Choosing between centralized and distributed control system designs


The power of the purse

There are upfront and long-term costs to consider with either a centralized or distributed design. From a control perspective, there are hardware costs of the controllers and the start-up costs of the control system. From an I/O perspective, there are hardware costs of the I/O platform and the installation costs to physically connect this hardware to the field devices. For both control network and fieldbus network installations, there may be additional third-party network certification costs.

Centralized control systems tend to have lower control hardware costs compared to distributed systems. This is simply due to fewer controllers in centralized systems. Centralized control system start-up costs are more complicated to determine. As noted in the previous example, installation phasing and operational expectations can impact these costs. It is import to determine upfront to what extent this might be the case, as it can have a big impact on the total lifecycle cost of your system.

From an I/O standpoint, centralized systems tend to achieve economy of scale on hardware components. To illustrate this, consider the I/O modules in a traditional hardwired I/O system. Assume a module controls 16 devices, and you have 200 devices to control. In a centralized system, you would need 13 I/O modules. If the same 200 points are distributed evenly across 10 process areas, you would need 20 I/O modules to meet the same requirement. The same principle applies in varying degrees to all the physical hardware in your I/O system. The more remote I/O areas in your system, the more I/O hardware you will tend to need for that system.

Counterbalancing this are labor and material installation costs. The more physically spread out the controlled equipment and sensors are from the I/O hardware, the higher these installation costs will be. Centralized I/O systems magnify these costs; distributed I/O systems mitigate them.

Your selection of I/O platform could have a large influence on the installation costs. Traditional hardwired I/O platforms require a separate wiring run between each field device and some I/O module. Network platforms may allow you to wire from device to device without the need for individualized runs back to the I/O system. You will tend to pay more for I/O hardware and cabling media, but your overall installation costs could be significantly lower.

On the fieldbus side, recent trends have been away from some of the open platforms, such as ControlNet, an open industrial network protocol for industrial automation applications. In that case, trending has been toward industrial Ethernet solutions. Alternatively, the AS-Interface is growing in acceptance and installations, and plays nicely as a partner network with higher level fieldbus platforms including those using industrial Ethernet.

Integration considerations

There are a few things to consider from a systems integration standpoint-physical and functional. The controllers in your system will often have to communicate with other process, visualization, and business systems. If your budget permits, a common broadband industrial network platform between these items will simplify configuration and maintenance over the life of your system, and allow for the most efficient communications between all systems.

The control, I/O, and components of your system will likely use some network features to a greater or lesser degree. Your network will only be as good as your cabling, connection, and switches. Industrial networks are often electrically noisy places. They are subject to electromagnetic interference, temperature ranges, dust, and humidity not found in office environments. Make sure you have the right cable for the job, with all of your network hardware properly installed and configured.

Older legacy systems usually contain low-speed and/or proprietary networks. If your control system is composed of these elements or must integrate with such a system, there is more to consider. Because of the slower speed of the network and communication protocols in this environment, special handling must be placed on how information is exchanged with other systems.

If there is process-critical real-time status that needs to be exchanged with other process systems, old-school hardwired signal exchange might still be appropriate under such circumstances. For other data, specialty network adapters can be found that convert between these proprietary platforms and more modern network protocols.

If there is a large amount of data to exchange with a host system, concentrating the data in a single location might be an option to consider. This will often yield the most efficient communication method under such circumstances. The trade-off is communication performance versus a single point failure. In a centralized system, it is relatively easy to concentrate the data. In a distributed system, you will need to select a controller to host and manually message the data from the other controllers in the system.

From our past, toward our future

The question of how much to centralize or distribute a control system has been around for a long time. It pertains to both the control and I/O aspects of the system. While certain hardware platforms may lean toward one direction or the other, it is a design concept independent of these platforms. Systems of any substance are rarely completely centralized or distributed. The best solution on this spectrum is driven by a variety of factors.

The normal operation of your equipment has a large influence on the degree to which you should centralize or distribute your system. You have to examine both the operational aspects and the physical layout of your equipment. How this equipment is installed over time, and how likely it is to change are also factors to consider.

Centralized systems tend to have lower hardware costs than distributed solutions, but often have higher installation costs. The opposite is true for distributed solutions. When putting it all together, remember to consider the system integration issues between your system and others.

