A slave to fieldbus technology

The growing use of fieldbus technology in AC drive installations is set to save industry considerable costs. But just what are the considerations that you need to make whenchoosing the right fieldbus for your application? GEOFF BROWN, consultant applications engineer at ABB, offers some practical advice

Standard AC drives look set to be one of the latest devices to benefit from fieldbus technology. While experience with fieldbus has been gained in traditionally demanding applications such as steel and papermaking, it is being increasingly applied to simple, everyday applications, accompanied by a wider availability of fieldbus devices.

By using a fieldbus to link drives with external control interfaces, application designers have never found it easier to design control systems for drives. Fieldbuses bring drives and their associated motors fully into the world of digital, distributed control systems. Process, manufacturing and building automation product vendors and systems end-users have been fast to recognise the benefits of digital field communications for drives.

Choosing carefully among the many fieldbus options now available can bring immense benefits to even the simplest drive application.

A fieldbus is a fully digital 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, a fieldbus usually links together a larger number of devices, each of which can take an active role in the exchange. Because a fieldbus transfers information serially, it is often referred to as serial communications.

Fieldbus differs fundamentally from point-to-point, because it is easy to add a new device to an existing system. However, strict rules, known as a protocol, are laid down, governing topology and information flow.

Master and slave

Each device on a system is described as a node, the node being either a master or a slave. AC drives are nearly always slaves. Each has a unique node number or address and the master can send information/commands to a number of slaves, either singly or together. The master, usually a PC or PLC, also receives information from the slaves and usually contains a fully application-specific program.

The method of communication from master to slaves varies according to the protocol selected. Each communication link will also have a transmission speed (baud rate), in either Hz or bits/second, usually determined by the master device, and a specific make up of the data configuration.

The technology to make simple connections between field equipment and a host controller has been around for two decades, but it has only recently been widely applied to AC drives.

Part of the delay has been the large number of product vendors which makes it unlikely that an industry-wide single fieldbus standard will ever be agreed upon. This has resulted in many different industrial protocols, including Profibus, Modbus, DeviceNet, InterBus-S and LonWorks, each of which has specific benefits and drawbacks. Apart from open fieldbuses there are also many proprietary fieldbuses, although the creation of any more new fieldbuses seems to have slowed. However, many products can be configured to utilise more than one fieldbus protocol and therefore offer end-users far more freedom of choice when it comes to adding extra equipment.

These fieldbus standards differ in such things as cable lengths and requirements, noise immunity, topology, transmission speed, message configuration, number of interconnecting levels and their applicability to differing solutions. Management and configuration tools are also different, along with the type of administration.

Drive requirements

When it comes to AC drives, the aim is to describe how AC drives will react on a particular fieldbus and what information will be transferred to and from the master. All open fieldbus organisations define these requirements.

The reasons for users choosing a specific fieldbus include:

Capability of controlling the process, and providing sufficient data to a user.

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

• I/O cabling cost
• Versatility of control and diagnostics
• Degree of standardisation
• System expansion potential
• Reliability.

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

• Simple to configure
• Lower control cabling costs
• Lower power consumption
• Fewer discrete 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 in the complexity of their task, in the availability of specific components and in the support for development.

So how do you find the ideal fieldbus for your needs? There is no single fieldbus that can fulfil all requirements for every application. There can be considerable differences in communication requirements for different applications, therefore the fieldbus must be selected according to these requirements.

Most users want to select a simple communication method that fulfils all the requirements for their application and their site. The selected standard must have appropriate devices and tools for the application.

Apart from ensuring that the chosen device can accept multi-vendor products, users should consider the following:

• Topology
• Access to the fieldbus
• Physical features
• Protocol
• Access rights in multimaster systems
• Noise immunity
• Transmission speed.

Consideration also needs to be given to the technical, communications and system safety requirements.

ABB
f103@industrialnetworking.co.uk