Hey everyone, I'm in search of effective techniques for creating functional specifications and test sheets for equipment interlocks. In my experience, using ladder logic is the most effective way to outline a specific process or equipment interlock. Another approach involves defining signals, such as A for Tank High Level and B for Mains Phase Failure, and then representing them as PMP005_INTERLOCK = A | B. Any suggestions or insights on this topic would be greatly appreciated!
In my experience, utilizing a cause and effect matrix in a spreadsheet has been a valuable tool for programming and documentation purposes. However, it should not be relied upon as a comprehensive function test document. Developing effective test plans requires a significant investment of time and a deep understanding of the testing process.
I have encountered a situation similar to this in Excel, where managing chained conditions like A OR (B and (D AND NOT C)) OR (F AND C) becomes cumbersome very quickly. Due to this, we switched from Excel to a Word document in portrait format and focused on text instead. In certain cases, when the individual setting up the interlocks comprehends the entire process, they may choose to define them as the result of an interlocked block. This approach helps in breaking down the complexity of the interlock logic into various process elements.
Utilizing Structured Text in PLC programming can provide benefits even if the program is inactive. By incorporating the ST program within the same PLC project as the 'normal' program, you can easily synchronize both versions. Additionally, including the specification in text pseudocode as a comment header can enhance clarity. It is also helpful to define signals in plain text for easier understanding, ensuring fail-safe operation. For example, specifying Tank High Level as a normally closed signal clarifies its function in the process.
Can you explain the concept of functional specification and test sheets? I typically incorporate permissives and interlocks into my programming, using the output as a condition for the subsequent rung.
An example of a spec could be: When the tank level hits 98%, the HH alarm will trigger, halting the fill pump, and then closing the fill valve after 3 seconds. The pump and valve will remain closed until the tank level drops below 95% or until authorized personnel bypass the interlock. Any bypass will be displayed on the HMI. Test procedures will include calibration of the instrument and interlock testing. For instance, calibrate the instrument following the xyz procedure, ensure the tank level is below 95%, close the discharge valve, open the fill valve, start the pump, and wait for the pump to forcefully stop. Record the time between pump stoppage and valve closure, ensuring it is within 6 seconds (+/- 1 second). Also record the tank level, ensuring it is at 98% (+/- .5%). Check the HMI for any indication of the interlock being bypassed. Remove the bypass, open the discharge valve, and confirm that the interlock is no longer active. This is just a brief example, with some test procedures being more intricate, such as when testing 2oo3 voting for interlocks on SIS systems. SIS validation is crucial whenever any part of the system is altered, requiring validation to ensure proper functioning.
I've found that using Function Block Diagrams (FBD) has worked really well for me. As opposed to ladder logic, which is generally linear, FBD allows you to visually structure the interconnections of various systems which I believe makes it easier to understand how each part contributes to the operation of the whole system. I also see the merit in your signal definition approach as well, especially in terms of simplicity and readability. But don't forget to maintain comprehensive documentations, it will go a long way to make your interlock system easier to troubleshoot and audit.
I've found success using a state diagram method to map out equipment interlocks. It provides a clear visualization of the process and helps to pinpoint where an interlock might actually take place. This technique is efficient at identifying inputs, outputs and how they relate to one another in a system's structure. As for functional specifications, I recommend utilizing V-model practices and creating a checklist of the system's requirements, ensuring that your specifications correlate directly with them. This way, you can keep on track and avoid any possible oversights.
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Answer: 1. What are some effective techniques for creating functional specifications and test sheets for equipment interlocks? - Using ladder logic and defining signals, such as A for Tank High Level and B for Mains Phase Failure, are effective ways to outline a specific process or equipment interlock.
Answer: - Ladder logic is considered effective as it provides a visual representation of the logic flow and sequence of operations in a clear and structured manner.
Answer: - Signals like A and B can be represented in the form of logical expressions, for example, PMP005_INTERLOCK = A | B, to define interlocks and their conditions.
Answer: - It's important to consider the specific requirements of the equipment and processes involved, as well as ensuring that the functional specifications are detailed and comprehensive to accurately capture the interlock logic.
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