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I haven't had any special courses or training in writing and some may say it shows. I wouldn't advocate anyone following my style, especially if you trying to promote your products or ideas. I tend to lead the reader on path of discovery by laying out the situation and the problems and then some interesting ideas. Maybe it is the user in me (33 years at Monsanto and Solutia), the scientist in me (Physics), or my approach to writing that wants to leave it up to the reader to make the judgments and assessments.

However, people today want to know upfront the bottom line. They may not have time or the background to come to useful conclusions. Plus my emphasis on detailing problems can be formatted with a more positive approach of presenting opportunities and solutions. Next month I will offer a short introductory overview of my recent articles that emphasizes the scenario, essence, and value of the ideas developed.

The main point of this blog like all of my writing is to share what I have learned. My goal for next year is to help prevent significant expertise and knowledge in process automation from being lost forever. I would guess 100 or more automation professionals are retiring each year who have published at best an infinitesimally small portion of their expertise for posterity. Also, new engineers are facing special challenges. My sense is the new kid in the control room doesn't have the mentors or the internal technical training programs I took for granted. They may be thrown into the midst of a difficult problem with no guidance.

Beginning this Fall I will be making presentations at Local ISA sections and interviewing young and seasoned automation professionals recommended by the sections to get a better idea of new and lost process control expertise. The first stop may be the ISA Boston section. I lived in Cambridge when I was overseeing a project at Badger. I am looking forward to returning to Harvard Square and Legal Seafood Market. I can dream of returning to Fenway Park. The interviews will be published in my monthly Control Talk column in Control magazine. I enjoyed doing the 3-part series in Control Talk on "The Secret Life of pH Electrodes" based on interviews at Broadley-James and Rosemount Analytical in Irvine California (nice place to visit).

In the mean time here is what I have learned about writing in no particular order except what pops into my brain (kind of the way I do my first draft).

(1) An outline is a good idea but it is just a starting point. I don't truly know where the article or book will take me at the beginning. The value I get out of writing is the discovery process. I find concepts and ideas along the way. I gain knowledge besides sharing knowledge. In my next book, I realized after about 5 pages into the first chapter that an important message is the dramatic change in the performance of modern measurements. The installed accuracy of key measurements has improved by one to two orders of magnitude compared to my days in E&I design and construction. A smart closed coupled coplanar DP with static pressure and temperature compensation has 0.02% installed accuracy. A radar level gauge can detect changes as small as 0.04 inches in level. A Coriolis liquid flow meter can have an installed accuracy of 0.05% with a rangeability of 200:1. The bench top and installed accuracy of pressure, level, and flow measurements in 1970s and 1980s was typically 0.5% and 2%, respectively. I have an article from that era that quantifies the deterioration from uncompensated process and ambient operating conditions. Then there was the noise introduced by wiring problems (see May 19 entry). Smart wireless instrumentation offer a whole new ball game if you don't screw it up with a bad installation. The limit to control loop performance is not the measurement but more than ever is the control strategy, the final element, and how well you tune the controller.

(2) The most difficult thing is writing the first sentence. The second most difficult thing is writing the second sentence. The third most difficult thing is writing the first paragraph. The fourth most difficult thing is writing the first page. The lesson here is to just get started. Writing is an iterative process. My problem is that I get bored rehashing ideas I have unleashed and want to move on to the next page, article, column, or book. I don't iterate enough. Here is where a person who knows the subject can help by reading and commenting on your drafts. Beware of technical writers or copy editors who want to rewrite the whole thing because it often results in a loss or change in meaning and intent.

(3) Once you get going, don't stop. A flow is important. After about 10 pages, the thoughts start to flow fast and free. At this point, music (particularly the best of Concrete Blonde, The Goo Goo Dolls, Josh Groban, Don Henley, Meat Loaf, Matchbox 20, Bruce Springsteen, and U2) adds inspiration and makes writing more fun for me. I think this was the only way I was able to write a dozen serious technical books, a half dozen funny technical books, fifty articles and papers, and 8 years of Control Talk column. It also helps to have an understanding management and spouse. Can you envision getting approval of humorous books, columns, and "top ten lists" through the official channels of a big corporation? Can you imagine your spouse letting you spend 8 hours writing on a weekend? Lastly, I do the diagram and figures last. This is mind numbing work for me that would sap my creative energy and interrupt the flow.

