Search by property

This page provides a simple browsing interface for finding entities described by a property and a named value. Other available search interfaces include the page property search, and the ask query builder.

List of results

• Passive Control of Flutter and Forced Response in Bladed Disks via Mistuning  + (Mistuning or blade to blade variation in j … Mistuning or blade to blade variation in jet-engine bladed-disks can</br>lead to large changes in engine performance. Even the small random</br>mistuning associated with manufacturing tolerances can significantly</br>change both stability boundaries and forced response. This thesis</br>addresses two questions. Analysis: given any mistuning (random or intentional),</br>what is the resulting change in performance? And passive control: can intentiona</br>l </br>mistuning be used to improve stability and forced response in a robust</br>manner? </br><p></br>A general framework based on symmetry arguments and eigenvalue/vector</br>perturbations is presented to answer both questions. </br>Symmetry constrains all facets of mistuning behaviour and provides</br>simplifications for both the analysis and control problems. This is combined</br>with the eigenvalue/vector perturbation which captures the nonlinear</br>mistuning dependence and solves the analysis problem. It is shown that</br>intentional mistuning can provide robust damping and so guarantee</br>improved stability and forced response under fixed manufacturing</br>tolerances. Results are demonstrated on a high-fidelity low-order</br>model derived from computational-fluid-dynamic data. </br><p>ived from computational-fluid-dynamic data. <p>)
• Time-Optimal Navigation in Uncertain Environments with High-Level Specifications  + (Mixed observable Markov decision processes … Mixed observable Markov decision processes (MOMDPs) are a modeling framework for autonomous systems described by both fully and partially observable states. In this work, we study the problem of synthesizing a control policy for MOMDPs that minimizes the expected time to complete the control task while satisfying syntactically co-safe Linear Temporal Logic (scLTL) specifications. First, we present an exact dynamic programming update to compute the value function. Afterwards, we propose a point-based approximation, which allows us to compute a lower bound of the closed-loop probability of satisfying the specifications. The effectiveness of the proposed approach and comparisons with standard strategies are shown on high-fidelity navigation tasks with partially observable static obstacles.ith partially observable static obstacles.)
• Formal Verification of an Autonomous Vehicle System  + (Model checking is a widely used technique … Model checking is a widely used technique for formal verification of distributed systems. It works by effectively examining the complete reachable state space of a model in order to determine whether the system satisfies its requirements or desired properties. The complexity of an autonomous vehicle system, however, renders model checking of the entire system infeasible due to the state explosion problem. In this paper, we illustrate how to exploit the structure of the system to systematically decompose the overall system-level requirements into a set of component-level requirements. Each of the components can then be model checked separately. A case study is presented where we formally verify the state consistency between different software modules of Alice, an autonomous vehicle developed by the California Institute of Technology for the 2007 DARPA Urban Challenge.nology for the 2007 DARPA Urban Challenge.)
• Model Predictive Control of a Thurst-Vectored Flight Control Experiment  + (Model predictive control (MPC) is applied … Model predictive control (MPC) is applied to the Caltech ducted fan, a</br>thrust-vectored flight experiment. A real-time trajectory generation software based</br>on spline theory and sequential quadratic programming is used to implement the</br>MPC controllers. Timing issues related to the computation and implementation of</br>repeatedly updated optimal trajectories are discussed. Results show computational</br>speeds greater than 10 Hz, 2.5 times that of the actuator dynamics. The MPC</br>controllers successfully stabilize a step disturbance applied to the ducted fan and</br>compare favorably to LQR methods. fan and compare favorably to LQR methods.)
• Robustness guarantees for structured model reduction of dynamical systems with applications to biomolecular models  + (Model reduction methods usually focus on t … Model reduction methods usually focus on the error performance analysis; however, in presence of uncertainties, it is important to analyze the robustness properties of the error in model reduction as well. This problem is particularly relevant for engineered biological systems that need to function in a largely unknown and uncertain environment. We give robustness guarantees for structured model reduction of linear and nonlinear dynamical systems under parametric uncertainties. We consider a model reduction problem where the states in the reduced model are a strict subset of the states of the full model, and the dynamics for all of the other states are collapsed to zero (similar to quasi-steady-state approximation). We show two approaches to compute a robustness guarantee metric for any such model reduction—a direct linear analysis method for linear dynamics and a sensitivity analysis based approach that also works for nonlinear dynamics. Using the robustness guarantees with an error metric and an input-output mapping metric, we propose an automated model reduction method to determine the best possible reduced model for a given detailed system model. We apply our method for the (1) design space exploration of a gene expression system that leads to a new mathematical model that accounts for the limited resources in the system and (2) model reduction of a population control circuit in bacterial cells.lation control circuit in bacterial cells.)
• Robustness Guarantees for Structured Model Reduction of Dynamical Systems  + (Model reduction methods usually focus on t … Model reduction methods usually focus on the error performance analysis; however, in presence of uncertainties, it is important to analyze the robustness properties of the error in model reduction as well. In this paper, we give robustness guarantees for structured model reduction of linear and nonlinear dynamical systems under parametric uncertainties. In particular, we consider a model reduction where the states in the reduced model are a strict subset of the states of the full model, and the dynamics for all other states are collapsed to zero (similar to quasi-steady state approximation). We show two approaches to compute a robustness metric for any such model reduction — a direct linear analysis method for linear dynamics and a sensitivity analysis based approach that also works for nonlinear dynamics. We also prove that for linear systems, both methods give equivalent results.ems, both methods give equivalent results.)
