Adaptive Tune Addresses Control Needs
by Dave Meyer, Watlow, www.watlow.com [1] To begin to understand adaptive tune control, it is important to discuss what isn’t adaptive tune. 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. How dynamic is the process? Are there overshoot problems? Are tighter control and increased accuracy important? Can consistent control help reduce scrap? 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 Solution
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 proportional, integral, derivative (PID) values to respond to the change in the process.When applied properly, it 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 tune in virtually any process because the user no longer needs to be a tune expert, nor do they need to call one. Even an expert cannot feasibly tune some processes, because they require re-tune 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 tune will over-tune an application. Most adaptive algorithms will not over-tune a loop; however the issue may merit questioning 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 tune only when needed.
Built-in Expertise Improves Tune
Adaptive tune is ideal where a tune 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 tune, 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 mis-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 types 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. Adaptive tune takes the experience of control experts and packages it in the algorithm, making it straightforward and easy for the user to implement.
The Difference is in the Algorithms
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 their 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, TRUTUNE+ adaptively tunes the PID parameters. (See Image 2.) When the process variable is outside this band, no tune is performed. This prevents undesirable de-tune 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-setpoint, 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 6 with most aggressive response and most potential for overshoot (highest gain).
A Case Study
A leading manufacturer of trailer mounted portable decontamination systems, including 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°. 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 to the very chemicals that the process was designed to remove. The company developed three- and four-boiler trailers to increase decontamination capacity, but control of this number of boilers proved to be difficult. A multi-loop temperature controller with an 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 tune loops more precisely than autotune features, and provided stable control through set point and load changes. Earlier systems with more than two boilers experienced unacceptable water temperature fluctuations when showers were turned ON or OFF. Prior attempts with other products failed to control this very dynamic system at a test facility.When deployed, the decontamination equipment has to provide precise water temperature control regardless of whether the emergency happens in the blazing temperatures of New Orleans, or the frigid cold of Minnesota.
Don’t Get Burned in DC/DC Thermal Management
by Carl Schramm, RECOM, www.recom-international.com
The efficiency of any energy conversion process is always less than 100 percent. Waste is a fact of life, and a part of any of the energy used in a system goes astray and is converted into heat. Ultimately this waste heat must be removed. Since the laws of thermodynamics state that heat energy can only flow from a warmer to a colder environment, if the internal heat of a device is to be dissipated the ambient temperature must be lower than the internal temperature. The smaller this difference is, the less heat energy will be dissipated.
Temperature Terms
Which temperature specifications should you consult for your calculations? RECOMs declare two values in its datasheets: the operating temperature range with or without derating and the maximum case temperature. Some manufacturers would say that these two values are the same. The maximum case (surface) temperature of many DC/DC modules is typically given as +100°C or +105°C. This value appears at first to be very high; however this figure includes not only the self-warming through internal losses but also the ambient temperature itself. Remember that the smaller the difference between case surface and ambient, the less heat lost. If a converter has higher internal losses, it will be more affected by a smaller temperature difference than a converter with lower internal losses. The internal losses occur mainly through switching losses in the transistors, rectification losses, core losses in the transformer, and resistive losses in the windings and tracks. The maximum allowable internal temperature is determined by the Curie temperature of the transformer core material, the maximum junction temperature in the switching transistors and rectification diodes and the maximum operating temperature of the capacitors.
Exploring Solutions
If the thermal dissipation calculations reveal that the DC/DC module will overheat at the desired ambient operating temperature, there are still a number of options available to reach a solution. One option is to derate the converter, i.e., use a higher power converter running at less than full load. The derating diagrams in the datasheets essentially define the maximum load at any given temperature within the operating temperature range. The derating curves are in reality not as linear as they are declared in most datasheets. However, reliable manufacturers will always err on the safe side so that the values given can be safely relied on in practice. If the converter has a plastic case, the next largest case size with the same power rating could be chosen to increase the available surface area. However, care must be taken not to compromise on efficiency; otherwise no net gain will be made. If the converter has a metal case, adding a heat sink can be very effective, particularly in conjunction with a forced-air cooling system. If a heat sink is used with fan cooling, the thermal resistance equation becomes:
where:
RTHcase-ambient Thermal
impedance from the case to the ambient surroundings
RTHcase-HS Thermal impedance
from the case to the heat sink
RTHHS-ambient Thermal
impedance from the heat sink to ambient
The value of RTHHS-ambient incorporates the thermal resistance of the heat sink as well as the thermal resistance of any thermally conductive paste or silicon pads used for a better thermal contact to the case. If these aids are not applied, then a value of approximately 0.2 K/W must be added to the thermal resistance of the heat sink alone.When establishing of the value of RTHHSambient it is also necessary to know how much air is being blown across the heat sink fins. These values are most often given in lfm (linear feet per minute) and declared by the fan manufacturer. The conversion to m/s is 100 lfm = 0.5 m/s. If the results of your calculations or measurements are borderline, then the issue must be examined in more depth. Sometimes a simple fix is enough, and that there is a difference in thermal performance between vertically and horizontally mounted modules, between static air, moving air, and air at low atmospheric pressures.
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