1. Introduction In today's rapidly developing science and technology, in order to meet people's needs for diversified and exquisite product packaging, the requirements for automation and position tracking accuracy of packaging printing transmission systems are getting higher and higher. Applying microelectronics technology, information processing technology, new sensing technology, laser technology, as well as new processes and new materials to printing machinery to achieve intelligent, highly automated, and efficient production is the development direction of modern printing machinery. 2. Current Status of Packaging Printing Machinery China's packaging printing machinery, especially gravure printing production lines, started relatively late and has a relatively weak foundation. According to relevant statistics, almost all of the approximately 300 existing cigarette packaging gravure printing production lines in China are imported from industrialized countries such as Europe, America, Japan, or Australia. Most of them were introduced in the 1990s and generally use technology from the 1980s and 1990s, and some even use technology from the 1960s and 1970s. The equipment has the following characteristics: (1) The control circuit of the equipment generally adopts programmable logic controller (PLC) or computer control, which is relatively advanced. (2) The transmission device generally adopts DC speed-regulating motor, and some equipment from the 1960s and 1970s still uses slip differential speed-regulating motor. The main drive drives a mechanical shaft through a DC speed-regulating host (or an AC speed-regulating host), and then connects the color group units and the die-cutting unit together through this mechanical shaft, rotating synchronously. This mechanical through-shaft structure is still the transmission form commonly used in roll paper gravure printing machines. (3) The registration device generally adopts a stepper motor with a reducer mechanism (including differential, continuously variable transmission, etc.). (4) The tension control device also generally adopts a stepper motor with a reducer mechanism. In recent years, electronic technology and microelectronic technology have developed at an astonishing speed, which has also driven the progress of AC drive technology. The development of modern AC drive technology has gone through more than 20 years and is now becoming the mainstream of electrical drive. Many fields that were previously occupied by DC drive have been occupied by AC drive. 3. Disadvantages of mechanical through-shaft structure The roll paper cigarette pack six-color gravure printing machine can print 1-6 colors on roll paper. Because roll paper gravure printing is fast, has rich tonal reproduction and good texture, it has been widely used. However, it has very high requirements for mechanical manufacturing precision. Especially since paper is easily deformed and expanded due to temperature and humidity, even if the registration adjustment mechanism is manufactured very finely and the movements of each part are highly coordinated and accurate, misregistration is still difficult to avoid if the degree of automation is not high, and product quality cannot be guaranteed. Most domestic roll-to-roll gravure printing equipment generally adopts a mechanical through-shaft structure, which requires manual pre-positioning of the registration, making the operation complex, with large positioning errors and slow dynamic adjustment. Inaccurate pre-positioning leads to a long paper-carrying time for the equipment, resulting in a lot of waste. 4. Advantages of AC Servo Drive To solve the above problems, some advanced printing press manufacturers in Europe and the United States have developed electronic shaftless roll-to-roll gravure printing machines using existing AC servo drive technology. Compared with traditional shafted (mechanical shaft) drive gravure printing machines, electronic shaftless roll-to-roll gravure printing machines eliminate the mechanical through-shaft structure between printing units. Each printing unit is independently driven by an AC vector frequency converter motor, which directly drives the printing plate cylinder and adjusts the phase of the printing plate cylinder to achieve longitudinal registration. At the same time, a stepper motor drives the lateral movement of the printing plate cylinder to achieve lateral registration. This design brings several significant advantages. (1) It eliminates many mechanical transmission links, minimizing the possibility of reduced registration accuracy due to mechanical wear, increasing reliability, and reducing maintenance costs. (2) The registration time is very short and the response speed is extremely fast. (3) It provides a unique high-precision pre-registration function. After the printing plate cylinder is loaded into the printing unit, the zero mark on the printing plate cylinder is detected by the sensor, and the drive motor will automatically rotate the printing plate cylinder to the preset zero position. The equipment can achieve pre-registration operation by running the motor idle, without the need for paper, so there is very little waste in the pre-registration process. In addition, since the shaftless transmission system has high control accuracy of the printing plate cylinder phase and fast information exchange, it can achieve high-precision high-speed registration. Therefore, it can still achieve much higher registration accuracy than mechanical shaft gravure printing machines in high-speed printing conditions. 5. Modification Plan Our company is a cigarette packaging printing plant invested and built by Hongta Group. It has an NL650 six-color gravure inline die-cutting printing production line imported from Komori-Shangbang Company in France. The technology used in this printing press basically reflects the technological level at that time. The control circuit uses Siemens PLC, and the overprinting circuit uses Shangbang's RNP93 system. The main drive uses a DC speed-regulating motor to transmit power to each printing color group and die-cutting station through a mechanical shaft. The overprinting (position follow-up and compensation) of each unit adopts a stepper motor plus differential structure, thus it cannot avoid the disadvantages of the above-mentioned mechanical through-shaft gravure printing press. In addition, there are the following shortcomings. (1) The main motor uses a DC speed-regulating motor, and the maintenance cost of carbon brushes and commutators is high. (2) The gearbox and continuously variable transmission are not only easy-to-wear parts with high maintenance costs, but also inevitably have mechanical backlash. When the adjustment made by the stepper motor according to the overprinting command reaches the printing plate through these mechanisms, there will be some loss, which can cause oscillation in severe cases. (3) The above equipment has been used for nearly 10 years, and many parts manufacturers no longer produce them, which brings great difficulties to maintenance. These problems have affected the long-term development of the company. This article aims to explore the possibility of using an AC servo system to transform this printing press. Compared with stepper motors, AC servo motors have the following obvious superior performance. (1) The control accuracy is greatly improved. (2) The low frequency characteristics are enhanced. (3) The torque-frequency characteristics are good. (4) The speed response performance, control performance (closed-loop control) and overload capacity are greatly improved. 