Introduction to the
Introduction to the SERCOS interface®
By Ronald Larsen, Managing Director, SERCOS N.A.
is an acronym for
ystem, a digital motion
control bus that interconnects motion controls, drives, I/O and sensors.
It is an o
intelligent digital device interface, designed for high
communication of standardized closed
loop data in real
time over a noise
optic cable. (A third generation of the SERCOS interface, now in development, will
industrial Ethernet as a transport mechanism
both copper and fiber optics.)
The SERCOS interface® was created in the 1980s
by a group of European machine tool
builders and control/drive vendors
to specify a digital open interface that would ease the
transition from analog to digital drive technology. It was originally intended to be a drive
interface for advanced machine tool applications, but
today is in wide use in all servo
controlled automation disciplines. The SERCOS interface standard covers con
that are common to all servo applications, as well as topics that are industry specific. It is
estimated that the majority of SERCOS interface axes in the field today are on
applications such as web
fed printing presses; packaging, converting &
machines; assembly, handling and robots; semiconductor processing equipment;
metalforming and welding machines; textile processing; fiber winding; simulators and
test equipment; plastics machines and many other types of special machines.
The SERCOS interface is a real
time communication system that 1) defines a
standardized physical layer and 2) offers a rich set of more than 500 standardized
parameters (called Idents) that describe the interplay of drives and controls in terms
of any manufacturer. It offers advanced motion control capabilities and it
includes features for I/O control that often allow a machine builder to dispense with the
need for a separate I/O bus.
The SERCOS interface is the only internationally standardize
d open digital interface for
motion control and I/O with the performance required to synchronize high
axis motion control systems.
The SERCOS interface was designed to include
mechanisms to ensure the high level of determinism required wh
multiple axes of digital drives, while some other buses attempt to ensure determinism via
brute speed. The SERCOS interface is defined with a minimum of overhead, in order to
ensure maximum throughput. A lab test has determined that a 16 M
interface system provides equivalent throughput to a 100 Mbit/s Ethernet system, because
of the difference in overheads.
Much of the information in this document was adapted with permission from data published on
the SERCOS N.A. website, www.sercos.com.
Introduction to the
Like most digital busses, the SERCOS interface greatly reduces connectivity problems in
control systems. The SERCOS inter
face can connect up to 254 drives to a control using
one fiber optic cable ring, compared to a traditional analog servo system with 8 axes of
motion that may require over 100 wires between the drive and control. This reduces
system cost, eliminates many ty
pes of noise problems and helps machine designers get
motion control systems up and running quickly.
SERCOS interface is an International Standard
The SERCOS interface is a set of standard specifications that may be incorporated into
any company’s produc
ts, with each control and drive maintaining its own functions and
features. Because it is an international standard (IEC/EN 61491), it allows any
manufacturer’s SERCOS interface
compatible digital control to talk to any other
gital servo drive, digital spindle drive, hydraulic system,
digital I/O or sensors over a well
defined fiber optic link (or standard Ethernet
technology, coming in SERCOS
III). Controls and drives conforming to the standard
comply with a standard medium fo
r transmission, topology, connection techniques, signal
levels, message (telegram) structures, timing and data formats.
Most of the other digital drive interfaces on the market are proprietary and can only be
used with the manufacturer's own drives or hav
e been “opened,” but have been developed
and are controlled by a single manufacturer, with a small number of other firms providing
products using that bus. The SERCOS interface is the only motion control interface that
is standardized, guaranteeing both a
standardized hardware platform and a standardized
protocol open to use by all manufacturers.
Differences and Similarities with other Communication Busses
Fieldbus protocols (DeviceNet, Profibus and others) were designed for I/O and are
intended for low
such as sensors, motor starters and inverters
mainly turned on and off, or which require an asynchronous analog command value. A
number of these busses can be used for control of loosely coupled drives
control is no
t their primary function.
