1 Introduction
With increasing demands for efficiency and precision in self-adhesive label printing, intermittent rotary printing presses have become a development trend. Compared to satellite and stacked printing presses, they offer faster printing speeds and greater material savings. Currently, most companies rely on imported intermittent rotary printing presses for self-adhesive labels, which are expensive and inconvenient to operate and maintain. While some domestic companies are working on independent development, current methods still cannot fully meet the control requirements of intermittent rotary printing presses, resulting in insufficient registration accuracy and misalignment during acceleration and deceleration. This is mainly due to the lack of precise synchronization between printing axes and changes in paper tension caused by variations in printing speed and roll size. To address this issue, we have developed an intermittent rotary printing press control system based on a multi-axis motion controller .
2. Working principle and process of intermittent rotary machine
2.1 Working principle of intermittent rotary machine
High-performance self-adhesive label intermittent rotary printing presses generally adopt a shaftless structure, that is, there is no drive shaft , and the printing shaft and the material feeding shaft are driven by their own motors. It mainly consists of a paper feeder, a material feeding shaft (including a paper pulling shaft and a paper feeding shaft), a printing shaft, and a paper take-up machine. Its structural principle diagram is shown in Figure 1.
Figure 1. Schematic diagram of the intermittent rotary machine.
2.2 Working process of intermittent rotary self-adhesive label machine
The main motion of an intermittent rotary press is intermittent motion. Within one printing cycle, its operation is as shown in Figure 2. The printing axis rotates uniformly in the same direction. When the printing plate contacts the printing material, the paper pull shaft, paper feed shaft, and printing shaft remain synchronized (the instantaneous linear velocity is zero), and printing begins. During the printing process, these three shafts remain synchronized. When the printing plate detaches from the printing material, printing is complete. At this point, the paper pull shaft and paper feed shaft first decelerate to zero, stop moving for a period of time, and then rotate in the opposite direction. When the printing shaft rotates until the printing plate contacts the printing material again, the second printing cycle begins.
Figure 2. Action relationships of each axis of the rotary machine
3 Control System Design
3.1 System Hardware Design
3.1.1 Motion Controller
The motion controller used in the control system is the Eur0209 from Trio Systems, UK. It is a European-style digital motion controller with a high-performance DSP core for real-time data processing, generating a 16-bit resolution 10V speed command signal. It integrates a 1Mbyte memory, an Ethernet port, an RS232 port, 16 24V inputs, 8 24V outputs, 2 12-bit resolution (0.10)V analog inputs, and a 24V/100mA watchdog relay output. This controller can control servo or stepper axes from 1 to 8. It can operate independently offline and supports multi-tasking (up to 7 programs can run simultaneously according to priority).
3.1.2 Touchscreen
A user-friendly human-machine interface can be created using a touchscreen, enabling functions such as product counting, setting printing speed (manual or automatic), setting wet printing plate length and printing speed, online tension adjustment, and abnormal alarms. Considering performance and price, the Weintek MT506 touchscreen was chosen, as its built-in EasyBuilder500 configuration software is simple to use and powerful.
3.1.3 Servo Motor
Based on the power and speed requirements, Yaskawa Σ-II series SGMGH servo motors from Japan were selected. A total of 8 motors were used, with 6 connected to the printing shaft and 2 connected to the material drag shaft.
3.2 System Software Design
3.2.1 Multi-axis synchronous control
Multicolor rotary printing presses require synchronized operation between the various color printing axes. Traditional rotary printing presses maintain synchronization by using a long mechanical shaft to transmit motion to each printing axis, but its accuracy and efficiency are limited by the mechanical transmission device. Using electronic virtual axis functionality overcomes these shortcomings, achieving axisless synchronous operation without the need for mechanical transmission shafts. The Eur0209 motion controller, in addition to controlling up to eight physical axes, also supports two electronic virtual axis functions. By superimposing the motion of the electronic virtual axes onto each printing axis through ADDAX instructions and then performing position compensation, synchronized operation between the various color printing axes is achieved.