5.1 Overall modification scheme (1) Remove all main drive and stepper servo drive components such as DC speed control host, mechanical through shaft, differential gearbox, and stepper motor, and replace the above components with 7 independent AC servo motor units to realize the transmission and position compensation functions. The 7 units mentioned here are 6 color groups plus 1 die-cutting unit. If 2 tension units are included, there should be 9 units. Therefore, the number of axes will vary depending on the modification scheme of different models. (2) The 7 AC servo motor units are directly connected to the printing plate shaft or die-cutting shaft through a high-precision maintenance-free reducer (speed ratio of 5:1 or 10:1), minimizing the mechanical transmission links between the motor and the plate shaft, and realizing 7-axis independent servo drive. 5.2 Specific Modification Scheme Figure 1 Schematic diagram of DC speed-regulating motor + mechanical through-shaft structure As shown in Figure 1, the transmission and overlay structure before the equipment modification consisted of a 35kW DC speed-regulating motor connecting six printing units together via a long shaft. The modification scheme involves removing the structure shown by the dotted line in Figure 1 and modifying it into the structure shown in Figure 2. Figure 2 Schematic diagram of independent AC servo drive structure (after modification) The printing plate motor drive unit will adopt Rexroth's Ecodrive intelligent AC servo drive and a matching MHD high-performance AC servo motor. The servo drive internally includes a current loop, a speed loop, and a position loop (the position loop is not used in this modification). The motor shaft speed and position detection elements are the servo motor's built-in 2500 rotary encoder (4x frequency). The second encoder interface on the servo drive is used to achieve multi-axis speed following and synchronization, and the synchronization structure is shown by the dotted line in Figure 2. In fact, the synchronous control of a printing press includes two aspects: the synchronization of the rotational speeds of each printing plate shaft and the synchronous control of printing registration. This involves using a photoelectric scanner to detect the actual position of the printed code lines and comparing it with the theoretical position, outputting pulse signals (forward or reverse rotation) to the actuator (previously a stepper motor) for position adjustment. This constitutes the outer loop of the synchronous control. Using a servo controller to control the printing plate motor in speed mode only achieves good dynamic characteristics. However, during the printing process, due to differences in the characteristics of the servo drivers of each motor, accumulated errors inevitably occur over long-term operation. The aforementioned outer loop control aims to solve this problem. The internal speed loop manages the synchronization of the speeds of each shaft, requiring good dynamic performance. Errors introduced by various disturbances into the internal speed loop can be compensated for by the outer loop control. The external position loop ensures stability and registration accuracy, as shown in Figure 3. Figure 3: Schematic diagram of inner and outer loop structure. This modification scheme retains the signal detection, processing, and transmission links of the original system for registration control, only replacing the stepper motor as the actuator with an AC servo motor. Therefore, how to convert the pulse signals previously sent to the stepper motor into signals controlling the AC servo motor is a major research problem. The current idea is to solve this problem by adjusting the operating parameters (actually PID parameters) of the original overprinting system and the PID parameters on the AC servo controller. 5.3 Overprinting Principle In multicolor printing, the printing position is generally adjusted by changing the rotation angle of the printing plate cylinder. Its closed-loop control principle is shown in Figure 4. Figure 4 Overprinting Control Principle Block Diagram According to the principle of the rotary encoder, the more lines it has, the more pulses it generates per revolution of the printing plate, and the higher the positional accuracy. Assuming the circumference of the printing plate is 500mm and the encoder has 500 lines, then 500mm corresponds to 500 pulses, 1mm corresponds to 1 pulse, and 20mm corresponds to 20 pulses. The actual printing position is detected by converting the detected color mark signal into electrical pulses using a photoelectric scanner. These electrical pulses, along with the pulse train generated by the rotary encoder, are simultaneously sent to the overprinting system computer. The given distance between each color mark is 20mm, meaning that when two colors are registered, the distance between the two color marks is 20mm, corresponding to 20 encoder pulses. As shown in Figure 5, the count during the pulse triggering time of color mark 1 and color mark 2 is 18 pulses. If color 1 is taken as the reference, it indicates that the printing plate shaft speed of color 2 is too fast. To slow it down and correct the positional deviation, the servo motor should be reversed by an angle equivalent to 2 pulses. Assuming the pulse triggering time of color mark 2 and color mark 3 is 23 pulses, if color 2 is taken as the reference, it indicates that the printing plate shaft speed of color 3 is too slow. To increase its speed and correct the positional deviation, the servo motor should be forward rotated by an angle equivalent to 3 pulses to eliminate the deviation. Figure 5: Schematic diagram of detected pulses.