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Through a detailed case study the authors demonstrate, for the first time, the capability of using advanced modeling techniques to correctly simulate the transient temperature response of a convective flow-based thermal shipper design. The objective of this case study was to demonstrate that simulation could be utilized to design a 2-inch-wall polyurethane (PUR) shipper to hold its product box temperature between 2 and 8 °C over the prescribed 96-h summer profile (product box is the portion of the shipper that is occupied by the payload). Results obtained from numerical simulation are in excellent agreement with empirical chamber data (within ±1 °C at all times), and geometrical locations of simulation maximum and minimum temperature match well with the corresponding chamber temperature measurements. Furthermore, a control simulation test case was run (results taken from identical product box locations) to compare the coupled conduction-convection model with a conduction-only model, which to date has been the state-of-the-art method. For the conduction-only simulation, all fluid elements were replaced with "solid" elements of identical size and assigned thermal properties of air. While results from the coupled thermal/fluid model closely correlated with the empirical data (±1 °C), the conduction-only model was unable to correctly capture the payload temperature trends, showing a sizeable error compared to empirical values (ΔT6 °C). A modeling technique capable of correctly capturing the thermal behavior of passively refrigerated shippers can be used to quickly evaluate and optimize new packaging designs. Such a capability provides a means to reduce the cost and required design time of shippers while simultaneously improving their performance. Another advantage comes from using thermal modeling (assuming a validated model is available) to predict the temperature distribution in a shipper that is exposed to ambient temperatures which were not bracketed during its validation.Thermal packaging is routinely used by the pharmaceutical industry to provide passive and active temperature control of their thermally sensitive products from manufacture through end use (termed the cold chain). In this study, the authors focus on passive temperature control (passive control does not require any external energy source and is entirely based on specific and/or latent heat of shipper components). As temperature-sensitive pharmaceuticals are being transported over longer distances, cold chain reliability is essential. To achieve reliability, a significant amount of time and resources must be invested in design, test, and production of optimized temperature-controlled packaging solutions. To shorten the cumbersome trial and error approach (design/test/design/test …), computer simulation (virtual prototyping and testing of thermal shippers) is a promising method. Although several companies have attempted to develop such a tool, there has been limited success to date. Through a detailed case study the authors demonstrate, for the first time, the capability of using advanced modeling techniques to correctly simulate the transient temperature response of a coupled conductive/convective-based thermal shipper. A modeling technique capable of correctly capturing shipper thermal behavior can be used to develop packaging designs more quickly, reducing up-front costs while also improving shipper performance.

In this paper we define a switching strategy for hybrid systems where both the actuating devices and the corresponding control signals are changing at the onset of a given time interval. This actuator-plus-controller switching is dictated by the need to address spatiotemporally varying disturbances in spatially distributed systems. Stability measures utilizing Lyapunov functions are employed to define the switching strategy at each time interval. To introduce a certain level of optimality, these Lyapunov functions are chosen as the optimal values of associated linear quadratic regulator functionals. To avoid the computational burden associated with an optimal strategy for a finite horizon quadratic functional, a suboptimal quadratic index is considered instead which takes the form of a cost-to-go of an infinite horizon index with a changing lower limit. This immediately imposes the solution to algebraic as opposed to differential Riccati equations. Due to this suboptimal measures, the proposed policy is applicable to linear systems with spatiotemporally varying disturbances. To validate the proposed switching policy, a flexible structure with multiple piezoceramic patches as actuators is considered. The objective then becomes that of suppressing the plate vibration that is subjected to spatiotemporally varying disturbances. Extensive simulation studies are performed to exhibit the viability of using actuator-plus-controller switching to minimize the effects of spatiotemporally varying disturbances.

