From Control in the Field to Control in the Edge

The Spanish version of this article is available here.

CIF

One of the most innovative features offered by the IEC 61158-2 compatible FOUNDATION Fieldbus protocol, was the one known as Control in The Field, or CIF.?

This technology’s goal was to allow the inclusion of control function blocks into FOUNDATION Fieldbus field devices electronics, so control loops could be configured without the need of a controller.

This local control proposal initially looks like a return to the control model used in old pneumatic control systems. While it feels like a throwback to ancient eras, the concept offered interesting application possibilities.?

The DCS control paradigm concept was to concentrate the processing of the control loops in a few high-performance controllers. Most DCS applications use one or two controllers.

Under this control model, it would be advantageous to avoid overloading the main controller’s processor, and this objective could be achieved by the decentralization that was allowed by the FOUNDATION Fieldbus CIF functionality.

Traditional DCS topology

An example of a CIF application

A simple example of a CiF application could be the following:

Let us imagine a valve that controls the flow of liquid to a tank. The valve is connected to the same Foundation Fieldbus segment as a level sensor located in the tank that monitors the liquid’s level inside the tank. If the control valve’s firmware included a PID function block, it was possible to configure it to use the level sensor data to close a control loop that, for example, was responsible of maintaining a continuous level in the tank or avoiding the overfilling or the emptying of the tank.

Control in the Field topology

CIF technology could be employed in a variety of control configurations

It could be used either in combination with the controller or independently. That is, the control loop could be configured in the main controller and in the field as a redundancy measure. Or it could be implemented only in the field to either save processing power at the controller or to ensure a faster response time.

Typical demonstration videos of the fieldbus era showed the simulation of a controller’s failure in a FOUNDATION Fieldbus application and how the control loop continued to work afterwards, with the function block being run in the field.

Fully redundant CiF topology

So, why is that this feature was not used widespread?

There are few reasons, but the main ones can be resumed as follows:

? There was never an agreement on which kind of function blocks were supposed to be available in which kind of field devices, so in the event of a device failure only an exact replacement unit could maintain the device’s role in the CIF application.

? FOUNDATION Fieldbus devices of that era had comparatively powerful processors when compared, for example with 4-20 mA or HART devices, but they were not powerful enough to implement advanced control strategies.

? The slow transmission rate of IEC 61158-2 compliant fieldbuses (31,25 kbps) implied that a careful approach at cycle timing configuration was necessary.

? A decentralized control architecture requires training and expertise, both from engineering and maintenance personnel.

An opinion

But, from my personal point of view, the main reason of the eventual demise of FOUNDATION Fieldbus technology was due to the refusal of control system suppliers to adopt the protocol’s implementation of Industrial Ethernet (HSE or High-Speed Ethernet) as a backbone network in their control systems. DCS suppliers had spent massive amounts to develop real time, proprietary deterministic Ethernet protocols, therefore there was little or no interest in replacing these protocols by an open implementation that would bring interoperability between competitors’ offerings.?

The core business of DCS suppliers is not actually selling control systems but selling control systems upgrades and after sale services. Systems interoperability goes against this business model.

Because of this incomplete adoption of FOUNDATION Fieldbus technology, all Foundation Fieldbus installations were in fact small FF islands interconnected by the proprietary Industrial Ethernet backbone networks used by DCS and control system suppliers.

The original FOUNDATION Fieldbus concept, integrating HSE and H1 physical layers

Simpler things tend to last longer than complex things

Therefore, CiF capable FOUNDATION Fieldbus technology came and went by. Its contemporary rival, PROFIBUS PA, has enjoyed a longer lifespan thanks to its less pretentious Master/Client architecture, the availability of a migration path towards Industrial Ethernet provided by PROFINET technology and the inclusion of a standardized method for the integration of legacy networks into PROFINET using Proxy technology.

Legacy and 3rd party fieldbuses can be integrated into a Profinet network using proxies

At the same time, the IT world was developing the concept of edge computing.

