Auto-tuning Of Linear Active Disturbance Rejection Controllers

In industrial process control,the auto-tuning of controller is regarded as an effective method to improve the automation of control system.The steps of controller auto-tuning is to obtain the output data of the plant through an appropriate excitation first,and then use these simple sampling data to obtain the important characteristic parameters of the system.Finally the controller parameters can be obtained by these characteristic parameters.At present,ABB,Siemens,Honeywell and other companies have used the auto-tuning technology of PID in their own products,and the effect is good.Although PID control is widely used in industrial field,the effect is not always good by some researches.Due to the limitation of PID structure(error feedback),it is also a fact that the PID control is not good for large delay-time plants and nonlinear plants.Linear active disturbance rejection control(LADRC)is developed by PID and modern control theory.The internal uncertainty and external disturbance are regarded as the total disturbance,which is estimated by extended state observer(ESO)and compensated by state feedback in real time,so as to eliminate the disturbance quickly.It can be seen that LADRC is an,"upgraded" edition of PID,so it is very important to study the auto-tuning of LADRC.Firstly,several common identification methods are introduced.The relay feedback identification method and the two-point method of step response are selected as the basic identification methods of the auto-tuning of LADRC through some comprehensive considerations.Secondly,the structure of LADRC is analyzed.The "integral" part of LADRC is included in the estimation of generalized disturbance of ESO.The relationship between PID and LADRC is further explained.And then the conversion between the parameters of LADRC and the parameters of PID is studied.This section proposes that second-order LADRC can be approximated by actual PID,and the concrete approximate formula is given.Finally,two auto-tuning formulas are derived for second-order LADRC by curve fitting:The relay feedback auto-tuning formula and the step response auto-tuning formula.These two formulas are only related to the characteristic parameters of relay feedback experiment and step response experiment.As long as a relay or step experiment is conducted on the controlled plant,the parameters of LADRC can be obtained by "one key".The simulations show that these two auto-tuning formulas proposed in this paper have good control effect and high practical application value.It also provides a good theoretical basis for the parameter auto-tuning of LADRC.