Batting for Profibus

Andy Verwer set up and ran the first accredited Profibus International Competency Centre (PICC) for the UK. He is no stranger to Australia and is closely involved in the training activities of Profibus Association of Australia and the local PICC.

Verwer will be visiting five cities in Australia later this year, between 29 October and 7 December, where he will be conducting a series of accredited training courses.

What are the key reasons behind the spread of Profibus?
Initially, when it was introduced in the early 90s, Profibus did not dominate the fieldbus market. However, it incorporated all the requirements for implementing reliable multi-vendor systems.

The basic specification for Profibus decentralised periphery, or DP, was simple to use and provided a platform that allowed manufacturers to easily develop a fieldbus interface for their products.

Because of this Profibus soon became the dominant fieldbus for manufacturing and general automation in Europe. However, other technologies dominated America, the Middle East and Asia, particularly in gas and oil.

Profibus International (PI), the user organisation, was very active in developing and extending the standard, adding new features that tightened specifications and allowed Profibus to be used in new areas and applications.

PI ensured that all changes and extensions were totally backwardly compatible. Other fieldbusses were also trying to extend their specification of course, but they often made the error of not achieving backward compatibility.

The Process Automation, or PA specification was a major addition to the Profibus family that was added in the mid 1990s.

This was aimed at the 4-20mA process control market giving power and data over the same cable, but with Profibus functionality. Profibus PA provides a simple solution for Intrinsic Safely, where there is a risk of explosion.

But Foundation Fieldbus (FF) also has power over the bus and can be used in hazardous environments. Why not just use FF?

Well, actually there is no difference in FF and PA at the physical or wiring level. They both use the same Manchester Bus Powered specification; they both use the same simple concept for protecting against ignition in explosive atmospheres.

However, the communication protocol is quite different. FF has its own protocol which is incompatible with other fieldbusses. PA, on the other hand, uses the same protocol as DP.

DP and PA can operate on the same network. With FF, a second, high-speed communication is invariably used at higher level. In fact in many instances FF is used together with Profibus DP.

Many industries use both process control and higher speed automation. Good examples are found in food, brewing, household product manufacture. Most industries have control systems that extend into both areas and you will often find a mixture of low speed process control and monitoring and high speed materials handling, packaging, storage and warehousing.

Are there any other differences?
Yes, a major difference between FF and PA is that the FF specification defines a mechanism for the controller to be implemented within the instrument or actuator. PA always implements control at the controller. In practice, this difference is not important, because most users of FF actually do not use the controller in the instrument.

For security, reliability and ease of controller parameter access for tuning, users generally prefer to implement control from the control room. The simplicity of the basic Profibus technology means that PA systems are generally easier to set up and perhaps more importantly, to maintain.

Surely, different manufacturers' devices will have different features. How do these fit in with the PA profile?
Yes, of course a particular vendor can incorporate features in their products that that differentiate them from other manufacturers'. But, the PA profile defines how this is to be done in a manufacturer independent way.

This keeps the user's view of devices standardised and simple, yet allows additional features to be used in particular devices.

A good example is the feature of blocked impulse line detection on differential pressure transmitters. This clever feature allows a DP transmitter to detect the subtle sign of a blocked impulse line to one or both the low or high pressure cell connections.

These additional parameters are added within the standard pressure transducer block, which is defined in the PA profile, but in the manufacturer specific parameter area. Another example is the feature of echo curve characterisation in ultrasonic level transmitters.

Profibus seems to have had a fair bit of success in mining and in water, globally and even in Australia. Why do you think this has happened?
The water industry seems to have similar requirements to process: devices mounted outside exposed to the weather; extensive installations with devices spread over a large area; generally quite slow process reaction times.

However, we often see mechanical devices used on screens, filters, for solids removal, cleaning. These devices quite often have similar requirements to manufacturing. Again the hybrid industry argument applies.

Mining, again has its own special requirements. However, again the wide range of devices available with a Profibus interface means that generally devices can be procured from a range of suppliers that will meet the need of the industry in a simple and cost effective way.

Perhaps these industries have come to realise that a common technology allows common tools and training to be applied across the whole plant.

