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D
A cooperative control system consists of multiple, autonomous
components interacting to<br>
control their environment. Examples include air tra±c control systems,
automated factories,<br>
robot soccer teams and sensor/actuator networks. Designing such systems requires
a combination of tools from control theory and distributed systems. In this
article, we review some of these tools and then focus on the Computation and
Control Language, CCL, which we have developed as a modeling tool and a programming
language for cooperative control systems. +
F
A feedback controller closes the loop from vision to wing motion to stabilize forward flight in a simulation of Drosophila Melanogaster. +
M
A generalized model predictive control (MPC) formulation is derived that extends the existing theory to a multi-vehicle formation stabilization problem. The vehicles are individually
governed by nonlinear and constrained dynamics. The extension considers formation stabilization to a set of permissible equilibria, rather than a unique equilibrium. Simulations for three vehicle formations with input constrained dynamics on con¯guration space SE(2) are performed
using a nonlinear trajectory generation (NTG) software package developed at Caltech. Preliminary results and an outline of future work for scaling/decentralizing the MPC approach and applying it to an emerging experimental testbed are given. +
A master equation describes the continuous-time evolution of a probability distribution, and
is characterized by a simple bilinear structure and an often-high dimension. We develop a
model reduction approach in which the number of possible confiurations and corresponding
dimension is reduced, by removing improbable configurations and grouping similar ones. Error
bounds for the reduction are derived based on a minimum and maximum time scale of interest.
An analogous linear identification procedure is then presented, which computes the state and
output matrices for a predetermined configuration set. These ideas are demonstrated first in
a finite-dimensional model inspired by problems in surface evolution, and then in an infinite-
dimensional film growth master equation. +
S
A new technique for stabilizing nonholonomic systems to trajectories is presented. It is well known that such systems cannot be stabilized to a point using smooth static state feedback. In this paper we suggest the use of control laws for stabilizing a system about a trajectory, instead of a point. Given a nonlinear system and a desired (nominal) feasible trajectory, the paper gives an explicit control law which will locally exponentially stabilize the system to the desired trajectory. The theory is applied to several examples, including a car-like robot. +
A
A numerical algorithm for computing necessary conditions
for performance specifications is developed for nonlinear
uncertain systems. The algorithm is similar in nature and
behavior to the power algorithm for the mu lower bound,
and doesn't rely on a descent method. The algorithm is
applied to a practical example. +
A reactive safety mode is built into a robust model predictive control algorithm for uncertain nonlinear systems with bounded disturbances. The algorithm enforces state and control constraints and blends two modes: (I) standard, guarantees re-solvability and asymptotic convergence in a robust receding-horizon manner; (II) safety, if activated, guarantees containment within an invariant set about a reference. The reactive safety mode provides robustness to unexpected, but real-time anticipated, state-constraint changes during standard mode operation. The safety-mode control policy is designed offline and can be activated at any arbitrary time. The standard-mode control has feedforward and feedback components: feedforward is from online solution of a finite-horizon optimal control problem; feedback is designed offline to provide robustness to system uncertainty and disturbances and to establish an invariant âstate tubeâ that guarantees standard-mode re-solvability at any time. The algorithm design is shown for a class of systems with incrementally-conic uncertain/nonlinear terms and bounded disturbances. +
A reactor for the deposition of superconducting \ybcolong\;thin films is modeled and
studied from a control perspective to determine the heat transfer dynamics of the reactor
under active thermal control. A nonlinear wavelength-dependent heat transfer model is
developed to predict reactor heat transfer throughout the film growth process, and
preliminary component testing is conducted to validate the model. The model is linearized
about a typical operating point and analyzed with linear feedback control methods to
determine the performance of the reactor under observer-based feedback control with film
growth disturbances. The controller and observer are selected through linear quadratic
regulator and linear quadratic estimator methods. Rates of convergence of the controller
and observer are determined through examination of the eigenvalues of the linearized
system, and disturbance rejection is assessed with ${\mathcal H}_2$ and ${\mathcal
H}_\infty$ norms. The eigenvalue and norm analysis is applied to varying reactor design
parameters to quantify performance tradeoffs. The maximum errors associated with control
and with estimation of a nominal design case are both 21 K, and the longest time scales
are 45 seconds and 10 seconds for the controller and observer, respectively. +
L