On the other hand, the SERCOS interface was designed for high
speed control of tightly
coupled motion axes, where deterministic control
is of high importance. In a high
As defined in an article entitled “Motion’s Digital Future”, May 24, 1001, from the
website, Determinism is knowing the slowest possible response time with
absolute certainty. In a deterministic network, each node communicates in the same order,
whether it has new data or not. Determinism means that a response to an event always happens
within a certain time frame
for example, 50 msec. If the system always responds in that time,
it's considered deterministic to within 50 msec. True determinism in a network or bus ensures that
each node, such as multiple servo axes, operates in perfect ti
me with the others. This is done via
some means of synchronization, such as a clock tick or a token.
Introduction to the
packaging line or multi
axis machine tool, sub
second synchronization accuracy of
multiple axes in the system is required.
Although the SERCOS interface was not designed primarily to be a fieldbus, it has the
ability to decentralize I/O via multiple I/O nodes that connect to the field devices. The
requirements may be satisfied via the SERCOS interface, negating the need for a
separate fieldbus. However, in high
power applications, such as flexible machine
systems, both a fieldbus and SERCOS interface often exist in the same control system.
ision of whether to use a fieldbus, SERCOS interface, or both, depends on the
application. If only loose coupling between axes is required, a series of single
position drives connected via a fieldbus may be adequate. However, in a synchronized
application, the SERCOS interface is required; and I/O can be handled over the
SERCOS interface, depending on the requirements and complexity of the system.
number of manufacturers offer SERCOS interface I/O systems.
This figure illustrates a system wit
h Ethernet for communication to a higher
network, a fieldbus for I/O and the SERCOS interface for real
time motion control.
As previously stated, there are a number of competitive motion control busses on the
market, several offering high
speed fiber optics. However, none of them is standardized,
Introduction to the
none offers a rich set of more than 500 motion and I/O commands and none enjoys the
wide vendor base of the SERCOS interface.
Three Generations of the SERCOS interface
The first two generations u
tilized an ASIC (Application Specific Integrated Circuit) as a
hardware processing platform, fiber optic transmitters/receivers, and a fiber optic cable as
the transmission medium.
The first generation operated at 2 and 4 Mbit/sec, using the SERCON410B ASI
and fiber optic cabling. The SERCON410B is no longer produced.
The second (current) generation operates at 2/4/8/16 Mbit/sec, using the
SERCON816 ASIC and fiber optic cabling.
In 2004, work was begun on the third generation of the SERCOS interface. It
the existing high
performance SERCOS interface with industrial Ethernet to
form a new generation of SERCOS, named SERCOS
The new version
the determinism of the original SERCOS interface with the high bandwidth of
thernet for the best of both worlds.
III maintains backward compatibility with previous versions in regard to
profiles, synchronization and message structures. It retains the set of more than 500
standard parameters that describe all aspects of rea
time motion and I/O control.
III operates at up to 100 Mbit/sec utilizing a Field Programmable Gate Array
(FPGA) or a General Purpose Communication Controller (GPCC) and is based on
standard Industrial Ethernet hardware. The first SERCOS
ucts are expected to be
available in late 2005.
More than 50 control manufacturers and 30 drive manufacturers worldwide offer first and
second generation SERCOS interface products, with hundreds of thousands of systems
installed. Several manufacturers off
er SERCOS interface I/O stations plus hardware and
software development tools.
Conformance Testing for SERCOS interface Products
Products with the SERCOS interface are tested at the Institute for Control Engineering of
Machine Tools and Manufacturing Unit
s (ISW) at the University of Stuttgart. This
laboratory is authorized by Interests Group SERCOS interface e.V. (IGS) to carry out the
testing per international standard IEC 61491.
The aim of the conformance test is to ensure the compatibility and interope
SERCOS interface devices from different vendors in multiple vendor environments.
Testing to the SERCOS interface Conformance Class A and B levels, plus testing for
conformance to the SERCOS Packaging Profile, are presently available. Tests to h
conformance classes are in development.
