I have developed a PLC program for a CO2 cooling system, but I am facing issues with getting the valves of the gas cooler to function properly. I am using an M340 Schneider Electric PLC for this system. Everything else in the system is operating correctly. There are two condensers for cooling the CO2 gas, each equipped with a regulating valve that requires a 0-10v signal input. The valves have valve drivers, specifically the CCMT-24 type, controlled by superheat controller EKE 1A from Danfoss. Despite having PID controllers in the PLC programming for these valves, there is a noticeable delay of 1-2 seconds before the valves start moving to the desired position after a change in the signal input. This delay is causing the pressure to increase rapidly, surpassing the safety limit set at 105 bar, with the pressure setpoint at 90 bar. Any assistance or additional queries regarding this issue are highly appreciated. Thank you in advance.
If the rate of change in your PV surpasses the physical CV device, it may pose a challenge. While tuning the PID for optimal responsiveness is an option, a 2-second lag in the device can hinder performance. Consider upgrading to faster drivers for improved results. Are the models inherently slow to react? Upon reviewing the EKE specifications, it appears you are operating it in driver mode with position input from the PLC. Verify that the mode is correct. The EKE includes its own PI controller - have you considered utilizing it instead of the PLC? Regardless, there is no indication in the specifications of an intrinsic delay in response. In fact, the 485 section specifies a response time of 50ms.
Thank you for your response regarding the EKE in driver mode. The product was purchased by the customer who requested us to make it work, leading to potential challenges. We have had discussions with Danfoss, receiving mixed feedback regarding the delay issue. Some suggest there should be no delay, while others believe we have the wrong product due to the delay. Despite recommending valve changes, the customer is hesitant to make adjustments. The 2-second lag could pose difficulties in resolving the issue. Operating at 90 bar in the gas cooler evaporators and 40 bar out from the valves complicates finding suitable replacements for the valves. Seeking a programming solution to address this problem, as utilizing the EKE as a controller is at the discretion of the customer. Our ability to make physical changes to the machine is limited without their approval.
This issue of excessive deadband in positioner setup is a common occurrence that leads to poor control performance. Many users face this dilemma of sacrificing accurate control to prevent hunting. However, a top solution for this problem is offered by Rexa. Despite this, some customers may opt to cut costs and deal with subpar control despite the negative impact on system efficiency.
Tom Jenkins frequently encounters the issue of excessive deadband in positioners, leading to poor control performance. While some may opt for this setup to avoid hunting, it ultimately results in subpar control. In such cases, Rexa offers a superior solution. However, some customers prioritize cost savings over optimal control, leaving them stuck with inadequate performance. For the past three decades, we have relied on Rexa Hydraulic controllers for their reliable and high-quality performance.
Thank you for your response. Today, we will be implementing the feed forward function to improve system stabilization. By monitoring the pressure in the tank that the compressors draw from, we can anticipate and adjust for changes in pressure. This will, in turn, regulate the speed of the compressors and the pressure over the gas coolers. I will provide an update here on the effectiveness of this approach. Thank you to everyone who provided their input.
It sounds like a challenging problem you're dealing with. One possible issue could be the response time of your PID loop. A delay of 1-2 seconds may seem insignificant, but it can be substantial, especially in a pressure control system that typically needs to react quickly to changes. You may need to optimize your PID controller settings, particularly the Integral and the Derivative parameters, to reduce your system's reaction time. Additionally, it might help to check the status of your valve drivers and the wiring connections if you've already optimized your controller settings. They may be causing the delay in response time. Also, check relay settings as well as the compatibility of your controller EKE 1A with the PLC and valves.
It seems like you've got a classic issue of control lag affecting your system's response time. PID tuning could possibly alleviate this problem but making sure your PLC's scan time is optimized can also be crucial. Remember - every additional instruction in your PLC program increases the scan time, which can lead to a delay in signal inputs. Beyond that, you might want to check the EKE 1A's responsiveness; make sure its settings are correct and its feedback mechanism is well-tuned. If everything is fine there, consider checking your valve drivers and their communication with the PLC. Hope this helps shed some light and moves you towards a resolution!
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Answer: - The delayed valve response could be due to the PID controllers in the PLC programming or the characteristics of the valve drivers being used.
Answer: - The delay in valve response leading to slower regulation of the CO2 flow could be causing the pressure to rapidly increase beyond the safety limit.
Answer: - You can try adjusting the parameters of the PID controllers, checking the communication between the PLC and the valve drivers, and ensuring proper integration of the superheat controller EKE 1A from Danfoss.
Answer: - You may need to fine-tune the PID controller parameters, review the signal input accuracy, and verify the compatibility of the valve drivers with the PLC system to improve the valve response time.
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