(4) Use a lot of sub headings and bullet lists. This gets the reader interested and helps to cherry pick what is of greatest importance. My article "Maximizing PAT Benefits from Bioprocess Modeling and Control" is a good example of missing lists and sub headings. In my defense, the article was a last minute deal. I had just a couple of days and was just doing a core dump of what I thought was important.

(5) Go back and improve the introductory paragraph to each section and the introductory sentence to each paragraph. This is a good idea. I am going to try it when I have time.

(6) Develop the solution, include the assumptions, and detail the verification. I tend to do the first two parts of this suggestion and provide evidence of the last part. However, it would help the readers to make suggestions on how to prove out the idea for their application. No solution is universally true. There are always exceptions. I have benefited from the best minds in process control but I have noticed that the bigger the mind the bigger the ego. A blind spot is developed that makes experts unwilling to acknowledge when their solutions don't work in a particular application. Maybe it is my science background or an ego deficiency but I am always looking for exceptions and I never think any of my ideas as perfect. You learn more from things that don't work.

(7) Don't make statements that are to be accepted as fact. I am particularly sensitive to statements made to be accepted as true that are mostly untrue. For example, "Thermocouples (TCs) are faster than RTDs." This statement is generally accepted as true but is it useful and is it in fact misleading? What if someone chooses a TC instead of a more accurate RTD because he or she thinks the TC is faster? If you had a bare element there might be a difference of a couple of seconds in the response but the uncertainty in the time lags of most temperature systems are an order of magnitude larger. Once you put the element in a thermowell, the construction and fit and length of the thermowell determines a time lag for the assembly that is an order of magnitude larger as well (no pun intended). Also most controllers are tuned so slowly, you don't see the effect of small changes in time lags. The bottom line is that you will probably never see the difference in speed between a TC and an RTD. Having said that I can envision exceptions, such as the temperature control of inline mixing of streams with an aggressively tuned controller. I have just never seen this application in practice.

(8) Use short clear sentences and paragraphs that build on each other. Providing qualifications can result in long sentences. Also, one thought for me quickly leads to another and to another and to another, which leads to run-on sentences. I need to constantly go back and split my thoughts into separate sentences with a building block approach. I am currently trying in my first draft to break thoughts up. So far it doesn't seem to interrupt the flow, which was my original concern.

(9) Don't get hung up on perfect grammar or a perfect piece. If you don't give copy editors something to do, they will start some serious messing with the sentences. Everyone wants to feel like they are contributing and doing their job. Also, what copy editors are looking for in terms of commas and hyphens may change. Finally, obsession with sentence structure can lock up your mind. I talked to a copy editor who said he needs to find another job so he can write a book. As a copy editor, he thinks too much about the technical details of writing.

(10) Think of writing as if you were having a conversation with a good friend. This makes the whole writing process less intimidating and lets you be more frank and less formal.

(11) Use plenty of examples and illustrations. It takes time but sure drives the point home. I have seen a lot of misinterpretations of an idea or its intent. The fault is really my own and not the reader. For example, in my article "Is Wireless Process Control Ready for Prime Time" a reader thought there was a concern being expressed on noise from variable speed drives on wireless transmitters when what I meant was the opposite. Wireless transmitters should eliminate these and other noise problems associated with wiring. I no longer assume any concept is obvious.

(12) Use free association and both sides of your brain. This allows you to take creative leaps you probably didn't even realize before you started writing. In my case it also enables me to add humor such as "Top Ten Lists" and the cartoon descriptions for my illustrious friend Ted Williams.

I write wherever I get the inspiration. Headphones are a given. Right now I am listening to Crosby, Stills and Nash's "Déjà Vu", which seems especially appropriate since this is a flashback to my career in instrumentation. When I am in a groove, I enjoy the detail of every note and lyric and the thoughts flow to the page as fast as the music.

I seem to get better insights when I can get away to places particularly beautiful and relaxing. For the November 1998 issue of Chemical Engineering I wrote the feature article "Trouble free Instrumentation" from Pensacola Beach while enjoying the sugar white sand, emerald waters, and tropical drinks as shown on the attached cover.

