September/October 2012
Process Automation

Modernization projects reduce field service cost

Modernization projects increase process availability and uptime while lowering ongoing maintenance costs applying FOUNDATION fieldbus embedded control

Fast Forward

  • Control system modernization decisions have a big impact on future automation efficiency and strategy for a long time to come.
  • Modernization projects that are "like for like" replacement result in conventional automation technology that has been around for 30 years providing no advantage.
  • Fieldbus technology provides an advanced path to truly embedded automation for process control.
 
By Larry O'Brien

Modern1Many end users are dealing with the issue of process automation system migration and modernization. The installed base of systems 20 years old or older is staggering-around $65 billion according to ARC Advisory Group. Chances are the decisions you make in your control system modernization will have a big impact on your future automation efficiency and strategy for a long time to come. Technology and the value it can bring to your process automation strategy have improved considerably since the days of the late 1980s/early 1990s, when "openness" mostly existed on paper and we were still using transparencies to deliver our presentations. The one thing you want to avoid in your modernization project is a "like for like" replacement that will leave you with conventional automation technology that has been around for 30 years.

FOUNDATION fieldbus provides a lot of advantages. Since this is an article on "embedded automation," let's talk about the unique ability of FOUNDATION fieldbus to provide control in the field or "embedded control." If you are planning a modernization project, you should consider the amount of truly embedded automation you are implementing in the new system from the perspective of the process  automation system and your instrumentation assets, including process field instrumentation, control valves, and analytical devices. Fieldbus technology provides an advanced path to truly embedded automation for process control. Created from the ground up to conform to process end-user requirements for control and automation, FOUNDATION fieldbus can provide a truly embedded automation solution for your modernization application that can significantly reduce hardware space requirements and the need for service trips to the field. The embedded control-in-the-field capability of FOUNDATION fieldbus can greatly increase process availability and uptime, and there are many documented cases where the implementation of embedded control has resulted in the avoidance of an unplanned shutdown.

Digital age already here

FOUNDATION fieldbus is a truly digital network that conforms to the IEC 61158 standard. Because FOUNDATION fieldbus is a purely digital protocol, that means you can do a lot of the good and useful things you can do with a digital network, such as time synchronization, time stamping, data management, and more. It seems natural that the world of device networks should evolve into the digital age. The rest of the world went digital a long time ago; from the telephone networks to your TV, it is universally acknowledged that digital networks give you better access to data, better data manageability, and more bandwidth. Yes, digital networks need to be managed somewhat differently from conventional analog technology, but in most cases, the initial investment in training is minimal. We will talk about that later.

Being digital alone, however, is not enough to fulfill the requirements of process automation end users. FOUNDATION fieldbus is also a peer-to-peer protocol. That means that digital field devices can communicate with each other without the requirement of a host command to initiate communications. This peer-to-peer communication is important because it allows devices that are experiencing a problem to broadcast an alarm or alert that the end user sees immediately. It is also important for supporting the function of embedded control.

User layer, block structure provide foundation for embedded automation

FOUNDATION fieldbus is a lot more than just a network. FOUNDATION fieldbus conforms to the well-known ISA seven-layer stack model, including the Application layer, Data Link layer, and so on. FOUNDATION fieldbus, however, has an additional user layer that was requested by the end users during the development efforts of the original ISA50 fieldbus committee. The users specifically requested that this user layer be added because it gives them the capability to use a block structure in conjunction with the network. Function blocks are perhaps the earliest known instances of object-based programming, where multiple bunches of code are all bundled into one "block" to accomplish a specific function. This object-oriented block structure allows FOUNDATION fieldbus to truly embed PID control in devices and actuators and provide true control in the field. Aside from the Proportional/Integral/Derivative (PID) function block, FOUNDATION fieldbus supports Analog Input (AI) and Analog Output (AO) blocks; Digital Input (DI) and Digital Output (DO) for things like on/off valves. Resource Blocks describe characteristics of fieldbus devices. Transducer blocks are used to configure devices and read a sensor's value and command an output value.

FOUNDATION fieldbus ensures deterministic transfer of real-time data between devices. At the same time, it allows for queued and event-based communications. Multiple function blocks can execute in parallel in different devices simultaneously. Signals are transmitted with associated measures of data quality. Is the signal of good, bad, or uncertain quality? That could influence the behavior of a control loop.

