Abstract: Based on the technological characteristics of printing machinery, this paper designs a multi-servo motor synchronous control system based on CAN fieldbus for the synchronous system of traditional printing presses powered by long mechanical shafts. The paper proposes a mechanism for generating synchronous motion data of the host computer, a clock synchronization mechanism and a synchronization interface protocol between the host computer and the driver, and verifies the results.
Keywords: Shaftless drive; Synchronous control; CAN bus
0 Introduction
Synchronous control of multiple motors is a crucial issue in the printing machinery industry. Due to the special process requirements of printing products, especially multi-color printing, high slip rates (generally ≤0.02%) are required for each motor to ensure printing registration accuracy (typically ≤0.05mm). Traditionally, printing machinery has largely employed synchronous control schemes using long mechanical shafts as power sources. However, these schemes are prone to oscillations and mutual interference between units, and the presence of numerous mechanical parts makes system maintenance and operation inconvenient. With the development of mechatronics technology, fieldbus technology has been widely applied in various fields. This paper proposes and validates a synchronous control solution based on CAN fieldbus for the synchronization requirements of unit-type printing machinery.
1. Synchronization Requirements of Shaftless Printing Press Control System
A roll-to-roll printing press typically consists of a paper feeder unit, a printing unit, a tensioner unit, a processing unit, and a rewinder unit. In traditional shaft-driven printing presses, the power source is an asynchronous motor that drives a long mechanical shaft (approximately 10-20m) via a belt pulley. This shaft then drives the gears, cams, connecting rods, and other transmission components of each unit, which in turn drive the actuators to complete the input and output tasks of the equipment.
Roll-to-roll printing presses require a printing speed of 300 m/min and a registration accuracy of ≤0.03 mm. To achieve this registration accuracy, the positioning accuracy of each unit must be ≤0.03 mm. During the printing process, the shafts of each unit must maintain a certain synchronous movement relationship with the main shaft. The ability to achieve this synchronization directly affects the printing speed and registration accuracy. Specifically, the paper feeder and printing press units must maintain a certain proportional relationship with the main shaft's rotation speed; the tension unit adjusts its tension coefficient according to different printing speeds; the processing unit needs to maintain a cam motion relationship with the main shaft; and the rewinding unit's motion pattern must decrease as the paper roll diameter increases.
We take the long mechanical axis as the main axis (reference axis) and each printing press axis as the driven axis, as shown in Figure 1. Each driven axis and the main axis must satisfy the synchronous relationship θ1=f1(θ), θ2=f2(θ), θ3=f3(θ) ···, where θ is the position angle of the main axis and θ1, θ2, θ3··· are the position angles of the driven axes.
Figure 1 Master-slave axis synchronization relationship
2. Design of Synchronous Control System
Considering the complex synchronous motion relationships in printing presses, high registration accuracy, numerous and dispersed printing press points, multiple operation substations, and long printing production lines, a fully distributed, fully digital, and fully open fieldbus control system (FCS) is adopted, with the CAN bus selected as the bus.
To achieve the complex synchronization relationship between the various printing press units, the main controller and the servo drivers of each motor are connected to the CAN bus, forming a CAN fieldbus system with the printing press controller as the core, as shown in Figure 2.
Both the controller and servo drives are equipped with communication adapter cards for the SJA1000 CAN bus controller and PCA82C250 transceiver. Through these CAN communication adapter cards connected to the printing press controller, the controller can easily and quickly communicate with each servo drive, sending control commands and position setpoint commands to each servo unit, and obtaining real-time status information for each servo motor. Servo parameters can be modified in real-time as needed, and each servo unit can also exchange data promptly via the CAN bus. Each servo drive closely follows its own position reference command after receiving it. Because the controller's position commands are directly input to each servo drive, each servo drive receives synchronized motion control commands, unaffected by other factors; that is, no single servo unit is affected by disturbances from other servo units. In this system, the controller and each servo drive act as network nodes, forming a CAN control network. Furthermore, due to the use of a fieldbus control system, the number of network nodes can be expanded according to the printing scale.
Figure 2 Synchronous Control System Diagram
3. Selection of Encoder and Servo Motor
In high-inertia-load printing systems, the selection of encoders and servo systems is particularly important. Taking the BF4250 web printing press as an example, its load rotational inertia is very large, with the flexographic unit having a rotational inertia of 0.13 kg·m2 and the offset unit having the largest rotational inertia of 0.33 kg·m2.
Because the system positioning accuracy requirement is ≤0.03mm, and considering the large inertia of the load, the control cycle is set to 2ms, requiring a steady-state error of ±1 pulse in the position loop. Based on the positioning accuracy and steady-state error, the encoder line count can be calculated to be 17,000 lines. However, considering that the positions of different units need to be constantly adjusted during the actual printing process, if the encoder resolution is selected to be 17,000 lines, the large rotational inertia of the printing rollers will generate a large angular acceleration, resulting in a large torque. For example, for offset printing units, adjusting the angular acceleration exceeds 700 rad/s², and the adjusting torque exceeds 200 N·m, which ordinary motors cannot meet.
Taking all factors into consideration, an encoder resolution of 40,000 lines was selected. This reduces the motor's adjustment acceleration during the adjustment process, thereby reducing the adjustment torque. For example, in offset printing presses with the largest load inertia, the adjustment angular acceleration is 78.6 rad/s², and the adjustment torque is 26 N·m. The 90M series servo motor from Kage Electric Co., Ltd. can fully meet the requirements.
4. Clock synchronization mechanism
In a distributed shaftless synchronous control system, all printing presses need to work in a unified and coordinated manner. Therefore, each press must have a unified timing system to ensure that they work in a coordinated manner and complete the printing task.