Over the years, thanks to Moore's law, computing hardware capabilities have been growing exponentially, so much in fact that the computing power available in common day devices such as smartphones or game consoles rivals the performance of the supercomputers that were available 20 years ago. As an example of how powerful current hardware is just consider this snippet of data: The first computer to achieve 1 teraflop was Intel’s ASCI Red in 1997. In 2008 an ATI Radeon 4800 GPU surpassed that 1 Teraflop capacity. In the last 30 years computing power has grown about ten thousand times.

Moore's law of processing power

A matter of bandwidth

Meanwhile, as predicted by Nielssen's law, available bandwidth for internet access has also experienced dramatic growth, but if we compare bandwidth growth with processing power over the same amount of time (last ten years) we can find that bandwidth increased by a factor of 57 and computing power by a factor of 100.

Nielssen's law of internet bandwidth

This means that the limiting factor for the ongoing process of digitalization of our processes is bandwidth, not computing power. The current tendency of performing data processing in cloud-based platforms found real life limits when the cost of uploading raw data became significative. All the additional data generated by IoT and IIOT applications was going to surpass the bandwidth capacity of the internet.

Edge computing

Edge computing place in the cloud hierarchy

The solution came with a new computing paradigm that became known as edge computing. The idea was to use a distributed computing model that made available computing and data storage resources closer to the sources of data. The idea was originated on the content distributed networks that were created in the 1990’s to serve web and video to end users.

Edge computers manage data processing in the last mile of the network; therefore, they experience significative less lag than cloud computers. The purpose of edge computing is to reduce latency while performing real time operations.

The arrival of hybrid controllers

The automation industry version of edge computing took advantage of the development and availability of hybrid controllers or PACs (Programable Automation Controllers).

There is still an ongoing debate on this denomination, but the general idea is that a hybrid controller is a device that can fulfil the role of a PLC, which is to perform standard control functions involving digital and continuous or analog signals, and additionally are able to perform advanced control tasks using techniques such as adaptative control, artificial intelligence, machine learning or fuzzy logic.

To do this, hybrid controllers feature high performance CPUs and large memory resources that surpass those of traditional PLCs, and which required DCS type of controllers from a few years ago.

A PAC example

Edge computing in process automation

In the last decade there has been an increasing interest in the use of distributed control architectures in the automation industry with the purpose of increased functionality, modularity, and scalability.?

The edge computing paradigm, applied to the process control industry, generated initiatives like the OPAS (Open Process Automation System) initiative. This approach to process control relies in the use of the Distributed Control Node (DCN), as the basic component of the Open Process Automation concept.

Simplified architecture of the OPAS concept

DCNs

The DCN description looks like an industrial edge device, or device that can manage local control duties and is also able to pre-process data near its source. By doing this, only relevant data is transferred to cloud-based data repositories therefore reducing the required bandwidth and eliminating the inherent latency of cloud-based applications that makes them unsuitable for real time purposes.

The complete OPAS architecture

Back to the future

By this time, you may have made the relation between edge computing and FOUNDATION Fieldbus CiF. Both technologies have a similar common purpose: to decentralize control functionalities, making them available closer to the field. The substantial difference, and great advantage, that the use of edge computing has over older concepts is that the processing capabilities of modern hybrid controllers are exponentially superior to whatever was available in the processors of field devices available in the FOUNDATION Fieldbus era.

The upcoming Ethernet-APL technology will eliminate the other bottleneck that affected CiF: bandwidth. Ethernet-APL technology offers data transfer rates of 10 Mbps for field devices. In comparison, the 31,25-kbps data transfer rate of IEC 61185-2 fieldbuses looks simply anachronic.

The Control in the Field concept was too demanding for the contemporary hardware, but it looks like the Control in the edge concept will be able to disrupt the current automation paradigm.

• If you would like to know more about the OPA (Open Process Automation) initiative, you can check here.
• If your feel curious about Ethernet-APL, check here.
• And finally, if you are interested in how HART technology is related with Ethernet-APL, check here.

Mirko Torrez Contreras?is a Process Automation consultant and trainer with a sense of expectation about the upcoming avalanche of technological innovations that are going to appear in the following years. he has an intense curiosity about what the future keeps on hold for us.

Phoenix Contact?sponsors this article. The opinions exposed in this article are strictly personal. All the information required for and employed in this article is of public?knowledge.