• Linear models for control of cavity flow oscillations  + (Models for understanding and controlling o … Models for understanding and controlling oscillations in the flow past a rectangular cavity are developed. These models may be used to guide control designs, to understand performance limits of feedback, and to interpret experimental results. Traditionally, cavity oscillations are assumed to be self-sustained: no external disturbances are necessary to maintain the oscillations, and amplitudes are limited by nonlinearities. We present experimental data which suggests that in some regimes, the oscillations may not be self-sustained, but lightly damped: oscillations are sustained by external forcing, such as boundary-layer turbulence. In these regimes, linear models suffice to describe the behaviour, and the final amplitude of oscillations depends on the characteristics of the external disturbances. These linear models are particularly appropriate for describing cavities in which feedback has been used for noise suppression, as the oscillations are small and nonlinearities are less likely to be important. It is shown that increasing the gain too much in such feedback control experiments can lead to a peak-splitting phenomenon, which is explained by the linear models. Fundamental performance limits indicate that peak splitting is likely to occur for narrow-bandwidth actuators and controllers.arrow-bandwidth actuators and controllers.)
• Synthesis and Validation of Control Software For A Vehicular Electric Power Distribution Testbed  + (Modern aircraft increasing rely on electri … Modern aircraft increasing rely on electric power, resulting in high safety-criticality and complexity in their electric power generation and distribution systems. Moti- vated by the resulting rapid increase in the costs and duration of the design cycles for such systems, we investigate the use of formal specification and automated, correct-by-construction control protocols synthesis for primary distribution in vehicular electric power networks. We discuss a design workflow that aims to transition from the traditional âdesign+verifyâ approach to a âspecify+synthesizeâ approach. We give an overview of a subset of the recent advances in the synthesis of reactive control proto- cols. We apply these techniques in the context of reconfiguration of the networks in reaction to the changes in their operating environment. We also validate these automatically synthesized control protocols on high-fidelity simulation models and on an academic-scale hardware testbed.and on an academic-scale hardware testbed.)
• An Aircraft Electric Power Testbed for Validating Automatically Synthesized Reactive Control Protocols  + (Modern aircraft increasingly rely on elect … Modern aircraft increasingly rely on electric power for sub- systems that have traditionally run on mechanical power. The complexity and safety-criticality of aircraft electric power systems have therefore increased, rendering the design of these systems more challenging. This work is mot vated by the potential that correct-by-construction reactive controller synthesis tools may have in increasing the effectiveness of the electric power system design cycle. In particular, we have built an experimental hardware platform that captures some key elements of aircraft electric power systems within a simplified setting. We intend to use this plat- form for validating the applicability of theoretical advances in correct-by-construction control synthesis and for study- ing implementation-related challenges. We demonstrate a simple design workflow from formal specifications to auto- generated code that can run on software models and be used in hardware implementation. We show some preliminary results with different control architectures on the developed hardware testbed.ectures on the developed hardware testbed.)
• Fast Automatic Verification of Large-Scale Systems with Lookup Tables  + (Modern safety-critical systems are difficu … Modern safety-critical systems are difficult to formally verify, largely due to their large scale. In particular, the widespread use of lookup tables in embedded systems across diverse industries, such as aeronautics and automotive systems, create a critical obstacle to the scala- bility of formal verification. This paper presents a novel approach for the formal verification of large-scale systems with lookup tables. We use a learning-based technique to automatically learn abstractions of the lookup tables and use the abstractions to then prove the desired property. If the verification fails, we propose a falsification heuristic to search for a violation of the specification. In contrast with previous work on lookup table verification, our technique is completely automatic, making it ideal for deployment in a production environment. To our knowledge, our approach is the only technique that can automatically verify large-scale systems lookup with tables.</br></br>We illustrate the effectiveness of our technique on a benchmark which cannot be handled by the commer- cial tool SLDV, and we demonstrate the performance improvement provided by our technique.nce improvement provided by our technique.)
• Bistable State Switch Enables Ultrasensitive Feedback Control in Heterogeneous Microbial Populations  + (Molecular feedback control circuits can im … Molecular feedback control circuits can improve robustness of gene expression at the single cell-level. This achievement can be offset by requirements of rapid protein expression, that may induce cellular stress, known as burden, that reduces colony growth. To begin to address this challenge we take inspiration by ‘division-of-labor’ in heterogeneous cell populations: we propose to combine bistable switches and quorum sensing systems to coordinate gene expression at the population-level. We show that bistable switches in individual cells operating in parallel yield an ultrasensitive response, while cells maintain heterogeneous levels of gene expression to avoid burden across all cells. Within a feedback loop, these switches can achieve robust reference tracking and adaptation to disturbances at the population-level. We also demonstrate that molecular sequestration enables tunable hysteresis in individual switches, making it possible to obtain a wide range of stable population-level expressions.ge of stable population-level expressions.)
• Robust Model Predictive Control for Signal Temporal Logic Synthesis  + (Most automated systems operate in uncertai … Most automated systems operate in uncertain or adversarial conditions, and have to be capable of reliably reacting to changes in the environment. The focus of this paper is on automatically synthesizing reactive controllers for cyber-physical systems subject to signal temporal logic (STL) specifications. We build on recent work that encodes STL specifications as mixed integer linear constraints on the variables of a discrete-time model of the system and environment dynamics. To obtain a reactive controller, we present solutions to the worst-case model predictive control (MPC) problem using a suite of mixed integer linear programming techniques. We demonstrate the comparative effectiveness of several existing worst-case MPC techniques, when applied to the problem of control subject to temporal logic specifications; our empirical results emphasize the need to develop specialized solutions for this domain.lop specialized solutions for this domain.)
• Reactive motion planning with probabilistic safety guarantees  + (Motion planning in environments with multi … Motion planning in environments with multiple agents is critical to many important autonomous applications such as autonomous vehicles and assistive robots. This paper considers the problem of motion planning, where the controlled agent shares the environment with multiple uncontrolled agents. First, a predictive model of the uncontrolled agents is trained to predict all possible trajectories within a short horizon based on the scenario. The prediction is then fed to a motion planning module based on model predictive control. We proved generalization bound for the predictive model using three different methods, post-bloating, support vector machine (SVM), and conformal analysis, all capable of generating stochastic guarantees of the correctness of the predictor. The proposed approach is demonstrated in simulation in a scenario emulating autonomous highway driving.ario emulating autonomous highway driving.)