This question will continue to be with us in the foreseeable future. As devices continue to get smarter, fieldbus and industrial network installations become more prevalent, and control platforms continue to evolve, the question of how to group or distribute these items remains. Understanding the core principles involved will help you develop the right design for your control system-now and in the future.

David McCarthy is president and CEO of TriCore Inc., a national systems integration firm based in Racine, Wis., with offices in Glendale, Calif., and Mesa, Ariz. Before he founded TriCore in 1991, McCarthy served in various capacities at Alfa Laval/Tetra Pak, including manager of engineering for its U.S.-based food engineering company. McCarthy, who has more than 30 years of experience in automation, is a computer scientist from Rochester Institute of Technology.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering

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Ravi Prakash , AE, India, 07/30/14 12:29 AM:

It is indeed refreshing to get into titbits of centralised and distributed control system after a long time.
Whatever said by author is true and my 30 years of experience in automation do confirm the same .
The bottomline is no system is fully centralised or fully distributed.
Anonymous , 07/30/14 02:25 AM:

I/O subsystem like I/O cards etc. has to be protected against rain, dust, other contaminant, humidity, high temperature, low temperature, or corrosive atmosphere etc. Therefore traditional I/O cards are usually kept indoor in local equipment room or field auxiliary room / satellite rack room etc. Personally I see a trend to towards using digitally networked sensors and actuators instead of traditional hardwired 4-20 mA and on-off signals. That is, a trend to eliminate I/O cards by networking field instrumented instead: bus or wireless.

I personally agree that using fieldbus for field instruments removes the need for individualized wire runs back to the I/O system for each signal of each device. This dramatically reduces the overall installation cost.

Keep in mind that 1 device is not 1 signal. Therefore a 16 channel I/O card cannot handle 16 devices. Simple devices like pressure and temperature transmitters have only 1 signal, but Valve positioner (limit switch feedback) has 3 signals, Valve positioner (position transmitter feedback) has 2 signals, Electric actuators (motor operated valve, MOV) has up to 16 signals, On-Off valve has 3 signals, Coriolis mass-flowmeter has 3 signals, Magnetic flowmeter has up to 4 signals, Gas chromatograph has up to 25 signals, pH analyzer has 2 signals, Guided wave radar level transmitter has 2 signals, and Process gas analyzer has 5 signals. That is, on average devices have 3 signals each – more if you want to use their full capability. That is, 16 devices require more like 48 I/O, or 16 I/O is 5-6 devices. This is where running digital networking all the way from the very first meter from the sensors/transmitters and positioner/actuator/valve becomes so powerful because you can run all these device signals over the same two wires. For instance, a bus with 10 field instruments with an average 3 signals each takes the place of 30 signals: 30 pairs of wires. This dramatically reduces the amount of device wiring, reduces I/O cards, eliminates marshalling, and reduces cabinet footprint.

Footprint is particularly reduced if the fieldbus power is built into the interface card:

For remote-I/O cards in field cabinets may depending on the ambient conditions have issues with rain, dust, other contaminant, humidity, high temperature, low temperature, or corrosive atmosphere etc. Therefore remote-I/O is rare in outdoor plants.

I agree that only “registered” network cable that meet the noise shielding requirements should be used, and only “registered” couplers (junction box terminals) that are encapsulated able to handle the humidity and dust like you say. These are readily available.

Use standard protocols like FOUNDATION fieldbus (FF), not proprietary protocols found in some electric actuators / motor operated valves (MOV) and tank gauging systems etc. FF protocol is now available in all kinds of devices so there is no need to use proprietary protocols.

I personally agree that devices continue to get smarter and that fieldbus installations are becoming more prevalent. We are seeing more diagnostics in fieldbus devices including diagnostics in on-off valves and continuous diagnostics in control valves as well as more diagnostics in temperature transmitters. We are seeing 8 input temperature transmitters taking the place of 8 regular temperature transmitters as well as their 8 pairs of wires, 8 safety barriers, and 8 input card channels and so on. These things would not be possible with 4-20 mA. We see fieldbus devices with more computational capability allowing remote seals to be eliminated and so on. It is pretty amazing what can be achieved when you run digital communication all the way from the sensors and actuators at the very first meter.
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