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Drives, motors and power transmission, couplings, clutches
News Release from: Siemens Automation and Drives | Subject: Industrial Ethernet and motion control
Edited by the Manufacturingtalk Editorial
Team on 23 December 2005
Industrial Ethernet moves into motion
control
Having solved its problems of determinism and network latency, Peter Stott believes that Ethernet is the way forward for motion control and is probably the fastest emerging technology today.
Combining the flexibility of office based systems with the robust environmental and real time needs of an industrial fieldbus has spawned a variety of solutions, which all use the name industrial Ethernet And although these solutions appear to be similar, a browse through industrial Ethernet websites will soon reveal phrases like 'real-time communication, 'standard TCP/IP' and 'completely open networks'
This article was originally published on Manufacturingtalk on 9 Aug 2000 at 8.00am (UK)
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All these claims tell you that you only need one network and that is Ethernet.
So are these claims realistic and do they really hold true for all versions of industrial Ethernet? Like most things in life, the devil is in the detail.
For Ethernet to be an effective fieldbus, certain limitations must be overcome.
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In office based communications the problem of Ethernet not being deterministic - never being sure of how long the data will take to arrive at its destination - is at worst trivial and mostly insignificant.
In motion control the determinism errors of Ethernet are significant and they can appear on the network in a number of ways.
* Data collisions - the most common forms of Ethernet, such as 100BaseTX and 1000BaseT, use the standard IEEE 802.3 carrier sense media access /collision detection (CSMA/CD) which aims to prevent data collisions by looking for activity on a particular Ethernet segment prior to transmitting data.
Figure 1 shows a server and four host devices all connected through an Ethernet hub.
The server attempts to send data to host A, at the same time host D also starts sending data.
When a hub receives incoming data it automatically forwards this data on all of its ports, regardless of whether the data is wanted there or not.
Consequently there is a collision in the segment containing host D, the collision will be detected by host D and it stops further network activity by transmitting a jamming signal.
Clearly this type of a shared media network is not usable if we need to transmit data in real time.
When Ethernet switches receive incoming data, they read part of it to see where the data is destined.
If the switch recognises the destination as being a device on one of its ports it will forward the data on that port only.
If, at the same time, the switch receives more incoming data on another port the switch will buffer this data until the outgoing port is free for a new transmission.
This method overcomes collisions and has the advantage of not needing to employ special protocols, but the reading and buffering of the data by the Switch causes a delay which highlights the problem of network latency - the delays in the network structure.
Network latency is the time taken for data to travel from the transmitting device to the receiving device.
Adding switches helps reduce collisions but, as we have seen, switches increase network latency.
Some implementations of industrial Ethernet claim network determinism down to +/-1 microsec.
Clearly in these networks latency has been overcome.
PROFINET isochronous real time (IRT) employs special switches that overcome the problems of latency and avoids data collisions, giving the best of both worlds.
The Siemens ERTEC 400 is an intelligent four port switch for use with PROFINET IRT, which allows connected devices to measure the latency between themselves and devices sending real time data.
They remember the latency figure and add it to any real time data they receive.
This allows devices to be self compensating for any delays caused by segments of copper conductors or the action of switches.
Having overcome the problems of data collisions and latency, the ERTEC 400 is left with one remaining issue; how to give priority to real time data.
* Real time Data versuss non-real time data - a completely open network would allow devices sending real time data to coexist harmoniously in the same domain as devices sending non-real time.
Real time data cannot use transport protocols like transport control protocol (TCP) and user defined protocol (UDP), the data takes too long to traverse these protocol stacks, making special protocols necessary for real time data.
Some versions of industrial Ethernet are entirely reliant upon these special protocols to marshal the flow of real time and non-real time data.
The real time protocol buffers TCP and UDP access to the network.
Therefore devices not supporting these special protocols must be excluded from real time domains.
This point is even more important for versions of industrial Ethernet that rely on low latency hubs in real time domains.
Without these protocols the real time mechanisms fall down as the domain would see collision after collision.
Implementing industrial Ethernet in this way means you can not take a standard PC, only equipped with TCP and UDP, plug it in a real time domain and expect the real time mechanisms to work.
PROFINET IRT is not solely reliant on special transport protocols to discriminate real time and non-real time data, this discrimination is also made by the switches.
The ERTEC 400 employs Store/Forward and Cut-Through Ethernet switching methods for marshalling of data in the network.
The ERTEC 400 uses the Frame Identifier, part of the PROFINET frame, to see if the data contained in the frame is real time or non-real time.
The ERTEC 400 stores non-real time data allowing real time to cut through and exit ahead of non-real time.
This makes a very secure mechanism for priority passing of real time data, even attempts to flood the real time domain would be marshalled by PROFINET IRT.
* Real time - for real - Siemens has incorporated the ERTEC 400 into its latest motion controller, SIMOTION D, and recently completed testing PROFINET IRT at MAN Roland in Augsburg, Germany.
During the test programme, seven motion controllers were used to synchronise 30 servo axes over four printing units.
When used with SIMOTION D, PROFINET IRT can synchronise over 100 axes of motion in a 1 millisecond cycle, leaving a network bandwidth of over 6 million bytes/sec still available for TCP/UDP communication.
* About the author - Peter Stott is a product manager for Siemens Automation and Drives.
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