A synthetic cell-cell signaling circuit should ideally be (1) metabolically lightweight, (2) insulated from endogenous gene networks, and (3) excitable rather than oscillatory or bistable. To accomplish these three features, we propose a synchronized pulse-generating circuit based on the design of published synchronized oscillators. This communication module employs a pulse generator built using Lux-type quorum sensing components and an IFFL transcriptional circuit. Both the input and output of this module are AHLs, the quorum sensing signaling molecule. Cells bearing this module therefore act as an excitable medium, producing a pulse of AHL when stimulated by exogenous AHL. Using simulation and microscopy, we demonstrate how this circuit enables traveling pulses of AHL production through microcolonies growing in two dimensions. Traveling pulses achieve cell-cell communication at longer distances than can be achieved by diffusion of signal from sender to receiver cells and may permit more sophisticated coordination in synthetic consortia. +
O
AbstractIn this paper, we consider the problem of
optimal Linear Quadratic Gaussian control of a system
in which communication between the sensor and the
controller occurs across a packet-dropping link. We first
prove a separation principle that allows us to solve this
problem using a standard LQR state-feedback design,
along with an optimal algorithm for propagating and using
the information across the unreliable link. Then we present
one such optimal algorithm, which consists of a Kalman
filter at the sensor side of the link, and a switched linear
filter at the controller side. Our design does not assume
any statistical model of the packet drop events, and is thus
optimal for any arbitrary packet drop pattern. Further,
the solution is appealing from a practical point of view
because it can be implemented as a small modification of
an existing LQG control design. +
S
AbstractWe examine the problem of distributed estimation
when only one sensor can take a measurement per time step.
We solve for the optimal recursive estimation algorithm when
the sensor switching schedule is given. We then consider the
effect of noise in communication channels. We also investigate
the problem of determining an optimal sensor switching strategy.
We see that this problem involves searching a tree in general
and propose two strategies for pruning the tree to minimize the
computation. The first is a sliding window strategy motivated
by the Viterbi algorithm, and the second one uses thresholding.
The performance of the algorithms is illustrated using numerical
examples. +
E
AbstractâIn this paper, we consider a state estimation
problem over a bandwidth limited network. A sensor network
consisting of N sensors is used to observe the states of M
plants, but only p · N sensors can transmit their measurements
to a centralized estimator at each time. Therefore a suitable
scheme that schedules the proper sensors to access the network
at each time so that the total estimation error is minimized is
required. We propose four different sensor scheduling schemes.
The static and stochastic schemes assume no feedback from
the estimator to the scheduler, while the two dynamic schemes,
Maximum Error First (MEF) and Maximum Deduction First
(MDF) assume such feedback is available. We compare the
four schemes via some examples and show MEF and MDF
schemes are better than the static and stochastic schemes, which
demonstrates that feedback can play an important role in this
remote state estimation problem. We also show that MDF is
better than MEF as MDF considers the total estimation error
while MEF considers the individual estimation error. +
L
Linear DNA for rapid prototyping of synthetic biological circuits in an Escherichia coli based TX-TL cell-free system +
Accelerating the pace of synthetic biology experiments requires new approaches for rapid prototyping of circuits from individual DNA regulatory elements. However, current testing standards require days to weeks due to cloning and in vivo transformation. In this work, we first characterized methods to protect linear DNA strands from exonuclease degradation in an Escherichia coli based transcription-translation cell-free system (TX-TL), as well as mechanisms of degradation. This enables the use of linear DNA PCR products in TX-TL. We then explored methods to calibrate linear DNA to plasmid DNA by concentration. We also demonstrated assembly technology to rapidly build circuits entirely in vitro from separate parts. Using this strategy, we prototyped a four-piece genetic switch in under 8 hours entirely in vitro. Rapid in vitro assembly has applications for prototyping circuits of unlimited size when combined with predictive computational models. +
N
Nonlinear Control of Rotating Stall Using Axisymmetric Bleed with Continuous Air Injection on a Low-Speed, Single Stage, Axial Compressor +
Active control of rotating stall and surge using bleed valves has been
demonstrated on low and high speed compressors using high bandwidth
actuators. In this paper we provide a method to reduce the bandwidth
and rate requirements for control of rotating stall by combining an
axisymmetric bleed valve with continuous air injection. The addition
of the continuous air injection is modeled as a shift of both the
stable and unstable parts of the compressor characteristic and serves
to reduce the requirement of a bleed valve used for rotating stall
stabilization purpose. The results are demonstrated using a
low-speed, single stage, axial flow compressor. +
I
Integrase-mediated differentiation circuits improve evolutionary stability of burdensome and toxic functions in E. coli +
