Focus on Drives: Merging AC drives & fieldbus technology

First it was energy savings. Then new motor control platforms dominated the industrial news headlines. That was followed by developments in EMC (electromagnetic compatibility) standards and legislation. Then, the hot topic of conversation was the huge market potential for microdrives. Now, the latest feature of AC drives to catch industry’s interest looks likely to be the ever-increasing use of fieldbus technology.

Recently, more process, manufacturing and building automation product vendors and systems end-users have explored the benefits of digital field communications in the open systems world.

This is happening because, as fieldbus experience is gained in traditionally demanding applications, the technology is being increasingly applied to simple, everyday applications. Furthermore, the spread of fieldbus know-how is accompanied by a wider availability of fieldbus devices.

As the workplace becomes more automated and the need for real-time, on-line access and information grows, fieldbus technology that allows AC drives to be networked with controllers more easily and more cost effectively is gaining considerable interest.

What is fieldbus?

Fieldbus is a fully digital and duplex data transmission system that connects intelligent field devices and automation systems to an industrial plant’s network. Unlike point-to-point connections, which allow only two circuits to exchange data, fieldbus usually joins a larger number of circuits, each of which take an active role in the exchange.

A fieldbus transfers information sequentially (serial, one after the other) and is often referred to as serial communications.

Fieldbus topology differs from point-to-point, with higher installation flexibility, because it is easy to add a new circuit to the existing system. However, with fieldbus, access requires stricter rules to let information flow from producer to consumer. This set of rules is called a protocol.

A device on a system is usually described as a node. Globally, there are many hundreds of thousands of “nodes” now in use, spanning many industrial sectors. A node may be a master or slave. Each has a unique node number or address and the master can send information and commands to a number of slaves. The master also receives information from the slaves and usually contains an application-specific program, for example for a PC or a PLC.

AC drives are always slaves. The method of communication from master to slaves varies according to the protocol selected — for example, polling, cyclic, point-to-point, strobe, or query and answer. Each communication link will also have a transmission speed (baud rate), in either Hz or bits/second and the master device usually determines this.

Trends in fieldbus and drives

While the technology for simplifying the way field equipment is linked to the host controller has been around for over a decade, it is only recently that its focus has turned to AC drives.

Part of the delay has been the confusion and uncertainty relating to a firmly established international standard. Because of the complexity of component choice now offered by a still-growing number of international product vendors, it is becoming clear that an industry-wide fieldbus standard is unlikely.

This aside, vendors’ products often conform to the requirements of more than one fieldbus technology protocol and, as such, offer end-users far more freedom of choice in terms of add-on buying patterns. This has resulted in many different industrial protocols with the following being the most recognized today:

Profibus (generally dynamic industrial processes)

Modbus (generic Modicon)

Modbus Plus (generic Modicon)

DeviceNet (Allen Bradley-specific implementation of CANbus)

InterBus-S (Phoenix generic)

Allen Bradley Data Highway (DH & DH+) (Allen Bradley specific)

ESP (GEC specific)

LonWorks (generic Echelon)

CanOpen (generic)

Ethernet (PC networking — generic)

Apart from open fieldbuses there are also many proprietary fieldbuses. The organizations behind the major open fieldbuses, such as Profibus, InterBus-S, DeviceNet and CANopen, are in an era of consolidation, and are improving system performance by occasionally adding new features.

New open fieldbuses that have appeared lately are ControlNet, Profibus-PA and Fieldbus Foundation H1 (with H2 under development), although the creation of any more new fieldbuses seems to have slowed. The latter two are process fieldbuses, of which Profibus-PA is strongly represented in Europe, whereas Fieldbus Foundation H1 is strongly represented in North America.

How fieldbuses differ

What differentiates these fieldbuses are differences in cable lengths and requirements, noise immunity, topology, transmission speed, protocol, number of interconnecting levels and its application in different solutions, together with management and configuration tools, along with the type of administration.

With AC drives, the aim is to define functional profiles within the protocol which describe how AC drives will react on, for example, Profibus, and what information, as a minimum, will be transferred to and from the master. All open fieldbus organizations define these requirements.

