Motion Control Resources
What’s The Power Behind ETHERNET Powerlink
by John Mazurkiewicz, Baldor Electric
Motion Control & Motor Association Posted 04/04/2007
Automation designers, builders, and integrators are merging servo/motion technology with ETHERNET Powerlink, attaining higher levels of performance along with numerous additional benefits.
Servo/motion control has worldwide use in demanding applications, and modern industrial automation equipment using servo/motion packages have been proven to be powerful, flexible, and easy to use, while providing a host of capabilities. Additionally high-speed communication networks in factories are becoming commonplace, so it now becomes easy to integrate servo/motion into the automation process. This is accomplished by using ETHERNET Powerlink.
The success of ETHERNET Powerlink can be attributed to its openness – it does not require custom chipsets or ASICs – standard chips can be purchased from a wide choice of suppliers.
ETHERNET Powerlink protocol makes use of well-proven standard Ethernet technology, enhanced to provide real time control, deterministic behavior, and to provide network security as standard. Key ETHERNET Powerlink attributes are listed in figure 1.
Figure 1 - ETHERNET Powerlink Attributes
Real Time Control
For automation and motion applications, real-time control is required in order to handle the dynamic speed of machines, the sophisticated control loops, as well as the safety protocols. Real time control, or how fast the system reacts to events (such as operator HMI inputs or push buttons, sensor activation and fault or alarm conditions) must be consistent and rapid. Dynamic response and increased accuracy are requirements in high performance 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 servo drives. One factor affecting performance is the precision to which the control loops in each drive are synchoroized.
The synchronized loops must be timed consistently, since control systems work on a cyclic update principal, and variations in this timing has detrimental effects, such as introducing speed variation and ripple which effects the machines operation and final product quality. Variation of this timing is expressed as “jitter”, and ETHERNET Powerlink achieves less than 1 μs 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.
Additionally 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 a multiplexing). This allows for optimized timing, providing for shorter, faster communication cycle time and higher bandwidth. ETHERNET Powerlink provides cyclic update rates down to 200 μs for fast system response. This makes for excellent reaction time in modern automation applications.
The other important attribute is determinism – which in communication terms means that a message must reach its destination in a specific predicable time. Or in other words, time critical data transfer must be guaranteed within very short and precise configurable cycles (less time-critical data can be transmitted in reserved asynchronous time slots). Messages reaching destinations at guaranteed time is 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. ETHERNET Powerlink accomplishes determinism and avoids data collisions by coordinating the exchange of information between devices. It manages the access to the network within allocated time slots, and therefore by design prevents collisions and ensures data is exchanged precisely and on schedule.
On the application layer, ETHERNET Powerlink has also been combined with widely used, and well-known, CANopen device profiles. This approach was undertaken because CANopen functionality was well defined and supported by many vendors. Ethernet on the factory floor potentially integrates motion control and field device control on a common network, and in order to accomplish this, a broad range of field devices from multiple suppliers must be made available. Thus 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). Availability of CANopen devices provides wide variety of other flexible and proven solutions from many manufacturers.
A major advantage of ETHERNET Powerlink is that it provides visibility for all devices on the network. It is possible to transfer data to standard applications such as data bases and process control system. This allows any device to be monitored, configured, diagnosed, or upgraded from any accessible network point.
It is possible to route devices thru the IS infrastructure. However access from any network, if exploited by hackers, could be a weakness. To overcome this, ETHERNET Powerlink 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. ETHERNET Powerlink has clear barriers and only users with dedicated rights get access to the real-time domains. Thus the real-time characteristics will remain stable independent from the network load.
For security as well as standards purposes, to ensure ETHERNET Powerlink fits the needs of automation network topologies, it was designed to comply with numerous international and safety standards, such as IEEE, IEC, and other standards. These are outline in figure 2.
Figure 2 - ETHERNET Powerlink Supported Standards
|IEEE 802.3||Fast Ethernet|
|IEEE 1588||Distributed real-time domain synchronization|
|ISO 15745-4||XML based device description language|
|IEC 61784-2||Digital data communications|
|CANOpen EN50325-4||Standard device profiles|
|IEC 61508||Safety and safety integrity standard|
Today numerous plants are updating their control systems and employing ETHERNET Powerlink networks. Communication is between an operator HMI terminal and a controller and various devices that control equipment in the plant. The Ethernet touch screen becomes the primary interface for operators to monitor the system. The controllers are programmed and coordinated to work together to ensure the process is supplying the proper demand and all equipment is working together. Reactions occur in fractions of a second, and the ETHERNET Powerlink system ensures that all of the communication takes place quickly. Besides increasing plant efficiency, ETHERNET Powerlink enhances systems with benefits indicated in figure 3.
Figure 3 - ETHERNET Powerlink Benefits
In implementing ETHERNET Powerlink along with motion control into machinery, designers are learning that they are accomplishing major hardware savings from reduced wiring and the ability to use a single motion controller. Immeasurable cost savings are also realized because build time is faster, and commissioning is simpler. It has been estimated that the hardware savings possible by reduced wiring of ETHERNET Powerlink to be as much as $130 per axis. Factory/machine costs are reduced.
On the factory floor, machine response has improved, and throughput has increased. By splitting up the coordinating task of a centralize controller into small, fast decentralized tasks in the drive, the controller performance improves due to reduced network and CPU load. Easy to use, high level programming languages available in localized drives reduced what could be intimidating software project to a predictable task. Additionally servomotors provide increased acceleration, increased speed and are replacing pneumatics and hydraulics. The number of axes inside a machine is now determined by the maximum allowed number of nodes and the network bandwidth (see figure 4 for comparison).
Figure – Field-bus wiring
|DeviceNet||line, trunkline/dropline||Twisted-pair Based on CAN||64||
|Modbus||line, star, tree||Twisted-pair, RS232 & RS485||247||100 meters between switches|
|Profibus||line, star, ring||Twisted-pair or fiber optic||127||100 meters @ 12Mbps (12 km with fiber)|
|ETHERNET Powerlink||line, star, tree||Shielded CAT5e cable & RJ45 connector||240||100 meters per connection @100 Mbps|
ETHERNET Powerlink has numerous attributes including being license free; provides real-time control for dynamic machine speed; is deterministic via its’ applications protocol; and is secure operating as a protected segment. Powerlink enhances performance, as well as providing numerous system improvements.