Abstract: This paper reviews the application of CAN bus combined with PLC and IPC in the control of dyeing and printing equipment, analyzes the characteristics of CAN, and points out that CAN has great potential for development in China.
Keywords: CAN bus, PLC, IPC
1 Introduction
CAN bus is a type of fieldbus, initially used for data communication between sensing and actuating components inside automobiles. It has extremely strong resistance to harsh environments and interference. Due to its inherent characteristics, its applications have expanded from transportation to process control, CNC machine tools, robotics , intelligent buildings, medical devices, and other fields, and it is widely recognized as one of the most promising fieldbuses.
Unlike most fieldbuses (such as PROFIBUS and CC-Link) that use the RS485 master-slave protocol at their physical layer, CAN uses Carrier Sense Multiple Access (CSMA) technology for media access, allowing for multi-master operation. Furthermore, its non-destructive bus arbitration technology significantly reduces bus conflict arbitration time. Unlike most fieldbuses, CAN only has a physical layer and a data link layer, leaving the application layer to user development, giving users considerable flexibility. These advantages are so significant that some well-known fieldbuses (such as DeviceNet and CANopen) use CAN at their underlying layer. Many processor manufacturers integrate the CAN bus protocol into their CPU chips, such as the 51 series microcontrollers, 196 series microcontrollers, and motion control-specific digital signal processors (DSPs). Many frequency converter manufacturers offer CAN communication cards as options or integrate them into their frequency converters , such as the Lenze93 series, Siments6SE series, and Mitsubishi FR-A500 series frequency converters. Many programmable logic controller (PLC) manufacturers provide CAN communication card options or CAN bus interfaces, such as the B&R 2000 series PLC and the Siemens S5 series PLC. The CAN bus is simple to learn, easy to develop with, and has widespread manufacturer support, making it suitable for China's national conditions.
Pre-treatment equipment for dyeing and printing, such as combined desiccant-bleaching and scouring machines, mercerizing machines, straight roller mercerizing machines, and soaping machines, are long machines that use multi-motor distributed transmissions and require constant tension and synchronous speed regulation. Currently, the popular technology is to use a PLC to control multiple frequency converters and use tension frames or tension sensors to achieve synchronization across multiple machines. Similar technologies are also used in post-dyeing and printing equipment (such as hot air tenter frames and heat setting machines), paper production lines, and wet-laid felt production lines. Frequency converters are widely used here, and there are many connecting lines between them and programmable controllers. Frequency setting and various monitoring information (such as voltage, current, speed, torque, etc.) use analog signals, which are susceptible to interference. If fieldbus technology is introduced, the number of connecting lines between numerous frequency converters and programmable controllers is greatly reduced (effectively to two), analog frequency setting is changed to digital frequency setting, and various monitoring and operational information can be transmitted on the fieldbus, thus solving the aforementioned shortcomings.
2. Control Scheme for PCC De-cooking and Bleaching Combined Machine Based on CAN Bus
The core of the control unit is a B&R Programmable Computer Controller (PCC) BR2005, which connects to a touchscreen monitoring station (Provit2200) via a 422 bus. Operators input commands and monitor the overall machine's operating status through this station. The PCC connects to 29 Siemens frequency converters via a CAN bus, transmitting frequency command, starting and stopping the converters, and monitoring their operating status. Synchronization between the 29 variable frequency motors is achieved using a tensioning frame, not shown in the diagram. The PCC also performs logic control of the entire equipment and closed-loop control of temperature, pressure, flow rate, liquid level, pH value, and formula. See Figure 1 for details. Due to the use of fieldbus technology, the number of field connection cables for the frequency converters is greatly reduced; in practice, only two wires are used to daisy-chain the PCC to the 29 frequency converters. The B&R Programmable Computer Controller is a new type of industrial control device integrating computer technology, communication technology, and automatic control technology. Since its inception in the 1960s, programmable control technology has evolved through three generations: Programmable Logic Controller (PLC), Programmable Controller (PC), and now PCC, representing its third generation. The new generation of PCCs is capable of handling large-scale distributed control and complex process control. Its excellent compatibility, rich function sets, diverse hardware modules, use of advanced programming languages, and modular programming methods enable PCCs to meet various industrial control needs. This PCC's programming platform uses Automation Studio software provided by BR, featuring a Windows interface for ease of use. It has RS232, RS485, RS422, CAN, and Profibus fieldbus interfaces, facilitating the construction of a control system computer network. The monitoring station (Provit2200) is a 486 industrial PC equipped with CAN, RS485, RS422, and RS232 interfaces, a 5.7-inch color LCD touchscreen, and 16 keys, exchanging information with the PCC via RS422. In addition to a CPU and CAN communication module, this PCC is equipped with 5 digital input (5*16 points) modules, 3 digital output (3*16 points) modules, 2 analog input (2*8 points) modules, and 2 analog output modules. The logic control section uses ladder diagram programming, while the CAN communication and closed-loop control sections use Basic programming language, though C language programming is also an option. The entire program was completed separately by three people and placed under the same project. Inter-process relationships are achieved using global variables.
Figure 1. Simplified control diagram of the de-cooking and bleaching combined machine.
3. Control Scheme for Winding Section of PLC Wet-laid Felt Production Line Based on CAN Bus
The core of the control unit consists of a Siemens S5-95U programmable logic controller (PLC) and three Lenze93 series AC servo controllers (9326). The PLC exchanges information with the three AC servo controllers (9326) via a CAN bus to achieve variable tension winding control, as shown in Figure 2. In addition to the CAN communication module, the S5-95U also has 64 digital inputs and outputs.
