Abstract: This paper mainly introduces the application of B&R's human-machine interface, ACOPOS intelligent servo drive, and Ethernet POWERLINK real-time communication in the shaftless drive control system of a wide-format flexographic printing press. It describes the equipment composition and working principle of the printing press, as well as the hardware configuration, key technologies, and solutions of the control system. This system boasts high operating accuracy, paper savings, and significant cost advantages.
Keywords: Wide-format flexographic printing press; Shaftless drive system; Human-machine interface; ACOPOS servo drive; Ethernet POWERLINK industrial Ethernet
ApplicationofB&RIndustrialControlProductsintheShaft-lessDriveControlSystemofWide-rangeFlexographicPress
0. Introduction
The pursuit of excellence in every detail is what outstanding companies strive for, which is why we see increasingly exquisite packaging for Budweiser and Tsingtao beer in supermarkets. This is a key factor in attracting consumers. In addition to the quality of the beer, the exquisite packaging also reflects the producer's grasp and pursuit of quality in detail, which also brings new opportunities to printing presses.
In addition, the beer and beverage industry has stricter requirements for environmental protection. In order to reduce the harm to human health during the production process, environmentally friendly printing has also received more attention, which has made the advantages of flexographic printing over offset printing and gravure printing gradually apparent.
1. System Requirements
Due to its high precision requirements for machined parts, wide-format flexographic printing did not see significant development for a long time. Currently, this type of machine is mainly used for printing on corrugated paperboard, and this printing process is called "pre-printing technology." In contrast, printing after the corrugated board production is called "post-printing technology." Compared to post-printing technology, pre-printing technology has its unique advantages:
Pre-printing technology is highly efficient and produces high-quality results. For corrugated cardboard printing, pre-printing technology links the printed face paper with the corrugating line, allowing for direct production of cartons without the need to transfer the paper to a slotting printing press, as is required with post-printing technology. This results in higher efficiency. Furthermore, unlike post-printing technology, it does not put pressure on the corrugated paper, thus avoiding any impact on the strength of the corrugated carton.
Environmentally friendly. Flexographic printing uses water-based inks, which are more environmentally friendly than oil-based inks, and have less impact on air quality and workers in production workshops.
The printing format is larger. For wide-format flexographic printing, a printing format of 1200mm to 2200mm can provide surface printing for large packaging containers, and can also provide printing for multiple products simultaneously through slitting.
The system requires a printing speed of 300 m/min, a printing accuracy of +/- 0.1 mm, and a printing width of 1200 mm. Equipment composition and working principle.
The unit consists of an unwinding unit, a front traction unit, a rear traction unit, a printing color group (generally no more than 9 colors), and a rewinding unit, as shown in Figure 1.
Figure 1. Equipment Composition of Flexographic Printing Unit
2.1 Unwinding Unit
The unwinding section mainly consists of an air brake and a tension swing roller. The material roll paper core is placed on the unwinding frame through an air expansion shaft. When the air brake pressure is zero, the material roll and the air expansion shaft can rotate freely together. During normal operation, the braking force and the unwinding tension are adjusted by regulating the air pressure (proportional valve: 0~10VDC indicates that the air pressure increases from small to large). The specific working principle is shown in Figure 2.
Figure 2 Schematic diagram of the working principle of the unwinding unit
The tension roller maintains a constant thrust (the air pressure in the cylinder is constant). During normal operation, we adjust the air pressure of the air brake through PID control to keep the tension roller at the set position (the position of the tension roller is detected by a potentiometer installed on the roller, which is 0~10V, and the middle value is usually adjusted to 5V). This maintains constant tension during unwinding, which is the first stage of tension control before the material enters the printing process.
2.2 Front Traction Section
The front traction section consists of a front traction motor, a front traction roller, a front traction pressure roller, and a tension swing roller. The function of the front traction section is to feed the printing material into the printing section with constant tension. The working principle is shown in Figure 3.
