Modeling and Control

(10) Stop talking about process feedback. I promise to do this right after this blog.
(9) Listen intently to my wife’s instructions. Why does my mind still jump to weighty matters like what is next for dinner?
(8) Stop making cheap control valve jokes. Could the next final element reputation I hurt be my own?
(7) Help make smart diagnostics smarter. Do I need to de-fussify my fuzzy logic?
(6) Stop lusting in my heart for more computing power. Is it the PC or me that is the constraint?
(5) Turndown the volume on my headphones. What did you say?
(4) Stop drinking cheap wine. Does good wine ever come in a size large enough?
(3) Read a college text on control theory. Can I watch Star Trek without setting up the state space equations?
(2) Stop answering a question with a question. Why should a consultant do this?
(1) Spend more time with my wife than with Control magazine. Whatever happened to my January issue?

Last Chance to Learn About Process Feedback

Processes can have negative feed back, no feedback, or positive feedback. A negative feedback process is a self-regulating process and will decelerate to a new steady state. A process with no feedback is an integrating process and will continually ramp. A process with positive feedback is a runaway process and will accelerate until hitting a relief device or safety interlock setting. As in circuits, positive feedback is problematic. Control systems use negative feedback for regulation and provide a correction to the process in the opposite direction of the excursion from set point to compensate for load changes. The control system has to be progressively more aggressive for integrating and ultimately positive feedback processes. The following file shows the response to of these three types of processes to a change in controller output with the controller in manual (open loop responses of self-regulating, integrating, and runaway processes).

Types of Process Feedback

Over 90% of the processes are self-regulating. However, many of the continuous and fed-batch processes involving large back mixed volumes with the greatest direct economic benefits behave in the time frame and control region of interest like they have an integrating response and can be best treated as “near integrating” processes. The classic integrating process is a pure batch or level process. Less than 1% of the processes are runaway. When these exist, understanding the runaway response is critical in terms of safety and control because of the propensity to accelerate and reach a point of no return. Runaway responses are almost exclusively associated with highly exothermic batch reactors used in plastics and specialty chemical production. I was in a control room when a batch reactor reached the point of no return and was going to blow over to a flare stack tank. There was nothing the operators could do except call for the evacuation of the reactor area and make sure the flare stack was ready.

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.

I haven’t gotten any feedback on process feedback, which is really fortunate because it allows me to keep my new years resolution, so I will slog the blog along to the question that has been keeping people awake at night; “When is a batch a batch?”

Some say all processes like life are batches because there is beginning and an end.

Traditionally labeled “continuous processes” can significantly benefit in terms of process efficiency, time, and safety from the application of batch sequence technology for the automation of startups, grade transitions, and shutdowns. Conversely, batch processes can benefit from what has been classified as continuous control techniques.

There are some important advantages in terms of the application of PID and model predictive control by being more definitive in the distinction of batch and continuous operation. A process is best classified as “continuous” when there is both a feed flow and a discharge flow. Thus, the startup and shutdown of continuous processes, where the discharge and feed flows are zero, respectively, is better controlled if recognized as effectively a batch process. Also, fed-batch processes, which have a feed flow but no discharge flow, while termed by some as “semi-continuous” is better treated as batch.

Batch processes have an integrating (ramping) response as described in last week’s blog. Such responses have different tuning rules and settings. If there is a zero load (e.g. no heat loss for temperature or conversion for concentration), the response is one sided, which means the process variable can only go in one direction. These processes will overshoot if reset action is used. Also, model predictive control (MPC) doesn’t work because it assumes it can drive the controlled variable in both directions (up and down).

For more details implications for PID control of batch processes see Advanced Application Note 4, the April 2005 Control Talk column in Control magazine, and the article “Life is a Batch" in the June 2005 issue of Control magazine. For information on the need and method of translation of variables for model predictive control of batch processes, see chapter 4 in the book New Directions in Bioprocess Modeling and Control published by ISA in 2006.

