Machine Directional Register System Modeling for Shaft-Less Drive Gravure Printing Machines

Abstract

In the latest type of gravure printing machines referred to as the shaft-less drive system, each gravure printing roller is driven by an individual servo motor, and all motors are electrically synchronized. The register error is regulated by a speed difference between the adjacent printing rollers. In order to improve the control accuracy of register system, an accurate mathematical model of the register system should be investigated for the latest machines. Therefore, the mathematical model of the machine directional register (MDR) system is studied for the multicolor gravure printing machines in this paper. According to the definition of the MDR error, the model is derived, and then it is validated by the numerical simulation and experiments carried out in the experimental setup of the four-color gravure printing machines. The results show that the established MDR system model is accurate and reliable.

1. Introduction

Recently, with the development of high-speed and high-precision printing, shaft-less drive technology has been widely used in the latest gravure printing machines. In this technology, the printing rollers are driven by independent servo motors, and the register error is controlled through directly adjusting the speed of the pertinent servo motor instead of changing the position of the compensation roller, so the traditional mechanical shaft is eliminated to make printing faster and more accurate. This register mode has the advantages such as higher response frequency, better control accuracy, and lower additional cost. However, the register error in the shaft-less drive mode has a cumulative effect; namely, the adjustment of register error in upstream will affect all register errors in downstream. Hence, the mathematical model of the register system which can accurately reflect the variation of the register errors is needed to be studied to control the register errors accurately.

It is difficult to get an exact register system model because the register errors are affected by various factors, such as the web tension and speed, the precision of parts and assembling, and the web material and production environment. Register errors are defined as two-dimensional errors: machine directional register (MDR) errors and cross-directional register (CDR) errors. Brandenburg [1] derived a linear mathematical model of the MDR errors of a moving web in a first-order differential equation by using an equilibrium equation of mass which is transported by printing rollers, but this model was based on the mechanical shaft drive mode and could not adapt to the shaft-less drive mode. Yoshida et al. [2–4] developed the MDR system model for the mechanical shaft and shaft-less drive mode, but the model still ignores many influencing factors and is imperfect. Hua et al. [5] got the MDR errors model of gravure printing machines via the system identification method, but this method has poor universality and bad practicability. Although the MDR errors model of the two-color system was established in [6], the MDR system model of the multicolor system was not derived, some key deducing steps were ignored, and some wrong symbols appeared in some formulae, which makes some processes of derivation difficult to be understood for readers. The modeling and control of CDR system were studied in [7–9]. The CDR errors are controlled by the lateral motion of printing rollers, which is greatly different from the control of MDR errors. In addition, initial works of modeling and control in correlation between MDR and CDR errors were researched in [10, 11]. However, the in-depth research on the MDR system model based on shaft-less drive mode is still not enough in gravure printing machines.

Therefore, in this study, a mathematical model of the MDR system in shaft-less drive mode is derived for gravure printing machines by using an equilibrium equation of mass and considering the fluctuation of tension, speed, and other factors. This paper is structured as follows. Section 2 introduces the schematic diagram of shaft-less drive gravure printing machines. Section 3 establishes the MDR errors model of the two-color system and then obtains the MDR system model of the multicolor gravure printing machines. Section 4 verifies the derived model by simulation and experiments. The results show that the model is accurate enough to describe the MDR errors in shaft-less drive gravure printing machines.

2. Shaft-Less Drive Gravure Printing Machines

The schematic diagram of the n-color gravure printing machines is presented in Figure 1, and n stands for the total number of printing units. This system is composed of an unwinding unit, an infeeding unit, an n-color printing system, an outfeeding unit, and a rewinding unit. Shaft-less drive mode has been used in the machines; that is, all of the driving shafts are driven by independent servo motors. The n-color printing system consists primarily of n printing rollers, and c1 is the first printing roller whose main task is to maintain the system speed constant. In the printing subsystem, tension is same with that in infeeding zone at steady state. Load cells are installed at idle rollers in the middle of continuous process in both the infeeding and outfeeding units for tension pickup. Two passive dancer rollers are used to reduce tension fluctuations and measure the tension signals simultaneously in the unwinding and rewinding units. Photoelectric sensors are installed in the machine for registering errors pickup.

