and is fundamental to safety systems, representing the order of magnitude by which process risks can be reduced. Positive logic is generally used in DCS, with low safety requirements and minimal hazards. However, interlocks in systems such as SIS, ESD, and ITCC are all related to equipment damage or personal safety incidents, and negative logic must be used as per regulations. This prevents the control system from continuing to assume that the site is safe due to damage to the instrumentation equipment or open circuits, thereby ensuring that protection functions are maintained. For interlock output signals, when the line is open circuit, in the case of positive logic, the signal received at the field side remains 0, and no interlock action occurs. In the case of negative logic, any disconnection will trigger the interlock mechanism, thereby ensuring safety. This is the function of negative logic—it emphasizes safety and reliability.
I’m learning *fail-safe design; just like the original poster is learning*