• Synthesis of Embedded Control Systems with High Sampling Frequencies  + (Motivated by current technological advance … Motivated by current technological advances in the design of real-time embedded systems, this work deals with the digital control of a continuous-time linear time-invariant (LTI) system whose output can be sampled at a high frequency. Since a typical sampled-data controller operating at a high sampling frequency needs heavy (high-precision) computation to alleviate its sensitivity to measurement and computational errors, the objective is to design a robust hybrid controller for high- frequency applications with limited computational power. To this end, we exploit our recent results on delay-based controller design and propose a digital-control scheme that can implement every continuous-time stabilizing (LTI) controller. This robust hybrid controller, which consists of an ideal sampler, a digital controller, a number of modified second-order holds and possibly a unity feedback, can operate at arbitrarily high sampling frequencies without requiring expensive, high-precision computation. Later on, it is discussed how to find a continuous-time LTI controller satisfying prescribed design specifications so that its correspond- ing digital controller requires the least processing time.roller requires the least processing time.)
• Intermittent Connectivity for Exploration in Communication-Constrained Multi-Agent Systems  + (Motivated by exploration of communication- … Motivated by exploration of communication- constrained underground environments using robot teams, we study the problem of planning for intermittent connectivity in multi-agent systems. We propose a novel concept of information-consistency to handle situations where the plan is not initially known by all agents, and suggest an integer linear program for synthesizing information-consistent plans that also achieve auxiliary goals. Furthermore, inspired by network flow problems we propose a novel way to pose connectivity constraints that scales much better than previous methods. In the second part of the paper we apply these results in an exploration setting, and propose a clustering method that separates a large exploration problem into smaller problems that can be solved independently. We demonstrate how the resulting exploration algorithm is able to coordinate a team of ten agents to explore a large environment.ten agents to explore a large environment.)
• Stabilization of a Pitch Axis Flight Control Experiment with Input Rate Saturation  + (Motivated by problems in flight control, w … Motivated by problems in flight control, we present a technique for</br>stabilizing a chain </br>of integrators in the presence of rate limitations on the input. Our</br>technique improves on several existing techniques in the literature</br>and has a number of interesting features. The controller is</br>evaluated experimentally on a pitch axis flight control experiment at</br>Caltech. The experimental results show that even in the presence of</br>rate limits that cause a linear controller to go unstable, the</br>time-varying controller stabilizes the system with minimal loss in performance.e system with minimal loss in performance.)
• Effects of Magnitude Saturation in Control of Bifurcations  + (Motivated by problems such as active contr … Motivated by problems such as active control of rotating stall in compression systems,</br>an analysis of the effects of controller magnitude saturation in feedback stabilization of</br>steady-state bifurcations is performed. In particular the region of attraction to the</br>stabilized bifurcated equilibria is solved for feedback controllers with magnitude</br>saturation limits using the technique of center manifold reduction and bifurcation</br>analysis. It has been shown that the stability boundary is the saturation envelope formed</br>by the unstable (or stable) equilibria for the closed loop system when the controllers</br>saturate. The framework allows the design of feedback control laws to achieve desirable</br>size of region of attraction when the noise is modeled as a closed set of initial</br>conditions in the phase space. It is also possible to extend the techniques and results to</br>Hopf bifurcations.chniques and results to Hopf bifurcations.)
• Efficient reactive controller synthesis for a fragment of linear temporal logic  + (Motivated by robotic motion planning, we d … Motivated by robotic motion planning, we develop a framework for control policy synthesis for both non-deterministic transition systems and Markov decision processes that are subject to temporal logic task specifications. We introduce a fragment of linear temporal logic that can be used to specify common motion planning tasks such as safe navigation, response to the environment, surveillance, and persistent coverage. This fragment is computationally efficient; the complexity of control policy synthesis is a doubly-exponential improvement over standard linear temporal logic for both non-deterministic transition systems and Markov decision processes. This improvement is possible since we compute directly on the original system, as opposed to the automata-based approach commonly used for linear temporal logic. We give simulation results for representative motion planning tasks and compare to generalized reactivity(1). and compare to generalized reactivity(1).)
• Delay-Based Controller Design for Continuous-Time and Hybrid Applications  + (Motivated by the availability of different … Motivated by the availability of different types of delays in embedded systems and biological circuits, the objective of this work is to study the benefits that delay can provide in simplifying the implementation of controllers for continuous-time systems. Given a continuous-time linear time-invariant (LTI) controller, we propose three methods to approximate this controller arbitrarily precisely by a simple controller composed of delay blocks, a few integrators and possibly a unity feedback. Different problems associated with the approximation procedures, such as finding the optimal number of delay blocks or studying the robustness of the designed controller with respect to delay values, are then investigated. We also study the design of an LTI continuous-time controller satisfying given control objectives whose delay-based implementation needs the least number of delay blocks. A direct application of this work is in the sampled-data control of a real-time embedded system, where the sampling frequency is relatively high and/or the output of the system is sampled irregularly. Based on our results on delay-based controller design, we propose a digital-control scheme that can implement every continuous-time stabilizing (LTI) controller. Unlike a typical sampled-data controller, the hybrid controller introduced here---consisting of an ideal sampler, a digital controller, a number of modified second-order holds and possibly a unity feedback---is robust to sampling jitter and can operate at arbitrarily high sampling frequencies without requiring expensive, high-precision computation.ing expensive, high-precision computation.)
• Control System Analysis on Symmetric Cones  + (Motivated by the desire to analyze high di … Motivated by the desire to analyze high dimen- sional control systems without explicitly forming computation- ally expensive linear matrix inequality (LMI) constraints, we seek to exploit special structure in the dynamics matrix. By using Jordan algebraic techniques we show how to analyze continuous time linear dynamical systems whose dynamics are exponentially invariant with respect to a symmetric cone. This allows us to characterize the families of Lyapunov functions that suffice to verify the stability of such systems. We highlight, from a computational viewpoint, a class of systems for which stability verification can be cast as a second order cone program (SOCP), and show how the same framework reduces to linear programming (LP) when the system is internally positive, and to semidefinite programming (SDP) when the system has no special structure. when the system has no special structure.)
