Abstract: This paper introduces the application of servo control and network communication technologies in printing and packaging equipment. Focusing on an electronic axis control system built with B&R's PCC, ACOPOS servo drives, and Ethernet POWERLINK real-time industrial Ethernet, the paper analyzes the system's design concept and application scheme, emphasizing the hardware and software composition and working principle of the unified electronic axis control system. It also elaborates on the classification of electronic axis control systems for gravure printing presses and the technical characteristics of gravure printing presses employing unified shaftless drive control technology.
Keywords: Gravure printing press; Electronic shaft system; Servo control; Network communication
Chinese Library Classification Number: Document Identification Code : B
0 Introduction
Offset printing, flexographic printing, and gravure printing stand as a three-way balance, each with its own strengths—a natural consequence of the development of printing technology. Offset printing excels in producing fine halftone lines, flexographic printing has seen significant market growth due to the environmental advantages of water-based inks, while gravure printing is praised for its minimal color difference, full-page solid printing, seamless prints, wide applicability to various printing materials, and suitability for long-run, flexible printing. Given the characteristics of these three printing methods, the correct approach to this balance is to leverage their strengths and mitigate their weaknesses, embracing a comprehensive approach. With the development of control technology, global printing companies and equipment manufacturers are now focusing on emerging control technologies—electronic axis control systems. This is especially true for gravure printing presses, where its advantages are even more pronounced.
An electronic axis control system is an automated servo control system that uses independent servo drives for each printing unit of a gravure printing press. All printing units exchange data in real time using the fast response of the network. While achieving precise power transmission to ensure synchronous control, it also achieves pattern registration by finely adjusting the phase of each printing roller, forming a virtual electronic spindle to replace the traditional mechanical main drive shaft. At the same time, it simplifies the mechanical transmission structure and omits the color compensation mechanism.
In contrast, traditional printing presses rely on a single mechanical spindle to transmit power to each printing unit. The mechanical spindle and the printing units are connected by worm gears. The power of the mechanical spindle comes from a three-phase asynchronous motor controlled by a vector control frequency converter. The entire system and structure result in poor transmission accuracy and a complex mechanical structure.
Moreover, in the crucial aspect of printing presses—the control of registration of printed images—traditional printing presses almost always achieve this by changing the material length between colors. To change the material length, it is necessary to control a specially designed floating roller compensation mechanism.
Currently, numerous servo control systems are used in my country's printing and packaging equipment, including those from B&R, Rexroth, Siemens, Schneider Electric, Sumitomo, and Yaskawa. Many equipment manufacturers are also exploring the use of these servo control systems to develop electronic axis control systems. The application of electronic axis control systems in printing presses has become a hot topic in the industry, ushering my country's printing and packaging equipment industry into the era of electronic axis control.
1. Types of Electronic Shaft Control Systems
With technological advancements, electronic shaft control systems for gravure printing presses have evolved into two main schools of thought. The first is the composite school, where the drive system and color registration control are independent; the second is the integrated school, where the drive system and color registration control are integrated into one unit. These two schools of thought have naturally led to the three main categories of electronic shaft control systems for gravure printing presses in practical applications.
(1) Sub-electronic axis control system
This is a control system that simply eliminates the traditional main drive shaft, with each printing unit employing an independent drive mechanism to ensure synchronized printing roller speeds. It retains the floating roller structure required for the color registration system, which is completely independent and unrelated to the transmission. Due to the relative independence of the transmission and registration control systems, achieving stable and consistent control is difficult. Furthermore, the necessary floating roller mechanism still complicates the mechanical structure, and the paper feed length between color groups increases, making it difficult to achieve high-speed, high-precision printing processes.
It is the predecessor of the electronic shaft control system of all unit-type gravure printing presses.
(2) Quasi-electronic axis control system
It is a control system that eliminates the traditional mechanical main drive shaft and the floating roller structure required by the color matching system. Each printing unit adopts an independent drive mechanism to ensure the printing plate roller speed is synchronized. At the same time, the relatively independent color matching system provides the color matching error signal that has been processed. The shaftless control system combines the speed synchronization signal to coordinate the work.
It represents the development and evolution of sub-electronic axis control systems.
(3) Unified electronic axis control system
This system uses a virtual electronic shaft drive based on servo control technology to replace the traditional mechanical main drive shaft, completely eliminating the floating roller structure required by the traditional color registration system. It achieves pattern registration by finely adjusting the phase of each printing plate roller, integrating servo drive and color registration into one electronic shaft control system that directly acts on the printing plate roller.
