Data Mining of Historic Data for Process Identification, Report no. LiTH-ISY-R-3039

Performing experiments for system identi cation is often a time-consuming task which may also interfere with the process operation. With memory prices going down, it is more and more common that years of process data are stored (without compression) in a history database. The rationale for this work is that in such stored data there must already be intervals informative enough for system identi cation. Therefore, the goal of this project was to nd an algorithm that searches and marks intervals suitable for process identi cation (rather than performing completely automatic system identi cation). For each loop, 4 stored variables are required; setpoint, manipulated variable, process output and mode of the controller. The proposed method requires a minimum of knowledge of the process and is implemented in a simple and e cient recursive algorithm. The essential features of the method are the search for excitation of the input and output, followed by the estimation of a Laguerre model combined with a chi-square test to check that at least one estimated parameter is statistically signi cant. The use of Laguerre models is crucial to handle processes with deadtime without explicit delay estimation. The method was tested on three years of data from more than 200 control loops. It was able to nd all intervals in which known identi cation experiments were performed as well as many other useful intervals in closed/open loop operation.