• A geometric and structural approach to the analysis and design of biological circuit dynamics: a theory tailored for synthetic biology  + (Much of the progress in developing our abi … Much of the progress in developing our ability to successfully design genetic circuits with predictable dynamics has followed the strategy of molding biological systems to fit into conceptual frameworks used in other disciplines, most notably the engineering sciences. Because biological systems have fundamental differences from systems in these other disciplines, this approach is challenging and the insights obtained from such analyses are often not framed in a biologically-intuitive way. Here, we present a new theoretical framework for analyzing the dynamics of genetic circuits that is tailored towards the unique properties associated with biological systems and experiments. Our framework approximates a complex circuit as a set of simpler circuits, which the system can transition between by saturating its various internal components. These approximations are connected to the intrinsic structure of the system, so this representation allows the analysis of dynamics which emerge solely from the system’s structure. Using our framework, we analyze the presence of structural bistability in a leaky autoactivation motif and the presence of structural oscillations in the Repressilator.uctural oscillations in the Repressilator.)
• Inverse Abstraction of Neural Networks Using Symbolic Interpolation  + (Neural networks in real-world applications … Neural networks in real-world applications have to satisfy critical properties such as safety and reliability. The analysis of such properties typically involves extracting informa- tion through computing pre-images of neural networks, but it is well-known that explicit computation of pre-images is intractable. We introduce new methods for computing compact symbolic abstractions of pre-images. Our approach relies on computing approximations that provably overapproximate and underapproximate the pre-images at all layers. The abstraction of pre-images enables formal analysis and knowl- edge extraction without modifying standard learning algo- rithms. We show how to use inverse abstractions to automatically extract simple control laws and compact representations for pre-images corresponding to unsafe outputs. We illustrate that the extracted abstractions are often interpretable and can be used for analyzing complex properties. be used for analyzing complex properties.)
• Resource optimisation in a wireless sensor network with guaranteed estimator performance  + (New control paradigms are needed for large … New control paradigms are needed for large networks of wireless sensors and actuators in order to efficiently utilise system resources. In this study, the authors consider the problem of discrete-tie state estimation over a wireless sensor network. Given a tree that represents the sensor communications with the fusion centre, the authors derive the optimal estimation algorithm at the fusion centre, and provide a closed-form expression for the steady-state error covariance matrix. They then present a tree reconfiguration algorithm that produces a sensor tree that has low overall energy consumption and guarantees a desired level of estimation quality at the fusion centre. The authors further propose a sensor tree construction and scheduling algorithm that leads to a longer network lifetime than the tree reconfiguration algorithm. Examples are provided throughout the paper to demonstrate the algorithms and theory developed.trate the algorithms and theory developed.)
• Finding stationary solutions to the chemical master equation by gluing state spaces at one or two states recursively  + (Noise is indispensible to key cellular act … Noise is indispensible to key cellular activities, including gene expression coordination and probabilistic differentiation. Stochastic models, such as the chemical master equation (CME), are essential to model noise in the levels of cellular components. In the CME framework, each state is associated with the molecular counts of all component species, and specifies the probability for the system to have that set of molecular counts. Analytic solutions to the CME are rarely known but can bring exciting benefits. For instance, simulations of biochemical reaction networks that are multiscale in time can be sped up tremendously by incorporating analytic solutions of the slow time-scale dynamics. Ana- lytic solutions also enable the design of stationary distributions with properties such as the modality of the distribution, the mean expression level, and the level of noise. One way to derive the analytic steady state response of a biochemical reaction network was re- cently proposed by (Mélykúti et al. 2014). The paper recursively glues simple state spaces together, for which we have analytic solutions, at one or two states.</br><p></br>In this work, we explore the benefits and limitations of the gluing technique proposed by Mélykúti et al., and introduce recursive algorithms that use the technique to solve for the analytic steady state response of stochastic biochemical reaction networks. We give formal characterizations of the set of reaction networks whose state spaces can be obtained by carrying out single-point gluing of paths, cycles or both sequentially. We find that the dimension of the state space of a reaction network equals the maximum number of linearly independent reactions in the system. We then characterize the complete set of stochastic biochemical reaction networks that have elementary reactions and two-dimensional state spaces. As an example, we propose a recursive algorithm that uses the gluing technique to solve for the steady state response of a mass-conserving system with two connected monomolecular reversible reactions. Even though the gluing technique can only construct finite state spaces, we find that, by taking the size of a finite state space to infinity, the steady state response can converge to the analytic solution on the resulting infinite state space. Finally, we illustrate the aforementioned ideas with the example of two interconnected transcriptional components, which was first studied by (Ghaemi and Del Vecchio 2012). first studied by (Ghaemi and Del Vecchio 2012).)
• Recursively constructing analytic expressions for equilibrium distributions of stochastic biochemical reaction networks  + (Noise is often indispensable to key cellul … Noise is often indispensable to key cellular activities, such as gene expression, necessitating the use of stochastic models to capture its dynamics. The chemical master equation (CME) is a commonly used stochastic model of Kolmogorov forward equations that describe how the probability distribution of a chemically reacting system varies with time. Finding analytic solutions to the CME can have benefits, such as expediting simulations of multiscale biochemical reaction networks and aiding the design of distributional responses. However, analytic solutions are rarely known. A recent method of computing analytic stationary solutions relies on gluing simple state spaces together recursively at one or two states. We explore the capabilities of this method and introduce algorithms to derive analytic stationary solutions to the CME. We first formally characterize state spaces that can be constructed by performing single-state gluing of paths, cycles or both sequentially. We then study stochastic biochemical reaction networks that consist of reversible, elementary reactions with two-dimensional state spaces. We also discuss extending the method to infinite state spaces and designing the stationary behaviour of stochastic biochemical reaction networks. Finally, we illustrate the aforementioned ideas using examples that include two interconnected transcriptional components and biochemical reactions with two-dimensional state spaces.actions with two-dimensional state spaces.)

