Cover story: Control loop monitors help optimize controller performance
In a word, control loop monitoring and optimization is all about savings—saving energy, raw materials, and operator effort while improving quality and profitability in a plant that depends on feedback controllers to produce a product. Commercial control loop monitors help the plant’s operators meet those objectives by automatically collecting and analyzing massive amounts of loop data from which to identify opportunities for improvement.
John Caldwell, DeltaV product marketing manager for advanced control and simulation products at Emerson Process Management, explains: “Manufacturers lose millions of dollars each year due to process variability and poor control performance, often unaware they even have a problem. Control engineers are typically overworked and ill-equipped to monitor and maintain the hundreds of control loops and instruments for which they are responsible. And contributing to the problem, traditional control systems don’t provide a systematic way to monitor, diagnose and improve process control performance.”
Hence the need for a control loop monitor that can provide “a systematic approach to improving control by monitoring control performance, identifying and diagnosing problem loops, recommending tuning and maintenance improvements, and continuously adapting to changing conditions to optimize plant performance,” says Caldwell.
Control loop monitoring software is typically installed in a server networked to a plant’s existing control system. It collects a variety of statistics from each loop then calculates how well the controllers are functioning and which of them might be able to do a better job.
George Buckbee, VP of marketing and product development at ExperTune, elaborates: “Control loop performance monitoring tools extract meaningful diagnostics from normally occurring process data. These diagnostics go far beyond the traditional realm of controller tuning by addressing such concerns as instrument failures, control valve performance, equipment sizing, process interactions, lost profit potential, quality management, and even operator loading.”
Key performance indicators
Specific performance indicators collected and reported by a control loop monitor can include statistics such as variance, standard deviation, and integrated error of the process variable from which clues about process variability, loop tuning, and loop interactions can be derived. (See sidebars.)
Michinao Takamuku, general manager of promotions at Yokogawa’s VigilantPlant Services Center, also cites time in control (a.k.a. “loop controllability” or “service factor”), time with the manipulated variable out of limit, and time in preferred mode as key performance indicators that a control loop monitor can track. He adds that in an ideal world, “The results are presented on a daily/weekly/monthly basis as customers like by an automated comprehensive report function which also provides a ranking of loops to help operators and maintenance staff find loop problems easily.”
The ability to flag control loops that have the most room for improvement is an important economic benefit of control loop monitoring. John Cunningham, director of advanced process services at the RoviSys Company, notes that, “In problematic processes that have many issues, loop performance monitoring can help you to identify the ‘low-hanging fruit’—those issues which have the biggest impact on process yield, production rate, and product quality. Performance monitoring software can help you prioritize the problems, identify the nature of the problem, and verify that the process or control measures that you take are actually making a difference. By historizing the statistics of your control loops’ performance, you can track the efficacy of the improvements and tie them to economic metrics.”
So what kinds of problems can a control loop monitor deduce from the statistics it collects? Steve Howes, product manager for PiControl Solutions, cites several: “Oscillations, control valve stiction and hysteresis, excessive control valve output movement, need for filtering the process variable, sluggish control, signal problems such as spiking, excessive mode changes, low service factors, and suboptimal tuning parameters,” among others.
He also notes that many of these problems develop over time, and “human beings unaided by automated monitoring tools just cannot keep up,” especially when there are hundreds or thousands of control loops to be monitored. Even if the plant involves only a few control loops that can all be optimized when they’re first installed, “control loops that once were performing well start to deteriorate as their output elements get worn out. Also, changes in process, operating, and economic conditions can result in changes in process dynamics or process characteristics that can cause loops to oscillate or become sluggish.”
Or, as Alan Hugo, lead development engineer for Control Arts, puts it, “Valves can stick, process dynamics can change, etc., so regular monitoring is required. Often these problems are easy to fix. Finding them is the challenge.”
Howes concludes that “plants need online, automated control monitoring tools if they want to run efficiently, maximize profits, and stay competitive in today’s competitive global market.” And the economic benefits can be significant. “A control loop monitoring project can increase the plant’s profits or production rates typically by 2% to 5%. Spectacular results—even as high as 10%—have been observed in some cases. Recurring annual monetary benefits of $50,000 to $3 million per plant can be achieved,” according to Howes.
Dennis Nash, president and CEO of Control Station, agrees. “PID loop tuning should not be left as a checkbox for that once-a-year maintenance review, but isolating individual ‘bad actors’ has proven an overwhelming task. That’s why solutions are now on the market to continually monitor individual PID loops and analyze their interaction with others in a very sophisticated way. In addition, PID loop tuning has become very sophisticated—far beyond the functionality available with auto-tuning. Tuning can be automated as part of a broader monitoring capability where PID loop analysis can be conducted within normal and even oscillatory process conditions through advancements in non-steady-state tuning technologies.”
Paul Botzman, marketing and sales manager at ControlSoft, sums up the benefits of control loop monitoring: “These solutions provide a course of action in real time (in the form of prioritized performance indicators, user notifications, customizable and meaningful reports) to the specific managers, engineers, or plant personnel who can make the most use of this information and act on it. No longer are plant personnel required to decode and decipher huge amounts of data to generate hypotheses. Targeted reports allow plant personnel to focus and prioritize resources to save time and money, based on areas of the process that need immediate attention due to performance and economic priorities.”
Vance VanDoren is contributing content specialist for Control Engineering. Reach him at controleng(at)msn.com.