From the Beach

The article is a summary of everything I learned installing, checking out, and starting up automation systems while in instrument construction and tackling dynamically challenged process control loops for Engineering Technology during my career at Monsanto and its spin off Solutia. The article is a core dump of 11 pages on the selection, installation, maintenance, and troubleshooting of instrumentation and control valves. The article lists major causes of measurement and valve errors and failure, best practices, dos and don'ts of maintenance, diagnostic and knowledge based monitoring techniques, and rules of thumb.

Stay tuned for an electronic copy. If you want to hear about the highlights and lowlights of my career that lead to this article, checkout my "Control Talk" column starting in the August issue of Control magazine titled "One Man's Story."

http://www.controlglobal.com/articles/2007/234.html

The control design and commissioning of a continuous process often focuses on the requirements associated with operating conditions found at normal plant throughput. Thus, during the startup of the process, many controls are left in manual and it is up to the operator to get the process to the point where the controls can be placed in automatic. From a practical standpoint, it makes sense that the design address normal operating conditions since this is where the process will operate most of the time. Thus, the problem of non-linear installed characteristics of final control elements, limited rangeability of measurement elements, etc are often ignored. The startup of a batch process has many of the same challenges associated with startup of a continuous process. However, since the startup phase of a batch process often represents a significant portion of the total batch cycle time, it is important to automate this phase of the operation. From this perspective, the design and commissioning of batch process controls can often be much more demanding than for a continuous process. Addressing the challenges associated with the wide range of operation during the startup phase of a batch process can pay benefits in terms of reduced cycle time. For processes that are not limited by market demand then any reduction in cycle time directly translates to increase production and associated profit. In some cases, manufactures have added features to the PID that may be used to address some of the requirements associated with the startup of a batch process.

It is common for one or two loops associated with a batch operation to exhibit a slow response. For example, the multiple lags associated with a critical temperature measurement may introduce the equivalent of a signification delay in the process response. Thus, this slow response may require that a long reset time be used in the temperature control. This slow tuning may be appropriate once the unit has reached the design processing conditions. However, during the batch startup this tuning may limit how quickly the unit may be brought up to design processing conditions. The time required for the startup phase of a batch may often be reduced using batch logic at the start of the batch to switch the control to manual and then to position the temperature controller output to a value required for normal operations. Once the impact of the change in the heating valve starts to be reflected in the batch temperature, then batch logic can be used to switch the control to Automatic when the temperature reaches a certain point. However, such a procedure is often complicated by the fact that a different valve position and point to switch the control to automatic may depend on the initial process conditions e.g. feed temperature or the product grade.

To assist a batch process in reaching normal operating condition in minimum time without the need for special batch logic, the DeltaV PID has two added parameters which were named anti-reset windup high and low limit, ARW_HI_LIM and ARW_LO_LIM. The PID is designed to address reset windup independent of these limits. Thus, the real purpose of these ARW parameters is to allow a batch process to quickly be brought on-line. For batch applications, these limits may be configured inside the controller output limits. When this is done, the controller reset time is reduced by a factor of 16X if the controller output is outside the ARW limit and the sign of the control action is driving the output toward the ARW limit. Thus, the control may be switch to Automatic mode at the start of a batch and the control output changes due to reset action will occur 16 times faster until the output reached the ARW limit and then the control will revert to normal reset action. This control action is illustrated in the following.

Response Using ARW limits

For batch applications that require this type of action during startup, the ARW limits should be set to values that are outside the range that will be required during normal batch operations. The ARW limit value may be adjusted to reach setpoint in minimum time without overshoot.

In some continuous applications, the process may be operating at saturation i.e. full open or closed valve under certain operating conditions. When the operating conditions abruptly change, then it may be necessary for the control to quickly position the valve of a slow action loop to avoid the associated control parameter from overshooting. In such applications, the ARW limits may be set to improve recovery from these process saturation conditions and get to setpoint without the use of special logic.

