Industrial communication powers up

Oct. 1, 2007
Modern industrial automation equipment employing servomotion has become powerful, flexible, and easy to use, while providing an abundance of capabilities.

Modern industrial automation equipment employing servomotion has become powerful, flexible, and easy to use, while providing an abundance of capabilities. Speedy communication networks are also becoming commonplace, making it easier to integrate servomotion into the automation process. This is increasingly accomplished using Ethernet Powerlink (EPL).

The success of this industrial communication system can be attributed to its openness. EPL does not require custom chipsets or ASICs; standard chips can be purchased from a wide choice of suppliers. EPL protocol makes use of well-proven standard Ethernet technology, enhanced to provide real-time control, deterministic behavior, and network security as standard.

Timing is everything

For automation and motion applications, real-time control is required to handle the dynamic speed of machines, sophisticated control loops, and safety protocols. Real-time control — which implies how fast a system reacts to events (such as operator HMI inputs, sensor activation, and alarm conditions) — must be consistent and rapid.

High dynamic response and increased accuracy are requirements in more and more applications such as machine tool, packaging, and printing. In these applications, the coordination of motion may be shared between a multi-axis motion controller and several servodrives. One factor affecting performance is the precision to which each drive's control loops are synchronized.

Synchronized loops must be timed consistently because control systems work on a cyclic update principal. Deviations in timing can have detrimental effects, such as introducing speed variation and ripple that affect the machine's operation and final product quality. Variation of this timing is expressed as “jitter,” and EPL achieves less than 1 µsec cyclic jitter, which makes it superior to many other approaches to real-time machine motion control. All drives synchronize their control loops to this cyclic time reference.

In addition, to optimize the cycle time for motion control, the real-time communication system allows important signals, such as command and feedback and machine I/O, to be transmitted each cycle, while other less critical devices can be updated over a number of cycles (referred to as multiplexing). This allows for optimized timing — a shorter, faster communication cycle time and higher bandwidth. EPL provides cyclic update rates down to 200 µsec for fast system response. This makes for excellent reaction time in modern automation applications.

How predictable

EPL's other important attribute is determinism — which means that a message must reach its destination in a specific, predictable time. Time-critical data transfer must be guaranteed within short and precise configurable cycles, while less time-critical data can be transmitted in reserved asynchronous time slots. Messages reaching destinations at guaranteed times are critical for motion control and some machine I/O devices.

Although widely used, “standard” Ethernet is not deterministic because of characteristics such as data collisions, message inquiries, and timing delays introduced by routers tied onto the system. In contrast, EPL accomplishes determinism and avoids data collisions by coordinating the exchange of information between devices. It manages access to the network within allocated time slots, and therefore prevents collisions and ensures data is exchanged precisely and on schedule.

On the application layer, EPL has also been combined with widely used, well known CANopen device profiles. This approach was undertaken because CANopen functionality is well defined and broadly supported. Ethernet on the factory floor potentially integrates motion control and field device control on a common network, but in order to accomplish this, a wide range of field devices from multiple suppliers must be available.

The Powerlink group worked closely with the CAN in Automation (CiA) group to adopt the CANopen application layer and device profiles, such as DS402 that describes the operational behavior for positioning drives.

Safe and secure

A major advantage of EPL is that it provides visibility for all devices on the network. It's even possible to transfer data to standard applications such as databases and process control systems. This allows any device to be monitored, configured, diagnosed, or upgraded from any accessible network point.

It's also possible to route devices through the IS infrastructure. However, access from any network, if exploited by hackers, could be a weakness. To overcome this, EPL operates as a protected segment. It will connect to a non-deterministic Ethernet network via a gateway/router device, which acts as a defensive barrier against attacks with features such as MAC (Media Access Controller) address filtering and a built-in firewall.

EPL has clear barriers and only users with dedicated rights get access to real-time domains, so that real-time characteristics remain stable, independent from the network load.

For security as well as standards purposes, and to ensure that EPL fits the needs of automation network topologies, it was designed to comply with numerous international and safety standards, such as IEEE, IEC, and others.

Application advantages

Today, numerous plants are updating their control systems and employing EPL networks. Communication is often between an operator HMI terminal, a controller, and various devices that control equipment in the plant. An Ethernet touch screen becomes the primary interface for operators to monitor the system. Controllers are programmed and coordinated to work together to ensure the process is supplying the proper demand and all equipment is getting along. Reactions occur in fractions of a second, and the EPL system ensures that all communication takes place quickly.

In implementing EPL along with motion control into machinery, designers often find significant hardware savings from reduced wiring and the ability to use a single motion controller. Cost savings are also realized because build time is faster and commissioning is simpler. Estimated hardware savings due to reduced wiring of EPL are as much as $130 per axis. Factory and machine costs are also reduced.

On the factory floor, machine response has improved, and throughput has increased. By splitting the centralized controller's coordinating tasks into small, decentralized tasks in the drive, performance improves due to reduced network and CPU load. Easy to use, high-level programming languages available in localized drives reduce what could be an intimidating software project to a predictable task.

EPL has also unshackled servomotors, in many cases, letting them replace pneumatics and hydraulics. Servomotors have always been fast enough, but have been limited by the number that could be connected to a single motion controller. With EPL, the number of axes inside a machine is now determined by the maximum allowed number of nodes and network bandwidth.

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Ethernet Powerlink attributes

  • Products available today
  • Open standard, no intellectual property rights, no patents
  • License free
  • No ASICs (special purpose microchips) required
  • Widely supported by numerous manufacturers
  • Built on international standards
  • No PC necessary; can operate with standalone motion controller
  • Centralized or decentralized intelligence
  • Deterministic communication; precise timing
  • Network security
  • Information exchange between any network nodes

Ethernet Powerlink benefits

  • Simplifies design
  • Digital signals — no analog issues
  • Shorter design cycles
  • Reduced hardware
  • Simpler commissioning
  • Communication between all devices
  • Reduced cabling complexity
  • No additional fieldbus required
  • Total network visibility
  • Enhanced performance

Fieldbus wiring

Bus Topology Physical media Max nodes Max distance DeviceNet Line, trunkline/dropline Twisted-pair based on CAN 64 500 m@125 kbps Modbus Line, star, tree Twisted-pair, RS232 & RS485 247 100 m between switches Profibus Line, star, ring Twisted-pair or fiber optic 127 100 m@12 Mbps (12 km with fiber) Ethernet Powerlink Line, star, tree Shielded CAT5e cable & RJ45 connector 240 100 m per connection@100 Mbps

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