Research On Auto-tuning Controller Based On Frequency Characteristic

PID controller is widely used in the industry control process, but the parameters tuning is still a problem that people concerned. The skill is more important than science in the PID parameters tuning.Auto-tuning normally is sorted into two means: one is based on modern control theory, which needs self-contained knowledge about the object. However, we always don't know exactly the structure of the object in the process control domain, so, it is difficult to build the model of the object-under-control. Another one is based on classical control theory, and it is needs the model that is in the function range of P PI PID control systems. The effect of the control will be ok, even if there is a little difference in the high frequency part.We always use the second means in the project. Especially, in the use of the critical scaling limited method, vibrate period-Tc and amplitude-Kc can be gotten for controller parameters. Because it is difficult to make stable critical vibrate and difficult to test parameters online. There is another means called relay-tuning method, which is put forward by Astrom and Hagglung. It ensures the stable closed loop vibrate and testing online, which is the one of simplest methods.Although relay-tuning method is used widely in the industry, it can take in interfeence and can't produce symmetrical vibrate.Aim at the problem above, referring a new method to get the characteristic of object-under-control, which is called sf(simple frequency) algorithm. SF arithmetic can get the characteristic of object-under-control easily, and then to tune the parameters with Z-N tuning formula.This algorithm carries out object characteristic with FFT,and get -180 degree traversing and the corresponding amplitude. Have similar step respond character with Z-N method. The test of fieldbus control system validates that it is available to carry out object characteristic with sfalgorithm.