Figure 1 

Schematic diagram of the n-color gravure printing machines.

Control of the gravure printing machines is basically composed of tension control and register control. Register control is required for high printing resolution in the multistage printing systems. Hence, an accurate MDR system model is essential to ensure adequate printing performance for the gravure printing machines.

3. Mathematical Modeling

3.1. Basic Assumption

In the gravure printing machines, the MDR error of a moving web is defined in two adjacent printing rollers as the relative difference of the distance between the previous printed pattern by the upstream printing roller and the later printed one in the downstream printing roller. To remove unnecessary interference and simplify the mathematical model, the following assumptions are made in the derivation of model.(1)There is no slippage between web material and rollers and no mass transfer between the web material and the environment.(2)The web material is isotropic, and its properties do not change with temperature and humidity.(3)The web cross-section in the unstretched state does not vary along machine direction in a web span.(4)The density and modulus of elasticity of the web in the unstretched state are constant in the cross-section.(5)The strain in web is small (much less than unity) and uniform within the web span.(6)The printing system is in a stable state, in which there are no disturbances from tension and other factors in time 𝑡≤0.

3.2. MDR Error Model of the Two-Color System

The mathematical MDR error model of the two-color system in shaft-less drive gravure printing machines is illustrated in Figure 2 by the coupling between the ith and (i+1)th units, and the ith unit is register errors reference. As shown in Figure 2, a coordinate axis along the machine direction is set up and the position of the ith unit is set as the origin of the coordinate axis. Figures 2(b) and 2(c) show that the color marks on printing roller ci and ci+1 are printed on the web at the printing instants t₁ and t₂, respectively.

Figure 2 

MDR error model of the two-color system.

The MDR error in the moving coordinate system is to calculate the difference between the printing point on the roller and the printed point on the web representing the same point of the pattern at the printing instants. Therefore, according to the moving coordinate system, the MDR error is just the deviation from the coordinate of color mark on the printing roller 𝑋_𝐹𝑖+1 and that on the web 𝑋_𝐵𝑖. At instant t₂, the MDR error is defined by 𝑒_𝑖𝑡₂=𝑋_𝐵𝑖𝑡₂−𝑋_𝐹𝑖+1𝑡₂. (1) It is to be noted that the movement of the printing roller can be viewed as a combination of two movements: one is the rotation of the roller and the other is the virtual translational movement of the roller along the machine direction which is caused by the variation of the web length. This variation can be affected by many factors such as dryer system, tension sensors, or some other disturbances.

By using assumptions (1) and (2), the following conclusion can be obtained: 𝑑𝑑𝑡^𝐿𝑖₀𝜌_𝑖(𝑥,𝑡)𝐴_𝑖(𝑥,𝑡)𝑑𝑥=𝜌_𝑖−1(𝑥,𝑡)𝐴_𝑖−1(𝑥,𝑡)𝑉^𝑅_𝑖(𝑡)−𝜌_𝑖(𝑥,𝑡)𝐴_𝑖(𝑥,𝑡)𝑉^𝑅_𝑖+1(𝑡). (2) Hence, the mass of the web material entering the printing unit i in the time interval from t₁ to t₂ can be derived as follows: 𝑑𝑑𝑡^{𝑋𝐵𝑖(𝑡2)}_{𝑋𝐵𝑖𝑡1}𝜌_𝑖(𝑥,𝑡)𝐴_𝑖(𝑥,𝑡)𝑑𝑥=𝜌_𝑖−1(𝑥,𝑡)𝐴_𝑖−1(𝑥,𝑡)𝑉^𝑅_𝑖(𝑡). (3) According to assumptions (3) and (4) and the definition of strain, the relationship between the stretched and un-stretched webs can be written as follows: 𝜌_𝑖(𝑥,𝑡)𝐴_𝑖(𝑥,𝑡)𝜌_𝑢𝐴_𝑢=11+𝜀_𝑖(),𝑥,𝑡 (4) where u means the unstretched state of the web.