• Nonlinear Control of Mechanical Systems: A Reimannian Geometry Approach  + (Nonlinear control of mechanical systems is … Nonlinear control of mechanical systems is a challenging discipline that lies at the</br>intersection between control theory and geometric mechanics. This thesis sheds new light</br>on this interplay while investigating motion control problems for Lagrangian systems. Both</br>stability and motion planning aspects are treated within a unified framework that accounts</br>for a large class of devices such as robotic manipulators, autonomous vehicles and</br>locomotion systems.</p></br></br><p>One distinguishing feature of mechanical systems is the number of control forces. For</br>systems with as many input forces as degrees of freedom, many control problems are</br>tractable. One contribution of this thesis is a set of trajectory tracking controllers</br>designed via the notions of configuration and velocity error. The proposed approach</br>includes as special cases a variety of results on joint and workspace control of</br>manipulators as well as on attitude and position control of vehicles.</p></br></br><p>Whenever fewer input forces are available than degrees of freedom, various control</br>questions arise. The main contribution of this thesis is the design of motion algorithms</br>for vehicles, i.e., rigid bodies moving in Euclidean space. First, an algebraic</br>controllability analysis characterizes the set of reachable configurations and velocities</br>for a system starting at rest. Then, provided a certain controllability condition is</br>satisfied, various motion algorithms are proposed to perform tasks such as short range</br>reconfiguration and hovering. </p></br></br><p>Finally, stabilization techniques for underactuated systems are investigated. The</br>emphasis is on relative equilibria, i.e., steady motions for systems that have a conserved</br>momentum. Local exponential stabilization is achieved via an appropriate splitting of the</br>control authority.abilization is achieved via an appropriate splitting of the control authority.)
• Bifurcation Control of Rotating Stall with Actuator Magnitude and Rate Limits  + (Nonlinear qualitative analysis is performe … Nonlinear qualitative analysis is performed on the Moore-Greitzer </br> model to evaluate the tradeoff of fluid noise, actuator magnitude saturation, </br> bandwidth, rate limits, and the shape of compressor characteristics in active </br> control of rotating stall in axial compressors with bleed valve actuators. Model </br> order reduction is achieved by approximating the dynamics on the invariant manifold </br> that captures the bifurcations and instabilities. Bifurcations and qualitative </br> dynamics are obtained by analyzing the reduced system. The operability enhancement </br> is defined as the extension of operating range for which fully developed rotating </br> stall is avoided. Analytic formulas are derived for the operability enhancement </br> as a function of noise level, actuator saturation limits, and the shape of the </br> compressor characteristic, which is the major nonlinearity in the model. The </br> shape of the compressor characteristic, especially the unstable part, is critical </br> to the rate required for robust operability near the peak for the closed loop </br> system. Experiments are carried out on a single-stage low-speed axial compressor </br> using different level of steady air injections to generate different compressor </br> characteristics. The theoretical formulas give good qualitative estimates to </br> experimental data and simulations using a high fidelity model (37 states).s using a high fidelity model (37 states).)
• Fault detection and isolation from uninterpreted data in robotic sensorimotor cascades  + (One of the challenges in designing the nex … One of the challenges in designing the next genera- tion of robots operating in non-engineered environments is that there seems to be an infinite amount of causes that make the sensor data unreliable or actuators ineffective. In this paper, we discuss what faults are possible to detect using zero modeling effort: we start from uninterpreted streams of observations and commands, and without a prior knowledge of a model of the world. We show that in sensorimotor cascades it is possible to define static faults independently of a nominal model. We define an information-theoretic usefulness of a sensor reading and we show that it captures several kind of sensorimotor faults frequently encountered in practice. We particularize these ideas to the case of BDS/BGDS models, proposed in previous work as suitable candidates for describing generic sensorimotor cascades. We show several examples with camera and range-finder data, and we discuss a possible way to integrate these techniques in an existing robot software architecture.n an existing robot software architecture.)
• Modeling the Effects of Compositional Context on Promoter Activity in an E. Coli Extract based Transcription-Translation System  + (One of the fundamental challenges in synth … One of the fundamental challenges in synthesizing complex biocircuits from existing biocircuit components is understanding how the spatial arrangement of biocircuit components impacts component behavior. In this paper we develop a set of synthetic biology parts for systematically probing the effects of spatial arrangement on transcriptional expression. Our initial experimental assays prove that even the rearrangement of two biocircuit parts (comprised of a promoter, coding sequence, and terminator) into three spatially distinct orientations (convergent, divergent, and tandem orientation) can exhibit significantly different levels of transcriptions. These findings motivate the need for mathematical models to describe these spatial context effects. We pose a novel nonlinear mass-action kinetics based model that enables the integration of knowledge about spatial or compositional context and canonical descriptions of transcriptional dynamics. Our findings suggest that compositional context is a key factor in determining bio- circuit part performance and thus represent another important piece in biocircuit interconnection theory.iece in biocircuit interconnection theory.)
• Effects of Noise and Actuator Limits on Active Control of Rotating Stall and Surge  + (Operability enhancement is one of the majo … Operability enhancement is one of the major goals for active control of rotating stall</br>and surge in aeroengines. The model developed by Moore and Greitzer exhibits the</br>qualitative behavior of rotating stall and surge dynamics and thus can be used for</br>controller designs. Based on this model, we derive a normal form from which explicit</br>relations between the stall and surge inception process and the shape of compressor</br>characteristics are obtained via bifurcation analysis. Analysis for the normal form with</br>bleed valve actuator dynamics shows that under certain circumstances the optimal control</br>is the "bang-on" control law that drives the bleed valve to open against its</br>rate limit once the disturbances grow out of the noise level. disturbances grow out of the noise level.)
