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Staying in tune: adaptive tuning can help address PID control needs, but there are factors to consider.

When considering whether adaptive tune control is right for an application, there are several key aspects to explore. How dynamic is the process? Are there overshoot problems? Are tighter control and increased accuracy important? Can consistent control help reduce scrap?

There are importance and features of adaptive tuning, and factors to consider when considering adaptive tuning for an application.

First, let's look at what adaptive tune is not. Often when "auto tune" capabilities are mentioned, it is typically "predictive tune" where the algorithm calculates what the proportional, integral and derivative values should be for the process loop to be controlled. Once the values have been set, the control of the process variable is achieved by varying only the process output percentage. As long as the process is stable, this works reasonably well.

Adaptive tune, as the name indicates, adapts to the dynamics of the process and will tune "on-the-fly," responding to certain process criteria as determined by the specifics of the adaptive algorithm being used. It changes the PID values to respond to the change in the process. When applied properly, adaptive tuning is of great value in taming hard to tune process loops. It will also tune a typical process loop more precisely.

Adaptive control algorithms can improve tuning in virtually any process because the user no longer needs to be a tuning expert, nor do they need to call one in. Even an expert, however, cannot feasibly tune some processes, because they require re-tuning as conditions change. This is true for processes that are operated at a wide range of set points such that the PID parameter values must be different at different set points. It is also true for processes that routinely undergo load changes, such as exothermic chemical reaction or shear heat that results from a plastic extrusion process. For such processes, adaptive control will provide a better match of PID parameters that are automatically optimized.

The question often arises about whether adaptive tuning will over-tune an application. Most adaptive algorithms will not over-tune a loop, however it is a question that may merit the question of the provider on how that particular adaptive algorithm tunes. If the provider can't explain how theirs works or is vague, it may require digging further to ensure it provides what you are looking for. The ideal situation is when the algorithm continuously monitors the process performance and adjusts the tuning only when needed.

Adaptive tune is ideal where a tuning expert is not available because it applies "built-in" expertise. All the operator must do is set up the sensor type and output type (such as time proportioning or burst fire), set a set point and set the control mode to tune. Then, the algorithm takes charge.

Most applications are not so dynamic that they require adaptive tuning, but virtually any process can be better tuned. The resulting PID settings (proportional band, integral reset and derivative/rate) will better reflect the thermal characteristics of the process.

When a process is well tuned, processed materials are kept closer to the target setting, and that improves yield and reduces scrap and rework of wrongly processed material. In addition, when the process variable tracks the set point better, the process spends less time warming up and stabilizing, so it is available and productive more of the time, which helps save capital and energy costs.

Most adaptive algorithms will work well across a range of different type of processes. That is, faster or slower responding loops. A faster responding process often calls for a higher proportional value, a lower integral value, and in some cases even turning the derivative to zero, whereas a slow responding loop will typically call for a lower proportional value, and higher integral value. The adaptive tune will automatically compensate for these differences in requirements.

The bottom line--adaptive tune provides "expertise-in-a-box." It takes the experience of control experts and packages it in the algorithm, making it straightforward and easy for the user to implement.

While there are similarities between the different adaptive tune algorithms, they each have their differences. The following shares some insight as to how Watlow implements its adaptive tune, called TRU-TUNE+. We will look at a couple of features called "Tune Band," and "Tune Gain," what they mean, and what they do for users.

* "Tune Band" used in this algorithm describes the process when the variable is within this band around the set point. When this occurs, TRU-TUNE+ adaptively tunes the PID parameters. When the process variable is outside this band, no tuning is performed. This prevents undesirable de-tuning of the PID parameters.

* "Tune Gain" is the parameter that determines how responsive the algorithm will be to deviations from set point and set point changes. Since the responsiveness is actually a user preference dependent upon the relative importance of preventing overshoot and minimizing time-to-set-point, this parameter is not set automatically and may be changed by the operator. There are six settings ranging from 1 with the least aggressive response and least potential overshoot (lowest gain) to six with most aggressive response and most potential for overshoot (highest gain).

The success of adaptive tuning has enabled the technology to be incorporated in more products that are being introduced to the market. Watlow recently announced that its new EZ-ZONE ST integrated control loop, which combines multiple control functions in a single package, incorporates adaptive tuning technology.

Technology in general has been advancing exponentially the last several years. Some changes are just that, changes with no limited advancement. The adaptive tune functionality brings changes that provide tangible benefits to industry and ultimately the consumer by improving quality, reducing defects and overall making products more user-friendly.

Adaptive Tuning in Action

Houston-based Advanced Containment Systems Inc (ACSI), a manufacturer of trailer mounted portable decontamination systems, which include heated showers, needed precise boiler temperature control for water used to decontaminate large numbers of people quickly in response to Hurricane Katrina in 2005.

For this application, it was critical that the shower water temperature be maintained at precisely 92[degrees]F. If the water was too cool, the hazardous material may not be successfully removed. If the water was too hot, people could be scalded or the pores of their skin could open, increasing their exposure the very chemicals that the process is designed to remove. ACSI developed three- and four-boiler trailers to increase decontamination capacity, but control of this number of boilers proved to be difficult.

Watlow's CPC400 multi-loop temperature controller with the TRU-TUNE+ adaptive algorithm was able to tune the loops automatically, minimizing setup time and effort. In addition, it was able to provide optimal performance by fine tuning loops more precisely than auto-tune features, and provided stable control through set point and load changes. ACSI's earlier systems with more than two boilers experienced unacceptable water temperature fluctuations when showers were turned on or off. Because the system includes numerous showers that can be independently turned on and off, the water flow can change quickly, and by a large volume. Prior attempts with other products failed to control this very dynamic system at ACSI's test facility. When deployed, the decontamination equipment has to provide precise water temperature control of whether the emergency happens to be in the blazing temperatures of New Orleans, or the frigid cold of Minnesota.

With the adaptive tune algorithm, the controller recognized the changing dynamics of the system and adapted on the fly while maintaining the temperature well within the application's specifications, even with the different inlet water temperatures and changes in flow rates.

Dave Meyer is regional application manager at Watlow Electric Manufacturing Co., 12001 Lackland Rd., St. Louis, MO 63246. He has been in the controls industry for more than 30 years with experience ranging the PLC applications to process sensors and controls. More information is available by calling 800-928-5692 or at www.watlow.com.
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Title Annotation:Engineering Instrumentation
Author:Meyer, Dave
Publication:Product Design & Development
Date:Mar 1, 2007
Words:1309
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