Combining (3) with (4), we can obtain ^{𝑋𝐵𝑖(𝑡2)}_{𝑋𝐵𝑖𝑡1}11+𝜀_𝑖(𝑥,𝑡)𝑑𝑥=^𝑡2_𝑡1𝑉^𝑅_𝑖(𝑡)1+𝜀_𝑖−1(𝑥,𝑡)𝑑𝑡. (5) Under assumption (5), we can obtain 11+𝜀_𝑖()=1𝑥,𝑡1+𝜀_𝑖(𝑡)≈1−𝜀_𝑖(𝑡). (6) Combining (5) with (6), we can get 𝑋_𝐵𝑖𝑡₂−𝑋_𝐵𝑖𝑡₁1−𝜀_𝑖(𝑡)=^𝑡2_𝑡11−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡. (7) As shown in Figure 2(a), the initial phase angle of the color mark on the printing roller ci is negative and can be calculated by 𝛼_𝑖(𝑡)=𝛼_𝑖(0)+^𝑡₀𝜔_𝑖(𝑡)𝑑𝑡. (8) The coordinate of color mark on the printing roller ci 𝑋_𝐹𝑖 can be calculated by (9) including two parts, 𝑋^𝑅_𝐹𝑖 and 𝑋^𝑇_𝐹𝑖, which are caused by rotation and virtual translational movement of the printing roller ci, respectively: 𝑋_𝐹𝑖(𝑡)=𝑋^𝑅_𝐹𝑖(𝑡)+𝑋^𝑇_𝐹𝑖(𝑡)=𝑋^𝑅_𝐹𝑖(0)+𝑋^𝑇_𝐹𝑖(0)+^𝑡₀𝑉^𝑅_𝑖(𝑡)+𝑉^𝑇_𝑖(𝑡)𝑑𝑡. (9) Under assumption (6), the following equation could be obtained 𝑋^𝑅_𝐹𝑖(0)=𝑅_𝑖𝛼_𝑖(0)𝑋≤0,_𝑇𝐹𝑖(0)=0. (10) So, (9) can be rewritten as follows: 𝑋_𝐹𝑖(𝑡)=𝑅_𝑖𝛼_𝑖(0)+^𝑡₀𝑉^𝑅_𝑖(𝑡)+𝑉^𝑇_𝑖(𝑡)𝑑𝑡. (11) At t = t₁, as shown in Figure 2(b), the color mark on the printing roller ci is printed on the web, the following equation can be gotten 𝛼_𝑖𝑡₁=𝛼_𝑖(0)+^𝑡1₀𝜔_𝑖(𝑡)𝑑𝑡=0. (12) We can rewrite (12) as follows: ^𝑡1₀𝜔_𝑖(𝑡)𝑑𝑡=−𝛼_𝑖(0)⟹^𝑡1₀𝑉^𝑅_𝑖(𝑡)𝑑𝑡=−𝑅_𝑖𝛼_𝑖(0). (13) The coordinate of the color mark on the printing roller ci coincides with that of the color mark on the web at t = t₁, so combining (11) with (13), we can get the following: 𝑋_𝐹𝑖𝑡₁=𝑋_𝐵𝑖𝑡₁=^𝑡1₀𝑉^𝑇_𝑖(𝑡)𝑑𝑡. (14) Because the ith unit is MDR error reference, the printing roller ci has no virtual translational movement, so 𝑉^𝑇_𝑖(𝑡)=0, and we can obtain 𝑋_𝐵𝑖𝑡₁=^𝑡1₀𝑉^𝑇_𝑖(𝑡)𝑑𝑡=0. (15) Hence, (7) can be rewritten as follows: 𝑋_𝐵𝑖𝑡₂=11−𝜀_𝑖(𝑡)^𝑡2_𝑡11−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡. (16) Similarly, as shown in Figure 2, the coordinate of the color mark on the printing roller ci+1 can be derived by.