Hello everyone, I am currently dealing with some issues related to the PID and cyclic interrupt OB1 on my conventional rubber press with upper and under heating plates. I am using a Siemens 1510SP-1 controller. I have two PID compact temperature controllers for heating the plates - should they both be in one cyclic interrupt OB or in separate ones? Additionally, I need help determining the cycle time for the cyclic interrupt. The current cycle time of the CPU is as follows: Shortest = 0.973 ms, Current = 1.3 ms, Longest = 15.8 ms. The cyclic interrupt is set to 5000 us (5 ms) - is this the correct setting or should it be faster? Furthermore, after tuning the PID, everything works well for a few hours but eventually, the PID controller starts heating and the temperature exceeds the Setpoint. I am receiving error bits #0000_0800 (Sampling time error: PID_Temp is not called within the sampling time of the cyclic interrupt OB) - what does this indicate? Lastly, the Input warning High does not become TRUE even when the input temperature is 300 degrees while the high limit is 250 degrees. Could this be related to the aforementioned error? I would greatly appreciate any help in answering these questions. Thank you.
To optimize the performance of your system, consider placing both processes in separate cyclic interrupts and adjusting the phase offset to avoid simultaneous interruptions. The effectiveness of this approach may vary based on the response time of your system. For instance, I successfully implemented a project involving IR heating with 4 PIDs and 4 interrupts. One interrupt (OB30) had a cycle time of 300ms with a phase offset of 0ms, while the other (OB31) had a cycle time of 300ms with a phase offset of 75ms, resulting in a new interrupt every 75ms for different PIDs each time. If you are experiencing errors, it may be due to a PID process that exceeds a processing time of 5ms.
I appreciate your detailed explanation. Thank you for clarifying this information.
For many temperature control applications, a scan time of 5ms is usually faster than needed. The system's time constant plays a crucial role in determining the appropriate scan time. Typically, for systems with a time constant of 1 second, a scan time of 100ms should suffice. While most temperature systems operate in the minute range, equipment such as a rubber press may require faster scan times.
Thank you, Peter, for clarifying that. Your insight is greatly appreciated.
Patrix mentioned encountering issues with the PID controller after tuning it successfully. The temperature unexpectedly surpasses the Setpoint due to Errorbits #0000_0800 indicating a sampling time error. It seems that the PID_Temp is not being called within the cyclic interrupt OB's sampling time. As someone new to Siemens, I am facing the same problem and seeking advice on how to resolve it. Any assistance would be greatly appreciated. Thank you in advance for your help!
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Answer: - It is recommended to have each PID controller in a separate cyclic interrupt OB to ensure independent control and avoid interference between the controllers.
Answer: - The cyclic interrupt cycle time should be set based on the CPU cycle time. Ensure that the cyclic interrupt time is within the shortest and longest CPU cycle times for optimal performance.
Answer: - Error bits 0000_0800 indicate that the PID_Temp function is not called within the sampling time of the cyclic interrupt OB, leading to temperature overshooting. Check the cyclic interrupt settings and PID tuning to address this issue.
Answer: - The Input warning High not becoming TRUE could be related to the sampling time error and cyclic interrupt issues affecting the input monitoring. Addressing the sampling time error and ensuring proper cyclic interrupt configuration may resolve this problem.
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