Figure 3 Schematic diagram of the working principle of the front traction part
The traction roller is driven by a traction motor, and the traction pressure roller presses against the traction roller during normal operation to prevent material slippage. During normal operation, the speed of the front traction motor is adjusted via PID control to keep the tension swing roller at a set position, thus maintaining constant material tension. This is the second stage of tension control before the material enters the printing section. Simultaneously, the front traction pressure roller also acts as a tension isolation mechanism, isolating tension fluctuations in the unwinding section.
2.3 Printing Section
The printing section consists of color units; there are 7 color units for every 7 colors. Each color unit comprises a color unit motor, a bottom roller, a printing roller, and an inking roller. The color unit motor drives the bottom roller via a gearbox, and the printing roller, bottom roller, and inking roller are connected by gears. The working principle is shown in Figure 4.
Figure 4 Schematic diagram of the working principle of the printing part
The printing roller, bottom roller, and inking roller are connected by gears to ensure consistent surface linear velocity. Before printing begins, there is a gap between the printing roller and the bottom roller, and the printing roller is in a raised state. At this time, the surface of the printing roller is not in contact with the printing material. After the printing roller is lowered, the surface of the printing roller maintains light contact with the surface of the substrate. The contact pressure between the printing roller and the material can be adjusted according to the desired printing effect. Lowering the printing roller involves the printing roller and the inking roller descending together. The sequence of retracting the printing roller is as follows: first, stop the rotation of the inking roller for 2 seconds, then retract the printing roller, maintaining the printing roller retraction signal and the inking roller stop signal for 2 seconds, and then resetting both signals. During normal printing, all color groups are strictly synchronized in position, and all color group calibrations must be completely identical. Between the color groups is the post-printing heating section for each color group, using hot air heating. The hot air comes from the main blower; each color group has its own heater, with both fast and slow heating methods. Heating types include electric heating wire and steam.
2.4 Rear Traction Section
The rear traction section consists of a rear traction motor, a rear traction roller, a rear traction pressure roller, and a tension swing roller; the function of the rear traction section is to guide the printing material out of the printing section with constant tension. The working principle diagram is shown in Figure 5.
Figure 5 Schematic diagram of the working principle of the rear traction part
The tension swing roller is installed between the last color group and the rear traction. Like the front traction, during normal operation, the traction pressure roller also presses on the traction roller to prevent material slippage and at the same time play a role in tension isolation. The system adjusts the speed of the rear traction motor through PID according to the set position of the swing roller to keep the swing roller stable at the set position, so as to achieve stable rear traction tension.
2.5 Rewinding section
Flexographic printing presses use a surface friction winding method. The winding section mainly consists of a tension roller, a friction roller, and a passive air shaft. Its working principle diagram is shown in Figure 6.
Figure 6. Schematic diagram of the working principle of the winding section.
The friction roller is driven by a third-party asynchronous motor, which is powered by an ACOPOS or vector frequency converter. During normal operation, the take-up pressure roller presses the material onto the friction roller surface, and the take-up roll also presses against the friction roller surface. The system compares the set position and actual position of the tension swing roller and adjusts the speed of the friction roller drive motor via PID control to stabilize the tension swing roller at the set position, thus achieving constant tension winding. Both the friction roller and the pressure roller are soft rubber rollers.
2. Control system hardware configuration
The hardware configuration of the system is shown in Figure 7. It mainly consists of one PowerPanel—a human-machine interface that integrates control, display and operation, multiple ACOPOS intelligent servo drives, and EthernetPOWERLINK real-time communication Ethernet.
Figure 7 System Hardware Configuration
3. Key Technologies and Solutions
4.1 Integrated Color Matching System
4.1.1 Disadvantages of discrete color matching:
For printing systems, the key lies in the registration accuracy of the color matching system. However, the currently popular color matching system solutions on the market are a separate design, where the product supplier of the transmission system and the supplier of the color matching system are combined into one system. This has the following drawbacks:
The price is high; professional color matching systems typically cost thousands of dollars per color.