When DeltaV Predict (embedded model predictive control, MPC) was introduced in 2001, I developed some of the MPC lecture material and process simulations that are used in our advanced control education class, 7201. Also, I helped teach the first three of four classes. I always enjoy teaching in these types of classes since of the customers who attend this training often represent a variety of industries. In one of one of the classes, a student showed a particularly strong interest in MPC and discussed several areas of his process that would benefit from this technology. It turned out that the student was the head of instrumentation at a Pharmaceutical plant in Puerto Rico.

A few months after this class, our service department received an order from this pharmaceutical customer to install MPC in one process area. Since DeltaV Predict was new to our service organization, the advanced control team provided some support in the implementation and startup of this MPC application. The savings from this one application were substantial (in the seven digits) since the area was a bottleneck in plant production. We later shot a video at the plant site. In this video the customer discusses the installation and the benefits they realized for this installation of MPC technology.

Making a control change, such as the installation of MPC, in a pharmaceutical installation was at that time complicated by the fact that the target process was an existing validated installation. Fortunately, our service department had done the original installation and thus was very familiar with the regulator submissions needed to make this change in the process control. However, the paper work required to make this improvement using proven embedded technology still took a consider amount of work and time. Because of this paperwork, many manufacturing procedures in the pharmaceutical industry are often treated as being unchangeable.

Some time after this MPC installation was completed, the Process Analytical Technology, PAT, initiative was established by the Federal Drug Administration (FDA) to encourage the voluntary development and implementation of innovative pharmaceutical development, manufacturing, and quality assurance. As part of this framework, an innovative approach has been developed for helping the pharmaceutical industry address regulatory issues and questions. This new approach is discussed in Guidance for Industry, PAT – A Framework for Innovative Pharmaceutical Development, Manufacturing and Quality Assurance. Through this initiative, the pharmaceutical industry has been given an opportunity to more easily innovate and improve operations through the application of proven measurement and control techniques.

Time is precious so here is your chance to learn in five minutes what took me five weeks of investigation. While most of these thoughts were banging around in my mind for last couple of decades, they might never have congealed if not for some triggering thoughts from my colleagues Terry Blevins and Willy Wojsznis and some knowledge discovery in my favorite laboratory, the virtual plant. All of this stuff has been discussed to some degree in last year’s blogs with more detail available in my Control magazine articles and Control Talk columns, the book New Developments in Bioprocess Modeling and Control, and Advanced Application Notes 1-4. The notes and presentations based on my ISA books as they become available are free for the downloading from the website: http://www.EasyDeltaV.com/ControlInsights/

(1) All the most popular tuning rules reduce to the same equation for the controller gain for maximum load rejection.

(2) While the ultimate performance of a loop is proportional to the dead time squared, the actual performance is set by the tuning (reset time and controller gain).

(3) Nearly all studies on the beneficial effect of improving loop dynamics retune the controller for better performance. If the controller was not retuned, there would be no immediate recognizable benefit in most cases.

(4) You can estimate the amount of dead time you can add before the loop performance deteriorates for unmeasured disturbances by comparing the present controller gain to the maximum controller gain for maximum load rejection.

(5) I would be out of a job if there was no dead time or disturbances, because barring any extenuating circumstances the controller gain could be set higher than you have ever seen or the control valve just sequenced to predetermined positions.

(6) Continuous temperature, concentration, and pH control loops on large well mixed volumes are best treated as “near integrators” for tuning.

(7) The use of dynamic reset limiting and a delayed external reset can provide dead time compensation that is easier to implement and more robust than a Smith Predictor. If the valve position PV for single loops and the secondary loop PV for primary loops is used for external reset, it prevents the controller from outrunning the valve or secondary loop and the dead time compensation is more accurate.

(8) If the model dead time used for the Smith Predictor is 100% larger than actual, the Smith predictor can break out into rapidly growing oscillations. A model dead time that is too large besides too small can cause instability in this predictor.

(9) The controller gain setting must be significantly increased beyond the normal maximum controller gain to realize the benefit from dead time compensation.

(10) A zero discharge flow causes the mass to increase as a batch progresses, which causes concentration and pH control to have an integrating response. The integrating process gain here is inversely proportional to level. For vessel pressure control where the vent valve pressure drop is large or critical, the pressure response’s integrating process gain is proportional level because the vapor space volume is decreasing. However, for temperature control where there is significant heat release and cooling capability, vessel level has little effect on the controller gain except when it is above or below the heat transfer surfaces (e.g. coils) because the effect of more mass is cancelled out by more heat transfer area covered by liquid.

