1. Introduction
In today's rapidly developing world of science and technology, to meet people's diverse and exquisite needs for product packaging, the requirements for automation and position tracking accuracy in packaging printing transmission systems are becoming increasingly stringent. Applying microelectronics, information processing, new sensing, laser technology, and new processes and materials to printing machinery to achieve intelligence, high automation, and high efficiency is the development direction of modern printing machinery.
2. Current Status of Packaging and Printing Machinery China's packaging and printing machinery, especially gravure printing production lines, started relatively late and has a weak foundation. According to relevant statistics, almost all of the approximately 300 existing gravure printing production lines for cigarette packaging in China were imported from industrialized countries such as Europe, America, Japan, and Australia. The vast majority of these were introduced in the 1990s, generally using technology from the 1980s and 90s, and some even from the 1960s and 70s. These equipment have the following characteristics.
(1) The control circuit of the equipment is generally controlled by a programmable logic controller (PLC) or a computer, which is quite advanced.
(2) The transmission device generally uses a DC speed-regulating motor, and some equipment from the 1960s and 1970s still uses a 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, so that they rotate synchronously. This mechanical through-shaft structure is still the transmission form commonly used in roll paper gravure printing machines.
(3) The alignment device generally adopts a stepper motor with a reducer mechanism (including differential, continuously variable transmission, etc.).
(4) Tension control devices generally also use stepper motors with reducers.
In recent years, electronic and microelectronic technologies have advanced at an astonishing pace, driving progress in AC drive technology. Modern AC drive technology has developed over two decades and is now becoming the mainstream of electrical drives, with many fields previously dominated by DC drives now being taken over by AC drives.
3. Disadvantages of Mechanical Through-Shaft Structure A six-color gravure printing press for cigarette packs can print 1-6 colors on roll paper. Due to its high speed, rich tonal reproduction, and good texture, roll paper gravure printing is widely used. However, it requires very high precision in mechanical manufacturing. Especially since paper is easily deformed and stretched by temperature and humidity, even if the registration adjustment mechanism is very finely manufactured and the movements of each part are very coordinated and accurate, if the degree of automation is not high, misregistration is still difficult to avoid, and product quality cannot be guaranteed. Most domestic roll paper gravure printing equipment generally adopts a mechanical through-shaft structure, requiring manual pre-positioning of the registration, which is complex to operate, has large positioning errors, and slow dynamic adjustment. Inaccurate pre-positioning leads to a long paper-carrying time, resulting in a lot of waste.
4. Advantages of AC Servo Drives <br />To solve the above problems, some advanced printing press manufacturers in Europe and America have developed electronic shaftless web gravure printing presses using existing AC servo drive technology. Compared with traditional shafted (mechanical shaft) gravure printing presses, electronic shaftless web gravure printing presses eliminate the mechanical 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. Simultaneously, 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 alignment 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 realize the pre-registration operation by running the motor idle, without the need for paper to be carried, so there is very little waste in the pre-registration process. In addition, since the shaftless drive 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 a much higher registration accuracy than mechanical shaft gravure printing machines in high-speed printing conditions.
5. Modification Plan <br />Our company is a cigarette packaging printing plant invested and built by Hongta Group. It possesses an NL650 six-color gravure inline die-cutting printing production line imported from Komori-Shangbang Company of France. The technology used in this printing machine basically reflects the technological level at the time. The control circuit uses a Siemens PLC, and the registration 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 registration (position tracking and compensation) of each unit uses a stepper motor with a differential gear structure, thus failing to avoid the shortcomings of the aforementioned mechanical through-shaft gravure printing machine. In addition, there are the following deficiencies.
(1) The main motor adopts a DC speed-regulating motor, and the maintenance cost of carbon brushes and commutator is high.
(2) Gearboxes and continuously variable transmissions are not only easy-to-wear parts with high maintenance costs, but also inevitably have mechanical backlash. When the adjustments made by the stepper motor according to the overprinting instructions reach the printing plate through these mechanisms, there will be some loss, which can cause oscillation in severe cases.
