Advances in Control Loop Optimisation
05 September 2008
Software lifts users’ concerns from a single loop to the entire plant, which may have hundreds, or even thousands, of control loops running simultaneously.
There was a time when ‘control loop optimisation’ meant selecting the proportional, integral, and derivative tuning parameters for all of the PID controllers in a plant so as to optimise the closed-loop performance of each loop individually.
Loop tuning is still a critical element of loop optimisation, but thanks to recent advances in control theory and commercial software products, the control operations of an entire plant can now be optimised to maximise 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 optimisation 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.” (PlantTriage software by ExperTune identifies, diagnoses, and prioritises improvement opportunities according to economics.)
Getting better, but still with a lot of room for improvement:
A recent survey of PID controllers conducted by Honeywell Process Solutions showed that 64% were operating in closed loop with about half of those closed loops tuned to provide ‘excellent’ or ‘acceptable’ performance. That still leaves a lot of room for improvement, as shown in the chart. Yet, loop optimisation tools aren’t always the best way to realise additional controller performance.
Dr. Alireza Haji-Valizadeh, manager of technology development for ControlSoft, (www.controlsoftinc.com) adds that today’s loop performance monitoring and diagnostic tools can:
• Provide both automatic and manual loop tuning and analysis tools;
• Keep track of all the loops in the plant no matter what platform or vendor;
• Execute non-intrusive, continuous on-line performance analysis;
• Generate customisable loop performance indices such as error distribution, integral of absolute error, process variability, and economic impact; and,
• Disseminate loop performance reports to all levels of the plant’s business hierarchy and across the Web.
LOOP MAINTENANCE AND MORE
Dr. 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:
• How a loop’s performance foreshadows future maintenance requirements;
• How loop maintenance efforts should be allocated to maximise return on investment;
• Which of the problem loops have the most room for improvement; and,
• How to keep loops continuously optimised on a plant-wide scale.
Steve Johansen, global services marketing manager for advanced solutions at Honeywell Process Solutions points to Honeywell’s Loop Scout (www.loopscout.com) 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 describes DeltaV InSight (www.easydeltav.com) as an on-line process modelling tool that automatically learns the process based on normal day-to-day operations. It then monitors performance, identifies and diagnoses problem loops, and 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.
BETTER, BUT NOT PERFECT
Whether due to advances in loop optimisation 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 optimised.
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 optimisation tools aren’t always the best way to realise 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 recognise 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 recognised 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.’
—Vance VanDoren, Ph.D., P.E. Control Engineering firstname.lastname@example.org
LOOP OPTIMISER SOFTWARE PRODUCTS
Loop optimising software is available from a wide variety of sources. Some are designed to be universal, and some for specific control platforms.
Aprom from PiControl Solutions Co
Online software calculates more than 25 performance criteria per control loop then generates a single ‘grade’ factor between 0 and 100 to indicate overall loop performance.
BESTune from BESTune.com
Software determines if the plant’s PID loops are optimally tuned and how large the potential benefits might be to retune them.
Control Loop Assistant from Lambda Controls
Process control optimisation tools include loop tuning software, data acquisition hardware and data analysis software.
ControlMonitor, Model ID and PID Tuning from Control Arts
ControlMonitor automates the task of determining which loops need attention, delivering detailed reports on which loops aren’t performing and how to fix the problem. The Model ID and PID Tuning packages generate dynamic models of the process and quickly calculate the best PID parameters from a few simple plant tests.
ControlWizard and TuneWizard from Plant Automation Services
ControlWizard provides non-invasive identification of control problems, detects poor control loop performance, troubleshoots problems, targets maintenance efforts, and improves product quality. TuneWizard provides data acquisition, process and control valve diagnostics, process identification, controller tuning, control loop simulation and report writing.
Control Performance Monitor from Matrikon
A condition-based application monitors, identifies, diagnoses and remedies control asset issues across all plants layers and offers monitoring, tuning and modeling capabilities within one platform.
Loop-Pro Software from Control Station Inc.
A suite of software modules provide PID controller tuning, data analysis an modeling, and control loop strategy design, simulation, and training.
Loop-Pro Product Suite from Control Station
Intuitive process modeling and tuning modules simplify PID control so users can optimise plant operations and improve the performance of business-critical processes.
Optimise IT Loop Performance Manager from ABB
Software tools help control engineers and process operators start-up, diagnose and maintain control loops on ABB and competitors’ systems.
PCS 7 PID Tuner from Siemens
An option for the Siemens PCS 7 distributed control system helps users determine the optimum tuning parameters in defined steps for P, PI, and PID control loops.
PI Control Monitor from OSIsoft
Four basic measurements—process variable, setpoint, controller output, and on/off control flag—are monitored to alert operations personnel when a control system is in trouble.
Pitops-PID from Artcon Inc.
Process Identification and Controller Tuning Optimiser Simulator tunes all PID loops from millisecond time frame to several hours of time constant.
PlantTriage by ExperTune
Software identifies, diagnoses, and prioritises improvement opportunities according to economics. Connects to any DCS using OPC. Provides KPIs, reports, alerts, and dashboards with drill-down capability to solve root-cause issues and is fully integrated with loop tuning and analysis tools.
Protuner from Techmation
Hardware and software system records and analyses dynamic test data in order to troubleshoot, tune, and optimise single-loop and complex multivariable regulatory control systems.
RSLoop Optimiser from Rockwell Automation
Optimisation tools analyse and tune PID loops in Allen-Bradley PLC-5, SLC 500, and Logix controllers to reduce process variability, improve productivity, and maximise efficiency.
TOPAS from ACT
Software allows users to develop, tune, optimise and troubleshoot basic and advanced control loops to produce better performance in a shorter time.
Tune-Plus from Givens Control Engineering
Tuner analyses bump test data and recommends loop tuning parameters based on the Lambda tuning method and the user’s specification on control speed.
U-Tune PID Tuning package from Contek Systems Ltd.
Tuning algorithms account for excessive noise and disturbances and give the process control engineer direct input into the process identification phase.
Windows Servo Design Kit from Galil Motion Control
Software uses a closed-loop frequency response test to generate PID parameters that optimise the performance of Galil motion control systems.
Most Viewed Articles...