Standard technologies utilizing real-time Ethernet as a basis for
automation/motion networks are adding valuable layers of new software functionality. Leading network technologies including
Ethernet IP, PROFINET, SERCOS, EtherCAT and Ethernet-Powerlink are all
continuing to advance the state-of-the-art with a push to new standards.
The focus is less on high performance motion
control, where existing solutions are established, and more on detailed improvements
in network management and topologies, specific device profiles and networked safety.
A key unresolved issue that's
important to end users is interoperability, or at the very least co-existence
of multiple protocols from different vendors on the same physical Ethernet network. Especially with highly demanding
applications, there is debate within networking organizations on how these
systems should be implemented.
"The issue is between standard,
off-the-shelf Ethernet hardware and high speed protocols that aren't
necessarily compliant with all of the network infrastructure equipment of
devices carrying other Ethernet-type traffic," says Richard Harwell, manager of
connectivity for Eaton Corporation and chief technology officer for ODVA. "The difference is at the IP layer and whether advanced capabilities are
being built on top of the IP layer, or replacing the IP layer for efficiency of
transport."
Harwell says compliant
IP doesn't necessarily give users interoperability but does achieve effective
coexistence on the network. Work is
ongoing to develop rules about how messages go over the network and
prioritization about how they are handled on the network. Since switches and routers depend on IP to
manage network traffic, compliance at that layer allows users to leverage the
network infrastructure and run along the bandwidth curve as Ethernet technology
evolves and moves ahead.
"I would like to think
we will see in the near future more interoperability between networks," says
Scott Hibbard, vice president of technology for Bosch-Rexroth. "Users are frustrated that they need to make
a decision among the main suppliers and technologies, and it's an area that
needs more exploration and development. "
Hibbard says there is an expectation that
there will be one bus in the future but it may take longer than we think. The next step is networks that allow more
than one protocol to run on them, and users can select what networking
technology is most appropriate for their application.
"We need to realize the key to success doesn't necessarily require
interconnection between
different types of products," Hibbard says.
"If we look at the PC community, we can learn from its evolution and
realize the key to innovation is within individual products. We can do
a better job of documenting and
defining the application layer, and achieve higher predictability that
two
devices can work together. All of the
fieldbus groups have work to do on this issue."
While the
interoperability issue is a substantial longer term goal, technology standards
in a wide variety of areas are increasing the robustness and performance of
automation/motion networks. What follows
is an overview of recent other new technology developments and features that
are broadening the scope and functionality of automation networks.
SERCOS Master Software
Driver Library Submitted as Open Source
SERCOS International (SI) has announced it
will provide an open-source software driver library for the SERCOS lll master
implementation. Driver software will be available as source code without any
license fees and usage limitations. SI
is cooperating with the Open Source Automation Development Lab to disclose the SERCOS III
master driver.
"With
the availability of the SERCOS III master library it will become much easier
for manufacturers to develop a SERCOS III master and to benefit from future
improvements and extensions of the software," says Peter Lutz, managing
director of SERCOS International. With machine builders recognizing the demand
for open platforms based on real-time Ethernet, Lutz says Linux is establishing
itself as a standard for the industrial market. He says it is advantageous for vendors and
users that the SERCOS III master library will become part of mainline Linux and
that it now will be supplied under a license compatible with Linux.
Encoder Profile for
SERCOS III
A
specification for an encoder profile for SERCOS III is also being developed in
consultation with users and suppliers. The goal is availability in November 2009 along with a conformance test
being developed to ensure interoperability of different devices.
The new encoder profile ensures the
functions of an absolute or relative encoder are made available via clearly
defined vendor-independent interfaces. The profile defines the functions
supported by a device and how these functions may be used by other devices,
such as control systems or servo drives.
"When defining the profile we are
considering existing parameters for encoders built into servomotors that are
already part of the SERCOS drive profile," says Lutz. "We are also making sure that the requirements
of the encoder manufacturers are incorporated, especially with regard to their
experience with existing, comparable device profiles of other bus systems."
Ethernet IP Adds Quality
of Service
Quality of Service (QoS) mechanisms defined
for EtherNet IP implementations provide users an ability to achieve better
network usage, bandwidth and the right prioritization throughout the system. The concept is to use standard Ethernet
networking capabilities, protocols, infrastructure and the capabilities built
into the switches and routers, and apply the technology to the industrial
area.
"It's all about prioritization of message
handling by the network infrastructure and equipment, switches and routers,"
says Harwell. "But it's also critical
depending on how it is applied into devices themselves because the
prioritization of messages in the devices also contributes to responsiveness
and ability to solve critical applications."
By bringing Quality of Service into the
EtherNet IP networking standards, it leverages the capabilities of switches,
routers and general Ethernet equipment for use in the factory. "Plus, we can reinforce it within products
that conform to the specification and achieve the kind of performance needed to
address areas such as motion control and safety systems," he says.