(3) The above equipment has been in use for nearly 10 years, and many parts manufacturers have stopped producing them, which makes maintenance very difficult.
These problems have affected the company's long-term development, and this article aims to explore the possibility of using an AC servo system to upgrade this printing press.
Compared to stepper motors, AC servo motors have the following significant advantages.
(1) The control precision is greatly improved.
(2) Enhanced low-frequency characteristics.
(3) Good torque-frequency characteristics.
(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 the DC speed control host, mechanical through shaft, differential gearbox, and stepper motor, and replace them with 7 independent AC servo motor units to achieve transmission and position compensation functions. The 7 units mentioned here refer to 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 machine models.
(2) Seven 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 printing plate shaft, and realizing independent servo drive of 7 axes.
5.2 Specific renovation plan
Before the equipment modification, the transmission and overprinting structure consisted of a 35kW DC speed-regulating motor connecting six printing units together via a long shaft. The modification plan involved removing the structure shown by the dotted line in Figure 1 and replacing it with the structure shown in Figure 2.
The printing plate motor drive unit will utilize Rexroth's Ecodrive intelligent AC servo drive and a matching MHD high-performance AC servo motor. The servo drive internally incorporates a current loop, a speed loop, and a position loop (the position loop will not be used in this modification). The motor shaft speed and position detection elements are the servo motor's built-in 2500 RPM rotary encoder (4x frequency). Multi-axis speed following and synchronization will be achieved using the second encoder interface on the servo drive; the synchronization structure is shown by the dotted line in Figure 2.
In fact, the synchronous control of a printing press includes two aspects: one is the synchronization of the rotation speed of each printing plate shaft, and the other is the synchronous control of printing registration. That is, the actual position of the printing code lines is detected by a photoelectric scanner and compared with the theoretical position. The output pulse signal (forward or reverse rotation) is sent to the actuator (which was a stepper motor before the modification) for position adjustment. This constitutes the outer loop of synchronous control.
Using a servo controller to control the printing plate motor in speed mode can only achieve excellent dynamic characteristics. However, during the printing process, due to the differences in the characteristics of various motor servo drives, cumulative errors will inevitably occur over long-term operation. The aforementioned outer-loop control is designed to solve this problem.
The internal speed loop is responsible for synchronizing the speeds of each axis and requires good dynamic performance. Errors caused by various disturbances to the internal speed loop can be compensated by the outer loop control. The external position loop ensures stability and overprinting accuracy, as shown in Figure 3.
This modification scheme retains the signal detection, processing, and transmission stages of the original system for overprint control, only replacing the stepper motor as the actuator with an AC servo motor. Therefore, a key issue to address is how to convert the pulse signals previously sent to the stepper motor into signals controlling the AC servo motor. The current plan is to solve this problem by adjusting the operating parameters (actually PID parameters) of the original overprint system and the PID parameters on the AC servo controller.
5.3 Overprinting principle.
In multicolor printing, the printing position is usually adjusted by changing the rotation angle of the printing cylinder. The closed-loop control principle is shown in Figure 4.
According to the principle of rotary encoders, the higher the number of lines (i.e., the more pulses generated per revolution of the printing plate), 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 a photoelectric scanner that converts the detected color mark signals into electrical pulses. These electrical pulses, along with the pulse train generated by the rotary encoder, are simultaneously sent to the registration system computer. The given distance between the color marks in each color group 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 pulse count during the triggering time of color mark 1 and color mark 2 is 18 pulses. If color mark 1 is taken as the reference, it indicates that the printing plate shaft speed of color mark 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 that the pulse count during the triggering time of color mark 2 and color mark 3 is 23 pulses, if color mark 2 is taken as the reference, it indicates that the printing plate shaft speed of color mark 3 is too slow. To speed it up and correct the positional deviation, the servo motor should be forward rotated by an angle equivalent to 3 pulses to eliminate the deviation.