3.2.2 Constant tension control
During printing, the paper must maintain a certain tension. However, unstable tension can cause misregistration, ghosting, and even longitudinal wrinkles. Constant tension control is achieved through another electronic virtual axis and ADDAX commands in the Eur0209. Its main principle is to control the phase difference between the paper pull axis and the paper feed axis, that is, to perform position compensation based on the synchronization of the paper pull axis and the paper feed axis. This position compensation amount is adjusted online until the registration error is within the allowable range. Finally, the compensation amount is saved as a variable. Calling this position compensation amount in a timely manner during the printing process achieves the purpose of constant tension control.
3.2.3 Coordinated movement of each axis
Intermittent rotary printing presses require the paper pull shaft and paper feed shaft to complete one forward and reverse rotation cycle in each printing cycle (the time it takes for the printing shaft to rotate one revolution), and to complete the printing action while rotating forward and synchronizing with the main shaft. In the Eur0209 motion controller , the entire process can be implemented by MOVELINK instructions.
3.2.4 PID Control with Feedforward
This control system employs a PID control strategy with feedforward, combining feedforward and feedback control. This strategy combines the timely characteristics of feedforward control with the ability to compensate for deviations caused by multiple disturbances. Its structure is shown in Figure 3.
Figure 3 shows the PID control structure with feedforward.
In the multi-axis motion controller Eur0209, PID control with feedforward is implemented through axis parameters P_GAIN, I_GAIN, D_GAIN and VFF_GAIN.
4. Experimental Results and Analysis
After connecting the Eum209 motion controller in the system to the PC via an Ethernet interface, the system can be debugged online in the field. The software oscilloscope built into MotionPerfect2 is used to track the axis and motion parameters. The axis parameter window is used to monitor and change the motion parameters of the axes on the controller. The axis parameters P_GAIN, I_GAIN, D_GAIN, and VFF_GAIN affect the performance of the control system, and these four parameters also influence each other. Stable positioning is the most basic requirement for a self-adhesive label printing machine, and the axis parameters P_GAIN and VFF_GAIN have the most significant impact on it. To simplify the experimental procedure, the study is conducted by adjusting the parameters P_GAIN and VFF_GAIN with the axis parameters I_GAIN and D_GAIN as constants.
The accuracy of the control system was evaluated using the follower error FE. When the printing spindle speed was 33.6 r/min, the software oscilloscope sampling period was 500 s, and the axis parameters I_GAIN=0.01 and D_GAIN=0.02, the effects of axis parameters P_GAIN and VFF_GAIN on the follower error FE were studied. The experimental data are shown in Table 1.
Obviously, as the values of axis parameters P_GAIN and VFF_GAIN decrease, the follow-up error FE gradually increases. If FE is greater than the FE-LIMIT setting, the axis status parameter display will show an error, and the control card will issue an alarm. In Table 1, P_GAIN = 0.03, VFF_GAIN = 0.3, and the follow-up deviation FE = 0.0036, which does not exceed the allowable error range for self-adhesive printing. The stability of the system directly affects the stability of the printing registration, which can be measured by the smoothness and overlap of the oscilloscope output waveform (the degree of overlap between the output waveforms of the printing spindle and the material guide spindle). Under the same experimental conditions, the influence of axis parameters P_GAIN and VFF_GAIN on the oscilloscope waveform is shown in the figure (omitted). When P_GAIN = 0.03 and VFF_GAIN = 0.3, the overlap of the output waveforms of the printing spindle and the material guide spindle motors is the smoothest, and the waveform is the best at this time. Printing experiments have shown that the printing machine operates most stably and is less prone to misalignment.
5. Conclusion
A control system for an intermittent rotary printing press based on the Eum209 motion controller was proposed. Utilizing the powerful capabilities of the Eum209 motion controller, independent control of each printing axis was achieved. Compared to traditional self-adhesive rotary printing presses, this simplifies the mechanical components and improves printing accuracy. Integrated and intelligent control was realized. The system can achieve a maximum printing speed of 12,000 impressions per hour, print up to eight colors of labels, with a registration error within 0.03mm. Acceleration and deceleration do not cause misalignment, meeting the industrial control requirements for self-adhesive printing.