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.

Last year we had disturbing thoughts on how fast upsets were particularly disruptive and anything you can do to slow them down makes the job of a loop much easier. In real processes, step disturbances are quite rare. However, there are some noticeable cases (e.g. on-off level control, interlocked and sequenced valves, and compressor surge control) where stuff comes at the loop too fast.

If level switches are replaced with a Hart or Fieldbus properly applied level transmitter, correctly tuned level controller, and throttling control valve with a digital positioner, you will make everything smoother downstream. The cost of the better automation system will pay for itself in terms of better reliability and visibility and reduced variability.

On-off valves must in many cases be sequenced and interlocked. The effect of these valves may be underestimated. Even for large valves with slow stroking times, most of the time is spent on the flat portion of the installed characteristic. For example a reactor air feed isolation valve had been deliberately slowed down by a restrictor on the actuator to take 145 seconds to stroke to allow the air compressor surge control system time to open its vent valve. An analysis of the installed characteristic revealed that there was actually only 1.7 seconds between when the flow dropped below the anti-surge controller set point and the flow hit the compressor surge line. The total time on the steep portion of interest in the installed characteristic was less than 3 seconds. The speed of the upset could have been regulated much better by a programmed partial reduction in air feed flow set point followed by a fast closing of the on-off valve to prevent reverse flow. I am must make it very clear at this point that a control valve should not be considered as a replacement for an on-off valve, or vice-versa. They serve distinctly different purposes. A control valve needs to have minimal seating friction for throttling and an isolation valve tight shutoff for isolation, which may be conflicting objectives.

Once compressor surge starts, not much can be done by a flow controller because it is like going over a cliff. The precipitous drop in flow occurs in less than 0.03 seconds. This was mistakenly interpreted as requiring a special microprocessor with a scan time of less than 0.05 seconds when really the control valve on big compressors often didn’t do much of anything for a second or more because you physically couldn’t move enough air out of the big actuator for the fail open vent valve. Also, the feedback controller needed to do something before it hit the surge curve. Once a compressor is in surge, an open loop back up is used to get out of surge because a flow reversal occurs every second or so totally confusing the controller.

For more details on compressor surge control see the books Centrifugal and Axial Compressor Control and A Funny Thing Happened on the Way to the Control Room (reprints available through UMI). Next week we will talk about the use of a simple open loop backup configured in a DCS to assist a PI loop for those applications were you need fast recovery for property and environmental protection.

Compressor anti-surge control is an extreme case but there are many applications particularly for parallel trains of equipment and batch to continuous transitions where it is advantageous to slow down disturbances by a coordinated startup and shutdown of flow set points.

Pressure waves travel at the speed of sound in the fluid (e.g. 1100 fps) whereas composition changes travel at the pipeline fluid velocity (e.g. 5 fps). The pressure waves can also reflect back and forth (e.g. water hammer), which like surge can be totally disruptive. Whether you are talking about pressure or composition changes it is wise here as well to slow them down by ramping the flow controller set point in the DCS rather than restricting the air flow to the actuators of on-off valves in the field. It is also beneficial to use pressure transmitters instead of pressure gages. Operator typically cannot outrun a pressure wave to get to the right one. In some cases we don’t know even whether a pressure upset is originating upstream or downstream let alone where it specifically starts. If you think about it, some field pressure regulators are also best replaced by pressure loops in a DCS to provide more visibility and control over the propagation of pressure waves and the allocation of pressure drops to prevent interaction and cavitation.

Momentum balances, normally not a part of dynamic process models, are required to simulate pressure waves and surge.