With the introduction of digital field devices, manufactures of control systems and maintenance tools were faced with the challenge of how to access and display information in devices that were produced by different companies. Over the years, a number of approaches have been developed. However, the Electronic Device Description Language (EDDL) is the dominant technology used in the process industry to support interfacing to digital devices. There are more than 15 million installed devices that support access to diagnostic and calibration information through the use of EDDL. A device manufacture may use EDDL to fully describe the data that is accessible in a field device. Also, this language allows the manufacturer to define the user interface and operating procedures needed for calibration and diagnostics. Quite complex interfaces and interactions are fully supported since EDDL addresses such things as commands, menus and display formats. The latest version of EDDL fully supports the use of menus, windows, tabs and groups and graphic support for graphs, trends, charts and dial indicators. Device description files that are created using EDDL are known as Electronic Device Descriptions, EDD. An engineering station or handheld that is EDDL enabled is designed to use EDD files to support diagnostics and calibration of devices. New EDD's for device updates or new devices introduced by a manufacture may be added to an EDDL enabled control system or maintenance tools without worrying about software viruses, revision levels, etc. This is because the EDDL file is simply interpreted by these systems and there is no requirement to load software components such as dll's into these tools. This is the major advantage that EDDL has over competing technologies such as FDT/DTM that require executable software components to be incorporated into engineering systems and handheld device.

The EDDL capability that we have today is the results of a cooperation effort by Fieldbus Foundation, HART Communication Foundation, PROFIBUS Nutzerorganisation e.V., and the OPC Foundation. These organizations fully support the use of EDDL for device description. The latest version of EDD's for any device certified by these organizations can be downloaded simply by going to their web site. For more information on the support that is provided for EDD's, you can visit the Fieldbus Foundation, HART Communication Foundation, or Profibus International web sites. The Electronic Device Description Language is a recognized international standard, IEC61804. Even though IEC6180 is an international standard and is supported by most manufacturers, many end users are unaware of this technology or how it compares to competing technologies such as FDT/DTM. Thus, in late 2005 I submitted a proposal to ISA to adopt the IEC61804 standard as an ISA/ANSI standard. As explained in this proposal, the primary reason for establishing EDDL as an ISA/ANSI standard is to help raise awareness in the process industry of the important role that EDDL plays in the process industry today and to convey the advantages this technology has over competing technologies such as FDT/DTM.

In response to my proposal, ISA announced in early 2006 the formation of SP104. Since I submitted the proposal to create this committee and was the US expert on the IEC SC65E WG7 committee that wrote the IEC61804 standard, I was asked to be the committee chairman. The editor of the IEC61804 standard, Ludwig Winkel, Siemens, is the vice-char of SP104. There has been a great response within industry to the formation of the SP104 committee. The committee currently consists of 10 members from the US, China, Singapore, Germany and France. Each member brings a variety of experiences and knowledge of the process industry.

Since our first meeting in October, 2006, the SP104 committee has made significant progress. In our first meeting we agreed to adopt the IEC61804 standard and to distribute this document for vote. This document is currently in the stage of public review. In addition, key team lead positions within the committee have been filled:

 Marketing - Ed Ladd, HART Communication Foundation
 Education - Jonas Berge, Emerson
 Certification - Christian Diedrich, University of Magdeburg
 Liaison to IEC, ISO and Consortia groups - Ludwig Winkel, Siemens

Over the last few months, the SP104 committee has been working with ISA on the design of a web site. This web site will be dedicated to information and educational material on EDDL technology. Through this initiative, the SP104 committee will introduce a variety of new material on EDDL that may be easily accessed by anyone from industry. This site should be on-line by early spring. Also, the committee plans to sponsor sessions and workshops on EDDL at some of the major trade conferences scheduled for later this year. Thus, you should be hearing more about EDDL over the next few months.

One of the foundation pieces of measurement and control as utilized by the process industry is the concept of mode. The mode of a measurement inputs to a control system may be used to indicate if the associated device is in or out-of-service. For a control or output function in a control system, the plant operator typically uses mode to select the source of the setpoint or output. In some cases, mode may also be used to indicate if a calculation function is in or out-of-service. Thus, the IEC61804 international standard, Function Blocks for Process Control, specifies that all measurement, control and output function blocks must contain a mode parameter.

Mode has traditionally been defined in different ways by manufacturers of control systems and field devices. One of the things that the ISA SP50 User Layer Committee realized was that a consistent definition of mode is required to achieve control system interoperability with field devices. Therefore, the technical report produced by this committee defined the mode parameter. The mode parameter structure proposed by the SP50 committee was adopted with minor changes by the Fieldbus Foundation's function block team. As an integral part of the interoperability test performed by the Fieldbus Foundation, the mode parameter implementation is verified to be consistent with this Function Block specification.