Device Level Ring
Topology
Another recent
innovation with EtherNet IP is support for Device Level Ring (DLR) as an
appropriate network topology for industrial applications. Instead of connecting individual devices
through a multi-port switch, device level ring allows users to daisy chain from
one device to the next while maintaining a high degree of reliability.
"With a daisy chain,
the user can drop it straight down in a line but if you lose a device in that
line, you have lost all of the devices below it," says Harwell. "One solution is to utilize a ring topology,
so the message can pass in either direction."
Device level ring not
only enables that topology to occur and be able to recover within milliseconds
from any single failure point within the ring without impacting even very demanding
applications. A ring network of 50 nodes
implementing the DLR protocol offers a worst case fault recovery time of 3 msec.
PROFINET Wireless
Communication for Sensors and Actuators
PROFIBUS & PROFINET International (PI) has defined
that upcoming work for wireless coupling of sensors and actuators for use in
factory automation applications will utilize radio technology compliant with
IEEE 802.15.1.
Investigations showed
that different requirements exist for coupling sensors and actuators in factory
and process automation applications. Standardized protocols for HART sensors
and actuators (for process automation) and IO-Link (for manufacturing
automation) are optimized for the respective requirements.
According to Carl
Henning, deputy director of PROFIBUS - PROFINET North America Trade
Organization, the
question of coexistence is especially important. In addition to coexistence
with WirelessHART, it must also be guaranteed with WLAN, the standard for
wireless transmission in PROFINET systems.
Specification work for
the wireless communication of sensors and actuators in manufacturing automation
has already begun. The target for completion of the corresponding documents is later
this year. Alternative and integrated uses of the radio technology for process
automation (IEEE 802.15.4) and factory automation are being left open for the
future by PI.
Fast Start Up for
PROFINET I/O
Fast Start Up (FSU) has been specified for
PROFINET IO to make it possible for an I/O device on the network to go
instantly into a ‘power on' state following a cyclic data exchange with an I/O
controller.
In the latest specification of PROFINET I/O,
the basic rule defines that the time between the ‘power on' and receipt of the
first cyclic input data should not exceed 500 ms. Because the IO device is parameterized and
configured during the first start up, the benefits of FSU may be used only from
the second start up.
Protocol optimizations are standardized in
the following ways. Use of fixed
transmission parameters instead of automatic detection (only for copper wires) can
reduce start up time by up to three seconds.
Network addresses are not passed to the I/O device on each cycle, only
at first start up, and parameters are saved within the permanent memory of the
I/O device. On every following start up, parameters are retrieved from memory
and reused. I/O devices can also "announce"
their readiness to establish communication instead of waiting for the I/O
controller to search.
EtherCAT Safety Drive Profile
The EtherCAT Technology
Group is developing a
new Safety Drive Profile that permits open, vendor-independent configuration
and control of safety functions in drives.
With this drive profile, safety
functionality of EtherCAT drives with either the CiA402 (CANopen) or the SERCOS
drive profiles can be used and configured in a "manufacturer-independent" way.
The IEC 61800-5-2 standard defines
safety-relevant functions for drives. Safe stopping of a drive can be
implemented using Safe Torque Off (STO) or Safe Stop 2 (SS2) functions, while safe
monitoring of motion can be achieved using Safe Limited Speed (SLS). Using
these features, dangerous movements at startup or during manual interaction
with a machine can be avoided or limited in a safe manner.
Based on functions defined by IEC
61800-5-2, a control word is specified which enables the separate activation of
these functions within the drive. Each function is represented by a bit in the
control word. If a safety function is selected and operates within its
boundaries, it is reported back to the supervision safety logic with a status
word. All communication makes use of the underlying safety protocol. The
definition of a uniform control and status word allows the user to operate
safety drives from different vendors in the same way with their safety
controller.
ETHERNET-Powerlink
Safety
A significant area of
technology growth for real-time Ethernet automation/motion networks is
safety. The goal is to replace hardware-based
safety functions, more expensive cabling and limited diagnostic options.
The Ethernet POWERLINK Standardization
Group (EPSG) has defined a next generation safety protocol for real-time industrial
Ethernet called EPLsafety. The protocol
allows both publish/subscriber and client/server communication. It is suitable for communication cycles in
the microsecond range.
Safety relevant data is transmitted via an
embedded data frame inside of standard communication messages. Measures to
avoid any undetected failures due to systematic or stochastic errors are an
integral part of the safety protocol. EPLsafety is in conformance with IEC
61508. The protocol fulfills the requirements of SIL3, and within specific
architectures, also the requirements of SIL 4. Error detection techniques have
no impact on existing transport layers.
Detailed
information on EPLsafety is available on the EPSG website.
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