𝑋_𝐹𝑖+1(𝑡)=𝑋^𝑅_𝐹𝑖+1(𝑡)+𝑋^𝑇_𝐹𝑖+1(𝑡)=𝑋^𝑅_𝐹𝑖+1(0)+𝑋^𝑇_𝐹𝑖+1(0)+^𝑡₀𝑉^𝑅_𝑖+1(𝑡)+𝑉^𝑇_𝑖+1(𝑡)𝑑𝑡. (17) At t = 0, (18) can be obtained as follows: 𝑋^𝑇_𝐹𝑖+1(0)=𝐿_𝑢𝑖. (18) Derived with the same procedure as 𝑋_𝐹𝑖, the following conclusion can be obtained: ^𝑡2₀𝑉^𝑅_𝑖+1(𝑡)𝑑𝑡=−𝑋^𝑅_𝐹𝑖+1(0). (19) Combining (17), (18) with (19), we can get 𝑋_𝐹𝑖+1𝑡₂=𝑋_𝐵𝑖+1𝑡₂=𝐿_𝑢𝑖+^𝑡2₀𝑉^𝑇_𝑖+1(𝑡)𝑑𝑡. (20) From the literature [12], we can obtain 1−𝜀_𝑖(𝑡)𝑑𝐿_𝑖(𝑡)𝑑𝑡−𝐿_𝑖(𝑡)𝑑𝜀_𝑖(𝑡)𝑑𝑡=𝑉_𝑖^𝑅(𝑡)−𝑉^𝑅_𝑖+1(𝑡)−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)+𝜀_𝑖(𝑡)𝑉^𝑅_𝑖+1(𝑡). (21) Since 𝑑𝜀_𝑖(𝑡)/𝑑𝑡≈0, (21) can be rewritten as follows: 𝑑𝐿_𝑖(𝑡)=𝑑𝑡1−𝜀_𝑖−1(𝑡)1−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)−𝑉^𝑅_𝑖+1(𝑡). (22) Because the change of the web length mapped to the coordinate is equivalent to the speed difference between the virtual translational movements of the two printing rollers, it can be expressed as follows: 𝑑𝐿_𝑖(𝑡)𝑑𝑡=𝑉^𝑇_𝑖+1(𝑡)−𝑉^𝑇_𝑖(𝑡). (23) Due to 𝑉^𝑇_𝑖(𝑡)=0, combining (22) with (23), (24) can be obtained as follows: 𝑉^𝑇_𝑖+1(𝑡)=1−𝜀_𝑖−1(𝑡)1−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)−𝑉^𝑅_𝑖+1(𝑡). (24) Combining (20) with (24), (20) can be rewritten as follows: 𝑋_𝐹𝑖+1𝑡₂=𝐿_𝑢𝑖+^𝑡2₀1−𝜀_𝑖−1(𝑡)1−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)−𝑉^𝑅_𝑖+1(𝑡)𝑑𝑡. (25) By using assumptions (5), we can get the following: 11−𝜀_𝑖(𝑡)≈1+𝜀_𝑖(𝑡),𝜀_𝑖−1(𝑡)𝜀_𝑖(𝑡)≈0. (26) Therefore, (25) can be simplified as follows: 𝑋_𝐹𝑖+1𝑡₂=𝐿_𝑢𝑖+^𝑡2₀𝑉^𝑅_𝑖(𝑡)−𝑉^𝑅_𝑖+1(𝑡)−𝜀_𝑖−1(𝑡)−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡. (27) According to the definition of MRD error, and combining (1), (16), and (27), the MDR error between the ith and unit (i+1)th printing units at the instant t₂ can be calculated as follows: 𝑒_𝑖𝑡₂=11−𝜀_𝑖(𝑡)^𝑡2_𝑡11−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡−𝐿_𝑢𝑖−^𝑡2₀𝑉^𝑅_𝑖(𝑡)−𝑉^𝑅_𝑖+1(𝑡)−𝜀_𝑖−1(𝑡)−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡. (28) The running time 𝑡_𝑇𝑖 = 𝑡₂−𝑡₁ represents the time that a color mark on the web takes to move from printing roller ci to printing roller ci+1, so 𝑡₁ = 𝑡₂−𝑡_𝑇𝑖, (28) can be simplified as follows: 