Weilin Yang

Robust Model Predictive Control for Discrete-Time Takagi–Sugeno Fuzzy Systems With Structured Uncertainties and Persistent Disturbances

In this paper, robust model predictive control for uncertain discrete-time Takagi-Sugeno (T-S) fuzzy systems with input constraints and persistent disturbances is considered. The robust positively invariant set for T-S fuzzy systems is investigated. Based on this result, computation of the terminal constraint set is proposed, which is of crucial importance in the robust predictive controller design. A zero-step predictive controller is discussed first, which has a time-varying terminal constraint set. The recursive feasibility and input-to-state stability can be ensured. Then, a novel controller with N-step prediction is further proposed, which can be used to deal with the case of fixed terminal constraint set. The implementation of the N-step controller involves both online and offline computations. It is shown that a sequence of approximating robust one-step sets can be computed offline. Then, bisection searches are carried out online, as well as a constrained convex optimization problem. The N-step controller guarantees that the system state can be steered to the terminal constraint set in less than N-steps, if the initial state lies in a specific region. Simulation results are finally presented to show the effectiveness of the proposed controllers.

A novel approach to the analysis of squeezed-film air damping in microelectromechanical systems

Squeezed-film damping (SFD) is a phenomenon that significantly affects the performance of micro-electro-mechanical systems (MEMS). The total damping force in MEMS mainly include the viscous damping force and elastic damping force. Quality factor (Q factor) is usually used to evaluate the damping in MEMS. In this work, we measure the Q factor of a resonator through experiments in a wide range of pressure levels. In fact, experimental characterizations of MEMS have some limitations because it is difficult to conduct experiments at very high vacuum and also hard to differentiate the damping mechanisms from the overall Q factor measurements. On the other hand, classical theoretical analysis of SFD is restricted to strong assumptions and simple geometries. In this paper, a novel numerical approach, which is based on lattice Boltzmann simulations, is proposed to investigate SFD in MEMS. Our method considers the dynamics of squeezed air flow as well as fluid-solid interactions in MEMS. It is demonstrated that Q factor can be directly predicted by numerical simulation, and our simulation results agree well with experimental data. Factors that influence SFD, such as pressure, oscillating amplitude, and driving frequency, are investigated separately. Furthermore, viscous damping and elastic damping forces are quantitatively compared based on comprehensive simulation. The proposed numerical approach as well as experimental characterization enables us to reveal the insightful physics of squeezed-film air damping in MEMS.

Quasi-min-max fuzzy model predictive control of direct methanol fuel cells

Abstract Direct methanol fuel cells (DMFCs) are known as a promising power source in future. In this paper, we consider steering a DMFC plant to a desired operating point while optimizing the transient performance according to a quadratic cost function. Quasi-min-max fuzzy model predictive control (FMPC) with input constraints is proposed for the DMFC. In order to reduce the computational burden for real time implementation, a partial off-line quasi-min-max FMPC is also proposed. In this case, a bank of invariant sets together with the corresponding feedback control laws are obtained by solving some linear matrix inequalities (LMIs) off-line, leaving the online part a bisection search and a much simplified constrained optimization problem. Online computation complexity for both the quasi-min-max FMPC and the partial off-line one is also analyzed. Simulation results are given to show the effectiveness of the proposed controllers.

Insights into the Impact of Surface Hydrophobicity on Droplet Coalescence and Jumping Dynamics

Droplet coalescence jumping on superhydrophobic surfaces attracts much research attention owing to its capability in enhancing condensation for energy and water applications. In this work, we reveal the impact of the finite surface adhesion to explain velocity discrepancies observed in recent droplet jumping studies, particularly when droplet sizes are a few micrometers (1-10 μm). Surface adhesion, which is usually neglected, can significantly affect both droplet coalescence and departure dynamics. It causes oscillations on velocity and contact area in the droplet coalescence process, as observed numerically and experimentally. Comparing the increasing rate of jumping velocity with contact angle for three different droplet sizes, we show that smaller droplets exhibit higher sensitivity to the change of surface hydrophobicity. We also specify the range of surface superhydrophobicity where the jumping velocity monotonically decreases (θ ≳ 170°), increases (θ ≲ 160°), or changes non-monotonically in transition (160° ≲ θ ≲170°) with droplet size. As a result, there exists a broad jumping velocity range for micrometer-sized droplets on a superhydrophobic surface with a slight contact angle variation. This work offers an extended understanding of the droplet coalescence and jumping dynamics to resolve the discrepancies in recent experimental observations.