• Convex Optimal Uncertainty Quantification  + (Optimal uncertainty quantification (OUQ) i … Optimal uncertainty quantification (OUQ) is a framework for nu- merical extreme-case analysis of stochastic systems with imperfect knowl- edge of the underlying probability distribution and functions/events. This paper presents sufficient conditions (when underlying functions are known) under which an OUQ problem can be reformulated as a finite-dimensional convex optimization problem.e-dimensional convex optimization problem.)
• Synthesizing Combination Therapies for Evolutionary Dynamics of Disease for Nonlinear Pharmacodynamics  + (Our previous results proposed an iterative … Our previous results proposed an iterative scalable algorithm for the systematic design of sparse, small gain feedback strategies that stabilize the evolutionary dynamics of a generic disease model with linear pharmacodynamics. Here we use piecewise linear approximations to model the nonlinear drug effects and leverage results from optimal controller synthesis for positive systems to formulate the feedback synthesis problem as an optimization problem that sequentially explores piecewise linear subsystems corresponding to higher and higher treatment dosages.ng to higher and higher treatment dosages.)
• Rapid prototyping of biomolecular circuits through module characterization in cell-free expression systems  + (Over the past years, the field of syntheti … Over the past years, the field of synthetic biology has gained a significant array of tools and parts, making way for increasingly complex bio-molecular circuits to be constructed. The development of biocircuits can be facilitated by assembling parts in a less complex, cell-free, environment which contains only the machinery for gene transcription (TX) and translation (TL), which have been extracted from bacteria. In this project, a part library was collected and used to assemble DNA constructs for a newly designed biocircuit. An in vitro TX-TL extract was used to test the circuit modules using linear DNA, and in parallel with predictive modeling of the biomolecular reactions, the overall circuit design was evaluated. The results have given valuable insight into the performance of the circuit modules in a much shorter time than conventional in vivo cloning and testing would have achieved.o cloning and testing would have achieved.)
• Model of Paradoxical Signaling Regulated T-Cell Population Control for Design of Synthetic Circuits  + (Paradoxical signaling occurs when the same … Paradoxical signaling occurs when the same sig- naling molecule can trigger antagonistic cell functions. For example, T-Cells secret cytokine IL-2 which promotes T-Cell proliferation and also affects cell death. It has been shown that cells with this signaling capability have bi-stable population dynamics and can achieve identical levels of population homeostasis independent of initial cell concentrations. These capabilities are desirable in the context of synthetic population control circuits designed for application in therapeutic treatment of various diseases. It thus becomes important to understand the dependence of the cell system on the intracellular paradoxical components and to develop accurate models to provide insight into optimal design characteristics. Here, we create a model that integrates three IL-2 driven intracellular mechanisms that trigger 1) T-cell proliferation 2) T-cell apoptosis and 3) IL-2 production. Using this model, we are able to explore the internal mechanisms necessary for paradoxical signaling in T-Cells. It was shown that the intracellular mechanisms considered were sufficient to produce population dynamic characteristics of paradoxical signaling consistent with published systems level models and data. Furthermore, analysis of parameters revealed dependency of population homeostatic stability on the production and activation of the specific intracellular proteins considered.pecific intracellular proteins considered.)
• Robust Performance Analysis for a Class of Uncertain Nonlinear Systems  + (Performance analysis of a large class of nonlinear systems is proven to be equivalent to performance analysis of a constrained uncertain linear system, for which computable analysis methods have already been developed.)
• Quantitative Performance Bounds in Biomolecular Circuits due to Temperature Uncertainty  + (Performance of biomolecular circuits is af … Performance of biomolecular circuits is affected by changes in temperature, due to its influence on underlying reaction rate parameters. While these performance variations have been estimated using Monte Carlo simulations, how to analytically bound them is generally unclear. To address this, we apply control-theoretic representations of uncertainty to examples of different biomolecular circuits, developing a framework to represent uncertainty due to temperature. We estimate bounds on the steady-state performance of these circuits due to temperature uncertainty. Through an analysis of the linearised dynamics, we represent this uncertainty as a feedback uncer- tainty and bound the variation in the magnitude of the input- output transfer function, providing a estimate of the variation in frequency-domain properties. Finally, we bound the variation in the time trajectories, providing an estimate of variation in time-domain properties. These results should enable a framework for analytical characterisation of uncertainty in biomolecular circuit performance due to temperature variation and may help in estimating relative performance of different controllers.tive performance of different controllers.)
• The role of single occupancy effects on integrase dynamics in a cell-free system  + (Phage integrase-based circuits are an alte … Phage integrase-based circuits are an alternative approach to relying on transcriptional and translational repression for biomolecular circuits. Previous research has shown that circuits based on integrases can perform a variety of functions, including counters, Boolean logic operators, memory modules and temporal event detectors. It is therefore essential to develop a principled theoretical and experimental framework for the design, implementation and study of such circuits. One of the fundamental questions that such a framework should address concerns the functionality limitations and temporal dynamics of the integrases as regulatory elements. We have tested the functionality of several large serine type integrases from a recently published library in a cell-free transcription-translation (TX-TL) platform. In addition, we have explored experimentally and through mathematical modelling and simulations how integrase dynamics depends on the concentration of integrase and that of its binding sites.</br></br>We report that sequestration of integrase molecules, either in the form of monomers or dimers, by the integrase's own binding sites dominates integrase dynamics, and that the delay in the activation of the reporter is negatively correlated with integrase plasmid concentration. We have validated our sequestration hypothesis by building a model with MATLAB’s SimBiology toolbox, and running simulations with various integrase and binding sites concentrations. The simulation results qualitatively match the experimental results, and offer further insights into the system.nd offer further insights into the system.)
