Because the CANopen communications protocol can be used in a variety of industrial applications, Maxon Precision Motors chose to offer it on its latest controller, the EPOS digital positioning system. As a built-in feature, the protocol connects multiple axis drives and assists the EPOS controller with expedited transfers, fast data transmission, and process data object mapping functions.
The EPOS 70/10 shown here is a digital
motion controller capable of position, velocity, and current mode
operation. It also accepts external signals to operate in “step and
direction” mode or “master encoder” mode.
The EPOS connects to dc and electronically commutated motors with incremental encoders in modular packages. Compact, these controllers offer power ranges to 700W. Engineers can assemble a range of flexible drive and automation systems for positioning, speed, and current regulation.
A PLC often functions as a master unit on a CANopen network, which integrates with the position controller and transmits commands and data between the devices. Within the EPOS is a Service Data Object (SDO) transport protocol that reads from or writes to entries in the position controller’s Object Dictionary. Normal transfer is used for reading or writing objects of more than four bytes. In these situations, the SDO protocol is segmented, meaning the transfer is split into different SDO segments, or CAN frames. Expedited transfer is used for objects of four bytes or less. Most objects of the Object Dictionary can be read and written using this non-segmented version of SDO.
Fast Data Transmission
Process Data Objects (PDOs) are used to quickly transmit high priority data. They are unconfirmed services without protocol overhead. Consequently, they are a fast and flexible method of sending data from one node to any number of other nodes. PDOs can contain a maximum of eight data bytes compiled to suit user requirements. Each PDO has a unique identifier and is transmitted by only one node. But it can be received by more than one node.
PDO transmissions are driven by either remote requests or the Sync message received function. When another device sends a transmission request, it triggers an asynchronous PDO transmission. The Sync message initiates synchronous transmission triggering, useful for initiating simultaneous sampling of input values of all nodes.
Synchronous transmission takes place in cyclic and non-cyclic modes. In cyclic transmissions, a node waits for a Sync message, after which it sends its measured values. A PDO transmission number, which ranges from 1 to 240, determines the number of Sync messages the node will wait before transmitting its next values. Non-cyclic PDOs are triggered by a defined event. The node transmits its values with the next Sync message but will not transmit again until another application-specific event occurs.
The PDO mapping function defines which application objects are transmitted within a PDO, along with their sequence and length. A device that supports variable mapping must do so during the pre-operational state. All changes in the object dictionary used for programming are available through the EPOS Graphical User Interface. Examples are resident in the program to clearly explain each operation.
The default mapping of application objects as well as the supported transmission mode is described in the Object Dictionary for each PDO. PDO identifiers should have high priority to guarantee short response time, as transmission is not confirmed.
In an automated welding application, a PLC can serve as the central control, transmitting through CANopen control feed rate data with input from the EPOS system.
In packaging operations that require a precise ratio between two axes, a PLC can maintain overall system operation, leaving the EPOS position controller to affect the axes directly. The CANopen protocol would facilitate communications between the PLC and the EPOS.
Multiple EPOS digital positioning controllers can be used, alleviating specific or critical operations from a central PLC control.
For more information: Maxon Precision Motor, Inc. • www.maxonmotorusa.com • 650.697.9614
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