Don t Tune These Four Loops!

Transcription

1 Don t Tune These Four Loops! George Buckbee 2012 ExperTune Inc. Page 1

2 Don t Tune These Four Loops! George Buckbee, ExperTune Inc ExperTune Inc Summary Some control loops cannot be improved by tuning. In fact, you might be wasting your time, or you might even make matters worse by tuning. This paper discusses four types of PID control loops that you absolutely should NOT tune. Read on to discover how you can save time and get better results by NOT tuning. Overview Why NOT to Tune It may seem odd that a company named ExperTUNE is giving advice on when not to tune a controller. But there are many good reasons to avoid controller tuning, including: 1. Tuning will not make the problem any better. 2. Tuning will make the problem worse. 3. You might completely miss the real problems. This paper covers four specific cases of control loops that you should not tune. Furthermore, it shows you exactly what action you should take to resolve issues and make the control loop better. Page 2

3 Don t Tune If The Loop is at a Limit If the controller is already at a limit, then there is no point in tuning the controller. So, what do we mean by at a limit? Usually, it is one of the following: The Control Valve is 100% open The Control Valve is 100% closed The Variable-Speed Drive is at max speed The heater is on (or off) 100% of the time The control output is limited by a soft limit The Process Variable is at 0% or 100% of its span In other words, the controller has no ability to control, no matter what tuning you put in place. What to Do Instead The obvious answer is to find out why the loop is limited. Once you figure that out, try to get the controller back into a normal controllable range. Finding Limited Loops ExperTune s PlantTriage monitors the condition of all your controllers, 24 hours a day. By watching the control output signals, it can instantly find all controllers that are at a limit, as shown in the table below: There s another benefit to this exercise: Loops at a limit are often bottlenecks to production. When you resolve these problems, you may also be able to increase production rate. Page 3

4 Don t Tune If The Instrument is Failing Instruments fail in a lot of ways. And when they fail, they can affect the controller in many different ways. The table below shows several modes of instrument failure, their impacts on operations, and their impact on controls. Failure Mode Instrument Fails to Zero High Noise Levels Low Noise Levels Instrument Holds Last Good Value Instrument Spiking Impact on Controls Loops in AUTO open or close valve completely. Combined with high controller gain, may damage the control valve. Makes it difficult to tune, and difficult to tell if controller is working well. Has the instrument failed? Is the tap line plugged? Over-filtered? Control may become sluggish. Loops in AUTO will fully open or close control valve. Operator may not know there is a problem. Causes sporadic movement of control output. What to do Instead Fix the instrument. Finding Failed Instruments PlantTriage automatically finds instruments that have failed from any of the methods above, and displays the information in a prioritized list: Page 4

5 Don t Tune If The Loop is Already Well-Tuned While this seems obvious, it isn t really so obvious. Do you know how to tell if a loop is well-tuned? Is it: Fast response to a setpoint change? Quarter-amplitude damping? No overshoot? Slow, but steady? Any one of these might be correct, but you can t have all of them! The answer depends on the needs of your process. So, you have to start by asking the question What does good tuning look like?, then look at the current performance of the loop in question. If a loop is already well-tuned, then any changes will make it worse! What to Do Tuning is always a balance between fast response and robustness. Robustness is a measure of the loop s performance under changing process conditions. If the tuning is not robust, the loop could become unstable. In most process plants, it is wise to build in a safety factor to ensure robustness under a range of conditions. If a loop is well tuned, with a good balance between performance and robustness, leave it alone! Finding Loops with Poor Robustness The effect of tuning on robustness can be plotted. Each line on the plot represents a set of controller tuning applied to the process. The cross-hairs show the point at which the loop becomes unstable. So the blue line represents unstable tuning, and the red line represents tuning that is slower, more stable, and more robust. The light blue warning track can be used as a guide. Very aggressive tuning will just touch the top-right edge of the warning track. This tuning will perform well within a narrow range of process dynamics. If the plant runs at a different rate, or makes a different product, the tuning could become unstable. ExperTune recommends a safety factor that will keep your loop stable, even if there is a shift in process conditions. Page 5

6 Don t Tune If The Root Cause is Somewhere Else You can waste a lot of time trying to fix the wrong problem. Because PID control is a form of feedback control, you simply cannot tune out all the upsets that come to your controller. Any upset that comes along will upset your controller, no matter what the tuning! Process plants are complex places, full of dynamic interactions that spread from the source, throughout the plant. What you need is a way to find the true root cause of process upsets, so that you can eliminate the problem at its source. What to do Instead Find the true root cause of the variation, and eliminate the problem at its source. Finding the Root Cause In recent years, powerful new tools have developed. Using cross-correlation analysis on your existing historical data, the Process Interaction Map can pinpoint the upstream root cause of the problem. PlantTriage users have discovered that many process upsets originate in the utilities area. Fixing these disturbances has wide-ranging impact, often worth millions of dollars in savings. Page 6

7 Summary Don t Tune These Four Loops There are four types of PID control loops to avoid tuning. These are: 1. Loops at a Limit 2. Loops with Failed Instruments 3. Loops with Good Tuning 4. Loops with Another Root Cause In each case, there are alternate actions that can be taken to improve overall process performance. Get More Detail ExperTune has over 25 years experience in control loop evaluation. If you would like a personal review of your control system issues, please contact us at for a consultation. Page 7

8 Sponsored by ExperTune s PlantTriage This white paper is sponsored by ExperTune s PlantTriage. This Control Loop Performance Monitoring software optimizes your entire process control system, including instrumentation, controllers, and control valves. Using advanced techniques, such as Active Model Capture Technology, PlantTriage can identify, diagnose, and prioritize improvements to your process. Continuous Improvement Tools PlantTriage improves performance every day. It monitors the entire facility, looking for the biggest improvement opportunities. PlantTriage notifies you of the issues, then gives you deep-dive analysis, so you know exactly what the issue is. Expert Analysis Every PlantTriage system contains an expert system, developed by industry leaders like F. Greg Shinskey. It s like having a world-class expert looking at your plant, every day. Open System Compatibility PlantTriage works with your control system. For over 25 years, ExperTune has studied process control systems, and learned the intricacies of working with each one. PlantTriage knows how to work with over 700 industrial control algorithms. Also, PlantTriage is certified OPC-Compliant. It can connect to your control system via OPC, or to your data historian via OPC-HDA. A Complete System PlantTriage comes complete, with everything you need, including configuration services and training. You do not need to buy any other software. You will have unlimited users working with the true thin-client interface. Engineers, operators, managers, and technicians can all use PlantTriage to make their plant run better. Page 8

9 PlantTriage registered Trademark of ExperTune Inc. ExperTune registered Trademark of ExperTune Inc ExperTune Inc. About the Author George Buckbee is V.P. of Marketing and Product Development at ExperTune. George has over 25 years of practical experience improving process performance in a wide array of process industries. An experienced manager and instructor, George has also been elected a Fellow at ISA. George holds a B.S. in Chemical Engineering from Washington University, and an M.S. in Chemical Engineering from the University of California. About PlantTriage PlantTriage is Control Loop Performance Monitoring software that optimizes your entire process control system, including instrumentation, controllers, and control valves. Using advanced techniques, such as Active Model Capture Technology, PlantTriage can identify, diagnose, and prioritize improvements to your process. Page 9