THERE has always been a great need for communication within industry, but now the focal points have changed. We used to concentrate on the different levels in the automation process - namely the lowest level (where sensors and actuators communicate with a controller), the field level (where more intelligent field devices, remote I/O-stations, operator panels and small controllers talk to each other) and, above that, the cell level (where automation stations, operator stations and PC communicate).
On top of it all exists a slender link to management for orders, reports and quality statistics.
Communication for the different levels was solved separately: fieldbuses covered the main part and Ethernet, often with different ISO-protocols, the upper segment.
Now we have different demands. From beneath field devices with more intelligence, standalone production equipment and other automation parts need to communicate large quantities of data in a fast and deterministic fashion.
From above, we need to integrate production planning and control, quality management, traceability, maintenance systems and many other MES functions. We want to integrate transparently everything from sensors to the board of directors, from remote plants to local operator stations, from raw material handling to central process and storage. And we want to raise productivity, measure and control everything faster, run synchronised motors quicker and calibrate instruments better (even in runtime).
All this calls for better communications.
WHAT can Industrial Ethernet provide that fieldbuses do not offer already? Let’s examine the 14 principal Industrial Ethernet solutions to reveal their motivation and cardinal offerings in varying measures.
Potential users will be looking for:
Integration to the office world, IT functions, Internet/intranet, remote configuration. This is a world of TCP/IP on Ethernet with application protocols like SNMP, FTP, MIME, HTTP. Communications over routers and servers where IP-addressing and TCP-transport are mandatory.
More bandwidth and bigger data package for communications with more and more intelligent automation devices.
Faster real-time communication with synchronisation good enough for demanding motion control applications.
Connecting and addressing more devices over wider areas.
Homogenous networking mostly using Ethernet.
New functions like MES, online updating of firmware and remote configuration and error handling.
Integration of existing fieldbuses.
These properties provide the motivation for introducing Industrial Ethernet solutions. However, no single solution can absolutely replace a fieldbus unless it can also provide the functionality a current fieldbus: standardised profiles for different applications, bus powered communication to process instruments in hazardous areas (Ex-Zone 0, 1 and 2), safety communication, motion control, capacious diagnostics - and simple and cost efficient connection of remote I/O.
Of course, no Industrial Ethernet solution can presently provide all this. The different solutions concentrate on different parts.
IT function vs real-time conflict
STANDARD Ethernet communication with TCP/IP is non-deterministic and reaction time is often above 100ms. Remote I/O demands reaction in the 5-10ms region. Motion control places even higher demands on determinism with cycle times into the microsecond region.
The conflict between IT traffic using TCP/IP and the achievement of real-time performance is dealt with in different ways.
Embedded fieldbus or application protocol on TCP/IP. By using standard TCP/IP embedding, a fieldbus protocol at the application level maintains full IT openness. You just tunnel the fieldbus protocol on Ethernet. Reaction time is around 100ms, like standard Ethernet. In local segments with few devices and small data packages, the reaction time goes down to 20ms. By using UDP instead of TCP, the reaction time comes down to 10ms at best. Direct MAC-addressing in a local segment can reduce this closer to 1ms. Time synchronisation can be added, such as that obtained by applying IEC61588. Bandwidth for TCP/IP traffic remains at 90-100%.
Closed Ethernet segment and special DLL-layer for the real-time devices. Real time is achieved by special protocol, on Layer 2 in the OSI-model, in every device in the real-time segment. For fast real-time cycling down to 0.2ms, the segment is closed for IT traffic. The real-time cycle is divided into slots where one slot is open for normal TCP/IP traffic. With 0.2ms cycle time and eight devices in the segment, the slot time is about 22µs, which allows for only small TCP/IP messages (up to 200 bytes). Bandwidth available for TCP/IP is approximately 1%.
Application protocol on TCP/IP, direct MAC-addressing with prioritised messages for real time and hardware switching for fast real time. Standard TCP/IP messaging in parallel with IT traffic is used for everything that doesn’t need real-time handling. A parallel [virtual] channel is created by according higher priority and direct MAC addressing on packets for real-time operation down to 1ms. A third channel is created for fast real time down to 0.2ms, using a built-in switch in those devices that need fast real-time. These switches reserve one part of the cycle for real-time traffic but leave the major part open for normal TCP/IP IT traffic. Bandwidth for TCP/IP is 50-100%.
Real time on TCP/IP achieved by prioritised messages and time synchronisation. Solutions aimed at motion control in parallel with standard IT traffic running on TCP/IP. The real-time operation is achieved through prioritised messages with a time stamp understood by the synchronised devices. The time latency in switches (up to a few hundred microseconds) is compensated within the devices. Real time down to about 1ms with a jitter of 10µs. Bandwidth for TCP/IP is 90-100%.
Real time achieved on Ethernet physics with built-in electronics and special protocol from OSI Level 2 upwards. Special messages run on Ethernet physics in a ring or on double cables. The real time is achieved through built-in electronics (e.g. asic) plus special addressing and protocol. TCP/IP with small messages can be embedded in the special protocol message and then unpacked in a master for forwarding onto the normal Ethernet. Connection of normal Ethernet devices uses hardware gateways. Bandwidth for TCP/IP is about 1%.