Top Ten Stuff That Comes at You Too Fast

(10) On-off level controlled flows
(9) Sequenced and interlocked flows
(8) Strong acids and bases
(7) Pressure waves
(6) Compressor surge
(5) Dunk shots
(4) Ice pucks
(3) Late night car commercials
(2) Corporate Restructuring
(1) Retirement

The physical standard and technical report produced by ISA SP50 were used as a starting point for much of the work done by the Fieldbus Foundation. In particular, the Fieldbus Foundation function block specification team adopted some basic concepts such as the definition of mode and status with minor changes from the definitions contained in the SP50 User Layer technical report. However, as work progressed on the Fieldbus Foundation function block specification, we found it necessary to expand the block types that were documented in SP50 and to change the way blocks were structured, defined, and implemented. Much of this change was driven by input from control system and field device manufacturers.

The Foundation function block specification team initially collected information on measurement, calculation and control functions and associated parameters that were common to the major suppliers of process control system. Through an analysis of this capability, we were able to identify functionality and parameters that were common to these manufacturers. Based on this work, the specification was broken into two parts. Part one of the specification contains a description of the architecture and formal model of the Function Block application process. In this part of the specification we addressed the components that make up the function block application process. To provide precise definitions that are sufficient to support implementation, the model is based on an object oriented design. Part 2 of the specification contains a basic set of ten function blocks that utilize the model and architecture defined in part 1 of the specification. This initial set of blocks addresses a variety of common measurement and control applications.

The function blocks defined for measurement and actuation were base on the parameters found in major process control systems. However, a fieldbus device must also contain many other parameters to support measurement and actuator diagnostics and calibration. We initially pursued the concept of the manufactures adding parameters to the basic IO blocks for device diagnostics and calibration. However, it soon became clear that this would lead to IO function blocks that were specific to each manufacture. If we took this approach then it would be necessary to know upfront what devices would be used in a particular installation before it would be possible to do basic control configuration. Our goal was to allow measurement, calculation and control to be configured independent of the device manufacturer. Thus, the concept of a transducer block was introduced into the architecture and block model. The transducer block serves as a container of calibration and diagnostic parameters associated with IO processing. By taking this approach, it was possible to design the IO function blocks that are the same for all device manufacturers.

A third type of block, the resource block, was defined to contain parameters associated with the physical device or are global to all function blocks. Some example of these typed of parameter are the manufacturer identification number and selections to enable or disable write lock protection. The specification requires that all fieldbus device support a resource block. Part 2 of the specification defines the resource block’s parameters. Only one resource block may be defined in a fieldbus device.

The IEC61804 international standard, Function Block for Process Control, includes the three blocks types defined by the Fieldbus Foundation function block specification. The transducer block is referred to in the standard as a Technology block. Thus, because of this consistency in definition, Foundation fieldbus devices comply with this standard. If you have an interest in learning more about the Fieldbus Foundation’s function block specification or the IEC61804 standard, then copies of these documents may be purchased at the following sites:

Fieldbus Foundation Specification

IEC61804 Standard

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.

While approaching an optimum something can sneak up that catches the loop off guard. Because of the deadly foe dead time, by the time the loop sees and reacts, it may be too late, particularly if it was blind sided.

The classic example is compressor anti-surge control. When moving to a lower discharge pressure or recycle flow (lower energy use), an inaccurate surge curve or untimely dip in feed can cause a precipitous drop to zero or negative flow in 0.03 seconds followed by huge reversals in flow from surge. Just a few of these surge cycles can damage the seals enough to reduce the efficiency of an axial compressor.

Another impressive case can occur for exothermic reactor control. During the approach to a higher reaction temperature and higher reaction rate (lower batch time) a higher than expected raw material concentration or catalyst activity can initiate a runaway acceleration of temperature and reaction rate.

Not quite as dramatic but still important in terms of environmental scrutiny occurs for an approach to a lower pH set point in a static mixer (lower base reagent use). A strong acid upset from a batch operation or level switch controlled sump can cause a low RCRA pH violation within seconds. Even if it lasts a few seconds and therefore has no measurable affect on any decent downstream volume, it can be a recordable environmental violation. In one particularly large application, an interlock diverted the feed from the plant waste treatment system if the control system could not do its job and a violation was eminent.The open loop backup successfully eliminated nearly all of these diversions.

A much slower but still important situation can occur for a bioreactor. During the approach to a lower substrate (glucose) concentration with less substrate inhibition (greater yield), non-ideal mixing and a drop in substrate feed can trigger starved biomass to eat their own product (ugh).