Effect of Surface Wettability and Gas/Liquid Velocity Ratio on Microscale Two-Phase Flow Patterns

Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.Copyright

Lattice Boltzmann Simulation of Rarefied Gas Flow Along Moving Rigid Objects in Micro-Cavities

Rarefied gas flow plays an important role in the design and performance analysis of micro-electro-mechanical systems (MEMS) under high-vacuum conditions. The rarefaction can be evaluated by the Knudsen number (Kn), which is the ratio of the molecular mean free path length and the characteristic length. In micro systems, the rarefied gas flow usually stays in the slip- and transition-flow regions (10−3 10). As a result, conventional design tools based on continuum Navier-Stokes equation solvers are not applicable to analyzing rarefaction phenomena in MEMS under vacuum conditions. In this paper, we investigate the rarefied gas flow by using the lattice Boltzmann method (LBM), which is suitable for mesoscopic fluid simulation. The gas pressure determines the mean free path length and Kn, which further influences the relaxation time in the collision procedure of LBM. Here, we focus on the problem of squeezed film damping caused by an oscillating rigid object in a cavity. We propose an improved LBM with an immersed boundary approach, where an adjustable force term is used to quantify the interaction between the moving object and adjacent fluid, and further determines the slip velocity. With the proposed approach, the rarefied gas flow in MEMS with squeezed film damping is characterized. Different factors that affect the damping coefficient, such as pressure of gas and frequency of oscillation, are investigated in our simulation studies.Copyright

Analysis of squeeze film air damping in MEMS with lattice Boltzmann method

Squeeze film air damping has significant impact on the performance of microelectromechanical devices. In order to understand the squeezed-film damping mechanism, Reynolds equation and its derivatives have been used in previous studies. In fact, the Reynolds equation has limitations in quantifying MEMS characteristics because its assumptions on small amplitude and non-slip boundary condition may not be satisfied in practice. Advanced modeling approaches should be considered to capture detailed energy dissipation physics. In this paper, we study the squeeze film air damping in MEMS using lattice Boltzmann method, which is derived from classical Boltzmann transport equation. Our major focus is to reveal how the air film is squeezed by the side movement of a comb structure. By considering the slippage and amplitude effect, direct lattice Boltzmann simulations are performed to obtain the Q factor. Viscous damping and elastic damping, two contributors to the energy loss, are quantitatively compared to reveal the dominant damping mechanism.

Prediction of thin liquid film evaporation characteristics with a thermal lattice boltzmann method

Evaporation plays an important role in many industrial applications, such as power generation, cooling, and thermal management. Micro/nanostructured surfaces have the potential of enhancing evaporation heat transfer, but its heat and mass transport mechanism becomes more complicated because of the surface wettability and capillarity effects. It is imperative to understand the evaporation mechanism of thin liquid film for heat transfer enhancement. Effective evaporation occurs in the thin liquid film region, instead of in the adsorbed ultrathin film region or intrinsic meniscus region. Although some theoretical and experimental results have been reported, detailed thin film evaporation (TFE) physics, especially heat and mass transport at the liquid-vapor interface, is still unclear. In this paper, we provide a mesoscopic lattice Boltzmann method (LBM) to study the TFE problems under various heating scenarios. In our study, a thermal LBM approach is used to simulate TFE and predict some macroscopic properties. Our thermal LBM simulations agree well with the reported experimental data and theoretical results, which empowers our model to probe the interfacial TFE physics and design high-performance micro/nano engineered evaporators.

Output Feedback Model Predictive Control of Linear Parameter Varying Systems

In practical control systems, the plant states are not always measurable, so state estimation becomes essential before the state feedback control is applied. In this paper, we consider output feedback model predictive control (MPC) for linear parameter varying (LPV) systems with input constraints. We proposed two approaches to obtain the observer gain, that is to compute the gain in the dynamic optimization at each time instant (on-line), and to compute the gain in advance (off-line), respectively. By applying both approaches, the state estimation error goes to zero asymptotically, meanwhile, the state feedback gain is optimized. In fact, the on-line approach can help enlarge the feasibility region and improve the control performance. It has been shown that feasibility of both approaches can be maintained for the closed-loop control systems even in the presence of state estimation error. Finally, the proposed output-feedback MPC strategies are applied to an angular positioning control system and the control of a transcritical CO2 vapor compression refrigeration system.Copyright