Property:Abstract
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D
This paper describes a synthetic in vitro genetic circuit programmed to work as an insulating device. This circuit is composed of nucleic acids, which can be designed to interact according to user defined rules, and of few proteins that perform catalytic functions. A model of the circuit is derived from first principle biochemical laws. This model is shown to exhibit time-scale separation that makes its output insensitive to downstream time varying loads. Simulation results show the circuit effectiveness and represent the starting point for future experimental testing of the device. +
R
Reduction of Bleed Valve Bandwidth Requirements for Control of Rotating Stall using Continuous Air Injection +
This paper describes a technique for reducing the actuator rate
requirement in active control of rotating stall and surge on
compressors. Actuation of the compressor characteristic, via the use
of continuous air injection at the rotor face in a single-stage, low
speed axial compressor, has been experimentally verified to provide a
method of reducing the actuator rate requirement of bleed valve
control of rotating stall. With a compressor rotor frequency of 100
Hz, active control of stall with a high speed bleed valve is achieved
only when the compressor characteristic is actuated. Furthermore, the
experiments show that the bleed valve rate requirement is reduced from
approximately 145 Hz to below 10 Hz when the amount of compressor
characteristic actuation is increased. Theoretical tools based on a
low order model (1-3 states) and simulations based on a reduced order
distributed model (37 states) have been developed to estimate the gain
and rate requirements of the bleed controller. All of the analytical
formulas and simulations share the same qualitative trends with
respect to the second and third derivatives of the compressor
characteristic function evaluated at the peak, and the experiments.
The agreement implies that bleed valve control of rotating stall
depends crucially on the rate limit of the bleed valve which in turn
depends on both the stable and the unstable part of the compressor
characteristic. Actuation of the compressor characteristic is
concluded to be a valuable tool in circumventing the rate limitation
of bleed valves for control of stall. By combining the compressor
characteristic identification tools and the analytic relations,
insights for designing a compressor-bleed pair are provided. +
A
This paper describes algorithms to generate trajectories
for differentially flat systems with zero dynamics.
Zero dynamics in flat systems occur when the flat outputs
are not the tracking outputs. This means that the output
trajectories can be fully parametrized by the flat outputs,
but that there is some additional freedom left.
This freedom can be exploited to minimize a cost criterion.
We parametrize the differentially
flat outputs by basis functions, and solve for the parameters
so as to track a prescribed trajectory approximately
while minimizing a cost function.
We focus on implementation issues and point out the computational
cost involved in the various problems. +
E
Extending Exponential Stabilizers for Nonholonomic Systems from Kinematic Controllers to Dynamic Controllers +
This paper describes an approach for extending (time-varying)
exponential stabilizers for nonholonomic systems from controllers which
command input velocity to controllers which command input torques. Due
to the nondifferentiable nature of exponential stablizers, additional
structure is required in order to ensure that the extended controllers
generate continuous control actions. In this paper we show how to
extend homogeneous controllers which use a nonstandard dilation adapted
to the problem. +
D
This paper describes the application of differential flatness
techniques from nonlinear control theory to mechanical (Lagrangian)
systems. Systems which are differentially flat have several useful
properties which can be exploited to generate effective control
strategies for nonlinear systems. For the special case of mechanical
control systems, much more geometric information is present and the
purpose of this paper is to explore the implications and features of
that class of systems. We concentrate on several worked examples
which illustrate the general theory and present a detailed catalog of
known examples of differentially flat mechanical systems. +
This paper describes the design, modeling, synthesis and preliminary validation of a protein concentration regulator circuit. The circuit is designed to maintain the level of a target protein to a reference level, specified by the amount of another protein. This is implemented using a single negative feedback loop that inhibited the production of the target protein once its concentration was equal to the reference amount. A mathematical model consisting of a set of ODEs is derived from mass action laws and Hill function approximations of protein production. Steady-state analysis of the model is used to predict parameter sensitivity and experimental behavior. We implemented this circuit in ''E. coli'' using scaffold-based sequestration and transcriptional activation. Preliminary experimental results show the system matching predictions from our model and performing the expected task. +
I
Interfacing TuLiP with the JPL Statechart Autocoder: Initial progress toward synthesis of flight software from formal specifications +
