Investigation on the Applicability of Active Magnetic Bearings to High Speed Spindle Design

A novel concept applicable to the control of spindles at high speed is developed by using active magnetic bearings (AMBs) that are non-contact and of low vibration. Extensive literature reviews explicate that the broad applications of AMBs are severely hampered by the incomplete description of the underlying electro-magnetic-mechanical dynamics. The thesis considers the gyroscopic effect inherent of a flexible rotor and explores the geometry coupling of the electro-magnetic actuators to the formulation of a comprehensive nonlinear AMB-rotor model. The model provides the basis for the creation of a novel time-frequency control algorithm whose derivation requires no linearization or mathematical simplification of any kind, thus allowing the model system to retain its true fundamental characteristics. Unlike proportional-integral-derivative (PID) controllers that are dominant in most if not all AMB configurations, the controller developed for the research is inspired by the wavelet-based nonlinear time-frequency control methodology that incorporates the basic notions of online system identification and adaptive control. Wavelet filter banks and filtered-x least-mean-square (LMS) algorithm are two of the major salient physical features of the controller design, with the former providing concurrent temporal and spectral resolutions needed for identifying nonlinear states of motion and the latter ensuring the dynamic stability of the AMB-rotor system at all operating speeds subjected to the presence of external disturbances. It is shown in the thesis that the vibration of the rotor is unconditionally controlled by maintaining the mandatory 0.55 mm air gap at 150,000 and 187,500 rpm subject to a tight spatial constraint (tolerance) of the order of 0.1375mm. System responses with and without considering the gyroscopic motion and geometry coupling are studied to demonstrate the negative impact on misinterpreting the AMB-rotor dynamics when the two effects are neglected. The case of an impact of 5,000m/s2 in magnitude and 0.001 seconds in duration at 187,500rpm is also investigated to establish the robustness of the controller design. The time responses of all the cases considered are both temporally bounded and spectrally bandwidth-limited, thus demonstrating the effectiveness of the wavelet-based time-frequency controller design in mitigating the inherent instability of the AMB-rotor system at extreme speeds

Lateral Control for Autonomous and Connected Following Vehicles with Limited Preview Information

Lateral control of an autonomous and connected vehicle (ACV), especially in emergency situations, is important from the safety viewpoint. In these situations, the trajectory to be followed by an ACV must either be planned in real-time (e.g., for a possible evasion maneuver if the obstacle to be avoided is detected) or be communicated from its preceding vehicle. Typically, the trajectory information is available to the following ACV in the form of GPS time samples. From the viewpoint of lateral control, the lateral velocity information is not readily available, and the feedback structure must reflect this reality. In this work, we develop a methodology to synthesize a lateral control algorithm for a following ACV in a two-vehicle platoon in two steps: (1) From the limited preview information of the trajectory to be tracked via samples of GPS waypoints, and we estimate the radius of curvature of the trajectory using ``least-square'' estimation and (2) develop a fixed-structure feedback control scheme for following the predecessor by synthesizing the set of stabilizing gains corresponding to lateral position error, heading error and heading rate error. Numerical simulation and experimental results corroborate the effectiveness of the proposed feedback-feedforward schemes. Based on this proposed feedback-feedforward controller, we investigated Emergency Lane Change (ELC) control problem for a convoy of autonomous and connected vehicles. Typically, an ELC maneuver is triggered by emergency cues from the front or the end of convoy as a response to either avoiding an obstacle or making way for other vehicles to pass. From a safety viewpoint, connectivity of ACVs is essential as it entails obtaining or exchanging information about other ACVs in the convoy. This thesis assumes that ACVs have reliable connectivity and that every following ACV has the information about GPS position traces of the lead and immediately preceding vehicles in the convoy. This information provides a ``discretized'' preview of the trajectory to be tracked. Based on the available information, this thesis focuses on two schemes for synthesizing lateral control of ACVs based on (a) a single composite ELC trajectory that fuses lead and preceding vehicle's GPS traces and (b) separate ELC trajectories based on preview data of preceding and lead vehicles. The former case entails the construction of a single composite ELC trajectory, determination of the cross track error, heading and yaw rate errors with respect to this trajectory and synthesis of a lateral control action. The latter case entails the construction of two separate trajectories corresponding to the lead vehicle's and preceding vehicle's data separately and the subsequent computation of two sets of associated errors and lateral control actions and combining them to provide a steering command. Numerical and experimental results corroborate the effectiveness of these two schemes. For multiple vehicle control in a convoy/platooning, identifying vehicles and objects in the vicinity of platooning vehicles is critical for convoy/platooning safety. A neighboring vehicle may change lanes and enter a lane with platooning vehicles. Any deployable platooning system must be able to first identify the presence of a cut-in vehicle before performing any control actions on the vehicles. In this work, we present a sensor fusion algorithm that combines radar and vision data obtained onboard a moving truck to identify the states of the cut-in vehicle. We then present experimental results to corroborate the performance of the proposed algorithms. First, I investigate temporal patterns of user trust and reliance in XAI systems (Objective 3). My study results show that model explanations not only affected user final trust but also shape how user trust evolves over time; indicating the importance of user behavior for evaluating XAI systems. Lastly, I propose an open-sourced human-attention evaluation baseline for direct evaluation of saliency map explanations (Objective 4). I demonstrate my human-attention benchmark's utility for quantitative evaluation of model explanations by comparing it with single-layer feature masks baseline. My experiments also show the advantage of my evaluation baseline by revealing different user biases in the subjective rating evaluation of model saliency explanations

