Research On Cross Directional Basis Weight Control System For Paper Machine Based On Hydraulic Headbox With Dilution Water

Basis weight is the weight of paper per unit area and it is one of the most important parameters of paper quality. The requirements for uniform distribution of fibers(mostly basis weight) are stricter than ever, because of consumers’ needs for paper with good quality and the widely use of high-speed presses. Using machine directional control only could not make paper with good quality. Cross directional basis weight distribution has become an important concern of paper making companies. Advanced dilution water headbox control the basis weight by adding dilution water, and this brings CD control to a new stage. Putting forward practical control strategy based on new methods and new requirements is a research hotspot of paper industry. This work is partially supported by Innovative Engineering Projects of Shaanxi Province(2012KTCQ01-19). Aiming at the interference, strong coupling, large delay and other problems that occur during the cross directional control process, a complete basis weight control solution is proposed. The main work of this paper can be summarized as the following aspects:(1) Research on cross directional control strategy with dilution headbox.The structure and working mechanism of the hydraulic headbox with dilution water are analyzed, especially dilution water concentration adjustment method. Advantages of dilution water adjustment method are analyzed. The overall control scheme is proposed to ensure the real-time and save costs based on the analysis of many difficulties in the process of cross directional control. The field data of basis weight is got from existing QCS system. A cross directional basis weight control system is built to calibrate data, de-noising, run the algorithm and control dilution valves.(2) Research on online detection and processing of basis weight.Infrared, β-rays, X-rays measurement principle for basis weight are overviewed, and their transmissive measurement solutions are given. The process in which the original measurement signal of X-ray experimental platform is transformed into the real weight of paper is showed. In the industrial environments, scan head slight vibration, displacement, interference ash affect and temperature drift happens sometime. A denoising method based on improved EMD is proposed. The method combines with the moving average filter algorithm, and separates the noisy signal by frequency using the time-scale characteristics of EMD. Simulation results show that the proposed algorithm is better than traditional denoising methods. The algorithm is also proved effective in the field data application.(3) Controller design of the CD basis weight control system.A general cross directional control system model for dilution headbox dilution water is proposed, assuming coupling can be treat as static. Through analysis and decomposition of the model, two algorithms are used to solve the correlation matrix and large delay control problems respectively. A cross directional basis weight control algorithm is proposed based on Dynamic Matrix Control(DMC) and Gram polynomials. Gram polynomial is used to transform model, a non-square coupling matrix is transformed into coupling between Gram matrix, then conduct global decoupling. The DMC algorithm is applied to the SISO systems with large time delay. Two control schemes are analyzed and discussed. the lower cost solution at this stage is selected. The basis weight control problem is converted to Gram polynomials parameter control, greatly reduces the control loop. It is superior to conventional algorithms.(4) Implementation of the cross directional control system.An implementation of cross directional basis weight control system is given by using Siemens S7-300 controller hardware, Step7 V5.5 and Win CC V7.0 software development platform, combining with OPC protocol and Industrial Ethernet communication. The program combines existing QCS system, and the implementations of algorithms are based on Matlab. Engineering application results show that the program to be effective, the basis weight distribution improved significantly.