This paper describes the implementation of an in- terface connecting the two tools : the JPL SCA (Statechart Autocoder) and TuLiP (Temporal Logic Planning Toolbox) to enable the automatic synthesis of low level implementation code directly from formal specifications. With system dynamics, bounds on uncertainty and formal specifications as inputs, TuLiP synthesizes Mealy machines that are correct-by-construction. An interface is built that automatically translates these Mealy machines into UML statecharts. The SCA accepts the UML statecharts (as XML files) to synthesize flight-certified2 implementation code. The functionality of the interface is demonstrated through three example systems of varying com- plexity a) a simple thermostat b) a simple speed controller for an autonomous vehicle and c) a more complex speed controller for an autonomous vehicle with a map-element. In the thermostat controller, there is a specification regarding the desired temperature range that has to be met despite disturbance from the environment. Similarly, in the speed-controllers there are specifications about safe driving speeds depending on sensor health (sensors fail unpredictably) and the map-location. The significance of these demonstrations is the potential circumventing of some of the manual design of statecharts for flight software/controllers. As a result, we expect that less testing and validation will be necessary. In applications where the products of synthesis are used alongside manually designed components, extensive testing or new certificates of correctness of the composition may still be required. +
A
This paper describes the implementation and testing of Alice, the California Institute of Technology’s entry in the 2005 DARPA Grand Challenge. Alice utilizes a highly networked control system architecture to provide high performance, autonomous driving in unknown en- vironments. Innovations include a vehicle architecture designed for efficient testing in harsh environments, a highly sensory-driven approach to fuse sensor data into speed maps used by real-time trajectory optimization algorithms, health and contingency management algorithms to manage failures at the component and system level, and a software logging and display envi- ronment that enables rapid assessment of performance during testing. The system successfully completed several runs in the National Qualifying Event, but encountered a combination of sens- ing and control issues in the Grand Challenge Event that led to a critical failure after traversing approximately 8 miles. +
This paper describes the use of a domain-specific language, and an accompanying software tool, in constructing correct- by-construction control protocols for aircraft electric power systems. Given a base topology, the language consists of a set of primitives for standard specifications. The accompanying tool converts these primitives into formal specifica- tions, which are used to synthesize control protocols. We can then use TuLiP, a Python-based software toolbox, to synthesize centralized and distributed controllers. For sys- tems with no time involved in the specifications, this tool also provides an option to output specifications into a SAT-solver compatible format, thus reducing the synthesis problem to a satisfiability problem. We provide the results of our synthesis procedure on a range of topologies. +
C
This paper describes the use of time-delayed feedback to regulate the behavior of biological networks. The general ideas are demonstrated on specific transcriptional regulatory and neural networks. It is shown that robust yet tunable controllers can be constructed that provide the biological systems with model-engineered inputs. The results indicate that time delay modulation may serve as an efficient bio-compatible control tool. +
R
This paper details the application of a constrained receding horizon control techniques to
stabilize an indoor vectored-thrust flight experiment, known as the Caltech Ducted Fan, subjected to step commands. The result of the experiment sheds light on the theoretical stability
of the receding horizon control strategy as well as validates some efficient computation techniques for solving nonlinear optimal control problems with constraints. The receding horizon
control problem is formulated as a constrained optimal control problem, which was solved with
an e±cient computational method developed and implemented by Milam et al., that combines
nonlinear control theory, B-spline basis functions, and nonlinear programming. Characteristic
issues including non-zero computational times, convergence property, choice of horizon length
and terminal cost are discussed. The study confirms the applicability of real-time receding
horizon control for constrained systems with fast dynamics. +
This paper discusses a Cooperative Path Planning (CPP) design
methodology for multi-vehicle systems and a Nonlinear
Trajectory Generation (NTG) algorithm. Three scenarios
of multi-vehicle tasking are proposed at the CPP framework.
The NTG algorithm is, then, used to generate realtime
trajectory for desired vehicle activities. Given system
dynamics and constraints, the NTG algorithm first finds trajectory
curves in a lower dimensional space and, then, parameterizes
the curves by the B-spline basis. The coefficients of the B-splines are further solved by the sequential
quadratic programming to satisfy the optimization objectives
and constraints. The NTG algorithm has been implemented
to generate real-time trajectories for a group of
cooperative vehicles in the presence of changing missions
and constraints. +
C
This paper discusses a design methodology of cooperative
trajectory generation for multi-robot systems.