Introduction to the
How the SERCOS interface Works
The first generation SERCOS interface operated at 2/4 Mbit/sec using the SERCON410B
ASIC, now obsolete. The second generation SERCOS interface operates at 2/4/8/16
ng the SERCON816 ASIC. SERCOS
III operates at up to 100 Mbit/sec,
using a FPGA (Field
Programmable Gate Array) or a General Purpose Communications
Controller (GPCC) and standard Industrial Ethernet hardware.
ASIC, FPGA or GPCC
based SERCOS interface cont
rollers are normally integrated into
master motion controls as well as drives, amplifiers and I/O modules. They simplify the
task of the designer by automatically handling most SERCOS interface communication
A set of over 500 standard software
(called IDNs or Idents) defines standard
motion and I/O functions. In addition, the interface allows for manufacturer
IDNs, which can be used to define unique functions, not addressed by the standard IDN
In a SERCOS interface syste
m, all servo loops are normally closed in the drive. This
reduces the computational load on the motion controller, allowing it to synchronize more
motion axes than it otherwise could. In addition, closing all the servo loops in the drive
reduces the effec
t of the transport delay between the motion control and drive.
Functions of the SERCOS interface
The SERCOS interface exchanges data between control and drives, transmitting
command and actual values with extremely short cycle times.
It guarantees an exact
synchronization for precise coordinated moves with as
many axes as required.
The interface includes a service channel for non
cyclic data transmission, used for
the display and input of all control internal parameters, data and diagnostics.
ers can be downloaded and uploaded for storage via the service
The SERCOS interface supports four operating modes: torque, velocity, position
control and block mode.
The interface enables the use of controls and drives from different manufacture
in a system, by standardizing all data, parameters, commands and feedbacks
exchanged between drives and controls.
Ideal for Distributed Multi
axis Control Systems
The SERCOS interface is a foundation for building distributed multi
axis control systems
for a myriad of applications. Distributed control improves machine flexibility by moving
processing power and decision making from the CNC or motion control down into the
drives and sensors. These devices then become intelligent building blocks that can ea
be added to a machine or production line without major changes in hardware and
Introduction to the
The SERCOS interface is well suited for distributed control because it places axis
dependent control functions, such as loop closures, interpolation and registra
tion, in the
drives, not in the motion controller. Thus, motion controllers can concentrate on motion
control profiles and toolpaths independent of the axes. In operation, the control issues a
position command to the drive, which then closes its own loops
trajectory, based on previously downloaded parameters. The SERCOS interface also
integrates input/output functions such as limit switches, pushbuttons and various sensors.
The SERCOS interface provides machine manufac
turers with the flexibility to configure
vendor control systems with plug and play interoperability. Even more importantly,
the SERCOS interface has facilitated great advances in machine productivity. A
prominent U.S. food manufacturer states “Our SE
RCOS applications play an important
role in our plan for a dynamic enterprise that can flexibly adapt manufacturing for every
new product size, shape and packaging configuration. Along with this flexibility, we have
realized incredible timesaving advantage
faster delivery, faster installation, faster
setup, faster product changeover, and faster production speeds.”
The SERCOS interface allows manufacturers to create intelligent digital drives with
vastly improved capabilities and flexibility. A single d
rive can be designed to handle
multiple motors, such as permanent magnet servomotors, high horsepower induction
servomotors (vector drives) and linear motors, with the configuration set
parametrically. The SERCOS interface is also used with stepper moto
rs, hydraulic drives
and I/O systems.
Keeping the interface Up to Date
Updates to the SERCOS interface standard are defined by multi
Working Groups (TWGs) in both North America and Europe, which ensure that the latest
ed by SERCOS member companies are incorporated into the
specification. Updates that have been accepted by the TWGs since official adoption of
the IEC standard are published by the SERCOS interface member organizations and
made available to anyone for a mod
Hershey finds SERCOS best path to enterprise
, Keith Campbell, Instrumentation and
Control Systems, January 1999.