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Control in the field is embedded automation

FOUNDATION fieldbus can address a wide range of control requirements. FOUNDATION fieldbus specified 10 function blocks for basic control. These include:

  • AI, AO, DI, DO
  • PID, PD
  • BG (Bias & Gain)
  • RA (Ratio Ratio)
  • ML (Manual Loader)
  • CS (Control Selector)

FOUNDATION fieldbus also offers 11 function blocks for advanced control. These include:

  • AA: Analog Alarm
  • TMR: Timer
  • AR: Arithmetic
  • IS: Input Selector
  • SPG: Setpoint Ramp Generator
  • INT: Integrator
  • DT: Dead Time
  • LL: Lead/Lag
  • SC: Signal Characterizer
  • OS: Output Splitter
  • DC: Device Control

Blocks are incorporated into fieldbus devices to achieve the desired device functionality, as well as to define a wide range of features and behaviors that must work in a standard way for devices to interoperate. For example, a simple temperature transmitter may contain an AI function block. A control valve might contain a PID function block, as well as the expected AO block. Thus, a complete control loop can be built using only a simple transmitter and a control valve.

With our new FOUNDATION for Remote Operations Management specification, FOUNDATION fieldbus can also accommodate large point count remote I/O, wireless field devices on ISA 100.11a, WirelessHART networks, or wired HART devices. A Flexible Function Block (FFB) is a user-defined block. The FFB allows a manufacturer or user to define block parameters and algorithms to suit an application that interoperates with standard function blocks and host systems.

Link active scheduler ensures determinism, reliability

Control in the field, however, also means that steps must be taken to manage network traffic among fieldbus devices to ensure control is executed in a deterministic fashion. That is why FOUNDATION fieldbus also has a Link Active Scheduler (LAS) that serves as the "traffic cop" for a fieldbus segment. The LAS is an arbitrator that decides which devices will have access to the fieldbus network and when. Only one Link Master device on a FOUNDATION fieldbus H1 Fieldbus Link can be functioning as that link's LAS.

Communication classes increase reliability

There are several different types of communication classes associated with FOUNDATION Fieldbus. Primary distinctions are made between scheduled and unscheduled transfers of data. Scheduled transfers are completely deterministic, while unscheduled data transfers, such as those associated with device health, status, and alarms, are a separate class. Unscheduled data transfers happen only when there is enough available bandwidth after the scheduled transfers occur. It is good practice to ensure your networks are not overburdened with scheduled transfers to ensure some level of determinism for unscheduled transfers, but this is not likely to be a great issue in most applications. It is good practice not to use more than 50% of the scheduled scan time (the macrocycle) for scheduled transfers. Many suppliers recommend considering periodically requested unscheduled communications along with scheduled communications.

Many paths to redundancy

There are ways in which redundancy, reconfiguration, and fail-safe strategies can be built in to respond to certain failures in the system. Since function blocks can be executed in different devices, embedded control can be configured to take advantage of parallel computations or to minimize data traffic depending on a given application need.

Increased reliability, single-loop integrity

One class of control loops where the greater performance benefits of Control in the Field (CIF) could be considerable is where controllers are responding to one-off spurious events to maintain operations and avoid the activation of the safety system. A good example of this is the pressure control of fuel gas supply for a pair of gas turbines in a power generating plant. When the gas turbines are operating normally, the pressure control is not too demanding.

However, if one of the gas turbines trips, the fuel gas supply pressure will rise quickly, and the controller has to respond rapidly in order to reduce the pressure and prevent the remaining gas turbine from tripping on a high-fuel gas pressure limit. The improved speed of response provided by CIF may offer substantial benefits in such control loops.

From physical marshalling to virtual marshaling

There has been a lot of talk among the supplier and end-user community about reducing the hardware requirements for new projects through various alternatives to traditional I/O and "marshalling" technology. With FOUNDATION fieldbus, however, the goal was to eliminate a lot of the I/O and related hardware altogether. Why make physical marshalling smarter or better when you can eliminate it entirely? Many of the functions provided by hardware in a conventional analog process automation system are no longer required by a FOUNDATION fieldbus system or are handled through software instead.