When a system malfunctions, it requires the joint efforts of two or more suppliers to resolve the issue.
4.1.2 Advantages of integrated color matching:
B&R, relying on the powerful performance of its products and its experience in printing machinery application software, designed an integrated color matching system solution. This solution is a revolutionary change from traditional solutions, resulting in significant cost savings. Furthermore, the fact that the entire system is provided by a single supplier minimizes the inconvenience of system maintenance and upgrades.
B&R was able to implement the system well because of its own hardware advantages, but more importantly, it was able to leverage its accumulated software library in the industry.
4.1.3 Design of Integrated Color Matching Algorithm
The integrated color matching system inputs the deviations of each printing unit into the color matching system model. This model is designed and modeled based on fuzzy control, and its parameters include printing-related process parameters such as tension, temperature, friction coefficient, printing length, and speed. Since the printing system is a continuous printing process, any deviation adjustment of any unit will affect other systems. Through the coupling and decoupling operations of the multivariable system, the system provides the adjustment amount to the motion control system, which then makes real-time adjustments to maintain high printing accuracy.
Traditional PLCs cannot design such complex algorithms, while PCCs, due to their high-level language programming capabilities, can more easily perform software design of algorithms and can leverage various open libraries to support application development.
4.2 Color marks are directly input to the driver
The ACOPOS servo drive is a modularly designed drive system with intelligent features and strong processing capabilities. It is designed with multiple slots, including four slots for communication, encoders, and I/O. Typically, the first slot is used for communication, such as CAN or POWERLINK. Slots 2-4 can be used for encoders, I/O, or even directly for a CPU module. In this application, the signal from the color mark sensor is directly sent to the ACOPOS drive, where signal sampling, filtering, processing, encoding, and communication encapsulation are completed before being sent to the color matching system. This design offers a faster response speed than the traditional PLC + I/O module approach.
4.3 Virtual Spindle Technology
A virtual spindle refers to a mathematical model established between motion control axes. Since all motion control speed and position synchronization problems can be reduced to the mathematical relationship between servo axes, the virtual spindle does not refer to a specific axis, but rather the relationship between them. Because it does not exist but is an ideal axis, it has no wear, runout, or deviation. Other servo axes follow this virtual spindle with 1:1 electronic gear synchronization. ACOPOS drives can support electronic gear synchronization accuracy up to 32-bit resolution.
4.4 Ethernet POWERLINK Real-time Communication Technology
Real-time communication is a crucial part of the system. Whether it is the uploading of deviation signals to the color matching system, the adjustment amount given by the color matching system, or the electronic gear synchronization relationship between the motion control system, all require data exchange between the axes through high-speed data channels.
In this system, the printing speed is up to 300m/min, while the accuracy requirement is +/-0.1mm. Each color registration refresh cycle is completed within a few milliseconds. Usually, the communication refresh cycle needs to be higher than this order of magnitude to ensure high-precision synchronization. POWERLINK can achieve a refresh cycle as fast as 100uS. Considering the position ring of the driver and the task cycle matching of the color registration system, it is usually set to 400uS or 800uS, which fully meets the high real-time requirements of high-speed printing systems.
POWERLINK is a high-speed real-time Ethernet network with a speed of 100Mbps and jitter << 1µs. It provides a fast channel for the exchange of large amounts of data throughout the system, enabling the system to complete rapid data exchange and ensuring system synchronization accuracy.
4. Conclusion
This integrated color matching system virtually eliminates the cost of the original system, representing a significant cost saving for machinery manufacturers and making it undeniably competitive. More importantly, the system boasts high precision and saves paper. Utilizing pre-registration technology, registration is completed quickly within just a few paper transfers, allowing for stable operation and improved efficiency while minimizing paper waste.