Is there a case for retrofitting fieldbus?
Well many companies in the process sector have 4-20mA devices installed and many in manufacturing are using traditional dumb sensors and actuators based on 0-24V switching technology. These technologies were state of the art when installed, but modern intelligent devices have many advantages.

The most obvious advantage of modern fieldbus enabled devices is that it usually has extensive inbuilt diagnostics to report errors in a manufacturer-independent way. This provides very rapid diagnosis of problems and consequently rapid repair.

Both Profibus DP and PA support standardised diagnostics that can tell us communications and peripheral errors. They can provide helpful information on identifying and locating errors down to the device, module and channel level.

Profibus PA provides properly scaled process values that do not need any scaling or calibration in the controller software. The accuracy and reliability of Profibus device is therefore generally much better than traditional devices. Calibration is also generally much easier to perform and check.

What about functional safety systems?
Well this is really a different area which is concerned with health and safety and general accident prevention.

Most fieldbusses have tried to address this problem area. A new international standard, IEC 61508, provides a method for implementing safety related functionality using electronic and programmable technology.

The standard introduces so called Safety Integrity levels which describe the probability of a dangerous equipment or system failure.

Failures, of course, cannot be avoided. But failures which allow or cause potentially dangerous situations to develop must be reduced to a level which is as low as possible. The ProfiSafe Profile that was introduced in 2000, is based on the IEC61508 standard allowing safety systems to be developed that operate using Profibus technology.

Many Profibus devices are now available for factory and process automation which can be used in certified functional safety to SIL level 3, which is the requirement for many protection systems.

How does Profinet relate to Profibus?
Profinet is one of a number of Industrial Ethernet technologies that has been introduced to give robust and reliable control over Ethernet.

Profinet is actually totally standard Ethernet at the physical and basic protocol levels, but it uses a clever solution based on high priority messages to achieve real time operation. "Real time" means that the control system responds quickly and deterministically compared to the requirements of the process.

Just how deterministic is Profinet?
Well Profinet determinism is about the same as Profibus. There are actually several degrees of determinism for both Profibus and Profinet systems. For basic factory automation we typically require cycle times in the order of 5 or 10ms with determinism in the order of perhaps 1ms.

This can be achieved with standard Profibus DP or Profinet devices. For higher performance systems such as positioning systems, servos, robotics we need much better performance.

The Profibus and Profinet specifications support so called isochronous operation, which basically means constant cycle timing with application synchronisation. So when using multi axis servos for example, all the axes are tightly synchronised to all move together at the required time.

The performance of isochronous operation on Profibus and Profinet is very similar. It is possible to synchronise such systems to within less than a micro second. Quite remarkable performance, particularly over standard Ethernet!

What role can Profibus play helping industry minimise its carbon footprint?
This is another area that Profibus International has been busy developing. A relatively recently introduced profile called "ProfiEnergy" is specifically designed to minimise energy usage, particularly during slack or non-operational periods.

The idea is actually very simple, just to reduce energy usage by shutting down power circuits if they have not operated, or are not required for a certain time.

The key with the ProfiEnergy profile is to do this in a standardised way that can be applied across manufacturers. Initially ProfiEnergy has been developed for Profinet, but it is expected that the profile will be extended to Profibus.

Profinet, which is based on Ethernet, is not compatible with Profibus. What about compatibility?
You are partially correct; a Profibus device cannot be connected to an Ethernet cable. However, Profinet is compatible with Profibus.

Firstly, the interface to Profibus is defined within the Profinet standard. In fact, the Profibus spec is embedded within Profinet. So, it is quite possible to run existing Profibus installations from Profinet controllers by using a standard gateway.

Secondly, most of the application profiles that were developed for Profibus transport to Profinet without change, or perhaps with just minor tweaking.

For example, the profiles for drives, encoders and ProfiSafe can all operate on either Profinet or Profibus. For these reasons, the investment in Profibus is protected, even if you want to introduce Profinet control in the future.

Will Profinet overtake Profibus and totally replace it?
Profinet runs at 100Mbit/s and Profibus at 12Mbit/s. But actually, the speed of the two is about the same, because Profinet telegrams are significantly longer than Profibus telegrams. For a given number of devices and data throughput, the cycle time on Profibus and Profinet is about the same.