It is a true electronic axis control system. This is the main topic we will discuss below.
2. Composition and Working Principle of Unified Electronic Shaft Control System
The unified electronic axis control system is designed to simplify mechanical structure and improve control performance. It excels in all printing press control technologies due to its high precision, high printing speed, and high registration accuracy.
Figure 1 is a simplified schematic diagram of the shaftless drive control system of a unit-type gravure printing machine, which shows the hardware configuration and network structure of the system.
2.1 System Hardware Configuration
Different printing presses require different printing units, and the servo power configured in each printing unit also varies. The following example, an eight-color gravure printing press, illustrates a unified electronic shaft control system. The hardware components of this system are shown in Figure 2.
The entire system uses a B&R PP320 touchscreen as the HMI (Human Machine Interface), which can also be considered as the host computer. It realizes functions such as inputting and displaying system parameters, monitoring system status, monitoring printing status, prompting system faults, and managing system recipes.
The X20 series PCC (Programmable Logic Controller) system, acting as the lower-level machine, uses the X20CP1486 CPU, which is the core of the entire system. It features an additional I/O processor and a floating-point processor (FPU), with a single-step instruction cycle as fast as 0.01µs and a task loop cycle of up to 200µs. It supports multiple communication interfaces, including RS232, EtherNet POWERLINK, CANopen, X2X LINK, and USB.
The ACOPOS servo controller and servo motor serve as the drive units. The encoder used for position signal feedback generates 4 million effective pulses per revolution through hardware frequency multiplication, achieving a system control accuracy of up to 0.001 mm. Even at printing speeds up to 600 m/min, it achieves significantly higher registration accuracy than mechanical shaft gravure printing machines. Printing production time has proven that this technology makes it possible to improve registration accuracy from 0.1 mm to 0.05 mm.
The system employs a fieldbus approach for reliable data exchange, control, and broadcast communication. Communication between the lower-level and upper-level computers occurs via Ethernet (TCP/IP). The lower-level computer has analog and digital modules for logic I/O control and speed simulation interfaces. The X20CP1486 connects to each ACOPOS servo drive via an industrial hub (0AC808) through an EtherNet POWERLINK bus. Each servo drive is equipped with a communication module AC114, an encoder signal analysis card AC120, a color matching error analysis card AC132, and an I/O module AC130. The ACOPOS servo controller and servo motor are connected via dedicated servo cables and encoder cables.
As shown in Figure 3, in the electronic shaft system, the EtherNet POWERLINK industrial network bus accesses the servo station in the form of network encoding.
Real-time communication is a crucial component of the system. The electronic gear synchronization between all servo motion control systems in the system requires data exchange between axes via a high-speed data channel. POWERLINK is a 100Mbps high-speed real-time Ethernet network with jitter << 1µs. It provides a fast channel for the large amount of data exchange in the entire system, enabling rapid data exchange and ensuring system synchronization accuracy.
Ethernet POWERLINK is a secure industrial Ethernet technology built entirely on industrial Ethernet hardware, without relying on other specific hardware environments, and implemented solely using software protocols. It boasts high real-time performance and deterministic reliability. POWERLINK supports not only cyclic communication of PCC, distributed I/O, and motion control system data, but also asynchronous communication between real-time network nodes. It employs SCNM (Slot Communication Network Management) technology to solve the uncertainty problem in data exchange caused by data collisions in traditional Ethernet CSMA/CD communication. The main technical parameters of Ethernet POWERLINK are shown in Table 1.
It provides the system with a real-time, reliable, and secure data channel, meeting the high data response speed requirements of the unified electronic shaft control system for high-speed gravure printing presses.
2.2 System Software Platform
The system uses B&R's Automation Studio integrated software (hereinafter referred to as AS) as the programming software platform to realize the programming work of specific processes and data processing.
AS is an integrated software platform that supports application software development for B&R's entire range of control products, as well as the development and application of Ethernet POWERLINK, CANopen, and X2X LINK buses. It also supports almost all programming methods, such as Basic, ANSI C, LAD (Ladder Diagram), IL (Instruction List), ST (Structured Text), SFC (Sequential Function Chart), and more.
AS supports time-sharing multitasking, which can classify tasks according to the system's needs and then set appropriate task scanning times for different tasks.
The software development strategy for the unified electronic axis control system is as follows: ANSI C programming is used throughout, with control implemented in three cyclic task layers. The first cycle handles servo control communication, process control, printing unit control, and color matching system control. The second cycle handles tension control and remote station control. The third cycle primarily handles human-machine interface control. The B&R PCC processes these tasks using its time-sharing multitasking management method.