In each of these cases, there is a significant undesirable event that requires a slow approach to an optimum and a fast recovery from an inadvertent excursion into an extremely undesirable operating region. This is particularly true for the first three cases, which involve environmental and property protection. The last thing you want is to test the adequacy of your interlock system or have a recordable incident.

(1) Compressor Anti-Surge Control
(2) Exothermic Reactor Temperature Control
(3) Static Mixer RCRA pH Control
(4) Bioreactor Substrate Concentration Control

An open loop back up has been applied in the above applications to assist but not interfere with the PID controller trying to do its job. The calculation simply consists of incrementing the controller output from its last value via the ROUT mode every module execution when the process variable has exceeded a limit. The increment is stopped when the process variable has recovered beyond the trigger point plus some differential (e.g. noise band). It is normally only activated only when the controller is not in manual. There is a bumpless transition to PID action when the open loop backup is cleared.

For surge control, the clearing of the open loop back up has a time delay to insure the compressor is out of surge and the control system is not fooled by a flow reversal.

In each case, the need to get out of trouble as quick as possible overshadows any temporary loss in efficiency.

Another strategy is to use a fast opening but slow closing of the control valves for compressor vent or recycle flow, reactor coolant flow, pH reagent flow, and bioreactor substrate feed. This can be implemented by putting a rate (velocity) limit on a decreasing signal to the control valve. This can be implemented in the analog output block via the SP_RATE_DN parameter, which in this block is active on the set point even when the block is in the CAS mode. To insure the reset action in the PID block is not faster than the rate limiting in the AO block, the “Dynamic Reset Limit” option must be enabled in the PID and the “Use PV for BKCAL_OUT” option enabled in the AO block to use the working set point for the BKCAL_OUT. Any rate limit will affect tuning and must be implemented before running any tests to identify dynamics or tuning settings. The strategy also works on variable speed drives for reagent and substrate feeds to allow a fast increase but insure a slow decrease in speed.

The attached screen prints show a simple example of an open loop calculation and enabling of the above options. As with any new technique, the configuration should be thoroughly tested by a realistic simulation before used in an actual application.

Open Loop Backup and Slow Closing Valve Option

Another option is to schedule the controller reset action to be much faster (reset time much smaller) when the process variable approaches a risky region to promote a fast recovery. There may be some overshoot of the set point but a slow approach back should prevent a second crossing to the more eventful side of the set point. Scheduling a drastically higher controller gain may not be a good idea because it can cause a bounce back toward the undesirable region from proportional action before the process variable even gets near the set point. Some new DCS software, such as DeltaV Insight, can automatically identify process dynamics and schedule the corresponding tuning settings.

Sometimes the open loop back up is called a kicker. The following is an excerpt from the January 2005 Control Talk column in Control magazine that describes a kicker used by Terry Chmelyk to reduce the number of feed diversions required to prevent the violation of an environmental constraint. It is similar conceptually to the previously described RCRA limit application, but here the measurement was conductivity instead of pH.

Terry: In a multi-effect evaporator system, we used built-in and integrated model predictive control (MPC) and optimization to reduce variability in the product density from 2.8% to 0.3% and increase throughput by 6 to 8%. We also used innovative environmental constraint handling to increase the interval between diversions by an order of magnitude.

Greg: Environmental limits can come on suddenly and unexpectedly. My experience is that these involve unmeasured disturbances and scenarios you can not initiate to develop a model. There is nothing sadder than an advanced control engineer without a model. What did you do?

Terry: We added an external "kicker" algorithm around the MPC because of the highly non-linear characteristics of the constraint variables (in this case it was condensate conductivities). The environmental impact required us to take immediate and "substantial" action to eliminate the contamination in the condensates. In essence, we built a basic fuzzy algorithm that "kicked" the weak black liquor (WBL) feed to the evaporators during a significant upset.

The first slide in the attached file summarizes the achievements of the MPC/kicker application. The second slide shows how the "kicker" backed out the WBL flow on high condensate conductivity to prevent a diversion yet allowed the MPC to recover quite well from the disturbance.

Conductivity Kicker