The mode parameter support by Foundation fieldbus function blocks consists of four attributes rather than a single target attribute found in some traditional control systems.

 Target mode attribute
 Actual mode attribute
 Permitted mode attribute
 Normal mode attribute

The plant operator uses the target mode attribute to select the desired mode of operation. The target mode selections defined by the specification are;Out-of-Service (O/S), Automatic (Auto), Manual (Man), Cascade (Cas), Remote Cascade (Rcas), and Remote Output (Rout). In the past, different terms have been used by manufacturers for some of the target mode enumerations. For example, Cascade mode is the equivalent to Remote Setpoint (RSP) in some traditional systems. Remote Setpoint and Remote Output are referred to as Supervisory and DDC mode respectively.

Based on the status of inputs to a function block and other conditions that impact block operation, it may not be possible for the block to operate in the requested mode. For example, if the output track input to a control block is active, then the block will not continue to operate as request e.g. Automatic mode. The actual mode attribute is used to reflect the mode of operation that can be achieved. Thus, the actual mode attribute is calculation by the block each execution. Two actual modes are defined that may not be selected as the target mode.

 Local Override (LO) mode - the block track input is active.
 Initialization Manual (IMAN) mode - the downstream path to the process is broken.

Since a control application may only require a few of the target modes supported by a device, the user may configure what operation modes are appropriate for his application through the permitted mode attribute. When this is done, the function block limits the target modes to those that are permitted. Similarly, the mode the operator should choose during normal plant operation is configured in the block using the Normal mode attribute. Even though this parameter is not utilized by the function block, it may be useful to other applications, such as an operator station to flag loops that are not running in the normal mode of operation.

One the challenges that the Fieldbus Foundation function block specification team addressed was how to define target mode to support both single knob (Man, Auto, Cas, RCas, Rout) vs. dual knob interfaces (Auto/Man + Cas/Rcas/Rout). By defining the Target mode attribute (bitstring) to use multiple bits for each target mode selection (including bits to indicate previous mode) it is possible to support both type of interfaces. Because of this capability, it is easier for legacy systems that use a dual knob interface to support the installation of fieldbus devices. Some modern control systems have adapted the Fieldbus Foundation's definition of mode. In these systems, the mode parameter is used in a consistent manner independent of whether the associated function block resides in a field device or in the controller.

As noted in my December 4th posting, the Hart Communication Foundation has adopted the IEEE 802.15.4 physical layer for wireless HART. One of the technical challenges is that the 2.4 GHz spectrum defined by IEEE 802.15.4 is also used by Wi-Fi and Bluetooth devices. Also, some electrical devices found in industry generate noise in this frequency band. Thus, at times it is expected that a transmission will be corrupted. To help minimize the impact of these other devices on communications, the Time Synchronized Mesh Protocol (TSMP) selected for wireless HART uses frequency hopping. Even so, at times it is expected that multiple transmissions of a measurement used in control or multiple communications of control actions to an actuator may be lost. Thus, a few years ago we started looking at the control requirements under these conditions. In particular we examined the behavior during communication loss and after communications are re-established.

When the control measurement is lost, a standard PID may be expected to continue executing and thus could windup because of reset action. This condition might be addressed by changing the actual mode of the PID to manual on detection of a measurement loss. However, with either approach, the reset action taken by the PID under this condition will be disruptive to the control. If derivative action is utilized in the PID, then the abrupt transition in the measured value on recover of transmission may cause a spike in output since the derivate contribution is normally calculated based on the period of execution. However, by modifying the reset and derivative calculation to account for the time since the last measurement update, then it is possible to minimize the impact of loosing multiple measurement transmissions.

The loss of multiple transmissions from the PID to an actuator may also disrupt loop operation. A standard PID under these conditions would continue to takes control action even though these actions have not reached the actuator. Thus, under these conditions, the reset action would wind up and when communications are re-establish you would expect to see a significant bump in the process. However, by using feedback from the actuator in the reset calculation , as defined by the Fieldbus Foundation, then windup under this condition may be avoided.