Having truly embedded automation means your solution should be configurable. You should not have to rely too much on external hardware to determine the configuration of your devices and control schemes. You could say fieldbus provides a "virtual marshalling" alternative, because FOUNDATION fieldbus devices provide multiple signals over the same two terminals. For instance, a control valve has three signals: one setpoint plus open and closed feedback. All the signal linking (block to block) is done in software, not using hardwiring Fieldbus I/O, because most of the relationships are defined through software.

Improved reliability, availability

Control in the field is a key element in providing significantly enhanced process integrity for many applications and control loops. There is evidence to support that control in the field has an 80% increase in meantime between failures compared to traditional DCS control. The increased MTBF combined with the reduction in data transfers required substantially increases reliability and availability. The overall reduction in network traffic also increases network availability, in spite of the trade-off in the increased amount of device condition, status, and other data that must be passed to the DCS. There is also much more flexibility with control in the field. Once deployed, the function block execution can be transferred from one device or actuator to another.

Key performance benefits provided by control in the field

  • Improved control-loop performance
  • Increased reliability and availability
  • Improved loop integrity
  • Reduced loading on DCS/PLC and  network
  • Lower capital and installation costs
  • Reduced operating costs
 

You can use existing wiring

One of the objections we continuously hear from users considering fieldbus for a retrofit project is the need for rewiring. Users should be aware that there are tools available that can test existing wiring to make sure it is compatible with FOUNDATION fieldbus fairly quickly and easily.

Interoperability, testing, registration

The Fieldbus Foundation is one of the only automation industry organizations with a registration program requiring mandatory testing of critical elements of its technology. Today, our testing and registration effort encompasses FOUNDATION Fieldbus host systems and field devices, as well as physical-layer components, such as power supplies, cables, and device couplers. The Foundation does all its own testing and registration; we do not outsource to third parties.

One of the founding principles of the Fieldbus Foundation is the support of interoperability-the ability to operate multiple devices from multiple manufacturers, in the same system, without loss of functionality. The testing and registration process at the Foundation is the key to interoperability. With FOUNDATION Fieldbus, interoperability is made possible by the fact that devices and software must conform to the same standard.

Products bearing the FOUNDATION Product Registration symbol have undergone a series of common tests administered by the Fieldbus Foundation. End users can select the best device for a specific measurement or control task, regardless of the manufacturer, and they know that device will provide a consistent level of functionality and interoperability, regardless of the host system or other devices used. Testing and registration ensures you can achieve the best return on your fieldbus investment.

Different, not difficult

With proper training and the right partner, FOUNDATION technology actually reduces the risk in the engineering and operational phase of the project, which also increases the net present value of the capital investment. Implementing FOUNDATION technology does require different thinking and training. It is different, but not difficult. The accumulated knowledge governing FOUNDATION Fieldbus engineering, for example, has been combined into a single set of free downloadable guides from the Fieldbus Foundation.

The AG-181 System Engineering Guide can be downloaded from the End User Resources section at www.fieldbus.org. The guide is a comprehensive resource that contains best practices for the engineering, design, and installation of FOUNDATION Fieldbus systems. In addition to the System Engineering Guide, supplemental guides on wiring and installation, intrinsically safe systems, and a guide to function-block capabilities in hybrid and batch systems are available.

Many resources exist today for cost-effective training. With eight certified training centers around the world, users now have access to the best instructors for face-to-face, onsite, or remote training. To ensure success, training should start in the earliest phases of the project to ensure a smooth transition after project handover.

Conclusions

FOUNDATION fieldbus with embedded control allows for significantly reduced footprint and reduced marshalling requirements and can be an excellent fit for your process automation system modernization project. The User layer and block structure of FOUNDATION fieldbus enables the entire concept of control in the field, and the Foundation has done a lot of work to make sure embedded control is deterministic, reliable, and safe. In fact, there is a lot of evidence that shows embedded control has prevented numerous unplanned shutdowns and greatly increased reliability and uptime. Avoiding a single incident of unplanned downtime can pay for your entire automation investment.

ABOUT THE AUTHOR

Larry O'Brien joined the Fieldbus Foundation as Global Marketing Manager in April 2011.  Prior to his job at the Foundation, he was Research Director for Process Automation at ARC Advisory Group, where he began work in 1993. As an industry analyst and market researcher, O'Brien covered the topics of process fieldbus, distributed control systems, process safety, the automation services business, and intelligent field instruments. He has authored or co-authored numerous market forecast reports, strategic level advisory reports, and white papers for ARC and its clients, including all the major process automation suppliers.