The solutions you’ve been waiting for
SOME Industrial Ethernet solutions are already international standards. Others are in the voting phase while some as yet remain proprietary - but with plans of going open.
I have not made any distinction since they all share the ambition to become standards-based. I have found 14 protocols with open aspirations although the list is probably not definitive. They appear in alphabetical order with no relative judgement implied.
Ethernet for Plant Automation (EPA) is a Chinese proposal for a deterministic Ethernet communication in the process industry. It is also a trade name of Zhejiang Supcon Company. No group support and no market penetration.
Method: Time slicing in the MAC level for real time. All other traffic is done over normal TCP/IP and/or with Modbus.
Real time: Cyclic communication 10-100ms. No synchronisation.
Functions: Normal IT and process communication over TCP/IP. Extra function for deterministic communication between distributed function blocks in slow real time.
Ethernet Powerlink was developed by Bernecker + Rainer (B&R) in 2001 and supported by EPSG (EPL Standardisation Group). Main producer companies: B&R, AMK, Baldor, Fraba, Lenze, Pepperl+Fuchs , Port and Smart Network Devices. Powerlink was early to market with products for motion control and has taken a market share in this niche with more than 60,000 nodes installed.
Method: Closed segment with a master providing synchronisation and time slots. One of the slots open for small TCP/IP-messages (1% bandwidth).
Real time: Cycle time from 0.2ms with eight axes. Maximum 20 axes restricted by a maximum of 10 hubs between master and drive. Synchronisation <1µs.
Functions: Primarily motion control. I/O with CANopen integrated. Safety protocol planned but not yet ready and approved.
Developed by Beckhoff in 2003. Supported by the EtherCAT Technology Group with some 140 members. Main producer companies: Beck IPC (chips for masters); Beckhoff (servos for motion control, I/O-masters and slaves); ACS-Tech 80, Baumüller, Danaher, Lust, Servo Dynamics and Stöber (drives); Balluf, Fraba and TR Electronics (encoders).
Method: A master in a closed segment connects to slaves in a ring, using Ethernet physics. One single message is passed on from the master through every slave and, from there, back to the master. The message has an Ethernet frame but the protocol is totally different. Every slave has a mailbox that is read and written to on the fly. Every device incorporates gate array or asic silicon for handling real-time decoding. Normal TCP/IP devices have to be connected over gateways; then small TCP/IP datagrams can be embedded in the EtherCAT message, unpacked and put together in the master and hence to the outer world. Bandwidth for TCP/IP about 1%.
Real time: Cycle times from 30µs. 100 axes in 100µs. Synchronisation <1µs.
Functions: Primarily motion control, I/O and encoders.
(IP=Industrial Protocol). Defined by Rockwell . Supported by the ODVA with some 250 members. Main producer is Rockwell for controllers, I/O, HMI and drives; Accu-Sort Systems , Datalogic and Sick : barcode readers; Acromag, Phoenix and Wago: I/O; Bosch Rexroth , Parker Hannifin and SMC : valves. About 21 certified products. In spring 2004, General Motors declared that it would standardise on Ethernet/IP for its automation programs.
Method: Based on normal TCP/IP with alternative UDP/IP as an object embedding protocol, CIP (Common Interface Protocol), transports I/O data, configuration and diagnostics over normal Ethernet. Non-deterministic with reaction time down to 10ms. Synchronisation (CIPsynq - IEC61588) can be added. Bandwidth for TCP/IP 90-100%.
Real time: Cyclic communication 10-100ms. Synchronisation about 10µs.
Functions: Fieldbus migration over bridges for ControlNet and DeviceNet (installed base 2.5 metre nodes), which use the same CIP application protocol. Drive control with moderate cycle time and synchronisation. Safety protocol planned but not yet ready and approved.
High Speed Ethernet. Defined by Fieldbus Foundation for connecting FF networks via a gateway to Ethernet. Supported by Fieldbus Foundation. The only product is the link device to connect Foundation Fieldbus networks. Emerson Process Management , Endress+Hauser and Smar International are link manufacturers.
Method: Embedding of a fieldbus (FF) protocol in an Ethernet message. No TCP/IP traffic.
Real time: None.
Functions: Enables a DCS to skip the FF interface and use only Ethernet as the communication network.
While not an Industrial Ethernet solution for general automation, it specifies communication protocol used in power distribution and power generation i.e. between substations, etc. It uses an object-orientated application protocol on top of normal TCP/IP. Supported by UCA International User Group. Products are expected from all the main companies within power generation and power distribution.
Method: An application protocol on top of TCP/IP.
Real time: No.
Functions: Communication protocol for power distribution.
Developed by Jetter AG. No supporting group. Products such as servo systems only from Jetter AG.
Method: Normal TCP/IP is used. Real time is achieved with synchronisation according to IEC61588, timestamp and compensation for delays in switches.
Real time: A few axes with a cycle time of 1ms. Synchronisation about 10µs.
Functions: Primarily motion control. Can also handle I/O.