Details on the PID modifications to account for loss of the control measurement or the path to the actuator are described in detail in a paper that we presented at ISA2006, "Improving PID Control with Unreliable Communications". An overview of this work is provided in the following presentation:

PID for Unreliable Communications

In this presentation, the performance of a standard PID is compared to a modified PID. The modified PID uses actuator feedback and the time since last good communication in the reset and rate calculations. The modified PID provides a significant control improvement over the standard PID for the conditions that were considered in these tests.

Over the last few years the process industry has expressed a growing interest in the application of wireless technology for field measurements. The ISA-SP100 Committee was established in early 2005 to set standards and recommended practices for implementing wireless systems in the automation and control environment with a focus on the field level. Also, various industry consortiums have been established to promote the use of wireless technology. For example, the Hart Communication Foundation has adopted the use of IEEE 802.15.4 physical layer for the implementation of wireless HART. At the ISA2006 conference the HART Communication Foundation sponsored a booth in which wireless transmitters from multiple vendors were demonstrated. However, one of the technical challenges that manufacturers face in applying wireless technology to process measurements is how to reduce the power consumption to a level that can be supported for many years without the need for external power.

If the information from a wireless transmitter is only used to monitor slowly changing measurement values e.g. levels in a tank farm then the transmitter power requirements may be minimized by simply slowing down how often a measurement is made and communicated. However, if the measurement is used in control applications that respond in seconds rather than minutes, then simply slowing down how often a measurement is made and communicated will negatively impact control response. To provide best control, it is necessary to reduce the latency in control response to setpoint or load disturbances. In a traditional control system it is possible to minimize latency by over-sampling the control measurement used in control. However, such an approach is not an option if your objective is to minimize wireless transmitter power consumption.

One means of reducing the need for over-sample control measurements is to synchronize the measurement sample with control execution as is done in Foundation Fieldbus device. Using some of the proposed wireless protocols, such as Time Synchronized Mesh Protocol (TSMP), it is possible to synchronize a measurement sample and its associated communication with control execution done in another node. However, the traditional approach of executing control 4-10X faster than the process time constant still will create communication loads that are a barrier in applying wireless devices in faster process applications.

A few years ago we started looking at techniques that could be used to reduce wireless communication load without sacrificing control performance. It turns out that for many applications a 10X reduction in communications load can be achieved by following simple rules in communication and by restructuring the PID control to use non-periodic sample values. Much of this work is documented in a paper that we presented at ISA2005, Similarity-Based Traffic Reduction to Increase Battery Life in Wireless Process Control Network. An overview of this work is provided in the following:

Control Using Wireless Transmitters

If you would like to learn more about the wireless technology, then a good starting point is Protocols and Architectures for Wireless Sensor Networks (Hardcover) by Holzer Karl and Andreas Willig.

My first exposure to model predictive control, MPC, was in late 1979 when I attended a meeting called by Bob Otto, ISA Fellow. Bob had just returned from the AIChE 86th Annual National Meeting where he sat in on Charlie Cutler and Ramaker's presentation of their paper Dynamic matrix control-a computer control algorithm. This landmark work by Shell was the fore runner of modern day model predictive control, MPC. Bob's assessment was that this technology represented one of the most important developments he had seen in process control. The power of MPC technology comes from the fact that the controller is generated based on a process step response or impulse response model and is designed to minimize the control error over a prediction horizon. Control performance is determined by parameters that specify penalty on error and penalty on move. Soon after Shell's public announcement of their work on dynamic matrix control, Charlie went on to form the DMC Corporation. Since that time, major suppliers of MPC technology have successful addressed a variety of applications. The wide spread acceptance of MPC technology is well documented in the paper by Professors Joe Qin and Tom Badgwell, A survey of industrial model predictive control technology.

In the early-80's, Bob Otto lead an initiative within Emerson to explore the feasibility of embedding MPC technology within a distributed control system. This research focused primarily on single loop applications as documented in the paper Development of a Multivariable forward modeling controller by Bob Otto and Kelvin Erickson. Field trails were conducted using a prototype of single loop MPC. One of the technical challenges that prevented general deployment of this technology at that time was the need to provide a robust means of process identification. Also, it was not feasible at that time to embed general MPC in the controller because of the associated CPU and memory requirements.