𝑒_𝑖𝑡₂=^𝑡2₀𝑉^𝑅_𝑖+1(𝑡)−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡)−𝑑𝑡_{𝑡2−𝑡𝑇𝑖0}1−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡−𝐿_𝑢𝑖. (29) Extending (28) to any time interval, then we can get 𝑒_𝑖(𝑡)=^𝑡₀𝑉^𝑅_𝑖+1(𝑡)−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡−)𝑑𝑡_{𝑡−𝑡𝑇𝑖0}1−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡)𝑑𝑡−𝐿_𝑢𝑖. (30) The differential form of MDR error between the ith and (i+1)th units can be derived as follows: 𝑑𝑒_𝑖(𝑡)𝑑𝑡=𝑉^𝑅_𝑖+1(𝑡)−𝑉^𝑅_𝑖𝑡−𝑡_𝑇𝑖+𝜀_𝑖−1𝑡−𝑡_𝑇𝑖𝑉^𝑅_𝑖𝑡−𝑡_𝑇𝑖−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡). (31) In the shaft-less drive printing system, there is no change of the web length caused by compensation rollers, so Δ𝐿_𝑖(t) is mainly caused by load cells, dryer systems, and other disturbances. In practice, Δ𝐿_𝑖(t) ≪𝐿_𝑢𝑖, so we can obtain 𝐿_𝑖(𝑡)=𝐿_𝑢𝑖+Δ𝐿_𝑖(𝑡)≈𝐿_𝑢𝑖. (32) In (21), we substitute 𝐿_𝑢𝑖 for 𝐿_𝑖(t), and then (21) can be rewritten as follows: 𝐿_𝑢𝑖𝑑𝜀_𝑖(𝑡)=𝑑𝑡1−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖+1(𝑡)−1−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡). (33) Combining (31) with (33), the MDR error model of the two-color system can be obtained as follows:𝑑𝑒_𝑖(𝑡)𝑑𝑡=𝑉^𝑅_𝑖+1(𝑡)−𝑉^𝑅_𝑖𝑡−𝑡_𝑇𝑖+𝜀_𝑖−1𝑡−𝑡_𝑇𝑖𝑉^𝑅_𝑖𝑡−𝑡_𝑇𝑖−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖(𝑡),𝐿_𝑢𝑖𝑑𝜀_𝑖(𝑡)=𝑑𝑡1−𝜀_𝑖(𝑡)𝑉^𝑅_𝑖+1(𝑡)−1−𝜀_𝑖−1(𝑡)𝑉^𝑅_𝑖(𝑡). (34) Considering 𝑉^𝑅_𝑖(𝑡)=𝑅_𝑖𝜔_𝑖(𝑡) and 𝜀_𝑖(𝑡)=𝑇_𝑖(𝑡)/𝐴𝐸, we can obtain 𝐴𝐸𝑑𝑒_𝑖(𝑡)𝑅𝑑𝑡=𝐴𝐸_𝑖+1𝜔_𝑖+1(𝑡)−𝑅_𝑖𝜔_𝑖𝑡−𝑡_𝑇𝑖+𝑅_𝑖𝑇_𝑖−1𝑡−𝑡_𝑇𝑖𝜔_𝑖𝑡−𝑡_𝑇𝑖−𝑇_𝑖(𝑡)𝜔_𝑖(𝑡),𝐿_𝑢𝑖𝑑𝑇_𝑖(𝑡)𝑅𝑑𝑡=𝐴𝐸_𝑖+1𝜔_𝑖+1(𝑡)−𝑅_𝑖𝜔_𝑖(𝑡)+𝑇_𝑖−1(𝑡)𝑅_𝑖𝜔_𝑖(𝑡)−𝑇_𝑖(𝑡)𝑅_𝑖+1𝜔_𝑖+1(𝑡). (35) In the practical printing process of gravure printing machines, the change of the web cross-section in different web spans is very small and negligible, so we substitute average A for 𝐴_𝑖(t) in (35). Equation (35) shows that the MDR mathematical model of the two-color gravure printing machines is expressed by two equations representing the dynamics of tension fluctuation between units and the dynamics of the register error fluctuation, respectively.

