Читать книгу Engineering Autonomous Vehicles and Robots - Shaoshan Liu - Страница 34

CANopen is a communication protocol and device profile specification for embedded systems used in automation. In terms of the OSI model (shown in Figure 2.1), CANopen implements the layers above and including the network layer. The CANopen standard consists of an addressing scheme, several small communication protocols, and an application layer defined by a device profile [5].

The CANopen communication protocol has support for network management (NMT), device monitoring, and communication between nodes, including a simple transport layer for message segmentation and desegmentation. The lower level protocol implementing the data link and physical layers is usually CAN, although devices using some other means of communication (such as Ethernet Powerlink, EtherCAT) can also implement the CANopen device profile.

The basic CANopen device and communication profiles are given in the CiA 301 specification released by CAN in Automation. Profiles for more specialized devices are built on top of this basic profile, and are specified in numerous other standards released by CAN in Automation, such as CiA 401 for I/O modules and CiA 402 for motion control.

Every CANopen device has to implement certain standard features in its controlling software. A communication unit implements the protocols for messaging with the other nodes in the network. Starting and resetting the device is controlled via a state machine. It must contain the states Initialization, Pre-operational, Operational, and Stopped.

The transitions between states are made by issuing a NMT communication object to the device. The object dictionary is an array of variables with a 16-bit index. Additionally, each variable can have an 8-bit subindex. The variables can be used to configure the device and reflect its environment, i.e. contain measurement data.

The application part of the device actually performs the desired function of the device, after the state machine is set to the operational state. The application is configured by variables in the object dictionary and the data are sent and received through the communication layer.