By the later-90's, the availability of low cost memory and vastly improved processor performance made it feasible to fully embed MPC technology within the control system. By embedding MPC in the control system, a control system supplier can provide an environment that makes it easier and quicker to engineer and commission MPC applications. Also, by embedding MPC in the controller, it is possible to address applications that require faster control execution e.g. 1sec period of execution. In many cases, embedded MPC control is a valid alternative to the traditional PID based strategies for deadtime compensation, feedforward and override control. If you have no experience with MPC, then some examples of how MPC may be effectively used to replace traditional PID based strategies are contained in the following:

MPC for smaller applications

These examples are based on the DeltaV MPC capability introduced in 2000, DeltaV Predict. This initial capability was targeted at smaller applications (no larger in size than 8x8). The DeltaV advanced control team later developed DeltaV PredictPro to address larger applications (as large as 40x80 in size).

Since we tend to learn by examples and we can all relate to the challenges of parenting, I came up with the following discussion of the relative advantages of PID and model predictive control (MPC) of a teenager for my Control Talk column in the September issue of Control Magazine.

Would Proportional-Integral-Derivative (PID) or Model Predict Control (MPC) be better as a parental advisory control algorithm? The PID excels at handling the unknown upsets, non-stationary behavior, and unpredictable nature of a teenager's response and derivative action provides some preemptive action. However, the abrupt action by PID could amplify noise in a teenager's behavior. But then there is the dead band from backlash and the resolution limit from friction. Maybe a lot of dither from the PID could keep the teenager off balance and minimize inaction if it doesn't wear you out. There is inverse response to contend with where the teenager starts out doing the opposite of what was requested. Theoretically, MPC could build this behavior into its knowledge of the teenager's future trajectory. Also, the patience of a MPC is inline with modern parenting ideas. But I not sure your can do enough pseudo random binary sequences (PRBS) or know the time to steady state of a teenager so my initial thought is proportional plus derivative (PD) with an adjustable bias to deal with the potentially unstable response. But then what do I really know anymore? I am an empty nester heading out for one of many vacations without children. All I need to know now is how to enjoy the antics of grandchildren and quickly return them to their rightful owner for process control.

When designing a control strategy you may be faced with the challenge of there being an extra degrees of freedom. One of the most common examples is where one control parameter may be maintained at setpoint through the adjustment of two manipulated parameters. Often the solution is to address the control design using split range or valve position control. Through the use of these techniques, the two actuators appear as one actuator to the PID control. However, there are some significant differences in the resolution and dynamic response that may be achieved using either technique. An alternate approach is to implement a strategy that combines the best of split range and valve position control.

I once was responsible for the design of the 400 # header pressure controls for a new power house in a pulp and paper mill. Under normal operating conditions, the header pressure was to be maintained by the turbo-generator extraction to the 400# header. However, if the turbine was to trip or be taken off-line for maintenance, then two pressure reducing valves (normally closed) were to be used to let down steam from the 1475# header to the 400# header. Under a trip condition, it was important that the full dynamic range of the pressure reducing valves be used to make up for the steam that had been supplied by the turbine extraction. This objective could be achieved through the use of split range control. However, if the turbo-generator was to be off-line for an extended period of time for maintenance, then it would be advantageous to provide the precise pressure control that may be achieved by taking advantage of the operating characteristics of valve position control. After some work, I came up with a network that combined the best of split range and valve position control. I commissioned and tested the header controls as the power house was brought on-line. The 400# header control proved to be quite effective and after over 20 years is still in use at the plant- migrated to new controllers.

The work I did on the revised header pressure control strategy was documented in a paper that I wrote and presented shortly after the power house startup, "Improving PRV Pressure Control", ISA 31st Annual Southeaster Conference, April, 1985. The technique was later used within Emerson's pulp and paper group to address a variety of applications e.g. furnace draft control variable speed ID fan in combination with damper, variable speed pump in combination with a regulating valve for recovery boiler liquor flow control, forced-draft fan control pressure control using a variable speed fan with inlet vanes. Many of these applications were documented by Bill Love, Forney International, in an article "Innovative control technique that improves control rangeability and resolution in paper mill applications", Tappi Journal, February, 1994.

The tools that are available in most modern control systems are sufficient to implement the network that I originally designed for the header pressure control. The basic network design is shown in the following:

Combining Split Range and Valve Position Control

Also, this material includes an example of how the network may be implemented as a re-usable composite block in DeltaV. Some process examples are iprovided that allow you to compare the dynamic response of this network to that achieved using spilt range control and valve position control. If your control objectives can not befully met by valve position or split range control, then you may want to consider this network for your application.