DDAB-assisted synthesis of iodine-rich CsPbI3 perovskite nanocrystals with improved stability in multiple environments

© 2020 The Royal Society of Chemistry. All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) have attracted considerable attention due to their tunable optical properties and high optical quantum yield. However, their stability in various environments, such as different solvents, high temperature and UV light, remains to be addressed to enable their exploitation in devices. Here, we report on the synthesis of all inorganic CsPbI3 perovskite nanocrystals capped with didodecyldimethylammonium bromide (DDAB). Monodispersed DDAB-capped CsPbI3 NCs have enhanced stability with respect to their morphological and optical properties compared to conventional oleic acid (OA)/oleylamine (OLA) capped nanocrystals. The DDAB-CsPbI3 NCs retain an optical quantum yield >80% for at least 60 days. The enhanced stability is explained by the binding of branched DDAB ligands to the NC surface, leading to the formation of a halogen-rich surface, as confirmed by X-ray photoelectron spectroscopy, with an iodine to lead atomic ratio of I : Pb = 4 : 1. These perovskites were used in light-emitting diodes (LEDs) and have a maximum external quantum efficiency (EQE) of 1.25% and a luminance of 468 cd m-2, and demonstrated improved operational performance. The enhanced stability of DDAB-CsPbI3 in the environments relevant for device processing and operation is relevant for their exploitation in optoelectronics

Rational design of dibenzo[a,c]phenazine-derived isomeric thermally activated delayed fluorescence luminophores for efficient orange-red organic light-emitting diodes

It is an immense challenge to develop efficient long-wavelength (orange-to-red) thermally activated delayed fluorescence (TADF) materials due to the increasing nonradiative decay rates following the energy-gap law. Herein, two pairs of asymmetric isomers; DPyPzTPA and TPAPzDPy, and PyPzDTPA and DTPAPzPy based on electron-deficient moieties dibenzo[a,c]phenazine (Pz) and pyridine (Py) combined with electron-donor units of triphenylamine (TPA) were designed and synthesized. Their photophysical properties could be finely modulated by changing the position and number of Py groups as well as TPA fragments onto Pz cores. DPyPzTPA and DTPAPzPy possess much more rigidity and thus less geometry relaxation and non-radiative decay between ground states and excited states than those of PyPzDTPA and TPAPzDPy. Intriguingly, DPyPzTPA exhibits the highest relative photoluminescence quantum yield (ΦPL) and the fastest reverse intersystem crossing (rISC) rate among them owing to relatively stronger rigidity and spin-orbit coupling (SOC) interactions between the lowest singlet (S1) and energetically close-lying excited triplet state and therefore, the device showed the highest maximum external quantum efficiency (EQEmax) of 16.6% (60.9 lm/W, 53.3 cd/A) with Commission Internationale de I'Eclairage (CIE) coordinates of (0.43, 0.55), peak wavelength 556 nm. In stark contrast, due to its lower rigidity and extremely weak delayed fluorescence (DF) characteristic and thus the much lower ΦPL, TPAPzDPy-based devices are only half as efficient (30.8 lm/W, 27.5 cd/A, 8.3% EQE) despite the isomers possessing equal singlet-triplet energy gaps (ΔEST) of 0.43 eV. On the other hand, the device based on DTPAPzPy also demonstrated a strongly enhanced performance (59.1 lm/W, 52.7 cd/A, 16.1% EQE) than its isomer PyPzDTPA-based device (39.5 lm/W, 35.2 cd/A, 10.3% EQE). This work explicitly implicates that the asymmetric and isomeric molecular design is a potential strategy for promoting the development of highly efficient long-wavelength TADF materials