Objective of PM Control Systems

 Objective and General Terms of PM Control Systems

9.1.1
Objective
A typical paper machine has a few thousands remote or automatically controlled functions like positioning systems, drives, valves and more than a thousand addi¬tional measurement devices to monitor the process and the condition of the ma¬chine. It is impossible for an operator to oversee all these devices without the help of modern automation systems. For example, he is guided through the process of starting the machine, and many control sequences happen automatically after a “start” button is pressed and he gets detailed diagnostics if something fails. Com¬puters are also used to maintain the quality standards, however the operator is still needed to handle the exceptions and to decide how to proceed when something fails.
9.1.2
Explanation of Terms
Control systems for paper machines can be categorized in different ways. The following terms are those most commonly used :
• MCS, machine control system: Discrete logic, safety interlocks, sequential controls. The MCS ensures that the machine attendant can safely operate the machine using control desks with pushbuttons, switches, status lamps, etc. Many of these functions are usually also available on screen displays.

Examples of MCS func¬tions are movements of rolls, blades or other machine parts with involvement of limit switches or other safety interlocks. MCS functions usually require fast reaction by the MCS. For example: If a ma¬chine part reaches a limit switch during a controlled movement, it usually has to be stopped by the MCS in less than 20 ms. The cycle time of a MCS is therefore in the 5 to 20 ms range.
 .• DCS, distributed control system:
 .– Analog process controls Analog process controls are mainly simple standard controllers. Examples are level controllers in chests, pressure controllers in pipes, etc. Most process con¬trollers act much more slowly than the MCS controls. Actions happen in sec¬onds to minutes. The cycle time of typical DCS functions is therefore in the 1 to 4 s range.

 .– Wet end controls Some controls in the process are more complex than just PID control loops. One example is the wet end control which ensures uniform process conditions in the approach flow of the paper machine and in the forming section by maintaining constant consistencies, gas content, charge and other parameters. It involves
 .e. g. special software tools (soft sensors) using wet end data instead of direct measurements to predict basis weight during start up of the machine, when the paper has not yet reached the reel or the quality measurements.

Such, wet end controls not only reduce the start up time of the process, but also minimize the consumption of chemicals, i. e. only as much retention aid is used as is really needed to reach a required retention level in the forming section.
 .• QCS, quality control system:
 .– Paper quality measurement The QCS has on-line sensors for basis weight, ash content, moisture content, gloss and other paper quality parameters. The sensors are mounted on measure¬ment frames, which allow the sensors to move (scan) within 20 to 40 s across the paper width.

These sensors measure the paper quality parameters in cross ma¬chine direction (CD) and machine direction (MD). Each scan typically gives about 1000 measurement values (databoxes), equally spaced across the web width. An array of CD measurement values is also called a CD quality profile or CD profile. The average measurement of each scan gives one MD quality value. A time series of MD measurement values is also called an MD quality profile or MD profile. MD measurement values and CD profile measurements are available only every 20 to 30 s.

 .– Paper quality control in the machine direction The first target of quality control in MD is to keep the main quality parameters of the produced paper within the limits defined for the current production. As an example, basis weight control is achieved by continuously adjusting the stock valve, which feeds the blended stock to the machine.

 The second target is to ensure fast grade changes, i. e. to ensure a fast change of basis weight, moisture and ash content to meet the specification of the next produced grade with a short transition time. The third target is to allow the operator to change machine settings without too large an impact on paper quality. The most important example is the coordinated

9.1 Objective and General Terms of PM Control Systems
speed change control. This is a tool to allow machine speed changes without disturbing paper quality during the transition period. The effect of a speed change is compensated by coordinated adjustments of stock valve, ash valve and drying energy, based on a dynamic process model. MD controls are slow but complex control loops.

 Only two to three measure¬ments are available per minute. Using interpolation techniques or Kalman fil¬ters, control steps can be performed every 5 to 10 s. MD controls are complex because of the considerably large transport dead time between actuators (i. e. stock valve) and measurement (i. e. scanner at the reel), which is typically larger than 1 min. Additionally the quality parameters have some influence on each other. Opening the ash valve increases not only the ash content but also the basis weight and moisture. Increasing the basis weight leads to higher moisture content at the reel, if the drying energy is not increased simultaneously. Thus, a decoupling controller with dead time compensation is required.

