SERCOS interface
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Introduction
In the field of Industrial Control SystemsIndustrial Control Systems
Industrial control system is a general term that encompasses several types of control systems used in industrial production, including supervisory control and data acquisition systems, distributed control systems , and other smaller control system configurations such as skid-mounted programmable...
, the interfacing of various control components must provide a means to coordinate the signals and commands sent between control modules. While tight coordination is desirable for discrete inputs and outputs, it is especially important in motion control
Motion control
Motion control is a sub-field of automation, in which the position 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...
s, where directing the movement of individual axis of motion must be precisely coordinated so that the motion of the entire system follows a desired path. Types of equipment requiring such coordination are for example metal cutting machine tools, metal forming equipment, assembly machinery, packaging machinery, robotics, printing machinery and material handling equipment. The sercos (serial real-time communication system) interface is a globally standardized open 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 is designed to provide hard real-time, high performance communications between industrial motion control
Motion control
Motion control is a sub-field of automation, in which the position 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...
s and digital servo drive
Servo drive
A servo drive is a special electronic amplifier used to power electric servomechanisms.A servo drive monitors the feedback signal from the servomechanism and continually adjusts for deviation from expected behavior.-Function:...
s.
History
Until the early 1980’s the majority of servo 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 real-time communication system) interface, was released and later submitted to the IECInternational Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
, which in 1995 released it as IEC 61491. After the release of the original standard, original working group member companies including ABB, AEG
AEG
Allgemeine Elektricitäts-Gesellschaft was a German producer of electrical equipment founded in 1883 by Emil Rathenau....
, AMK, Robert Bosch
Robert Bosch
Robert Bosch was a German industrialist, engineer and inventor, founder of Robert Bosch GmbH.-Biography:...
, Indramat
Indramat
Indramat GmbH was an industrial control firm founded in 1958, based in Neuwied , Germany. Its name is a German abbreviation meaning “Gesellschaft zur INDustrialisierung-RAtionalisierung und AutoMATisierung“ .The core business was the production of industrial servo drives and motion controls...
, and Siemens
Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...
founded the "Interest Group sercos" to steward the standard. 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.sercos II was introduced in 1999. It expanded the data rates supported to 2, 4, 8 and 16 Mbit/s.
sercos III
SERCOS III
sercos III is the third generation of the sercos interface, a globally standardized open digital interface for the communication between industrial controls, motion devices, and input/output devices . sercos III merges the hard real-time aspects of the sercos interface with Ethernet. It is based...
merges the hard-real-time aspects of the sercos interface with the Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
standard.
Features of sercos interface
Important features of the sercos interface include:- Collision-free communication through the use of a time-slot mechanism.
- Highly efficient communication protocol (little overhead).
- Extremely low telegram jitterJitterJitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
(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).