Advances in Control Loop Optimization
Software takes users from simple tuning to plant-wide optimization.
Vance VanDoren, Ph.D., P.E. Control Engineering -- Control Engineering, 5/1/2008
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There was a time when “control loop optimization” meant selecting the proportional, integral, and derivative tuning parameters for all of the PID controllers in a plant so as to optimize the closed-loop performance of each loop individually. Loop tuning is still a critical element of loop optimization, but thanks to recent advances in control theory and commercial software products, the control operations of an entire plant can now be optimized to maximize overall profitability, not just individual loop performance.
As George Buckbee, vice president of marketing and product development for ExperTune puts it, “Control systems personnel are using control loop monitoring and performance supervision software to generate measurable economic benefits. Today’s loop optimization software focuses on providing meaningful, actionable metrics that deliver results to the bottom line and graphical tools to focus the users’ attention on the most economically important issues, thereby helping them to focus their time and effort on the areas where they can make the biggest improvement.” See the chart, “Typical improvements from loop optimization.” (PlantTriage software by ExperTune identifies, diagnoses, and prioritizes improvement opportunities according to economics.)
Dr. Alireza Haji-Valizadeh, manager of technology development for ControlSoft, adds that today’s loop performance monitoring and diagnostic tools can:
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Provide both automatic and manual loop tuning and analysis tools;
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Keep track of all the loops in the plant no matter what platform or vendor;
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Execute non-intrusive, continuous on-line performance analysis;
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Generate customizable loop performance indices such as error distribution, integral of absolute error, process variability, and economic impact; and,
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Disseminate loop performance reports to all levels of the plant’s business hierarchy and across the Web.
Loop maintenance and more
Haji-Valizadeh notes that his company’s InTune software (version 5) offers all of these features as well tools that are capable of addressing maintenance issues such as:
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How a loop’s performance foreshadows future maintenance requirements;
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How loop maintenance efforts should be allocated to maximize return on investment;
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Which of the problem loops have the most room for improvement; and,
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How to keep loops continuously optimized on a plant-wide scale.
Steve Johansen, global services marketing manager for advanced solutions at Honeywell Process Solutions points to Honeywell’s Loop Scout as a comprehensive loop management workflow solution that contains loop analysis tools, valve diagnostics, a robust control theory-based tuning package, and alternative techniques to PID control.
Loop Scout collects data on process values, setpoints, controller outputs, and control loop configuration data throughout the plant; then generates performance reports for each loop. Expert guidance tools walk the novice user through the data and offers additional tests to determine if there’s a maintenance issue (such as stiction in a valve), a tuning issue, or a problem with the control strategy itself.
“Loop Scout has evolved to the point where the user doesn’t need to understand the underlying technology. He only needs to drill down to the specific information that impacts decision-making. The user can choose which characteristic by which he wants to rank and sort his loops — the severity of each problem detected, the general performance of each loop (good, fair, poor, etc), or the greatest opportunity for improvement in terms of business metrics,” says Johansen.
John Caldwell, DeltaV product marketing manager at Emerson Process Solutions adds that DeltaV InSight is an on-line process modeling tool that automatically learns the process based on normal day-to-day operations then monitors performance, and identifies and diagnoses problem loops; it also recommends tuning and maintenance improvements. He also notes that new developments in adaptive control, advanced diagnostics, and performance reporting are planned for release later this year with DeltaV version 10.3
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| A recent survey conducted by Honeywell Process Solutions suggests that almost one-third of control loops are operating with at least "acceptable" performance. Extend that to a minimum of at least "fair," and you cross the half-way point. Of course there is still that 36% running in manual. |
Better, but not perfect
Whether due to advances in loop optimization technology or other causes, there has been some improvement in control loop performance across the process industries. The average respondent to a 1996 Control Engineering survey reported that approximately 55% of their control loops were running in closed-loop rather than manual or open-loop mode. Of those running in closed loop, 45% were believed to be optimized.
More recently, a survey of PID controllers conducted by Honeywell Process Solutions showed that 64% were operating in closed loop with 50% of those tuned to provide “excellent” or “acceptable” performance. That still leaves a lot of room for improvement, as shown in the chart. Yet, loop optimization tools aren’t always the best way to realize additional controller performance.
Manual tuning needed too
“I once used a commercial loop tuning package and didn’t find it very useful for the particular loop I was trying to tune,” says Matt Lyles, managing partner at SBL Systems, a control system engineering company. He suspects that the significant deadtime in his loop may have been the problem, so he resorted to a tried-and-true manual tuning technique.
“I can see where there could be some critical processes out there where there is limited time allowed to tune a loop and where either a good loop tuner or derivation would be necessitated,” Lyles added. “Typically the loops I have worked with were not critical and allowed me plenty of time and even wasted product to get the loop right.”
Process control consultant Barry Payne of Barry D. Payne & Associates also has mixed feeling about loop management software. “We have found that tuning software is useful if the control loop it is applied to is functioning properly, and the person using the software already understands what a 'normal response’ is for the process.
“If the loop is subject to significant unmeasured disturbances while the tuning software is being used, or if one or more of the end devices is malfunctioning, then the tuning software (at least that supplied by a few of the larger DCS manufacturers) may give unpredictable and frequently erroneous results.”
He cites an example where a DCS-based automatic loop tuner failed to recognize that the very low model gain it had determined for a particular process was due to an instrument malfunction rather than the actual process dynamics. “Anyone with experience tuning this type of simple loop would have quickly recognized that there was a fault. Auto-tuning software can be useful, but only when used in combination with experience in tuning a particular type of application. It is not an 'Easy button’ for those with little or no skill at loop tuning and troubleshooting.”
| Benefit | Typical range | Conservative range |
| Source: Control Engineering with data from ExperTune | ||
| Energy reduction | 0.5% to 2% | 0.5% to 1% |
| Production increase | 1 to 10% | 1 to 2% |
| Valve maintenance budget | 10% to 50% reduction | 10% to 20% |
| Quality improvement | 5% to 50% improvement | 5% to 10% |
| ROI from performance supervision | 3 to 9 months | 1 to 2 years |
Vance VanDoren is consulting editor to Control Engineering. Reach him at controleng@msn.com.
For more information, visit:
www.planttriage.com ExperTune
www.loopscout.com Honeywell Process Solutions
www.easydeltav.com Emerson Process Management
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