• Quantitative characterization of random partitioning in the evolution of plasmid-encoded traits  + (Plasmids are found across bacteria, archae … Plasmids are found across bacteria, archaea, and eukaryotes and play an important role in evolution. Plasmids exist at different copy numbers, the number of copies of the plasmid per cell, ranging from a single plasmid per cell to hundreds of plasmids per cell. This feature of a copy number greater than one can lead to a population of plasmids within a single cell that are not identical clones of one another, but rather have individual mutations that make a given plasmid unique. During cell division, this population of plasmids is partitioned into the two daughter cells, resulting in a random distribution of different plasmid variants in each daughter. In this study, we use stochastic simulations to investigate how random plasmid partitioning compares to a perfect partitioning model. Our simulation results demonstrate that random plasmid partitioning accelerates mutant allele fixation when the allele is beneficial and the selection is in an additive or recessive regime where increasing the copy number of the beneficial allele results in additional benefit for the host. This effect does not depend on the size of the benefit conferred or the mutation rate, but is magnified by increasing plasmid copy number.gnified by increasing plasmid copy number.)
• Pre-orders for Reasoning about Stability Properties with respect to Input of Hybrid Systems  + (Pre-orders on systems are the basis for ab … Pre-orders on systems are the basis for abstraction based verification of systems. In this paper, we investigate pre-orders for reasoning about stability with respect to inputs of hybrid systems. First, we present a superposition type theorem which gives a characterization of the classical incremental input-to-state stability of continuous systems in terms of the traditional epsilon/delta-definition of stability. We use this as the basis for defining a notion of incremental input- to-state stability of hybrid systems. Next, we present a pre-order on hybrid systems which preserves incremental input- to-state stability, by extending the classical definitions of bisimulation relations on systems with input, with uniform continuity constraints. We show that the uniform continuity is a necessary requirement by exhibiting counter-examples to show that weaker notions of input bisimulation with just continuity requirements do not suce to preserve stability. Finally, we demonstrate that the definitions are useful, by exhibiting concrete abstraction functions which satisfy the definitions of pre-orders.ich satisfy the definitions of pre-orders.)
• Control of Rotating Stall in a Low-Speed Axial Flow Compressor Using Pulsed Air Injection: Modeling, Simulations, and Experimental Validation  + (Previous results in the use of pulsed air … Previous results in the use of pulsed air injection for</br>active control of rotating stall have suggested that air injectors</br>have the effect of shifting the steady state compressor</br>characteristic. In this paper we analyze the effect of a compressor</br>characteristic actuation scheme for the three state Moore Greitzer</br>compression system model. It is shown that closed loop feedback based</br>on the square magnitude of the first rotating stall mode can be used</br>to decrease the hysteresis region associated with the transition from</br>unstalled to stalled and back to unstalled operation. The compressor</br>characteristic shifting idea is then applied to a higher fidelity</br>distributed model in which the characteristic shifting has phase</br>content in addition to the magnitude content captured by the three</br>state model. The optimal phasing of the air injection relative to the</br>sensed position of the stall cell is determined via simulation and the</br>results found to agree with those obtained via an experimental</br>parametric study on the Caltech low-speed axial flow compressor.} Caltech low-speed axial flow compressor.})
• Characterizing the Effects of Air Injection on Compressor Performance for Use in Active Control of Rotating Stall  + (Previous work at Caltech has developed an … Previous work at Caltech has developed an air</br>injection controller for rotating stall based on the idea of a</br>shifting compressor characteristic. To further understand the</br>properties of this controllers, a series of open loop tests were</br>performed to measure the performance characteristics of an axial flow</br>compression system when air was injected upstream of the rotor face.</br>The distance from the rotor face, the span-wise position, and the</br>angle relative to the mean axial flow were varied. These tests show</br>that the injection of air has drastic effects on the stalling mass</br>flow rate and on the size of the hysteresis loop associated with</br>rotating stall. The stalling mass flow rate was decreased by 10\% and</br>the hysteresis loop was completely eliminated under some conditions.mpletely eliminated under some conditions.)
• Combined Air Injection Control of Rotating Stall and Bleed Valve Control of Surge  + (Previous work at Caltech has developed a c … Previous work at Caltech has developed a controller for rotating stall</br>in axial flow compressors using pulsed air injection. In this work,</br>theory is developed for the combination of this air injection</br>controller with a bleed valve controller for the system's surge</br>dynamics. The controller analysis is based on the surge dynamics</br>acting on a slow time scale relative to the rotating stall dynamics.</br>Experiments demonstrating this controller design on the Caltech rig</br>are also presented.ign on the Caltech rig are also presented.)
• Temperature Dependence of Biomolecular Circuit Designs  + (Quantifying performance of biomolecular ci … Quantifying performance of biomolecular circuit designs across different environmental conditions is a key step in assessing their robustness. It is generally unclear how robust this performance is to the important environmental variable of temperature. Here, we address this issue for a transcriptional negative feedback circuit design that can speed up the response time using a combination of simple computational methods and dynamic experimental measurements. We use a simple two-state model of gene expression to illustrate different ways in which temperature dependence of reaction rate parameters can propagate through to the functional output. Next, we extend this analysis to the response time of a transcriptional negative feedback circuit design. Finally, we present experimental results characterizing how response time of a negative transcriptional feedback circuit depends on temperature. These results help to develop framework for assessing how functional output of biomolecular circuit designs depend on temperature.lar circuit designs depend on temperature.)
• Design of a Toolbox of RNA Thermometers  + (RNA thermometers mediate responses to temp … RNA thermometers mediate responses to temperature changes in various natural circuits, and have been developed in a synthetic context as well. However, a toolbox of RNA thermometers with diâµerent sensitivities to temperature is lacking. Here, we address this issue using a combination of computational and experimental methodologies. We analysed a set of available synthetic RNA thermometers through a quantification of their activity as a function of temperatures in a cell- free expression molecular breadboard system as well as through computation of their melt profiles. Based on this, we computed melt profiles of a library of RNA thermometers and found that the library contained RNA thermometers with a range of sensitivities and thresholds in their response to temperature. We constructed this library and found, through preliminary measurements, a wide range of responses to temperature, which in some cases matched the computational predictions. The constructed library represents a toolbox of RNA thermometers with different sensitivities and is foun- dational work towards synthetic biology applications such as efficient control of large volume chemical reactors, precise spatiotemporal control of gene expression as well as tools to engineer robustness to temperature in biomolecular circuits.s to temperature in biomolecular circuits.)