 .– Paper quality control in cross machine direction Many paper properties can be controlled in small control zones across the ma¬chine, to ensure uniform quality in CD. Such actuators are located e. g. in the headbox to control basis weight, in the press and in the dryer section to control moisture content, and in the calender section to control caliper across the paper width. CD controls act even more slowly than MD controls:

The measurement noise on zonal measurements is much higher than the noise on MD measurements. The measurement time of one zonal measurement (databox) is around 20 ms, whereas MD measurements are averages of typically 1000 databoxes (1 scan). The special complexity of CD controls is the coupling of adjacent control zones. Each control zone affects the quality of some neighboring control zones.

The controller needs to know exactly the spatial response of actuator movements in the CD. On the other hand the exact location of the response of a CD actuator in the paper may change over time e. g. due to changing shrinkage of the paper in the drying section, or by web wander to the left or right.
 .– Paper quality reports The QCS not only controls quality, but also documents the achieved quality, which is required according to ISO 9000 and other quality regulations. The quality reports also serve internally within the paper mill as a means of supervising the efficiency of the production process, by reporting per week, per day or per shift how much paper of which quality has been produced.

• MCC, motor control center:
 .– From these cabinets hundreds of motors on the machine are controlled. These are motors for pumps, movements of machine parts or others. The MCC is additionally used to control a few other consumers of electrical power, for example heating devices for oil in calenders.
 .– The MCC is not responsible for the roll drives in the paper machine. The drive system is usually an independent package, which is delivered by the ma¬chine drive supplier, and which receives its setpoints from the DCS.

 .• Monitoring systems:
 .– Bearing and vibration monitoring
Acceleration sensors are attached to bearings and close to roll shafts, to detect
bearings at risk of failing and roll roundness abnormalities before they have a
negative impact on the process. Thus bearing and roll changes can be done
during the next planned shutdown of the machine, and do not cause extra
downtime.

 .– Monitoring of periodic paper quality variations
Typically 30 to 100 process data are measured in parallel. The measured signals
include important quality data, taken from the QCS sensors. The target is to
detect the occurrence and the reasons for periodic paper quality variations.

 .– Web break monitoring
Web break monitoring is a camera-based tool to find the reason for paper breaks,
and as a consequence to improve efficiency of the production process.

 .– Web inspection system
Web inspection systems are also camera based. Cameras are mounted in the CD
direction across the web, and check the complete production for defects like
holes, dark spots, white spots.

 .– Information system:
An information system mainly fulfils three tasks:

1. 1. It collects on-line data of MCS, DCS, QCS and monitoring systems into acentral data storage.
2. 2. It provides a user interface to view the historical data of these systems, allow¬ing one to compare and correlate the data from the various origins. The most convenient user interface is based on web browser technology, similar to the Internet. Some of the data are presented to the user as time trends, others as statistical reports over a defined production period.
3. 3. It provides tools for data analysis. This usually includes the capability of show-ing the fourier transform or the cross correlation of selected data. More sophisti¬cated information systems contain expert systems or data mining tools, to auto¬matically detect relationships between data.
. • Automation system: All the systems above together form the complete automation system of a paper machine (Fig. 9.1). The automation systems are grouped around a fiber optical data network, which has a ring topology for redundancy reasons. The connected systems are operating stations, engineering stations for configuration or main¬tenance work, DCS cabinets, server computers and dedicated control computers for various monitoring and control purposes.

 The dashed lines indicate the dif¬ferent locations with automation equipment, such as control rooms for opera¬tors, electrical control rooms (rack rooms) with DCS or QCS computers, or the paper machine.

9.1 Objective and General Terms of PM Control Systems
 
To summarize, nowadays systems can be divided into the following categories
 .• DCS/MCS
 .– Control systems for analog and digital controls
 .– Capable of implementation of advanced model based controls which are needed for e. g. soft sensor or QCS applications
 .• QCS
 .– Quality measurements for paper properties (direct measurements)
 .– Measurements for felt and wire condition
 .– Indirect measurements of paper properties, also called soft sensors
 .– Quality controls (sometimes integrated into DCS systems)
 .• Monitoring systems
 .– Bearing and vibration monitoring
 .– Monitoring of periodic paper quality variations
 .– Web break monitoring
 .• Information system
 .– Process history database
 .– User interface
 .– Diagnostic tools
. • Automation system

– Includes all of the above systems
In the following chapters some elements of a paper machine automation system are discussed in more detail. The focus is on QCS and information systems, as they are mainly responsible for the ability of the mill to maximize quality tons on the reel.

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