CANopen fieldbus network protocol in the application
Fieldbus-based automation network technology research is a hot area of development. In a variety of industrial field bus, CAN bus with its low-cost, fast, real-time and reliability characteristics of higher has been rapidly applied to automotive electronics, medical, military and other fields. However, CAN bus is only the definition of the physical layer and data link layer, is not in itself complete, for this issue, CiA (CAN in Automation) through continuous development of the Association in 1995 published a complete CANopen protocol, which not only the definition of the application layer and communication sub-agreements, as well as programmable systems, different device, interface, application protocol defines the sub-frame, and fieldbus-based all-digital distributed control system and the wider use of integrated management systems to provide the solution.
This article Fieldbus network to improve the operation of communication system efficiency and real-time information processing capacity of the premise, and for machine control system Autoleveller the functional modules in the role of high-level CAN bus communication protocols CANopen, the realization of the function of data communication between modules. Through experimental tests to demonstrate that the system is stable, reliable and guaranteed all-digital distributed network system speed, accuracy and real-time.
2 CANopen protocol outlined
2.1 Device Model
CANopen by CiA association agreement is an agreement for the CAN is not defined by integrity and a higher level of agreement - Application layer protocol. A CANopen device module can be divided into 3 parts, as shown in Figure 1. Communication interface and protocol software to provide bus service to send and receive communications object. Different CANopen communication equipment are completed through the exchange of communication objects. This part of direct-to-CAN controller to operate. Object dictionary description of the equipment used by all the data types, communication objects and application objects. Dictionary object is located in the application procedures and communication between the interface to applications, applications to operate on the object dictionary CANopen communication can be achieved. Including part of the application and communication functions of communication in part through the operation of the object dictionary CANopen communications to achieve, and the functional part of the realization in accordance with application requirements.
2.2 Object Dictionary
Dictionary object (Object Dictionary, OD) is an orderly group of objects, each object using a 16-bit addressing of the index value, in order to allow access to the data structure of a single element, while the definition of a 8-bit sub-index, the structure of the object dictionary reference Table 1, the object dictionary description with reference to Table 2.
CANopen network in each node has a target of the only dictionary, the object dictionary of each node have the same structure, but according to the content of the different devices, including a description of the equipment and its network behavior all parameters.
2.3 Communication object
CAN network through the CANopen communication objects can be used to describe services and protocols. Categorized as follows:
(1) real-time data through the process of data objects (PDO) transmission protocol.
With 8 bytes of data fields to PDO mapping process data objects into a single CAN frame in order to transfer the application object. Each PDO has a unique identifier and a node can only send, but its more than one recipient. PDO in the object dictionary of each used to describe two objects: PDO communication parameters, the provisions of the PDO communication behavior, including the COB ID which will be the use of PDO, transmission type, the prohibition of the time and timer cycle; PDO mapping parameters, which provides object is mapped to a PDO, the producers and consumers need to know the mapping, to explain the contents of PDO.
(2) Service Data Objects (SDO) protocol used to read and write operations to the object dictionary.
SDO data object through the service can access a device's object dictionary. In order to enable communication to be confirmed, an SDO with the needs of two different CAN identifier of the data frame through the SDO, asked the two devices can establish peer-to-peer communication, the access object dictionary SDO communications device is a server, a device can support multiple SDO object, but at least one SDO in a supported device.
(3) agreement of special features for specific target applications, network synchronization, time stamp and emergency message transmission.
(4) Network Management (NMT) agreement to provide network initialization, error control state control services and equipment.
In the CANopen network, NMT to fulfill the need for a node of the Master (master node) function. Other nodes of the NMT as a Slave (slave nodes). NMT provides the following functions: control module (to initialize the network communication from the point); error control services (management of network nodes and the state); configuration control services (upload and download the network configuration, according to).
(5) layer set of services (LSS) protocol used to configure the CAN bus communication parameters and device parameters.
The definition of the use of CANopen protocol message LSS to achieve the configuration of CAN-bus network, including node ID, baud rate of communications, apparatus product number, serial number and so on. LSS message in the CANopen communication protocol is optional and can be directly specified node node initialization ID, communication parameters such as rate, its use can improve the system flexibility.
3 of the CANopen network protocol
3.1 Autoleveller system communication network structure
The machine and system Autoleveller keyboard display controller, dsp controller, A-hole servo motor controller, B-hole servo motor controller, PLC logic controllers and PC-monitors, each node in the CAN bus based on the use of CANopen protocol communications, the completion of pre-defined system functions. Figure 2 shows the A-hole ring-opening ceremony and Autoleveller machine schematic.
DSP controller part of the core of the whole system, is responsible for the entire communication network of information collection, data computation, control work. Inter alia, the following DSP controller CANopen protocol.
Integrated CAN module TMS 320F 2810 DSP to achieve, it has up to 32 mailboxes, occupied 512 B RAM, can be configured to send or receive mail, and the receiver has a programmable mask register. This will enable a more convenient and flexible data transfer, greatly increasing the amount of information. Figure 3 controller for the DSP part of CAN bus driver circuit, respectively TMS320F 2810 selection of the 64-pin and 65 pin CANTXA as a CAN bus CANRXA to send and receive pins. The use of high-speed digital isolators ADuM1100 an alternative to traditional optocoupler. MCP2551 used as a high-speed CAN transceiver, with a differential ability to transmit and receive, many nodes can be connected to the same network.
CANopen protocol software in accordance with the framework, respectively, set up a NMT message, SYNC message, SDO message, PDO and Heartbeat Message message, and from the node controller for the DSP and its relationship with the nature of the relationship between each node, set up the message of the COB - ID, see Table 3.
Keyboard in the system-based display controller node, through the liquid crystal display monitor of the information from the node, main node 4 by detecting from the node to send periodic heartbeat message (Heartbeat) to judge the work of each state from the node. Figure 4 for the DSP controller agreements from the CANopen node communication processes: power in the system, the first node to send to the main Boot-up message to inform the master node to enter the Pre-operational status, waiting for the main node of the SDO Packet Access Object Dictionary Upon the completion of the configuration of the object dictionary, the node is waiting to receive the main message of NMT into the Operational state, PDO at the beginning of the send and receive text to complete the exchange of information and data to achieve distributed network control.
4 test results
In order to ensure the system for each module will be able to work, the need to design a machine with the PC real-time communication a common board, to meet the various parts of the system environment in the laboratory testing in order to ensure the normal operation of the system. LabVIEW software design using PC-monitor panels, built-in signal acquisition, measurement analysis and data display, eliminating the traditional complexities of development tools, in addition to providing power at the same time also ensures that system flexibility.
Monitoring panel to set the node number (Node ID), master / slave node, set up to send the type of message (such as TPDOn, SDO), NMT, Heartbeat, LSS) and content (data length, content), select the communication port can also be observed that the received CAN message type, data length, node number and message content. Figure 5 shows the data state when the initialization settings, you can see in the data window to receive LSS node setting its parameters to set up and store the return message, and nodes in accordance with the provisions of the agreement CANopen Send Boot-up message , and after entering the pre-operational status of the Heartbeat message.
This article applies to CANopen protocol Autoleveller system, fieldbus control system for distributed network of digital, modular, and enhance the device scalability, improved reliability of data transmissions. Fieldbus-based application-layer standard protocol development and use, to improve the efficiency of the system, especially for the development of complex systems has a certain significance.
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