Cross-Directional Control Aids Paper Making

Paper-making is a precision operation. Weight, moisture, and thickness of paper must be uniform from the start to the end of a roll and across the width of the sheet. A variety of control techniques have been developed to maintain the quality of the paper not only in the direction the sheet is moving but in the cross-direction as well.

02/01/2003


KEY WORDS

 

  • Process control & instrumentation

  • Process control systems

  • Advanced control

  • Controllers

  • Instrumentation and process sensors


Suppliers

Paper-making is a precision operation. Weight, moisture, and thickness of paper must be uniform from the start to the end of a roll and across the width of the sheet. A variety of control techniques have been developed to maintain the quality of the paper not only in the direction the sheet is moving but in the cross-direction as well.

The accompanying illustration shows a paper machine with actuators and sensors requiring to perform a variety of cross-directional (CD) control functions. Each controller has a different goal and a different strategy for accomplishing its objectives.

Look at the variables

The weight of the paper determines its suitability for particular applications. A sheet of newsprint has a weight of about 45 gsm, a paperback book cover may have a weight of about 300 gsm, and cardboard may weigh 450 gsm. Variations in the weight of the paper sheet will also affect its other properties.

CD control of the paper's weight is accomplished by actuators at the headbox. The function of weight control actuators is to achieve an even distribution of the pulp fibers across the width of the wire belt supporting the sheet, despite changing pulp properties. Since weight control actuators are located furthest upstream, the dynamics of weight control must compensate for a significant transport delay or dead time that results as the paper sheet travels through the entire machine before reaching the scanning sensor.

Due to the nature of the raw materials, characteristics of the pulp stock change over time. Consistency and drainage properties of the delivered stock are kept as constant as possible by the approach system, but variability inevitably occurs. In addition, flow of the pulp stock through the headbox and on the wire belt can distribute wood fibers unevenly in the cross-direction. Profile control of the basis weight is important not only for reasons of paper strength, but also because poor quality weight profile will propagate downstream and appear as disturbances in the moisture and caliper (thickness) profiles.

Dealing with moisture

Moisture content of a sheet of paper is a very important factor in determining its strength. Typical moisture content targets are 5-9% of the total weight of the paper sheet. Overdrying a paper sheet will reduce its strength as fibers are damaged. An excessively variable moisture profile leads to a variable temperature profile and thus increases demand on the caliper profiling actuators.

Dewatering and drying of paper sheet as it passes through the paper machine is a very complex process and is affected by many factors. Fiber slurry exiting the headbox is approximately 0.5% fibers and 99.5% water. At the other end of the machine, paper that is wound up on the reel is about 95% fibers and 5% water. The goal of CD control of moisture is to perform the fine control and level out a variable moisture profile.

Checking thickness

The caliper or thickness of a sheet of paper is controlled by feeding the sheet through rotating rollers known as the calendar stack . The pressure that the rollers exert on the paper sheet can be adjusted by locally heating (cooling) one of the rollers. As temperature of the roller increases (decreases), its diameter also increases (decreases) due to thermal expansion. Thus, pressure on the paper sheet increases (decreases), leading to a decrease (increase) in the paper caliper.


Typical positions of various actuator arrays and scanning sensors are visible in this wide view of the paper machine.

Early CD caliper control was implemented through the use of hot and cold air showers on the roller. Modern caliper control is much more efficient and uses induction-heating actuators. A high-frequency alternating current is used to generate an oscillating magnetic field at the roller surface. The resulting eddy currents near the roller surface cause the roller temperature and the diameter to increase. Pressure on the paper sheet is increased commensurately.

Like all feedback controllers, cross-directional controllers must be adjusted or tuned to match their efforts to the behavior of the controlled process. There are three main components to tuning a cross-directional controller-alignment, dynamics, and spatial filtering.

Maintaining web control

Aligning a cross-directional control system is the process of assigning each portion of the measured sheet to the appropriate individual actuator in an array of actuators. Alignment is affected by sheet shrinkage due to drying.

The paper sheet may also wander back and forth as it proceeds through the paper machine. If a CD controller is incorrectly aligned, an actuator will attempt to control a measurement that actually belongs to its neighbor. This can lead to an unstable system.

There may be a significant dead time due to transport delay from the actuator array to the scanner. In addition, there may be a significant rise time due to the inertia of the actuators. Rise time represents the interval that the actuators require to implement a requested control action.

As with all feedback systems, process dynamics play an important role in tuning the controller. Symptoms of a poorly tuned controller include slow removal of disturbances or oscillatory control action.

Spatial filtering

Each actuator in the CD array will affect a certain spatial neighborhood around itself known as its footprint . Often this region is wide and overlaps the responses of neighboring actuators. An array composed of actuators with overlapping footprints must be treated with care. This overlap prevents the control system from removing narrow disturbances in the paper sheet.

The controller must be tuned to ignore the narrow wavelength, uncontrollable components of the measured paper profile. This is done through use of spatial filters whose bandwidth must be tuned to the controllable wavelengths of the process as determined by the actuator footprints.

If the spatial filters are not tuned correctly, then the actuators will 'see' the narrow wavelength components of the paper profile and attempt to remove them. However, the process will not respond, and the error will persist. Proceeding around the loop, the actuators will continue to move farther and farther as the error persists in the measurement, eventually resulting in an unstable control system.

Special-purpose controllers

Challenges of implementing and tuning cross-directional controllers are sufficiently specific to paper making machines that several vendors have created special-purpose controllers for the application.

Metso Paper Automation (Tampere, Finland), for example, offers IQControls, part of the PaperIQ product line. An IQController bases its control efforts on actual machinery response models. It is designed to provide sheet profiles of the highest precision and stability. Applications include conventional and dilution headboxes, pressing and drying section profilers, coating profilers, and online and offline calendering operations.

Honeywell Industry Solutions (Phoenix, AZ) has recently introduced IntelliMap version 3-a fully automated software tool that performs process model identification and complete cross-directional controller tuning, including all facets of alignment, dynamics, and spatial filtering.

IntelliMap allows the user to perform a bump test experiment, in which certain actuators are moved in a stepwise manner while the response is recorded. Upon completion of the bump test, IntelliMap automatically identifies the CD process model's time response and CD response shape for all CD actuators and processes.

The newest features of IntelliMap 3 incorporate recent advances in two-dimensional loop shaping for CD controller tuning. IntelliMap tunes the CD controller to perform aggressively and still respect the physical limitations of the process. This avoids the risk of an unstable actuator profile.

More options

ABB's Pulp and Paper Business Unit (Columbus, OH), in addition to offering an automated bump test function, has introduced Robust CD Control to further optimize its CD control application. Without performing a bump-test experiment on the actuators, Robust CD Control attempts to maintain a consistent profile performance under different process conditions.

Robust CD Control uses a set of expert rules to continuously monitor CD profile performance. Before the profile starts to degrade, Robust CD Control will detect any misaligned actuators and automatically readjust the alignments until a better performance is achieved.

-Comments? E-mail Vance at controleng@msn.com






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