Proceeding Articles

An examination of papermachine wet end fibre balance and stock flow equations reveals the interdependence of important papermaking parameters and the control strategy required to make changes in machine running conditions. To avoid breaks during any adjustment, the fibre and water balance of the wire must be controlled closely and any controlling device must incorporate some ` built in’ knowledge or mathematical model of the process . It is shown how analog computing elements can fulfil this requirement and thus provide a low cost alternative to a digital computer installation.

An analog computer control system has been in operation for 18 months on the No. 2 papermachine at the Grove Mill Paper Co. of New Mills. During this period, a 15 percent increase in production can be attributed directly to the use of the computer. Results given include a typical beta-gauge chart, showing the close control of basis weight and the sequence of events that occur during a grade change.

No. I machine at Chartham Paper Mills produces a high grade of tracing at substances ranging 40-160 g/m2 . Increased process understanding of the wet end system led to the idea of using a simple analog technique to obtain a more uniform and more efficiently produced product from this machine.

The computer system was installed in January 1968 and has been fully operational since then. It consists of two basic operations, feedforward and feedback control. The feedforward control uses process equations calculated from equilibrium conditions that link together stuff gate flow, breast box flow and efflux ratio with reel-up substance and dry end speed. The feedback control maintains the required substance by feedback to the stuff gate.

Performance analysis of the computer has shown that broke at substance changes has been significantly reduced and wet end conditions are considerably more stable . Present development work consists of incorporating computer dynamic substance control on this machine.

The response of a flow balance coating weight control system to changes in control settings and transient disturbances has been investigated by means of an electronic analog. A non-linear blow-off characteristic, determined experimentally, is shown to give rise to instability in certain conditions. The effect of pipe lags and valve action times can be demonstrated by use of the model; the validity of analog approximations to pure time delays is discussed and corroborated by results obtained with a digital computer simulation technique. The extension of the model to take account of preferential pick-up of moisture by the paper web is described.

Supervisory control of average basis weight and moisture in the machine-direction must be achieved by different control procedures, depending upon the importance of other objectives such as production rate and dimensional stability . Maintenance of close control of refining must be modified, depending upon the number and arrangement of refiners. The control of basis weight, moisture and refining all interact with each other to increase the complexity of the control models required, since the adjustment of a single input variable may influence all three areas of control. Some of the experiences of International Paper’s Southern Kraft Division control group in working with these control areas are described.

Automatic basis weight regulation on No. 1 fine paper machine at Empire Paper Mills has been achieved by using a substance gauge at the dry end, connected to a central control digital computer, this computer performing direct digital control (DDC) upon various areas of the papermachine.

The paper describes an information system applied to a newsprint machine as a functional addition to the operator’s capabilities . Examples of its use by the operators are given.

The system was designed, as part of an initial machine installation, after the intensive study of the mode of operation of two generally similar machines. The objective was to retain the skilled operator as part of the control system, but to provide him with more information more conveniently displayed to enable better and faster decisions to be made. The stored program system permits the operator to manage the machine on an information by exception basis.

The system is a Bailey 754 with alarm monitoring data display and future arithmetic capability. There are 150 analog inputs, 200 contact closures, 26 speed and draw measurements and 15 integrations. Data acquisition is continuous providing data to the parallel programmed and independently operating subsystems.

The paper examines the various stages in a project to implement control of a papermachine and its associated stock preparation plant using a digital computer. The project is looked at from the initial economic feasibility through to the commissioning and implementation stage.

A general description of the computer system and the control strategy employed is included. Particular mention is made of those aspects of the project not immediately associated with the control of the process, yet making important contributions to the whole system.

Reference is made to the more significant problems faced during the project, giving when possible indication of lessons learned for future applications.

My purpose is to give α brief description ofwhat we have achieved with our computer installation and to draw some comparisons in philosophy with the paper by Cyprus & Attwood.

Almost without exception, process control computer systems have been to some extent development projects. Most have been applied to commercially operated production units, for which the improved control of the process provides adequate financial justification for the computer installation. In such cases, conflict arises between the use of the computer for the basic control of a continuously running process and the development of new programs. One solution is to provide α comprehensive conventional standby control system so that the computer can be taken off line at any time; alternatively, to limit the system to supervisory set point control . This approach is very expensive and there is α tendency for the computer to become purely α research too l, imposed upon α rather unwilling production unit. Another common approach is to use a relatively expensive computer system with backing store and full time-sharing facilities. This second method is justifiable when the process control computer is required to carry out α large amount of off-line work such as scheduling and invoicing, but is unnecessarily expensive if the computer’s principal task is process control.

This paper discusses the current trends in the evolution of hardware for computer control and the implications of these trends for the papermaking industry. This discussion covers, in addition to computer main frames, the development of remote analog signal multiplexing, conversion and digital data transmission, the evolution of graphic displays as operator panel replacements and the elimination of long cable runs from instruments by the use of laser data links.

As an overview of the future, an indepth presentation is given of an integrated papermill computer control hierarchy (of the sort to be expected within five years). In this configuration, the actual process control is performed by a number of freestanding (that is, no special computer room), small (20 in x 20 in x 60 in) computers dedicated to the direct digital control of the process sub-units (for example, grinders, bleaching plant, head box, dryer section and coaters). These first level computers are subservient to a larger foreground/background computer, performing such functions as control parameter optimisation, management information reduction and presentation, as well as on-line optimum production scheduling.

This portion of the paper also includes a discussion of the control techniques and strategies that will be in common use at this date and the additional process variables that will be on computer control (gloss, air permeability, colour and caliper). A part of this discussion covers the future state of the art, which will be approaching the control of the distributed parameter aspects of the process (such as cross-machine moisture and basis weight control).