08/01/01 - Article: The influence of Ethernet TCP/IP to factory automation and the impact on AS-Interface


The influence of Ethernet TCP/IP to factory automation and the impact on AS-Interface


Summary

Independent from the speed of Ethernet installations penetrating the field level in automation solutions AS-Interface is the ideal sub system to network binary sensors and actuators. It does not matter if AS-Interface is a sub system to already existing fiedbus systems or to Ethernet. Today there is no alternative to AS-Interface. Users which invest today in AS-Interface can be sure that they have invested in a system following the requirements of the future. Ethernet and AS-Interface harmonize perfectly together. Customers can use AS-Interface today. AS-Interface will harmonize with future Etherent installations.


Foreword

Ethernet will find its place in factory automation. But it is a controversial issue how deep AS-Interface will penetrate the field level and which impact Etherent will have on existing automation solutions. The following article deals with the demand to Ethernet based solutions in automation and the impacts on AS-Interface.


Preface

Today Ethernet is no.1 topic in automation. Ethernet will find its place in tomorrow's automation solutions. The question is which automation problems will be solved with Ethernet and which will be solved with already existing fieldbus systems.
Will Ethernet replace all existing fieldbus systems like PROFIBUS, InterBus, CAN or AS-Interface?
Will AS-Interface as the only sensor-actuator-bus system for binary sensors and actuators disappear or is AS-Interface the ideal partner system in the sensor actuator level for Etherent installations?


Purpose of the use of Ethernet in automation

Although all leading opinions which work in the field of Ethernet based solutions for automation have got different basic approaches and different specifications the objective targets are the same. This is because all of them have got the same original position.
Today's automation solutions use fieldbus systems - PROFIBUS, Interbus, DeviceNet etc.- in combination with central control units. With that structure the engineering effort rises together with the rising requirements.
With the use of Etherent in automation the following targets should be fulfilled.

While the targets are the same, the methods of resolution are different, similar to the different fieldbus solutions. Every solution is "open", but the solutions - IDA (interface for distributed automation), PROFINET or Ethernet /IP etc. - are not compatible to each other.
But all try to share the intelligence in a lant to the various devices. Especially PLCs, robot controls, drives and operating panels.


Use of Web technology

With the help of standardized protocols out of the internet world - HTTP, FTP, SNMP - data shall be exchanged. Standard tools like webrowsers can be used to display the information. This enables remote maintenance via the internet.
Each device gets its own web server with own homepage. The information small devices like I/O modules etc. can be handled via a proxy server.
Different fieldbus systems can be connected to the Ethernet via gateways. These gateways act as proxy server. In that way the respective data of the various fieldbus nodes can be transmitted via Ethernet.


Real time data transmission

The problem of real time data transmission via Ethernet has been discussed during the last months. The problem has been solved within some limits. So automation devices can be connected with Ethernet used as fieldbus. The future will show how many fieldbus installations will be replaced by Ethernet installations. Within the near future both solutions will be used parallel for factory automation.


The impact on AS-Interface

While Ethernet was actually designed for office applications it is now coming down to the automation field level, AS-Interface comes from the very lowest level from networking binary sensors and actuators. That aspect has left in the discussion of Ethernet in automation. But it is important because the quantity of binary sensors and actuators is about 90% of all automation devices.

AS-Interface has been developed as intelligent cabeling system for binary sensors and actuators.The development focus was based on the requirements of the actuator sensor level. One target was that the commisioning of an AS-Interface installation has to be very simple. "Just Plug and Play!" this thought was very dominant. The easy handling and the quick connection of AS-Interface slaves to the yellow cable using the piercing technology are the big AS-Interface's advantages. Everything with just an unshielded two wire cable which is used for powering the AS-Interface slaves and for data transmission. Because of the possibilty to have AS-Interface modules with only a few I/O e.g. 2I/2O or integrated sensors or actuators there is no parallel wiring in machines and plants. That is not possible with Fieldbus systems or Ethernet. Ethernet I/O modules need at least 16 binary I/O to be efficient. Nobody can imagine a binarys witch within the next years with an Ethernet interface and own homepage. The big advantage of AS-Interface is the high cost efficiency because of real fast cabeling, commisioning but also diagnostics in the sensor actor level. Especially this advantage would get lost if the regress back to parallel wiring was made.


Coupling of AS-Interface to superior systemes

Because of the different approaches of Ethernet and AS-Interface it is useful to combine both systems to use advantages of both systems.

Figure 1: Connection of AS-Interface via a fieldbus to Ethernet

The coupling of AS-Interface to superior systems is today already solved. The mapping of the AS-Interface data into supierior fieldbus sytems is quite simple because only a limited number of bytes have to be mapped (e.g 32 bytes for 62 AS-Interface slaves).
Bihl+Wiedemann offers AS-Interface masters with interfaces to open fieldbus systems - PROFIBUS, InterBus, DeviceNet, CANopen, Modbus, Modbus Plus LON or Bitbus and so on. All AS-Interface gateways can be put into operation without any additional software. There is no additional engineerig effort. AS-Interface is already totally integrated in these fieldbus systems. The AS-Interface gateways act from the point of view of the superior fieldbus as modular fieldbus I/O modul. AS-Interface would be coupled to Ethernet via a higher fieldbus system. A proxy server transmits the fieldbus data into Ethernet and so also the AS-Interface data would be transmitted into the Ethernet world.

Figure 2: Direct coupling of AS-Internet to real time Ethernet

Today the direct connection of AS-Interface to Ethernet is also already possible. Applications with network complex devices via Ethernet can scan the binary sensors and actuators via AS-Interface. With the use of Bihl+Wiedemann's AS-Interface/Ethernet Gateway both systems can be combined in an optimal way. While control tasks are solved within the Ethernet level, the binary sensors and actuators are communicating via AS-Interface. The use of the gateway's internal Web server offers the posibility to display the all AS-Interface data with the help of a standard browser.

Figure3: AS-Interface/Ethernet Gateway by Bihl+Wiedemann


Protection of investments

The doubtlessness to have invested in a system with future is very important for the user of automation products. With a change to new system there must be ensured that the system is still available within 5 or 10 years. Otherwise the user has to change from technology to technology wich means big efforts with every change.
This point is very important for AS-Interface. AS-Interface is a world wide standard. With the new AS-Interface specification 2.1 AS-Interface is totally downwards compatible to the specification2.0. According to the concept AS-Interface Safety at Work there will be possibilities to have safe AS-Interface slaves together with standard AS-Interface slaves at the same cable and to realize safety concepts for machines with AS-Interface. This demonstrates that AS-Interface is fit for the future. The protection of inverstments concerning the coupling of AS-Interfacenterface and Ethernet has been explained. Even if a machine or plant has got an AS-Interface installation in combination with a fieldbus and the fieldbus shall be replaced by an Ethernet installation in the future the AS-Interface network remains. Just the AS-Interface master or AS-Interface gateway will be replaced by a gateway to Ethernet.



Author
Dipl.-Wirtsch.-Ing. Lutz Bieberstein
Bihl+Wiedemann GmbH
Floßwörthstr. 41, 68199 Mannheim
Fon: 0621/33996-0, Fax: 0621/3392239
eMail: lbieberstein@bihl-wiedemann.de
Mr Bieberstein is Head of sales and marketing at Bihl+Wiedemann GmbH

[back]