Closed Loop Tuning vs. Open Loop Tuning: Tuning all your Loops While the Process is Running is Now Possible
12 March, 2010 | White paper
Traditional methodology for optimizing and tuning PID loops (excluding ‘trial-and-error’) rely on ‘open-loop’ tests, whereby the loop is placed in manual mode and the controller output is moved, usually in a step-wise fashion. The issues with this ‘step’ test are numerous, primarily that the test will disturb the process. This is especially true for slow processes such as temperature, where a seemingly small output move could result in a large process change that can have serious consequences.
There are now tools and methods that enable one to safely optimize and tune PID loops in closed loop mode. Some of these tools can generate and send small, rapid setpoint changes to the controller, independently of any operator intervention. With the right understanding of these tools, it is now possible to optimally tune every PID loop in a plant with minimal time and risk. If planned correctly, process control personnel will define the boundaries and conditions and the tests will be done during night shifts or at any moment. Later, the process control engineer will analyze the results and decide on tuning objectives; software will calculate tuning parameters. Optimizing processes and tuning loops without spending hours in the control room reduces not only the resources needed but also production losses and the attention of operators.
This paper will discuss how these new tools may work in closed loop tuning mode, when and how they can be used, when they may fail, and the mistakes people may make when using them.
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