Model Predictive Control and Optimization for Papermaking Processes [chapter]

Danlei Chu, Michael Forbes, Johan Backstrom, Cristian Gheorghe, Stephen Chu
2011 Advanced Model Predictive Control  
Advanced Model Predictive Control 310 trajectories to quickly and safely transfer production from one grade to the next. Basic MD-MPC, economic optimization, and automatic grade change are discussed in this chapter. MPC for CD control was introduced by Honeywell in 2001 (Backström et al. 2001 . Today, MPC has become the trend of advanced CD control applications. Some successful MPC applications for CD control have been reported in (Backström et al. 2001 Chu 2010a; Gheorghe 2009 ). In
more » ... processes, it is desired to control the CD profile of quality variables such as dry weight, moisture, thickness, etc. These properties are measured by scanning sensors that traverse back and forth across the paper sheet, taking as many as 2000 or more samples per sheet property across the machine. There may be several scanners installed at different points along the paper machine and so there may be multiple CD profiles for each quality variable. The CD profiles are controlled using a number of CD actuator arrays. These arrays span the paper machine width and may contain up to 300 individual actuators. Common CD actuators arrays allow for local adjustment, across the machine, of: slice lip opening, headbox dilution, rewet water sprays, and induction heating of the rolls. As with the CD measurements, there may be multiple CD actuator arrays of each type available for control. By changing the setpoints of the individual CD actuators within an array, one can adjust the local profile of the CD measurements. The CD process is a multiple-input-multiple-output (MIMO) system. It shows strong input and output off-diagonal coupling properties. One CD actuator array can have impact on multiple downstream CD measurement profiles. Conversely, one CD measurement profile can be affected by multiple upstream CD actuator arrays. Therefore, the CD control problem consists of attempting to minimize the variation of multiple CD measurement profiles by simultaneously optimizing the setpoints of all individual CD actuators (Duncan 1989). MPC is a natural choice for paper machine CD control because it can systematically handle the coupling between multiple actuator and multiple measurement arrays, and also incorporate actuator physical constraints into the controller design. However, different from standard MPC problems, the most challenging part of the cross directional MPC (CD-MPC) is the size of the problem. The CD-MPC problem can involve up to 600 MVs, 6000 CVs, and 3000 hard constraints. Also, the new setpoints of MVs are required as often as every 10 to 20 seconds. This chapter discusses the details of the design for an efficient large-scale CD-MPC controller. This chapter has 5 sections. Section 2 provides an overview of the papermaking process highlighting both the MD and CD aspects. Section 3 focuses on modelling, control and optimization for MD processes. Section 4 focuses on modelling, control and optimization for CD processes. Both Sections 3 and 4 give industrial examples of MPC applications. Finally, Section 5 draws conclusions and provides some perspective on the future of MD-MPC and CD-MPC. Overview of papermaking processes A flat sheet of paper is a network consisting of cellulose fibres bound to one another. A paper machine transforms a slurry of water and wood cellulose fibres into this type of network. The whole papermaking process can be regarded as a water-removal system: the consistency of fibre solutions, called stock by papermakers, increases from around 1% at the beginning of a paper machine (the headbox) to around 95% at the end (the reel). www.intechopen.com Model Predictive Control and Optimization for Papermaking Processes 311 Brief description of papermaking processes In general a paper machine can be divided into four sections: forming section, press section, drying section, and calendering section. In the forming section, the stock flow enters the headbox to be distributed evenly across a continuously running fabric felt called the wire. The newly formed sheet is carried by the wire along the Fourdrinier table, which has a set of drainage elements that promote water removal by various gravity and suction mechanisms. These elements include suction boxes, couch rolls, foils, etc. The solid consistency of the paper web can reach 20% by the time the web leaves the forming section and enters the press section. Figure 1 illustrates the configuration of a Fourdrinier-type paper machine.
doi:10.5772/18535 fatcat:iyfzytyqszer3pen6djfbnkule