SERCOS (SErial Realtime COmmunication System) interface is a globally standardized digital interface for the communication between industrial controls, motion devices (drives) and input output devices (I/O). It is classified as standard IEC 61491 and EN 61491. The SERCOS interface was originally designed to provide highly accurate, synchronized, communications between industrial motion controls and digital servo drives. Motion control is a sub-field of automation, in which the position and/or velocity of machines are controlled using some type of device such as a hydraulic pump, linear actuator, or an electric motor, generally a servo. ...

History

Until the early 1980’s the majority of drive systems used to control motion in industrial machinery were based upon analog electronics. The accepted interface to control such devices was an analog voltage signal, where polarity represented the desired direction of motion, and magnitude represented the desired speed or torque. In the 1980s, drive systems and devices based on digital technology began to emerge. A new method needed to be devised to communicate with, and control such units, as their capabilities could not be exploited with the traditional interface method used with analog drives. The earliest interfaces were either proprietary to one vendor or designed only for a single purpose, making it difficult for users of motion control systems to freely interchange motion control and drives. The membership of the VDW (German Machine Tool Builders’ Association) became concerned with the implications of this trend. In response to that, in 1987 the VDW formed a joint working group with the ZVEI (German Electrical and Electronics Industry Association) to develop an open interface specification that was appropriate for digital drive systems. The resulting specification, entitled “SERCOS (Serial Realtime COmmunication System) interface, was released, and later submitted to the IEC (International Electrotechnical Commission), which in 1995 released it as IEC 61491.^[1] Over the history of the SERCOS interface, its capabilities have been enhanced to the point where today it is not only used for motion control systems, but also for the control of I/O on machinery, as a single machine network.

Versions

SERCOS-I was released in 1991. The transmission medium used is optical fiber. The data rates supported are 2 and 4 MBit/s, and cyclic update rates as low as 62.5 microseconds. A ring topology is used. SERCOS-I also supports a "Service Channel" which allows asynchronous communication with slaves for less time-critical data. ^[2]

SERCOS-II was introduced in 1999. It expanded the data rates supported to 2, 4, 8 and 16 MBit/s, and instituted a “Service Channel” for the exchange of asynchronous information.^[3]

SERCOS-III merges the hard-real-time aspects of the SERCOS interface with the Ethernet standard. Work began on SERCOS-III in 2003, with vendors releasing first products supporting it in 2005. It increases the data rates to 100 MBit/s, supporting cyclic update rates as low as 31.25 microseconds. Both line and ring topologies are supported, with added functionality for bump-less ring break recovery, support for hot-plugging new bus nodes, and cross communication. SERCOS-III uses standard Ethernet Frames (EtherType 0x88cd), allowing co-existence with other protocols, such as TCP/IP on the same network.^[4] Ethernet is a large, diverse family of frame-based computer networking technologies that operates at many speeds for local area networks (LANs). ... EtherType is a field in the Ethernet networking standard. ...

SERCOS interface Features

Important features the SERCOS interface include:

• Collision-free communication through the use of a time-slot mechanism.
• Highly efficient communication protocol (little overhead).
• Extremely low telegram jitter (specified at less than 1 microsecond, in practice as low as 35 nanoseconds).
• Highly developed standardized profiles agreed upon by multi-vendor technical working groups for dependable interoperability of devices from different manufacturers.
• Ability to control, for example, 70 axes of motion at an update of 250 microseconds for each and every drive (SERCOS-III).^[5]

Support

The SERCOS interface is supported globally by SERCOS International e.V. (SI) in Germany. Regional support is provided bySERCOS North America(USA) and SERCOS Japan. These organizations provide a forum for the continued development of the standard, as well as user support.

Conformance Testing and Interoperability

An important aspect of an open communications system is rigorous testing of products for adherence to the standard and their ability to operate in networks comprised of products from multiple vendors. SERCOS International e.V. supports a Conformance Laboratory at the University of Stuttgart’s Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW). Products successfully passing conformance testing may display a mark indicating they are conformance tested, and are publicized in an index of certified devices.

External Links

• SERCOS North America
• SERCOS International e.V.

Notes

1. ^ [1] History of SERCOS
2. ^ [2] SERCOS I & II Overview
3. ^ [3] SERCOS I & II Overview
4. ^ [4] SERCOS III Overview
5. ^ [5] SERCOS-III Performance Examples