• Layered Feedback Control Improves Robust Functionality across Heterogeneous Cell Populations  + (Realizing homeostatic control of metabolit … Realizing homeostatic control of metabolites or proteins is one of the key goals of synthetic circuits. However, if control is only implemented internally in individual cells, cell-cell heterogeneity may break the homeostasis on population level since cells do not contribute equally to the production or regulation. New control structures are needed to achieve robust functionality in heterogeneous cell populations. Quorum sensing (QS) serves as a collective mechanism by releasing and sensing small and diffusible signaling molecules for group decision-making. We propose a layered feedback control structure that includes a global controller using quorum sensing and a local controller via internal signal-receptor systems. We demonstrate with modeling and simulation that the global controller drives contributing cells to compensate for disturbances while the local controller governs the fail-mode performance in non-contributing cells. The layered controller can tolerate a higher portion of non-contributing cells or longer generations of mutant cells while maintaining metabolites or proteins level within a small error range, compared with only internal feedback control. We further discuss the potential of such layered structures in robust control of cell population size, population fraction and other population-dependent functions. and other population-dependent functions.)
• Controller Synthesis for Constrained Flight Systems via Receding Horizon Optimization  + (Receding horizon control allows a blending … Receding horizon control allows a blending of navigation and control functions</br>at the inner and outer loop levels and significantly enhances the ability of the control</br>system to react to complex dynamic and environmental constraints. In this paper, we</br>explore some of the limits of receding horizon control, including the extent to which</br>traditional control specifications can be cast as RHC problem specifications. Simulation</br>results for a planar flight vehicle with representative flight dynamics illustrate</br>the main features of the proposed approach.he main features of the proposed approach.)
• Nonlinear Control of Mechanical Systems: A Lagrangian Perspective  + (Recent advances in geometric mechanics, mo … Recent advances in geometric mechanics, motivated in large part by applications in</br>control theory, have introduced new tools for understanding and utilizing the structure</br>present in mechanical systems. In particular, the use of geometric methods for analyzing</br>Lagrangian systems with both symmetries and non-integrable (or nonholonomic) constraints</br>has led to a unified formulation of the dynamics that has important implications for a</br>wide class of mechanical control systems. This paper presents a survey of recent results</br>in this area, focusing on the relationships between geometric phases, controllability, and</br>curvature, and the role of trajectory generation in nonlinear controller synthesis.</br>Examples are drawn from robotics and flight control systems, with an emphasis on motion</br>control problems.th an emphasis on motion control problems.)
• Synthetic logic circuits using RNA aptamer against T7 RNA polymerase  + (Recent advances in nucleic acids engineeri … Recent advances in nucleic acids engineering introduced several RNA-based regulatory components for synthetic gene circuits, expanding the toolsets to engineer organisms. In this work, we designed genetic circuits implementing an RNA aptamer previously described to have the capability of binding to the T7 RNA polymerase and inhibiting its activity in vitro. Using in vitro transcription assays, we first demonstrated the utility of the RNA aptamer in combination with programmable synthetic transcription networks. As a step to quickly assess the feasibility of aptamer functions in vivo, a cell-free expression system was used as a breadboard to emulate the in vivo conditions of E. coli. We tested the aptamer and its three sequence variants in the cell-free expression system, verifying the aptamer functionality in the cell-free testbed. In vivo expression of aptamer and its variants demonstrated control over GFP expression driven by T7 RNA polymerase with different response curves, indicating its ability to serve as building blocks for both logic circuits and transcriptional cascades. This work elucidates the potential of RNA-based regulators for cell programming with improved controllability leveraging the fast production and degradation time scales of RNA molecules. degradation time scales of RNA molecules.)
• Vision as a Compensatory Mechanism for Disturbance Rejection in Upwind Flight  + (Recent experimental results demonstrate th … Recent experimental results demonstrate that flies posses a robust tendency to orient towards</br>the frontally-centered pole of the visual motion field that typically occurs during upwind flight.</br>In this paper we present a closed loop flight model, with a control algorithm based on feedback</br>of the location of the visual pole of contraction, which is affected by changes in wind direction.</br>The feasibility of visually guided upwind orientation is demonstrated with a model derived</br>from current understanding of the biomechanics and sensorimotor computation of insects. The</br>matched filter approach used to model the visual system computations compares extremely well</br>with open-loop experimental data.ely well with open-loop experimental data.)
• Quantifying Crosstalk in Biochemical Systems  + (Recent work has introduced biocircuit arch … Recent work has introduced biocircuit architectures that exhibit robust oscillatory behavior in organisms ranging from cyanobacteria to mammals. Complementary research in synthetic biology has introduced oscillators in vivo and in vitro suggesting that robust oscillation can be recapitulated using a small number of biochemical components. In this work, we introduce signaling crosstalk in biocircuits as a consequence of enzyme-mediated biochemical reactions. As a motivating example, we consider an in vitro oscillator with two types of crosstalk: crosstalk in production and degradation of RNA signals. We then pose a framework for quantifying crosstalk and use it to derive several dynamical constraints and suggest design techniques for ameliorating crosstalk in in vitro biochemical systems. As an example, we show that the balance between production and degradation crosstalk plays a key role in determining system stability, potentially leading to loss of oscillatory behavior. We demonstrate that the effects of crosstalk can attenuated through the effective tuning of two key parameters in order to recover desired system dynamics. order to recover desired system dynamics.)