John Mazurkiewicz
Baldor Electric Co.
Fort Smith, Ark.

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. Realtime 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 realtime 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 timecritical 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.