Specific clock synchronization methods can be categorized into hardware clock synchronization, synchronization message timing synchronization, and protocol timing synchronization.
(1) Hardware clock synchronization. Hardware clock synchronization refers to the synchronization between local clocks using certain hardware facilities (such as GPS receivers, UTC receivers, dedicated clock signal lines, etc.). The object of operation is the computer's hardware clock. Hardware synchronization can achieve very high synchronization accuracy (usually 10⁻⁹ seconds to 10⁻⁶ seconds).
(2) Synchronization message timing. At the beginning of each communication cycle, the master station sends a synchronization message in broadcast form. For example, in the SERCOS protocol data transmission layer, the start of each SERCOS communication cycle is marked by the master station sending a synchronization message MST. The data field of the MST is very short, occupying only 1 byte. The synchronization accuracy of the MST message is very high; if optical fiber is used as the transmission medium, the synchronization accuracy can be within 4 microseconds.
(3) Protocol-based time synchronization. Protocol-based time synchronization, also known as software-based time synchronization, refers to using a network to transmit the master clock source to other subsystems to achieve time synchronization of the entire system. By calculating the time difference between sending the master clock information and the target node receiving the information and generating an interrupt, the delay time can be obtained. Then, delay compensation is used to achieve time synchronization. Software-based time synchronization is low in cost, but due to the large delay and significant uncertainty in the transmission of synchronization information over the network, the time synchronization accuracy is low (usually 10⁻⁶ to 10⁻³ seconds).
Taking all factors into consideration, the clock synchronization scheme in this paper adopts hardware clock synchronization. Each node adjusts its clock according to the master clock specified in the system. Specifically, a hardware clock synchronization signal line CONCLK is added to transmit the time synchronization signal. The synchronization control signal period is 2ms, and the rising edge of the synchronization signal is used as the synchronization point. A synchronization signal generator is set in the controller, and a synchronization receiving unit is set inside each driver. After the synchronization receiving unit of the slave station detects the rising edge of the master's CONCLK, the clocks of each slave station are simultaneously reset to zero. This periodic reset not only maintains the consistency of the slave station clocks but also avoids the accumulation of synchronization errors. To improve the anti-interference capability of the module synchronization signal, a balanced differential drive method is used to transmit the synchronization signal. Optical isolation is used to ensure that the signals of the master station and slave stations do not interfere with each other. The master and slave station synchronization signal circuit is shown in Figure 3.
Figure 3. Synchronization signal circuit diagram of master station and slave station
5. Generation of synchronous motion data by the host computer
The task of generating synchronous motion data is placed in the soft PLC-Comac PLC system developed by Beijing Shouke Kaiqi Electric Technology Co., Ltd. The hardware system of the company's soft PLC system adopts an industrial computer platform, and the operating system adopts the WinCE embedded operating system launched by Microsoft. In this soft PLC system, fast logic tasks and slow logic tasks are established. Fast logic is used for situations with high time requirements, such as emergency handling and high-precision sampling. Slow logic tasks are mainly used for situations with low time requirements. A fast logic task is a task that needs to be executed at a time (similar to an interrupt service routine). This task must be completed within one system sampling cycle. A slow logic task is an infinite loop that can be completed within several system sampling cycles [2]. The fast logic task is timed by the timer controller 8254, with a timing period of 1 millisecond. During the execution process, the fast logic task is executed once in each sampling cycle to generate synchronous motion data. In order to maintain the synchronous relationship between each driven axis and the main axis, a motion reference data source is established to simulate the motion state of the main axis. In each system sampling cycle, based on the motion state of the virtual master axis and the synchronous motion requirements of each slave axis, the position information of each slave axis is calculated, and synchronous motion data of each slave axis is generated and placed into the transmission queue of the CAN controller for transmission, as shown in Figure 4. The motion data generation and calculation tasks are placed in a fast logic task to ensure the real-time nature of the generated motion data.
Figure 4. Generation of synchronous motion data
6 Synchronous Interface Technology Protocol
The system bus baud rate is set to 1 Mbps, and the bit transmission time τbit is 1 × 10⁻⁶ seconds. Each data frame consists of 8 bytes, with a fixed transmission frame length of 131 bits (29-bit identifier) and a feedback frame length of 99 bits. The data frame transmission time Cm = 131 μs. The synchronization control signal line CONCLK is used as the synchronization cycle signal line and the reference signal line for the message. The synchronization control signal period is 2 ms, active high, and the signal width is 10. During normal communication, the CAN network can transmit 16 synchronization data messages within one control cycle. The controller sends a command message within 50 μs after the rising edge of CONCLK, and the driver sends a feedback message within 100 microseconds after receiving the command message. The command message content includes the position command value and the logic interface signal input, where the position command occupies 4 bytes (32 bits) and the logic interface signal input occupies 1 byte. The logic interface signal input includes driver enable, reset, and other commands. The feedback message includes servo operating status information and fault information, and the communication timing is shown in Figure 5.
Figure 5 Communication Timing Diagram
7. Conclusion
This paper proposes a fieldbus control system centered on a controller for the synchronous control system of a traditional mechanical long-shaft printing press, using a CAN fieldbus to achieve communication between the controller and the servo units. This solution not only overcomes the shortcomings of various mechanical components in traditional mechanical long-shaft control schemes, but also has advantages such as good synchronization performance, no mutual interference between servo units, high control accuracy, and convenient maintenance.
The key feature of this synchronization method is that it leverages the high reliability, short transmission time, and strong anti-interference capability of the CAN bus, as well as the high positioning accuracy and fully closed-loop characteristics of digital servos.
The innovation of this paper lies in applying fieldbus technology to the printing press field and designing a CAN bus-based synchronous control system to replace the traditional mechanical long shaft transmission system.
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