3.3. MDR System Model of the n-Color Gravure Printing Machines

The schematic diagram of the n-color MDR system is presented in Figure 3. T₀(t) is the tension of the infeeding zone at steady state and can be measured by load cells.

Figure 3 

Schematic diagram of the n-color gravure printing machine.

According to (35) and Figure 3, the MDR system model of the n-color gravure printing machines can be expressed as follows: 𝐴𝐸𝑑𝑒₁(𝑡)𝑅𝑑𝑡=𝐴𝐸₂𝜔₂(𝑡)−𝑅₁𝜔₁𝑡−𝑡_𝑇1+𝑅₁𝑇₀𝑡−𝑡_𝑇1𝜔₁𝑡−𝑡_𝑇1−𝑇₁(𝑡)𝜔₁(𝑡),𝐿_𝑢1𝑑𝑇₁(𝑡)𝑅𝑑𝑡=𝐴𝐸₂𝜔₂(𝑡)−𝑅₁𝜔₁(𝑡)+𝑇₀(𝑡)𝑅₁𝜔₁(𝑡)−𝑇₁(𝑡)𝑅₂𝜔₂(𝑡),⋮𝐴𝐸𝑑𝑒_𝑛−1(𝑡)𝑅𝑑𝑡=𝐴𝐸_𝑛𝜔_𝑛(𝑡)−𝑅_𝑛−1𝜔_𝑛−1𝑡−𝑡_{𝑇(𝑛−1)}+𝑅_𝑛−1𝑇_𝑛−2𝑡−𝑡_{𝑇(𝑛−1)}𝜔_𝑛−1𝑡−𝑡_{𝑇(𝑛−1)}−𝑇_𝑛−1(𝑡)𝜔_𝑛−1(𝑡),𝐿_{𝑢(𝑛−1)}𝑑𝑇_𝑛−1(𝑡)𝑅𝑑𝑡=𝐴𝐸_𝑛𝜔_𝑛(𝑡)−𝑅_𝑛−1𝜔_𝑛−1(𝑡)+𝑇_𝑛−2(𝑡)𝑅_𝑛−1𝜔_𝑛−1(𝑡)−𝑇_𝑛−1(𝑡)𝑅_𝑛𝜔_𝑛(𝑡). (36) The 𝜔_𝑖(t) is composed of a stationary component ω* which corresponds to web feed speed and a variable component Δ𝜔_𝑖(t) determined by the MDR control input. In particular, ω₁(t) is used as a speed reference, so we can get (37) as follows: 𝜔₁(𝑡)=𝜔^∗,𝜔₂(𝑡)=𝜔^∗+Δ𝜔₂(𝑡),⋮𝜔_𝑛(𝑡)=𝜔^∗+Δ𝜔_𝑛(𝑡). (37) In (36), 𝑡_𝑇𝑖 is also known as time delay and is defined as follows: 𝑡_𝑇𝑖=𝐿_𝑢𝑖𝑅_𝑖𝜔_𝑖≈𝐿_𝑢𝑖𝑅_𝑖𝜔^∗. (38) Equations (36) and (37) show that the n-color MDR system in the gravure printing machine is a multi-input multioutput, nonlinear, strong-coupling, and time-delay system. In the system, 𝜔₂(t)~𝜔_𝑛(t) are the input signals, and e₁(t)~𝑒_𝑛−1(t) are the output signals.

4. Simulation and Experiments

4.1. Experimental Setup and MDR Simulation Model of the Four-Color System

To investigate the reliability of the derived MDR mathematical model, numerical simulation and experiments under open-loop control are carried out by using MATLAB and experimental setup for the four-color gravure printing machines. Figures 4 and 5 show the setup configuration of the shaft-less drive gravure printing machine.

Figure 4 

Experimental setup of the four-color gravure printing machines.

Figure 5 

Control configuration of the experimental setup.

