PID Tuning / Loop Control
Articles and white papers on PID tuning and loop control for automation, process control, and instrumentation applications.
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The White Water Butterfly Effect (Confessions of a Looney Tuner)
I still remember my very first paper machine control audit, seventeen years ago. The mill in question was in a typical remote location, and produced linerboard. We collected basis weight data, while the scanner was in fixed point in the center of the sheet (to the uninitiated, basis weight is the total weight of the paper, fiber and moisture). Collecting data in this manner shows the true Machine Direction (MD) variability. We quickly discovered that there was a pronounced 90-second cycle in the basis weight. It was clear that this cycle was one of the main culprits behind the machine’s poor runnability record. Read full article by Sylvain Millette, Millette Control Engineering Inc.
Improving PID Control with Unreliable Communications
By Jianping Song, et al, Emerson Process Management This paper identifies the poor dynamic response of standard PID algorithms in the case of lost communications. It proposes an enhanced PID algorithm to improve dynamic response under these conditions. The authors evaluated the enhanced PID algorithm under several wireless scenarios. Read full article
Implementing Performance Monitoring in Your Plant
Much of industry recognizes that optimized controller performance increases plant efficiency. There are now case studies in every industry showing the benefit of continual assessment and diagnoses of controller performance. But how do you get started with performance monitoring? This paper outlines the 5 steps. Read full paper by Tom Kinney and John Gerry of ExperTune.
"See no Evil, Hear no Evil, Speak No Evil" (Confessions of a Looney Tuner)
I firmly believe that, given a set of initial conditions (and enough time), everything will gravitate towards their natural state. In this month’s adventure, I was again giving a loop tuning/troubleshooting lecture in the field. In this case, my students and I were hooking up to the input and output signals of a control loop. Read full article
Which Tuning Method Should You Use?
There are many, many different tuning rules available. Quarter Amplitude Damping, Cohen-Koon, IMC, Trial-and-error, Lambda, Ziegler-Nichols, Shinskey, minimum IAE, to name just a few. Any tuning method will have proponents who push the merits of the method saying it guarantees this and that in terms of response and at the same time is robust and not very sensitive to the process changing. Which one should you use? Read full article
How to Control a Process With Long Dead Time
A process with a large dead time presents a special challenge for a controller - any controller. In a dead time process, the controller makes a change, then waits, waits, waits until the dead time has passed. Finally the controller finds out how its change effected the process variable. It is like trying to drive a car blindfolded with the passenger telling you what to do. You will need to go very slowly. Find out how to get the best control of large dead time processes. Read full article
The Fast Way to Tune Slow Loops
Tuning slow loops does not have to take a long time. To tune a control loop, analysis software must process some loop data. You must test or bump the loop somehow. For example a setpoint change in automatic or a controller output change in manual mode. Using the strategy outlined in this article and corresponding presentation, you can tune slow loops very quickly. Read full article
Choosing the Best Control Loop Filter
Choosing the proper filter for your control loops can have an impact in your company's bottom line. In general, the best type of filter is a second order Butterworth, but a simple first order filter can do much to reduce controller output jitter and save the life of your valve or variable frequency drive. There are always tradeoffs in process control and choosing the filter type and size are one of them. Read full article
The Best Sample Interval For Process Control
If you don't collect the data fast enough then you will be "hiding" data from the controller or analysis software. Imagine if you attempted to drive your car by looking at the road once every 5 seconds. It is better to err on the side of being too fast. This applies both for what your industrial digital controller should sample at and how fast to collect data for analysis, modeling and PID tuning. Read full article
Ziegler-Nichols Tuning Rules And Limitations
The standard reference for PID tuning seems to be the Ziegler-Nichols tuning rules developed in 1942 on a pneumatic controller. This article demonstrates how to tune a controller using these rules. Read full article
Eliminate Cycling in Your Plant
Remove cycling in your plant and you almost assuredly will improve the entire operation. You will probably get one or more of these benefits: better product quality, less waste, energy savings, raw material savings, or higher throughput. Read full article
Loop Stability - The Other Half of the PID Tuning Story
There is always a tradeoff between fast tuning and stability. Tune a control loop aggressively enough and it will oscillate. Time response is one side of the story. Robustness plots are the other. Read full article
Derivative Action - The Good, Bad, and Ugly
The Derivative parameter has a stabilizing effect on your control loop. The reason to use derivative (or D) is that you can use more Proportional and especially more Integral action. The result can be a much faster reacting control loop. Read full article
PID Tuning Tips - How to Linearize Your Process
A non-linear process will have different response across the operating range. It will be sluggish at one end of the range and comparatively faster or even oscillatory at the other. Linearize Your Process and Get Optimal Response at All Production Rates. More Info...
PID Controller Specification
There are no industry-wide standards for PID controllers. However, robust and optimal control of process loops requires PID controllers to have certain abilities and features described here. More Info...
Characterizers for Control Loops
Commercial controllers such as the PID series (proportional, integral, derivative, and their combinations) are linear devices within their normal operating range, i.e., within set limits of input and output signals. Yet most fluid processes which they are assigned to control are nonlinear to some degree, which means that their gain in response to control action is subject to change. Two examples are given here where the gain variation was sufficient to cause control problems, and was corrected by applying characterization. More Info...
Should You be Using Lambda Tuning
A tuning method based on achieving smooth set point response is becoming more popular. The method guarantees stability, robustness and no overshoot. Shouldn't you be using it? More Info...
Measure. Decide. Act. Applying Real-Time Feedback to Deliver Business Results
By George Buckbee, P.E., ExperTune Inc.
To control a business, key measures include sales, profit, productivity, and cash flow. This white paper discusses appropriate measures for control of single loops, unit operations, and entire businesses. The “right things” have a few common characteristics. Read full article