The trajectory of achieving cooperative tasks, i.e., with
temporal constraints, is constructed by a nonlinear trajectory
generation (NTG) algorithm. Three scenarios
of multi-robot tasking are proposed at the cooperative
task planning framework. The NTG algorithm is, then,
used to generate real-time trajectory for desired robot
activities. Given robot dynamics and constraints, the
NTG algorithm first finds trajectory curves in a lower
dimensional space and, then, parameterizes the curves
by a set of B-spline representations. The coe�cients
of the B-splines are further solved by the sequential
quadratic programming to satisfy the optimization objectives
and constraints. The NTG algorithm has been
implemented to generate real-time trajectories for a
group of cooperative robots in the presence of spatial
and temporal constraints. Finally, an illustrated example
of cooperative task planning with temporal constraints
is presented. +
L
This paper explores low observability flight path planning of unmanned air vehicles (UAVs)
in the presence of radar detection systems. The probability of detection model of an aircraft
near an enemy radar depends on aircraft attitude, range, and configuration. A detection model
is coupled with a simplified aircraft dynamics model. The Nonlinear Trajectory Generation
(NTG) software package developed at Caltech is used. The NTG algorithm is a gradient descent
optimization method that combines three technologies: Bsplines, output space collocation and
nonlinear optimization tools. Implementations are formulated with temporal constraints that
allow periods of high observability interspersed with periods of low observability. Illustrative
examples of optimized routes for low observability are presented. +
A
This paper explores the problem of finding a real--time optimal tra jectory for unmanned air vehicles (UAV) in order to minimize their probability of detection by opponent multiple radar detection systems. The problem is handled using the Nonlinear Tra jectory Generation (NTG) method developed by Milam et al. The paper presents a formulation of the trajectory generation task as an optimal control problem, where temporal constraints allow periods of high observability interspersed with periods of low observability. This feature can be used strategically to aid in avoiding detection by an opponent radar. The guidance is provided in the form of sampled tabular data. It is then shown that the success of NTG on the proposed low--observable tra jectory generation problem depends upon an accurate parameterization of the guidance data. In particular, such an approximator is desired to have a compact architecture, a minimum number of design parameters, and a smooth continuously--differentiable input-output mapping. Artificial Neural Networks (ANNs) as universal approximators are known to possess these features, and thus are considered here as appropriate candidates for this task. Comparison of ANNs against B-spline approximators is provided, as well. Numerical simulations on multiple radar scenarios illustrate UAV
trajectories optimized for both detectability and time. +
S
This paper explores the stability analysis problem for nonlinear systems which have general
linear feedback interconnections. Systems are often modeled in this manner in the study of
decentralized control because many communication topologies can be modeled and analyzed
using connections to graph theory. We present necessary conditions for stability of a classification
of interconnected systems, and we give some examples to provide insight into this problem.
These conditions are related to positive definiteness of matrices associated with the feedback
interconnection, and specialize to the common case where the Laplacian matrix of a graph
represents the communication topology of the system. +
L
This paper explores the tradeoffs and limits of performance in feedback control of interconnected multi-agent systems, focused on the network sensitivity functions. We consider the interaction topology described by a directed graph and we prove that the sensitivity transfer functions between every pair of agents, arbitrarily connected, can be derived using a version of the Mason's Direct Rule. Explicit forms for special types of graphs are presented. An analysis of the role of cycles points out that these structures influence and limit considerably the behavior of the system. The more the cycles are equally distributed among the formation, the better performance the system can achieve, but they are always worse than the single agent case. We also prove the networked version of Bode's integral formula, showing that it still holds for multi-agent systems. +
F
This paper focuses on RNA flux regulation for in vitro synthetic gene networks and considers architectures that can be scaled to an arbitrary number of species. Feedback loops are designed based on negative autoâregulation (which can minimize the potentially harmful amount of molecules not used to form useful products) and crossâactivation (which can maximize the overall output flux): transcription rate matching can be achieved through proper feedback constants; negative feedback is faster and maintains stability. A possible experimen- tal implementation of a three and four genes negative feedback architecture is also numerically studied. +
C
This paper focuses on a new geometric approach to fully actuated control
systems on the Riemannian manifold S^2. Our control laws exploit the basic
and intuitive notions of geodesic direction and of distance between points,
and generalize the classical proportional plus derivative feedback (PD)
without the need of arbitrary local coordinate charts. Even for the
stability analysis, the appropriate Lyapunov function relies upon the notion
of distance and its properties. This methodology then applies to spin-axis
stabilization of a spacecraft actuated by only two control torques:
discarding the rotation about the unactuated axis, a reduced system is
considered, whose state is in fact defined on the sphere. For this reduced
attitude stabilization problem our approach allows us not only to deal
optimally with the inevitable singularity, but also to achieve simplicity,
versatility and (coordinate independent) adaptive capabilities. +
R
This paper gives a brief introduction to some of the experimental
facilities at Caltech being used for instruction and research. Two
experiments are described: a flight control experiment using a vector
thrust engine, and a low-speed, axial flow compressor rig. A common
hardware and software infrastructure is used to control these
experiments, allowing easy transition from instructional to research
use. +
