1969 Volume 2

One of the problems encountered during the computerisation of No. 6 machine at Dartford was the need for α flow box control system to replace the existing Hornbostel hole arrangement. The circumstances were such that we required α control system implemented by the computer program (DDC) and the facilities for bumpless transfer to an analog controller arrangement in the event of computer failure.

To attain reliability and simplicity, we limited our investigations to the manipulation of the stock and air pressure supply valves to effect automatic control. Our control objectives were

1. The minimisation of level and total head variability.
2. The maintenance of stable control at all times.
3. The inclusion of efflux ratio control, in the computer controller, to reduce general variability and aid weight changes.

The dynamics of a pressurised flow box can be reasonably well described by simple transfer functions, as Dr Smith has shown. Fig. 1 represents the system as multi-variable (two input/two output).

The papers on this subject have described total head controlled by pneumatic valves as actuators. In the Chao & Sanborn paper, the authors mention that a time lag of 5 s or more must be tolerated when using these actuators. Ι suggest that somewhat more rapid control can be arranged by using a modern electronic controller.

A numerical optimisation technique has been used to show, computationally, that simple control trajectories can give considerably reduced grade change times. The models studied include one-pump and two-pump systems with and without changing wire rejection factors. Consideration has been given to some simple constraints.

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.

Entire papermachine control implies at least the regulation of all the product quality variables such as basis weight, moisture content, strength, opacity and porosity. Implementation of such a control scheme is generally done in a piecemeal fashion . The usual starting point is automatic control of basis weight by manipulation of stock flow. The remaining variables are on manual control.

This paper presents the Warkaus Mill computer system and is focused particularly on the paper web moisture profile control developed in our company. The aim is to make use of the possibilities given by the second and the third presses with controlled crown rolls. Special attention is directed to profile measurement data analysis techniques and an on-line continuous statistical variation partitioning method is developed . The information is then used for operator guidance control, based on a derived control law . The profile control is not yet integrated in the system, but the control parameters are being established.

The design of more responsive control systems for papermachine dryer sections requires a knowledge of the coupling between web moisture content and changes in basis weight and machine speed. The interactive effect of changes in basis weight and machine speed on the measured moisture content must be known to allow proper compensation.

An estimate of the steady state coupling coefficients was made by forming an approximate mathematical model and calculating the steady state response surfaces for average web moisture content versus machine speed and basis weight. The model was a realistic representation of the partial differential equations relating heat conduction and vapour diffusion within the web. Approximate, constant values of web properties and transport coefficients and the alternating boundary conditions characteristic of conventional cylinder drying were used . Basis weight of 26-90 lb/1 000 ft2 and speeds of 800-1800 ft/min were studied.

The calculated coefficients showed that, for typical production conditions of 421b board at 1 400 ft/min, a 1 lb change in basis weight would be expected to produce a 2 percent change in web moisture content; a speed change of 100 ft/min would produce a 5 percent change in moisture content. The dependence of the coupling coefficients on the level of basis weight and speed was also demonstrated. The coupling coefficients for basis weight and for speed both increased approximately linearly with the operating speed, whereas the increases in both coefficients through basis weight changes were less significant. These results are of use in the design of non-interactive control systems for papermachine dryer sections.

Although the calculated results were restricted to the steady state gains for this system, an indication is given of the possible use of a more complex model in studying the dynamic behaviour of the drying system.

Control systems, whereby a final customer may obtain a satisfactory product after it has passed through several stages of manufacturing, are characterised by considerable time delays in both feedforward and feedback paths. These systems are examples of the industrial dynamics problems investigated by Forrester and others. The matter under discussion in this paper is concerned with the establishment of sensible control systems and the maintenance of their reliability and accuracy. It specifically considers paper products used in data processing as punched cards and forms for optical character recognition.

Models of the behaviour of national economies have been developed and popularised for some time. The economic behaviour of whole industries has also been examined and statistical analysis has uncovered at least the main features of the working of a number of industries. There is no difficulty in principle in carrying this process of analysis on to the examination of individual products . The paper  discusses these topics and illustrations are provided of national and industrial economic models and the behaviour of several product groups in the U.K. paper industry are examined. The results show that statistical analysis can reveal important economic influences at work, but that it is necessary to beware of special factors, technical, political and social, which can be of overriding importance for particular products at particular times.