We consider the problem of monitoring and scheduling a network of intelligent sensors and actuators in large flexible structures. A hybrid controller that activates a subset of the devices in the sensor and actuator network is proposed and whose objective is to effectively address the effects of spatiotemporally varying disturbances. It is assumed that the flexible structure is subjected to disturbances and the intelligent control policy attempts to activate the actuators that, over the duration of a time interval, have spatial authority against the spatiotemporally varying disturbances. The hybrid controller switches the relevant actuators on and deactivates the ones that are not used over a given time interval. The basic idea behind the proposed supervisory scheme is to show that using a switched controller, it is possible to simultaneously improve controller performance and reduce the power consumption in the actuator/sensor network. Both analytical and experimental results on a flexible aluminum plate with eight pairs of collocated actuator/sensor PZT pairs are presented.

The problem of actuator and sensor placement in a flexible plate is revisited within the context of an intelligent control scheme. Instead of considering individual actuators and sensors, we consider groups of actuators and sensors that have the same capacity to address specific modes. The placement optimization procedure chooses actuators and sensors within a given group so that can collectively address a specific range of modal frequencies. Integrated into the control scheme is the ability to select, over a time interval of fixed length, a given group that can best address spatiotemporally varying disturbances in which the spatial distribution of disturbances changes with time. For the numerical studies on a thin aluminum plate, clamped on all sides and employing piezoceramic patches as collocated actuators/sensors, we consider four groups of PZT actuators/sensors wherein each actuator in each group is designed to have a high level of modal controllability with respect to a given modal shape. Incorporated into the above optimization is the influence of each PZT on the plate's modal shapes. The intelligent control then provides the switching scheme in which, at a given time instance, only one of the four groups is active with the remaining three being kept dormant in order to reduce power consumption.

In this note, we revisit the problem of actuator/sensor placement in large civil infrastructures and flexible space structures within the context of spatial robustness. The positioning of these devices becomes more important in systems employing wireless sensor and actuator networks (WSAN) for improved control performance and for rapid failure detection. The ability of the sensing and actuating devices to possess the property of spatial robustness results in reduced control energy and therefore the spatial distribution of disturbances is integrated into the location optimization measures. In our studies, the structure under consideration is a flexible plate clamped at all sides. First, we consider the case of sensor placement and the optimization scheme attempts to produce those locations that minimize the effects of the spatial distribution of disturbances on the state estimation error; thus the sensor locations produce state estimators with minimized disturbance-to-error transfer function norms. A two-stage optimization procedure is employed whereby one first considers the open loop system and the spatial distribution of disturbances is found that produces the maximal effects on the entire open loop state. Once this "worst" spatial distribution of disturbances is found, the optimization scheme subsequently finds the locations that produce state estimators with minimum transfer function norms. In the second part, we consider the collocated actuator/sensor pairs and the optimization scheme produces those locations that result in compensators with the smallest norms of the disturbance-to-state transfer functions. Going a step further, an intelligent control scheme is presented which, at each time interval, activates a subset of the actuator/sensor pairs in order provide robustness against spatiotemporally moving disturbances and minimize power consumption by keeping some sensor/actuators in sleep mode.

The objective of this investigation is to present a computational scheme for the optimal placement of actuating devices in flexible plates that utilize piezoceramic patches as actuators. In addition to controllability criteria, the candidate locations are required to also exhibit robustness with respect to the spatial distribution of disturbances. These PZT actuators are bonded at optimal locations that are determined by statically minimizing the optimal value of a disturbance-to-output transfer function with respect to the worst distribution of disturbances. Once the optimal actuator locations are determined, that in addition to performance specifications also satisfy a spatial robustness criterion, a suitable LQR-based controller is designed. At a given interval of time, only one PZT is activated and the remaining ones are kept dormant. The rationale of actuator/controller switching is to demonstrate the better vibration alleviation characteristics of switching between actuators over the use of a single actuator that is always in continuous use. The optimality of switching is made with respect to a cost-to-go performance index that corresponds to each actuating device. Extensive computer simulations with repeatable spatiotemporally varying disturbance profiles, reveal that this algorithm offers better performance over the non-switched case.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.