Product quality related process variables have significant role in advanced process control (APC). Online analyzers and software sensors can provide accurate and timely information for APC systems. In this paper we give an overview of data based soft-sensor development. We show that soft-sensor models of APC require maintenance and demonstrate that statistical quality control (SQC) techniques can be effectively used to automatize the related fault detection tasks.
I. INTRODUCTIONAdvanced process control systems should have the following functionalities to ensure and continuously improve product quality [1]:• prediction of product quality from operating conditions, • optimisation of operating conditions to improve product quality, and • detection of faults or malfunctions for preventing undesirable operation. These functionalities require timely and accurate information about process variables characterizing and influencing product quality. Offline laboratory tests mostly take more than two hours which time delay can cause control problems resulting economic loss. Online analysers have faster response time (1-4 minutes) and can eliminate the dependence on laboratory data by providing timely information for corrective action or real time control (see Fig. 1) [2]. However, due to the high instrumentation and maintenance costs and low reliability of online analyzers there is a need for an easily implementable, maintainable and robust alternative.