Figure 2. Working principle diagram of the winding section of the wet-laid felt production line.
Three LENZE-9300 series servo controllers (9326) were used to drive three variable frequency asynchronous motors (M) with rotary transformers (R). Among them, the roller servo controller 9326 (1) operates in speed mode. Its speed setpoint (1/2 end) comes from the analog output of the main control PLC of the production line, and the auxiliary speed setpoint (3/4 end) comes from the tension frame signal, so as to keep in sync with the production line. The roller 1 and roller 2 servo controllers (2/3) operate in torque mode and have internal roller diameter calculation function. They can perform closed-loop control on the tension setpoint information sent by the PLC through the CAN bus and the actual tension information sent by the tension sensor. There is no need to implement special speed control for roller 1 and roller 2. They can automatically float their linear speed to the required value. The linear speed information required for roller diameter calculation is sent by the roller servo controller through the special speed cascade interface X9-X10, and the rotational speed information required for roller diameter calculation is measured by the rotary transformer. Reel 1 and reel 2 work alternately to achieve continuous winding. A shaft-changing motor driven by a LENZE-8215 frequency converter (not shown in the diagram) performs the shaft-changing function. The CAN bus also sends the winding diameter information calculated by the servo controller (2/3) to the PLC. The PLC calculates the tension setting based on this information and then sends it back to the servo controller (2/3) via the CAN bus. The winding section requires the reels to be tight on the inside and loose on the outside, which necessitates a high initial tension. As the winding diameter increases, the tension gradually decreases according to a certain pattern. This application system fully meets these requirements. Actual operation has proven the reliability of the winding system, achieving dense and neat winding from 86 mm to 1200 mm in diameter, with a winding speed of up to 80 m/min.
4. Control Scheme for the Scraper Printing Section of an Industrial Computer-Controlled Flatbed Printing Machine Based on CAN Bus
Figure 3 illustrates a modification scheme for the control of the squeegee section of a BUSH-5V flatbed screen printer. The original scheme used RS-232 serial communication between the central controller and the squeegee units, which was slow and unreliable. Therefore, some critical operations still relied on traditional methods with direct wiring. The BUSH-7V uses RS-485 serial communication, improving reliability. In our scheme, a CAN bus is used to achieve serial communication between the central control IPC and the frequency converters of each squeegee unit, broadcasting start/stop commands and monitoring the operating status of each squeegee unit. The squeegee units can also communicate with each other, copying setting information and simplifying repetitive parameter settings. Given the high reliability of CAN, all control and status signals are sent via the bus, simplifying wiring and improving real-time performance.
Here, there are 18 scratch-printing units, using our self-developed DSP-based dedicated frequency converter. The motion control-specific TMS320LF2407 DSP chip integrates a CAN controller, so the dedicated frequency converter has CAN communication functionality without any additional hardware. The central control unit (IPC) is equipped with a CAN communication card.
There are many articles discussing the CAN bus, and the SJA1000 is a commonly used standalone CAN chip. 8-bit microcontrollers with integrated CAN controllers include the P8xC591, but the CAN controller integrated in the TMS320LF2407 is quite unique. It has six mailboxes: two transmit mailboxes, two receive mailboxes, and two selectable transmit/receive mailboxes. Each transmit mailbox has an independent transmit identifier, each receive mailbox has an independent receive acceptance code, and every two receive mailboxes share a receive mask. This multi-mailbox arrangement, compared to the SJA1000's equivalent of only two mailboxes (one receive mailbox/one transmit mailbox), greatly facilitates users in constructing more complex networks and achieving more flexible communication. It also simplifies the writing of communication protocols.
The ISO 11898 CAN communication protocol has only two layers: the physical layer and the data link layer. The essential application layer protocol is left for secondary developers to choose or design. Commonly used application layer protocols include CANopen, DeviceNet, and SDS. CANopen is more popular in Europe, while DeviceNet and SDS are more common in the United States. Considering that the inverter for our flatbed screen printing machine's squeegee unit is specialized, we did not adopt a general application layer protocol but instead customized one. The physical layer protocol is responsible for the transmission, decoding, bit timing, and bit synchronization of physical signals; the data link layer protocol is responsible for bus arbitration, information framing, data acknowledgment, error detection, and flow control; the application layer protocol is primarily responsible for identifier allocation, and secondarily for network startup or monitoring node processing. Since the CAN protocol does not specify the allocation of information identifiers, different methods can be used depending on the application. Therefore, determining the allocation of CAN identifiers is crucial when designing a CAN-based communication system and is a key aspect of the application layer protocol.
Figure 3 CAN bus control network of flatbed screen printing machine
5. Conclusion
From the above analysis, we can draw the following simple conclusion: With its unique characteristics, the CAN bus, combined with PLCs and IPCs, has already occupied an important position in the control of printing and dyeing equipment (including papermaking equipment). Considering that CAN is relatively easy to develop and its application layer protocol leaves room for secondary development, CAN offers an opportunity for China, which does not yet have its own fieldbus standard.
References
[1] Yang Xianhui, Fieldbus Technology and Its Applications, Tsinghua University Press, 1999.6
[2] Pro.Dr.-lng.K.Etschberger: Higher-level protocols and sub-protocols based on CAN
[3] Qi Rong, Programmable Logic Controller Tutorial, Northwestern Polytechnical University Press, 2000.9
[4] Jiang Simin, TMS320LF240* DSP Hardware Development Tutorial, Machinery Industry Press, 2003.6