2.3 Control Principle
The development of a unified electronic axis control system is based on servo control and network technology. We typically refer to each servo motor circuit as an "axis." Figure 4 shows the control principle diagram and wiring diagram of a single color group "axis" in the electronic axis control system of a unit-type gravure printing press.
The controlled objects of a unified electronic axis control system are several interconnected yet independent "axis" that control the printing plate roller.
The interconnectedness is manifested in the fact that the speed of each axis, which controls the printing rollers for each color, originates from a unified source: the real-time speed given by the virtual electronic axis of the system and the adjustment speed required to achieve registration. While ensuring the synchronization of the system's transmission, each axis must respond with appropriate color registration phase adjustment. For registration errors detected by photoelectric sensors (configured in all printing units requiring color registration control), feedback is sent to the servo driver ACOPOS via the color analysis card AC132. The servo driver ACOPOS then feeds this feedback to the PCC via the EtherNet POWERLINK bus. After unified real-time processing, the PCC instructs each interconnected axis to make corresponding phase adjustments; this is called "serial adjustment" or "color registration decoupling." The serial adjustment of the axes is the essence of color registration control in a unified electronic axis control system.
Its independence is manifested in the fact that each axis operates under the same control mode and framework, with the same operating mechanism, forming a self-contained system. In terms of control, the lower-level PCC performs specific process analysis and data processing. Simultaneously, upon initial power-up, the PCC is responsible for downloading the functional parameters and system functions required for each axis's operation via the POWERLINK bus to the servo driver ACOPOS for each axis. ACOPOS then initializes based on the received functional parameters and system functions, performs necessary data analysis and processing, and instructs the corresponding servo motor to control the axis. The actual operation of the servo motor is fed back from its internal encoder to the encoder signal analysis card AC120 on the servo driver ACOPOS, forming a closed-loop control for the axis itself.
3. Technical Features of Gravure Printing Machine Based on Unified Electronic Shaft Control System
Compared with traditional mechanical shaft gravure printing machines, unit-type gravure printing machines using a unified shaftless drive control system have the following advantages:
(1) The overall mechanical structure and wiring system are simple.
It eliminates the mechanical drive shaft and the mechanical floating roller mechanism配套 for the color matching system of traditional printing presses. The material feeding path of the machine body is reduced by 20%, which greatly saves material feeding time.
By adopting remote site control for external I/O wiring and communication via a bus, wiring is truly reduced, thereby greatly reducing system interference and failures and simplifying equipment maintenance and care.
(2) Fast and high-precision pre-fitting
With any printing roller installed, the system automatically adjusts the roller phase according to the material length between color groups, positioning it to the "printing position." The servo system's positioning accuracy can reach 1/1000 of 1 mm, with a repeatability of 1/100 of 1 mm. The automatic pre-registration range can be strictly controlled within ±5 mm, thus achieving truly high-precision pre-registration.
(3) High precision, high printing speed, and high registration accuracy
The system utilizes a high-speed PCC processor with strong data processing capabilities. The POWERLINK bus, forming the overall machine communication network, ensures simpler, more real-time, and more reliable communication. For the control of the relative positions of all printing rollers, the encoder, acting as position feedback, generates 4 million effective pulses per revolution through hardware frequency multiplication, achieving a system control accuracy of up to 0.001 mm. Practical application has proven that this technology can improve printing accuracy from 0.1 mm to 0.05 mm.
(4) Special printing processes are easier to implement
The system effectively solves the problem of freely switching between positive and negative printing for each printing color group without any changes to the electrical configuration. The implementation of differential coating and spot coating functions is simple yet reliable.
(5) Grouping of multicolor machines
This system facilitates the grouping of multi-color printers. Specifically, while some color groups are operating normally, users can prepare the plates and apply ink for the next batch of orders in advance for the remaining color groups. Furthermore, users can select whether or not to operate each printing color group based on production requirements; color groups not involved in operation can be placed in a single-action stop state to achieve energy savings.
4. Conclusion
The application of electronic axis control systems in printing presses has significantly improved their automation. It not only optimizes control coordination and stability but also enables high precision, high printing speed, and high registration accuracy. Its user-friendly operation, safe design, and energy-saving features further advance the design philosophy of printing and packaging equipment.
About the Author
Lian Dawei (1976~), male, from Pingliang, Gansu Province, is an engineer. Since joining Shaanxi Beiren Printing Machinery Co., Ltd., he has been engaged in the electrical design of printing presses. His main research area is the electrical control design and research of printing presses.
