NC State
S. Hem. Computer control of a papermachine using a linear state space model. In Papermaking Systems and their Control, Trans. of the IVth Fund. Res. Symp. Oxford, 1969, (F. Bolam, ed.), pp 541–557, FRC, Manchester, 2018.


Perturbation experiments on a Fourdrinier papermachine have confirmed that its dynamic behaviour can be represented adequately by a state space model in the form of a matrix difference equation. The basic equations involved have been treated in general terms, but the discussion on the model building and control system design is made more explicit by reference to a specific system.

Methods have been developed for investigating and describing the papermaking system as a process to which modern control theory can be applied. It has been shown how the model can be used to determine possible control strategies to change grade in such a way that t e grade change time is at a minimum and certain papermaking criteria are obeyed. The control objectives have been stated by analytical performance criteria in the form of quadratic cost functions.

Α simple grade change at constant machine speed was achieved by altering the thick stock flow according to a trajectory determined by the rate constraint of the flow valve. It was found necessary to manipulate and synchronise the thick stock and thin stock flows together with the machine speed in order to change grade at constant production rate.

Based on optimum control and filter theory, an on-line controller has been designed to manipulate the thick stock flow in order to minimise the variance of the measured basis weight . The developed  formulation incorporates optimum estimation of inaccessible state variables as an implicit feature. The control action is given by a proportional term together with a memory term to account for past values of control and basis weight. The controller has been implemented on a machine and is shown to have stabilised the system considerably.

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