The system includes an unwinding unit, an infeeding unit, an outfeeding unit, a rewinding unit, and a four-color printing subsystem, and all of the driving shafts are driven by independent servo motors. The printing rollers are driven by servo motors (YASKAWA SGMGH-44), and printing speed is obtained by the encoder on the servo motor. The register controller (KESAI ST3000) works in shaft-less drive mode, and the MDR errors are measured using photoelectric sensors (XINMAO XM08). Tension signals are tested by load cells (MITSUBISHI LX-030TD) and tension measuring amplifiers (MITSUBISHI LM-10TA) mounted on the shaft of idle rollers. The shaft-less drive system is controlled by multiaxis controller (GOOGOLTECH T8VME). The experimental web material is biaxially oriented polypropylene (BOPP). As shown in Figure 5, tension of the printing subsystem is same with that in the infeeding zone at steady state and is not controlled; in other words, T₁(t), T₂(t), and T₃(t) are not controlled. In the infeeding unit, T₀(t) is adjusted by changing the speed of the roller 2. After preregistration, e₁(t), e₂(t), and e₃(t) work in the open-loop control mode and are sampled once per revolution of the printing rollers. The parameters of the experimental setup are summarized in Table 1.

Table 1 

System parameters.

According to Table 1, we can get 𝑅₁=𝑅₂=𝑅₃=𝑅₄𝐿=𝑅,_𝑢1=𝐿_𝑢2=𝐿_𝑢3=𝐿_𝑢,𝑡_𝑇1=𝑡_𝑇2=𝑡_𝑇3=𝑡_𝑇. (39) Hence, combining (36) with (39), the MDR system model of the four-color gravure printing machines in the simulation can be expressed as follows: 𝐴𝐸𝑑𝑒₁(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₂(𝑡)−𝜔₁𝑡−𝑡_𝑇+𝑅𝑇₀𝑡−𝑡_𝑇𝜔₁𝑡−𝑡_𝑇−𝑇₁(𝑡)𝜔₁(𝑡),𝐴𝐸𝑑𝑒₂(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₃(𝑡)−𝜔₂𝑡−𝑡_𝑇+𝑅𝑇₁𝑡−𝑡_𝑇𝜔₂𝑡−𝑡_𝑇−𝑇₂(𝑡)𝜔₂(𝑡),𝐴𝐸𝑑𝑒₃(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₄(𝑡)−𝜔₃𝑡−𝑡_𝑇+𝑅𝑇₂𝑡−𝑡_𝑇𝜔₃𝑡−𝑡_𝑇−𝑇₃(𝑡)𝜔₃(𝑡,𝐿_𝑢)𝑑𝑇₁(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₂(𝑡)−𝜔₁(𝑡𝑇)+𝑅₀(𝑡)𝜔₁(𝑡)−𝑇₁(𝑡)𝜔₂(𝑡),𝐿_𝑢𝑑𝑇₂(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₃(𝑡)−𝜔₂(𝑡)+𝑅𝑇₁(𝑡)𝜔₂(𝑡)−𝑇₂(𝑡)𝜔₃(𝑡,𝐿_𝑢)𝑑𝑇₃(𝑡)𝜔𝑑𝑡=𝐴𝐸𝑅₄(𝑡)−𝜔₃(𝑡)+𝑅𝑇₂(𝑡)𝜔₃(𝑡)−𝑇₃(𝑡)𝜔₄(𝑡). (40)

4.2. Results and Analysis of the Simulation and Experiments

In order to ensure the consistency of the simulation and experiments, the parameters used in the numerical simulation are taken from Table 1. Because the simulation and experiments under open-loop control are carried out, the speed of all servo motors is the same in the four-color printing system. In order to verify the developed MDR system model of the n-color gravure printing machines, T₀(t) has a step change from 100 N to 110 N at 𝑡=5 s when ω₁ is equal to 100 r/min and 200 r/min. The responses of the MDR errors and tension signals in simulation and experiments are shown in Figures 6 and 7, respectively.

(a) Tension dynamics at 𝜔1=100r/min

(b) Tension dynamics at 𝜔1=200r/min

(c) MDR errors dynamics at 𝜔1=100r/min

(d) MDR errors dynamics at 𝜔1=200r/min

Figure 6 

Open-loop dynamics of the MDR errors in simulation.

(a) Tension dynamics at 𝜔1=100r/min

(b) Tension dynamics at 𝜔1=200r/min

(c) MDR errors dynamics at 𝜔1=100r/min

(d) MDR errors dynamics at 𝜔1=200r/min

Figure 7 

Open-loop dynamics of the MDR errors in experiments.