Advances in synthetic biology, bioengineering, and computation allow us to rapidly and reliably program cells with increasingly complex and useful functions. However, because the functions we engineer cells to perform are typically burdensome to cell growth, they can be rapidly lost due to the processes of mutation and natural selection. Here, we show that a strategy of terminal differentiation improves the evolutionary stability of burdensome functions in a general manner by realizing a reproductive and metabolic division of labor. To implement this strategy, we develop a genetic differentiation circuit in Escherichia coli using unidirectional integrase-recombination. With terminal differentiation, differentiated cells uniquely express burdensome functions driven by the orthogonal T7 RNA polymerase, but their capacity to proliferate is limited to prevent the propagation of advantageous loss-of-function mutations that inevitably occur. We demonstrate computationally and experimentally that terminal differentiation increases duration and yield of high-burden expression and that its evolutionary stability can be improved with strategic redundancy. Further, we show this strategy can even be applied to toxic functions. Overall, this study provides an effective, generalizable approach for protecting burdensome engineered functions from evolutionary degradation. +
F
Aircraft operate in different modes during flight, corresponding to different flight conditions and
control schemes. We use differential flatness of an approximate model of the pitch dynamics of a
thrust vectored aircraft to achieve fast switching between those modes. We investigate some
methods to compensate for the perturbations to flatness. Simulations and experimental data are
provided to validate the approach. +
M
An experimental investigation of acoustic mode noise suppression was conducted in a cavity using a digital controller with a linear control algorithm. The control algorithm was based on flow field physics similar to the Rossiter model for acoustic resonance. Details of the controller and results from its implementation are presented in the companion paper by Rowley, et al (2002). Here the experiments and some details of the flow field development are described, which were done primarily at Mach number 0.34 corresponding to single mode resonance in the cavity. A novel method using feedback control to suppress the resonant mode and open-loop forcing to inject a non-resonant mode was developed for system identification. The results were used to obtain empirical transfer functions of the components of resonance, and measurements of the shear layer growth for use in the design of the control algorithm. +
P
An ongoing area of study in synthetic biology has been the design and construction of synthetic circuits that maintain homeostasis at the population level. Here, we are interested in designing a synthetic control circuit that regulates the total cell population and the relative ratio between cell strains in a culture containing two different cell strains. We have developed a dual feedback control strategy that uses two separate control loops to achieve the two functions respectively. By combining both of these control loops, we have created a population regulation circuit where both the total population size and relative cell type ratio can be set by reference signals. The dynamics of the regulation circuit show robustness and adaptation to perturbations in cell growth rate and changes in cell numbers. The control architecture is general and could apply to any organism for which synthetic biology tools for quorum sensing, comparison between outputs, and growth control are available. +
S
System identification of phosophorylation based insulator in a cell-free in vitro transcription-translation system +
An outstanding challenge in the design of synthetic biocircuits is the development of a robust and efficient strategy for interconnecting functional modules. Recent theoretical work demonstrated that a phosphorylation based insulator implementing a dual strategy of high gain and strong negative feedback could potentially serve as a device to attenuate retroactivity. This research investigates the structural identifiability of the phoshorylation based insulator when implemented in an {\it in vitro} transcription-translation cell free expression system. We consider a complex model that provides an intricate description of all chemical reactions and leveraging specific physiologically plausible assumptions, we derive a rigorous simplified model that captures the output dynamics of the phosphorylation based insulator. We perform standard system identification analysis and determine that the model is globally identifiable with respect to three critical parameters. Our findings show the utility of the transcription-translation cell free expression system as a platform for system identification, as it provides extra control inputs for parameter estimation that typically are unavailable in vivo. The parameters estimated are then used as a basis for a simulation study in a parallel experimental work (also submitted to qBio-2014). +
N
Analysis and simulations are performed for a simplified model of a
commercially available variant on the skateboard, known as the Snakeboard.
Although the model
exhibits basic gait patterns seen in a large number of locomotion problems,
the analysis tools currently available do not apply to this problem. The
difficulty is seen to lie primarily in the way in which the nonholonomic
constraints enter into the system. As a first step towards understanding
systems represented by our model we present the equations of motion and
perform some controllability analysis for the snakeboard. We also perform
some numerical simulations of the gait patterns. +