Modbus/TCP with real-time extension RTPS
Defined by Schneider Electric . Supported by Modbus-IDA user group. The original Modbus protocol (e.g. RS485) in use since 1979. Migration to Ethernet easy to implement and widely spread. Probably the most used Ethernet solution so far. About 90 products mostly from suppliers of remote I/O who have multiple choice of interface.
Method: Based on normal TCP/IP embedding Modbus, a very simple protocol using a request/reply model. The solution is non-deterministic and reaction time is 20ms at best. Real time with RTPS (Realtime Publisher Subscriber) can be added. This uses UDP/IP to improve performance but not to true real-time standards. Bandwidth for TCP/IP 90-100%.
Real time: Cyclic communication 20-100ms. No synchronisation.
Functions: Connection of Modbus to Ethernet. Simple protocol for I/O and reading/writing in registers.
P-Net on IP
A proposal from the Danish national committee. A method of migrating P-Net on Ethernet using the same model as HSE. See section on HSE. No products, not realised yet.
Defined by Profibus International with more than 1200 members and regional organisations in 25 countries on all continents. More than 25 companies with 100+ products. Beckhoff: PLC and I/O; Comsoft: I/O-controller; Comtrol: I/O-controller and gateways; Danfoss , Rexroth, SEW : drives and motion control; HMS: I/O and I/O-controller, Interbus Proxy, Gateways; Hilscher: I/O and I/O controller, gateways and software; Phoenix Contact : Interbus proxy, I/O and I/O-controller; Siemens : PLC, drives and motion control, I/O and I/O-controller, PC-cards, asics; Wago: I/O; Yokogawa : PLC. German auto industry comprising Audi, BMW, Daimler-Chrysler and Volkswagen declared on Profinet.
Method: Normal TCP/IP is used for most functions. This includes configuration, parameterisation and CBA (Component Based Automation). No restrictions on TCP/IP traffic. For I/O and other real-time functions down to 1ms, direct addressing and prioritised messages are used (RT channel). No restrictions for TCP/IP traffic but shorter delays can occur in switches due to the priority.
For real-time demands below 1ms and with synchronisation <1µs, Profinet uses asic-based switches connected in a daisy chain or a ring to control traffic in the segment. Synchronisation according to IEC61588 and configured cycle time from 250µs. The switches use reserved framing in real time while permitting 50 to 100% of normal TCP/IP traffic including full length messages. With a 4-port switch, any normal TCP/IP device can connect to one of the two free ports. Bandwidth for TCP/IP 50-100%.
Real time: Cycle times from 250µs with 30 axes and 50% TCP/IP. 150 axes in 1ms. Synchronisation <1µs.
Functions: CBA, transparent migration for Profibus (installed base 13 metre nodes) and Interbus (installed base: 7.5 metre nodes). Other fieldbus migrations in progress. Safety protocol based on PROFIsafe available, with approval expected shortly. Other Profibus profiles for completion over the next two years. New profiles like MES functions, starting with maintenance, are implemented.
Developed from the Sercos bus for drives. Supported by the Intressengemeinschaft Sercos Interface e.V. with 66 members. Sercos has an established drive and motion control market and can be expected to provide a fulfilled area with its Sercos III product for drives controlled over Ethernet.
Method: A built-in switch in every device in the segment forms a daisy chain or a ring to control traffic. Cycles from 31.25µs are divided into real time and an open part. TCP/IP devices can be connected to the last switch at the end of the daisy chain. Bandwidth for TCP/IP up to 50%.
Real time: Cycle times from 31.25µs with eight axes/0% TCP/IP. 150 axes in 1ms/50% TCP/IP. Synchronisation <1µs.
Functions: Drive and motion control. I/O can be connected. TCP/IP for parameterisation and other traffic.
Developed by Motion Engineering, now owned and supported by Danaher Motion. Has a niche for motion control applications in semiconductor, electronics and medical OEM machinery. Products from AMC, Danaher, Glentech, Kollmorgen, Panasonic and Yasgawa. More than 60,000 axes installed.
Method: Built-in electronics in every device uses Ethernet physics running an alien protocol. Two Ethernet connections: one for incoming and one for outgoing messages. No TCP/IP or other Ethernet messages.
Real time: Cycle times from 25µs with four axes. Synchronisation <1µs.
Functions: Drive and motion control.
A proposal from Toshiba . First product is a transmission module for the Toshiba 3000 system.
Method: An Ethernet segment is closed in with routers. Cycle time and synchronisation taken from a master. A slave sends high, medium and low prioritised messages within its time slot. The low prioritised messages can be TCP/IP and are allowed to pass through the routers. The rest operates like EPL (see the section on Powerlink).
Real time: A similar performance to EPL is to be expected.
Functions: Not known.
Developed by Yokogawa in 2004. No supporting group.
Method: HTTP on normal TCP/IP with an extension, Real-Time and Reliable Datagram Protocol (RTP). This enables reaction times down to 10ms.
Real time: Cycle communication down to 10ms. No synchronisation.
Functions: Normal TCP/IP with applications on HTTP.
* Commentary by Lars Larsson, product manager, process automation, Siemens AB and chairman of the Swedish Profibus organisation.