On-Line Noise Estimation for Adaptive SPC Loop Control

Abstract

This paper describes a method of measuring the variance of a noisy process on-line, enabling the application of adaptive Statistical Process Control (SPC) to supervise a continuous process loop. A fundamental requirement of SPC techniques is that the data are independent. Process control loops typically generate autocorrelated data, and this tends to create an increase in the false alarm rate when attempting to apply SPC techniques. The method presented here produces an uncorrected sequence from potentially correlated data, overcoming this problem. The SPC supervisory unit acts externally to the control loop preventing any lags in feedback. When these SPC charts are used in a supervisory role with a conventional PID controller, application of individual PID terms achieves a much quieter control with little reduction in performance and without the need for prior off-line noise tests. The results of tests in simulation and on a steam-water heat exchanger are presented.

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“…Prior to its removal as part of building refurbishments, the heat exchanger had been used for development of modelling and control strategies, which had been published in two journals. [7] , [8] . The first paper [7] describes an automatic tuning method for the design of PID controllers and comprises accurate modelling and control design achieved by a combination of polezero cancellation and frequency response methods.…”

“…This was a simple first order filter with a 0.25 s time constant. In the simulation work described for the development of the adaptive statistical process control loop supervisor [8], a coloured noise signal had been added to the simulation control loop, as a signal was required that displayed similar characteristics to the noise present on the actual heat exchanger system, which appeared to be a noisy 'random walk'. This comprised many different components of various frequencies, including random noise from instrumentation, and cyclic variations from disturbances such as: changes in the steam pressure and quality from the boiler plant, changes in control air pressure, variations in the mains water temperature, variations in the ambient temperature, and variations in the water speed through the heat exchanger.…”

“…Prior to its removal as part of building refurbishments, the heat exchanger had been used for development of modelling and control strategies, which had been published in two journals. 7,8…”

“…The first paper 7 describes an automatic tuning method for the design of proportional-integral-derivative (PID) controllers and comprises accurate modelling and control design achieved by a combination of pole-zero cancellation and frequency response methods. The second paper 8 describes the process of measuring the variance of the heat exchanger in real time enabling the application of adaptive statistical process control to supervise the continuous process loop. Each paper describes the plant in detail and presents the modelling that had been carried out.…”