Performance analysis of cross-direction process control using multivariable and spectral models

Addresses two interrelated problems in cross-directional (CD) control of the paper making processes. The first problem is explaining paper property streaks observed in many CD control installations. The streaks usually correspond to the high-resolution error profile harmonics with wavelength close to twice the actuator spacing. Typically, the streaks of paper weight or moisture are very stable. A question that often arises is whether the streaks can be compensated by better tuning of the CD controller. The second, related, problem addressed in the paper is how the achievable high-resolution error profile and its spectrum depend on the CD controller tuning. The paper compares the performances of a mapped controller and an optimal multivariable controller. It also describes spectral characteristics of CD error profile around the Nyquist frequency. We demonstrate applications of the developed analysis methodology to paper mill data where weight streaks are a problem.