Motion Control with CANopen

The gymnast gets a running start, jumps, and somersaults with a half turn over the vaulting horse. Her motions are followed all the way by a rail-mounted camera that now captures her triumphant smile.

The drive that kept step with her every move is equipped with a CANopen (for Controller Area Network) interface through which the camera movements are controlled. The television audience can enjoy the sports event as if they were right beside the gymnast inside the gymnasium.

This kind of camera system was first used for the 16th Commonwealth Games in Kuala Lumpur, Malaysia, in 1998. However, the system stood the practical test in Sydney in the year 2000 when it was used for the Olympic Games. At the Games, mobile cameras networked with CANopen filmed high jumping and soccer matches, amongst other events.

In addition to its use at sporting events, the CANopen specification has been put to the test in manufacturing controls, building controls and on-board automotive electronics. The CANopen device profile for electric drives that was used for networking the cameras was already published in May 1997 by the CAN in Automation (CiA) international users’ and manufacturers’ group, based in Erlangen, Germany. Version 2.0 was published in July 2002 and is currently being revised.

Device profiles define process data, configuration parameters and diagnosis information. Part of the process data are the Control Word and the Status Word. The Control Word is used for controlling the drive-specific Status Machine.

The drive transmits the current status back to the application. That way, the application controller stays informed about any change of status at any time. Although the Status Machine definition (actually a “state machine”) may appear daunting, defined states are indispensable to ensure failure-free operation for synchronous motors.

The CANopen profile for drives describes servo controllers, as well as stepper motor controllers and frequency inverters. In order to accommodate the different control modes, the device profile defines a number of modes: profile positioning mode, interpolating positioning mode, profile speed mode, profile torque mode, speed mode and homing mode. The devices may switch control mode during operation in run-time.

The CANopen protocol architecture is layered, so that it is possible to implement the device profile for drives on other bus systems and networks. The only thing needed for this is to implement the CANopen Object Dictionary. The communication-specific objects differ depending on the bus system or network to be used. In CANopen, process data is transmitted without confirmation within a single CAN-frame (Process Data Object, or PDO, service). Configuration parameters are confirmed and can optionally be sent segmented in CAN message pairs (Service Data Object, or SDO, service). Emergency messages are unconfirmed single CAN messages. More detailed diagnosis data are transferred via SDO services.

When using other bus systems, such as IEEE 1394 Firewire, or other networks, such as EtherCAT or Ethernet-Powerlink, the corresponding consortium must define the mapping of the communication services. Process data and configuration parameters are bus system- and network-independent. The CANopen device profile for drives specifies a multitude of configuration parameters.

A new version of the CANopen device profile will be published this fall. This version will then be available not just for CANopen networks, but also for Ethernet-based and Firewire networks. The device profile for drives is already one of the winners. The transparency between different networks and bus systems will be enhanced, and for CANopen networks, the default-PDOs will be even better adjusted to the different drive systems. Gymnasts and athletes, together with their coaches, will study recorded movements in slow motion without knowing that the recording cameras were networked with CANopen. Neither will they know how the camera was configured. The system integrators at Camera Tracking Co., a United Kingdom-based firm that specializes in the technology, appreciate above all else the simple and easy functional extensibility possible with the help of a standardized communication system.

Holger Zeltwanger, zeltwanger@can-cia.org, is managing director of CAN in Automation, in Erlangen, Germany.

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