Quality Control System QCS for paper, board and tissue machines

A quality control system (QCS) refers to a system used to measure and control the quality of moving sheet processes on-line as in the paper produced by a paper machine. Generally, a control system is concerned with measurement and control of one or multiple properties in time in a single dimension. A QCS is designed to continuously measure and control the material properties of the moving sheet in two dimensions: in the machine direction (MD) and the cross-machine direction (CD). The ultimate goal is maintaining a good and homogenous quality and meeting users' economic goals.A basic quality measurement system generally includes basis weight and moisture profile measurements and in addition average basis weight of the paper web and moisture control related to these variables. Caliper is also one of the basic measurements.

Other commonly used continuous measurements include: ash content, color, brightness, smoothness and gloss, coat weight, formation], porosity, fiber orientation, and surface properties (topography).

QCS is used in paper machines, board machines, tissue machines, pulp drying machines, and other plastic or metal film processes

In modern systems QCS applications can be embedded to distributed control systems.

Sensors measuring the paper quality (online meters) are attached to a sensor platform that move across the web guided by the scanner beam. A typical crossing time for a sensor platform is 10–30 s (an 8 m web, 60 cm/s). The sensor platform scans across the paper web and continuously measures paper characteristics from edge to edge. It can also be directed and stopped to a specific, fixed point on the web to measure the machine-direction (MD) variation at a single point.

The QCS scanner beam is an essential part of a QCS system. Wide machines and accurate profile calculations require beam stability and accuracy of mechanical movement. As high accuracy in demanding and variable conditions is required, the sensitive sensors must be securely fastened. The most important goal is maintaining the exact respective position of the upper and lower measurement platforms in relation to their distance from each other, in MD and in CD . This is achieved through a robust construction and by reducing the effects of temperature and other environmental effects and through a moving mechanism with minimized backlash of the measurement platform.

Usually the scanner beam also contains all the cables and the air, cooling liquid and protection gas pipes. The base of the scanner beam contains elements that dampen vertical vibration.

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