Related content available at www.controleng.com/archive or via the individual URLs below:
Control loop performance monitoring can reduce process variability
One of the principal objectives that a typical loop monitor attempts to achieve is a reduction of variability in the conditions it’s measuring. Variability can result from poor loop tuning, uncontrolled loop interactions, and degraded performance of the sensors, actuators, and I/O devices that interface the control system to the real world. Many of these problems can be identified and remedied with the right loop monitoring tools.
The figure shows why minimizing process variability is important. If a process variable deviates less from its setpoint, the process can be operated closer to its alarm limits where productivity is usually higher. The trick, says Jim Huff, VP of Technology at PAS, is having “knowledge of the process and control systems, the right tools for the job, and importantly, a systematic approach to follow.”
Frequency analysis aids control loop optimization
Every commercial product for monitoring the performance of a plant’s feedback control loops approaches the underlying mathematical problems differently, but many rely on some sort of frequency analysis. By identifying variables that oscillate at the same frequency or with similar patterns, a control loop monitor can infer which of them are interconnected.
Frequency analysis is particularly useful for detecting subtle interactions between loops and for identifying the source of an otherwise unknown disturbance. Consider, for example, the simplified batch process shown here. One of the feedstocks entering vat no. 2 on the right happens to pass through a pipe above vat no. 1 on the left, and is subject to heating when the left-hand batch is cooking. A control loop monitor collecting data from both vats’ temperature controllers should be able to detect this unintended interaction by observing that the disturbance to temperature no. 2 follows the trajectory of temperature no. 1.
Armed with that revelation, the plant’s engineers might be able to eliminate the unintended heating by insulating the offending feedstock pipe, relocating it, or scheduling the vats to operate at different times. Yet as simple as these hypothetical remedies might seem, the problem could have proven much more difficult to solve had a loop monitor not been available to identify the source of the temperature disturbance to begin with.
Loop tuning and optimization software
PlantESP from Control Station
Actively monitors the performance of a production facility’s PID controllers, facilitating the isolation and correction of underperforming regulatory control systems.
RSLoop Optimizer from Rockwell Automation
Includes tools for loop-tuning, valve analysis (stiction, hysteresis, and excessive valve movement), process variable and setpoint filter analysis, and loop interaction analysis.
rCAAM from RoviSys
Analyzes current and historical control loop data to identify control, instrument, and process problems; ranks the problems and reports to engineers and operators in several formats, with minimal up-front engineering.
PSS Control Loop Optimization Software from PAS
Provides individual loop performance and interaction analysis, loop rankings to identify the best candidates for improvement, a map showing the health of each controller, and recommendations for improving poor performance.
Control Loop Assistant from Lambda Controls
Evaluates control loop health, optimizes controller tuning, and reveals process variability and clues to its cause.
Protuner from Techmation
Provides data acquisition, data analysis, and simulation for testing, troubleshooting, tuning, and optimizing installed single loop and complex multivariable regulatory control systems.
TOPAS from ACT
Helps improve new and existing controllers through PID tuning, optimization, and troubleshooting; controller selection; model parameter estimation; performance analysis; and cost-saving projections.
Control Loop Attendant from Algosys
Monitors a plant’s control loops in real time, detects changes in process behavior, and provides troubleshooting and diagnosis tools to help operators prevent inadequate process operations and anticipate equipment-related problems.
Loop Scout from Honeywell Process Solutions
Monitors control systems, detects and prioritizes problems, proactively notifies appropriate personnel to take action, and intelligently diagnoses and resolves root causes.
INTUNE+ from ControlSoft
Benchmarks loop and plant performance, highlights gaps between current and desired performance, identifies and prioritizes the root causes of process control problems, and differentiates between hardware and tuning problems.
PlantTriage from ExperTune
Tracks the performance of control loops in real time, identifies problem areas such as valve mechanical issues, finds the causes of oscillations and process upsets, then prioritizes and diagnoses improvement opportunities.
DeltaV InSight from Emerson Process Management
Automatically learns the process based on normal day-to-day operations then monitors performance, identifies and diagnoses problem loops, and recommends tuning and maintenance improvements.
PID Watch from AspenTech
Automatically monitors and evaluates the performance of PID loops and provides analysis, reporting, simulation, and tuning functions to correct performance problems and identify optimal tuning parameters.
ControlMonitor from Control Arts
Provides controller performance histories, controller performance reports, analysis to determine the source of underperformance problems, level controller analysis, process model identification, and controller tuning.
OptimizeIT Loop Performance Manager from ABB
Helps process managers recover profit margins through systematic monitoring and adjustment of the control system, including loop tuning and loop audits that show how loop performance is changing.
Apromon from PiControl Solutions
Calculates over 25 performance criteria per control loop then generates a single "grade" factor between 0 and 100 to indicate overall loop performance.
InsightSuiteAE from Yokogawa
Optimizes overall equipment effectiveness through digital device failure and fault diagnostics, control loop and valve diagnostics, and asset performance diagnostics.
Control Performance Monitor from Matrikon (Honeywell Process Solutions)
Uses control loop tuning, modeling, and multivariate step tests coupled with automatic data collection and analysis to improve and sustain control performance.
ProLoop Control Loop Monitor from Process IT Development
Computes how closely the process variable tracks the setpoint, identifies cyclic deviations, and determines the extent to which the controller is varying its output.