Growth in Ethernet

One new fieldbus system gaining strongly in market share is Industrial Ethernet. Industrial Ethernet provides proven technology (Ethernet is commonly used in PC networks and factory automation), but has not yet found an open standard with regard to the data protocol and device profiles.

So far the Ethernet TCP/Modbus interface has been adopted by some manufacturers, but without a functional profile. With Ethernet TCP/Modbus, the physical level is Ethernet. Embedded in the Ethernet message frame are the Transmission Control Protocol (TCP) frames and the actual data frames, constructed according to the Modbus protocol.

As an interpolation of this, one could use any of the existing open fieldbus data protocols instead of the current Modbus protocol (e.g. Ethernet TCP/Profibus). If these data protocols could additionally be handled by the microcontroller of the Ethernet interface, instead of the fieldbus-specific ASIC (Application-Specific Integrated Circuit), only one standardized hardware design would be needed. The handling of the data protocols could then be realized in the form of software modules. In this way, the handling of the large variety of fieldbus alternatives would be made much easier.

Cost savings with fieldbus

Traditionally, plant engineers and managers have accessed shop floor process data by using a host control system into which all “field” instrumentation and automated manufacturing cells are fed. The downside here is that all devices are attached by complicated, lengthy analogue signal wiring.

Fieldbus technology eliminates the bulk of costly analogue signal wiring by using a single cable system on to which field equipment may be hooked for greatly enhanced, digital data retrieval.

There are many other benefits of using fieldbus technology, including:

flexibility to extend the net and connect different modules on the same line

greater distances can be achieved than with traditional connections

elimination of drift (analogue signals)

wider range of applications can be covered

reduction of global costs

simplification of the installation and operation

reduction of engineering costs

availability of tools for installation and diagnosis, and the

possibility to connect products from different manufacturers.

The downside of using fieldbus is that users need to know what they are doing and must be prepared for a higher investment in equipment and tools for monitoring. These disadvantages, however, are only temporary, and disappear once experience of the new technology is gained.

Furthermore, it is estimated that “traditional” analogue signal processing/manufacturing systems cost producers, worldwide, in excess of US$60 billion annually in plant maintenance — a level of spending that is three times greater than the capital investment.

By comparison, the use of fieldbus in drive installations
provides a dramatic reduction in costs (see fig. 2) principally through the massively reduced length of wiring required.

Benefits continue to accrue well after plant commissioning. For example, the fieldbus cabling enables field device inter-communication that will facilitate bi-directional transfer of data between devices and the host.

It is not only the cost of the interface that is determining the future for fieldbus. Cost is also associated with the assembly and wiring of the device. For this reason the fieldbus interface is today usually integrated within the drive.

How does fieldbus affect you?

Fieldbus concerns three groups of industrial professionals.

Users — all those who must work on a system based on a fieldbus. Until everything works well, such people are often unaware of the underlying fieldbus system, and that is why they ignore the fieldbus they depend upon.

System integrators — those who build automation systems with fieldbuses created with material available on the market.

Product manufacturers — companies that make and sell products to connect to a fieldbus.

The requirements of these three categories are not the same. For consumers, the fact that the installed system works correctly and cheaply is the main thing: that this happens thanks to a fieldbus may not be important.

From a customer’s perspective, the reasons for choosing a fieldbus include:

Integration of intelligent field devices and automation systems independent of the manufacturer

Lower I/O cabling cost

Versatile control and diagnostics

High degree of standardization

Easy system expansion

Improved reliability.

An important consideration for system integrators when choosing a fieldbus is how easy it is to install. Therefore, they are interested in equipment to install, configure and monitor the net — a reduction in wiring implies a direct price advantage. A fieldbus, in fact, favours the realization of modular and easily expandable systems.

From a system integrator’s viewpoint, the main reasons for purchasing fieldbus are:

Lower I/O cabling costs

Lower power consumption

Fewer I/O channels in DCS/PLC systems

Less space needed

Less documentation

More information from the field level

Easier to expand, even during operation.

Finally, those who design equipment and communication interfaces for a fieldbus are interested instead in the complexity of their task, in the availability of specific components and in the support for development.

For more information, contact ABB Inc., Automation Technology Products Division, Saint-Laurent, QC, tel. 514-832-6573, visit www.abb.com/motors&drives, or use the reply card number.