As shown in Figure 6, the step change of T₀(t) causes the step change of T₁(t), T₂(t), and T₃(t), and with an increase in ω₁(t), the time that T₁(t), T₂(t), and T₃(t) take to reach the steady state value is gradually shortened. For example, when ω₁(t) is equal to 100 r/min and 200 r/min, T₁(t) reaches 110 N at 35 s and 20 s, respectively. The step change of T₀(t) also directly causes the change of e₁(t) and then the changes of e₂(t) and e₃(t) by coupling characteristics, and as the speed increases, the interval from the peak of 𝑒_𝑖(t) to the peak of 𝑒_𝑖+1 (t) is gradually shortened; for example, when ω₁(t) is equal to 100 r/min and 200 r/min, the interval from the peak of e₁(t) to the peak of e₂(t) is 3.11 s and 1.56 s, respectively.

Figures 6 and 7 illustrate that although there are some disturbances in the experiments, the experimental results are almost coincident with the simulation curves. For example, with an increase in ω₁(t), the time that tension signals take to reach the steady state value is gradually shortened and so on. Comparing with Figures 6(c), 6(d), 7(c), and 7(d), respectively, the peak of 𝑒_𝑖(t) in the experiments is slightly more than that in the simulation, such as when ω₁(t) is equal to 200 r/min, the peaks of e₁(t) in the experiments and simulation are −0.82 mm and −0.78 mm, respectively. There are two primary reasons: one is that there are external disturbances from environment in the experiments, and the other is that there are errors between the simulation parameters and real experimental parameters.

The results of the simulation and experiments indicate that the established MDR system model is accurate and reliable and can be used to design a MDR controller. At the same time, the results also offer a powerful proof that the MDR system of the multicolor gravure printing machines is a multi-input multioutput, nonlinear, strong-coupling, and time-delay system.

5. Conclusions

In the gravure printing machines, the register accuracy is the most important in performance. Therefore, in order to improve the control accuracy of MDR system, a nonlinear mathematical model of the MDR system in shaft-less drive mode is presented and derived in detail for the multicolor gravure printing machines in this paper. First, we introduce the schematic diagram of the shaft-less drive gravure printing machines. Secondly, we make a deep analysis of the causes and influencing factors of the MDR errors, then establish the MDR error of the two-color system, and further obtain the MDR system model of the n-color gravure printing machines. At last, the proposed model is verified by the numerical simulations and experimental studies. Therefore, it can be concluded that the MDR system model is accurate enough to describe dynamics of the MDR errors in multicolor printing system and is worthwhile to be developed as an effective MDR controller in future study.

Nomenclature

𝛼𝑖:	Phase angle of the color mark on the roller ci
𝑡𝑖:	Printing moment of the color mark on the roller ci
𝑡𝑇𝑖:	Transmission time of the color mark from ith to (i+1)th printing units
𝜔𝑖:	Angular velocity of rotation of the roller ci
𝑋𝐹𝑖:	Coordinate on x-axis of the color mark on the roller ci
𝑋𝐵𝑖:	Coordinate on x-axis of the color mark on the web
𝑒𝑖:	Register error between ith and (i+1)th printing units
𝑅𝑖:	The radius of the roller ci
𝐿𝑢𝑖:	Length of the free web between ith and (i+1)th printing units
𝐿𝑖:	Actual length of the web between ith and (i+1)th printing units
Δ𝐿𝑖:	The length variation of the web between ith and (i+1)th printing units
𝜀𝑖:	Strain of the web between ith and (i+1)th printing units
𝑇𝑖:	Tension of the web between ith and (i+1)th printing units
𝑉𝑇𝑖:	Virtual velocity of the roller ci
𝑉𝑅𝑖:	Web velocity of the roller ci
E:	The modulus of elasticity of the web material
𝜌𝑖:	Density of the web between ith and (i+1)th printing units
𝐴𝑖:	Cross-section of the web between ith and (i+1)th printing units.

Conflict of Interests

The authors declare that they have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product or company that could be construed as influencing the position presented in, or the review of, the paper.

Acknowledgments

This research is supported by the Program for Changjiang Scholars (IRT1172) and the National Science & Technology Pillar Program of China, under Grant no. 2012BAF13B06.