Professor Tom Edgar of the University of Texas and Joseph Alford from Eli Lilly and Co. have been collecting data and opinions regarding the current syllabus of the typical undergraduate Chemical Engineering Process Control Course and its relevance to the skills and knowledge needed in today's industrial process control environment. An upcoming issue of Intech magazine will have an article by Edgar on how process control education in the universities should be changed. There will be replies from a key group of professors and practitioners. My contribution is the first 250 words of the following:

Terry Tolliver and I have taught a course on dynamic modeling and control at Washington University (WU) in Saint Louis since 2002 that is a requirement for a degree in chemical engineering. The course uses an industrial virtual plant and the ISA book Advanced Control Unleashed. The students are very computer literate and pick up on the use of industrial software from just a few screen prints put into the laboratory exercises. The knowledge gained is generally applicable since the function blocks are based on Foundation Fieldbus used in millions of devices and by over a hundred manufacturers. The configuration environment is also consistent with the international standard IEC 61804. The students learn how to intelligently discuss and use an industrial process simulation, DCS, and data historian that form a virtual plant on their desk. There is companion course taught by Bob Heider where an actual hardware version of the same DCS is used to control the temperature, pressure, and level of vessels in a hardware lab. The three professors have a total of more than 100 years experience in industry.

Most of the chapters in Advanced Control Unleashed start with an introductory section on "Practice," continues with sections on "Opportunity Assessment" and "Application" and concludes with "Theory". The strategy is to provide the relevance and practical considerations before getting into the theory that offers a deeper understanding. For example, in Chapter 2 - "Setting the Foundation", the student gets an overview and perspective, list of opportunities, examples, application detail, and rules of thumb before getting into the theory where the focus turns to the set up of the differential equations for the material and energy balances to enable the student to learn the source of process time constants and gains in terms of process parameters. The students are not asked to solve or integrate these equations. Instead, the students graphically create a dynamic simulation of processes for unit operations commonly encountered on the job. Blocks for filters, dead times, noise, periodic disturbances, and backlash and sticktion are added to make the challenge of process control more realistic. Additionally the students configure an actual control system that can be downloaded into a real DCS. The students apply industrial embedded tools for auto tuning, statistical analysis, and model predictive control (MPC). The course centers on time response because this is what they see on the trend charts in the control room but there is a session to show how to go from the time domain to the frequency domain.

When I recently went back to WU and gave a guest lecture on the use of PID and MPC for fed-batch control of a fermenter, a student asked "what is a batch?" I knew that students were taught to think in terms of a steady state and the material and energy balances on a Process Flow Diagram (PFD) for continuous operations but I didn't fully realize the implications until the question.

I have had chemical engineers in industry ask, why do you need a PID or MPC when you can just set the flow shown on the PFD? In fact, the batch sequences defined by process engineers today often try to set a predetermined step sequence of flows instead of using feedback control to sort it out. I have also have had experienced instrument engineers ask why do you need a Coriolis density measurement when the composition is constant as shown on the PFD? I also see ads for pressure and temperature compensated differential pressure orifice meters that claim to offer an unqualified mass flow measurement. If only the composition in all the pipelines were constant. This would sure make life easy. Product quality would be a non issue. Obviously the importance of dynamics and disturbances for process measurement and control is often missing in action.

In a batch process, the product concentration follows a profile. In some case there is also a temperature profile and in almost every case where a PID or MPC is used, the transfer of variability for a constant set point means there is a profile in the controller output. This understanding is lacking when chemical engineers are taught to think steady state. The lessons from batch would also be useful for the automated startups and grade transitions in continuous operations.

To add a bit of levity, I offer the following Top Ten List:

Top Ten Reasons Why an ISA Book on Control is not a University Text

10. Costs less than $100
9. The authors spent too much time in industry
8. Contains top ten lists and cartoons
7. Shows flow sensors upstream instead of downstream of the control valve
6. Discusses stick-slip and backlash
5. Shows unmeasured load upsets as inputs to the process
4. Includes field implementation considerations
3. Estimates tuning settings to just two significant digits
2. Doesn't use tensor analysis for flow loops
1. Depicts signal lines as electronic instead of the pneumatic