Showing papers by "Richard M. Murray published in 2023"

Pacti: Scaling Assume-Guarantee Reasoning for System Analysis and Design

Abstract: Contract-based design is a method to facilitate modular system design. While there has been substantial progress on the theory of contracts, there has been less progress on scalable algorithms for the algebraic operations in this theory. In this paper, we present: 1) principles to implement a contract-based design tool at scale and 2) Pacti, a tool that can efficiently compute these operations. We then illustrate the use of Pacti in a variety of case studies.

Lipschitz Continuity of Signal Temporal Logic Robustness Measures: Synthesizing Control Barrier Functions from One Expert Demonstration

Abstract: Control Barrier Functions (CBFs) allow for efficient synthesis of controllers to maintain desired invariant properties of safety-critical systems. However, the problem of identifying a CBF remains an open question. As such, this paper provides a constructive method for control barrier function synthesis around one expert demonstration that realizes a desired system specification formalized in Signal Temporal Logic (STL). First, we prove that all STL specifications have Lipschitz-continuous robustness measures. Second, we leverage this Lipschitz continuity to synthesize a time-varying control barrier function. By filtering control inputs to maintain the positivity of this function, we ensure that the system trajectory satisfies the desired STL specification. Finally, we demonstrate the effectiveness of our approach on the Robotarium.

Reasoning over Test Specifications Using Assume-Guarantee Contracts

Abstract: We establish a framework to reason about test campaigns described formally. First, we introduce the notion of a test structure—an object that carries i) the formal specifications of the system under test, and ii) the test objective, which is specified by a test engineer. We build on test structures to define test campaigns and specifications for the tester. Secondly, we use the algebra of assume-guarantee contracts to reason about constructing tester specifications, comparing test structures and test campaigns, and combining and splitting test structures. Using the composition operator, we characterize the conditions on the constituent tester specifications and test objectives for feasibly combining test structures. We illustrate the different applications of the quotient operator to split the test objective, the system into subsystems, or both. Finally, we illustrate test executions corresponding to the combined and split test structures in a discrete autonomous driving example and an aircraft formation-flying example. We anticipate that reasoning over test specifications would aid in generating optimal test campaigns.

Development of cell-free transcription-translation systems in three soil Pseudomonads

Abstract: In vitro transcription-translation (TX-TL) can enable faster engineering of biological systems. This speed-up can be significant, especially in difficult-to-transform chassis. This work shows the successful development of TX-TL systems using three soil-derived wild-type Pseudomonads known to promote plant growth: Pseudomonas synxantha, Pseudomonas chlororaphis, and Pseudomonas aureofaciens. One, P. synxantha, was further characterized. A lysate test of P. synxantha showed a maximum protein yield of 2.5 μM at 125 proteins per DNA template and a maximum protein synthesis rate of 20 nM/min. A set of different constitutive promoters driving mNeonGreen expression were tested in TX-TL and integrated into the genome, showing similar normalized strengths for in vivo and in vitro fluorescence. This correspondence between the TX-TL derived promoter strength and the in vivo promoter strength indicates these lysate-based cell-free systems can be used to characterize and engineer biological parts without genome integration, enabling a faster designbuild-test cycle.

Rules of the Road: Formal Guarantees for Autonomous Vehicles With Behavioral Contract Design

Abstract: The problem of safe and fair conflict resolution among inertial, distributed agents—particularly in highly interactive settings—is of paramount importance to the autonomous vehicles industry. The difficulty of solving this problem can be attributed to the fact that agents have to reason over other agents' complex behaviors. We propose the idea of using a behavioral contract to capture a set of explicitly defined assumptions about how all agents in the environment make decisions. In this article, we present a behavioral contract for a specific class of agents that can guarantee the safety and liveness (i.e., progress) of all agents operating in accordance with it. The behavioral contract has two main components—an ordered behavioral rulebook that the agent uses to select its intended action and some additional constraints that define when an agent has precedence (or not) to take its intended action. If all of the agents act according to this contract, we can guarantee safety under all traffic conditions and liveness for all agents under “sparse” traffic conditions. The formalism of the contract also enables assignment of blame. We provide proofs of correctness of the behavioral contract and validate our results in simulation.

Context-Aided Variable Elimination for Requirement Engineering

Abstract: Deriving system-level specifications from component specifications usually involves the elimination of variables that are not part of the interface of the top-level system. This paper presents algorithms for eliminating variables from formulas by computing refinements or relaxations of these formulas in a context. We discuss a connection between this problem and optimization and give efficient algorithms to compute refinements and relaxations of linear inequality constraints.

21.1 A 65nm CMOS Living-Cell Dynamic Fluorescence Sensor with 1.05fA Sensitivity at 600/700nm Wavelengths

Abstract: Integrated, low-cost and miniaturized devices that can detect clinically relevant biomarkers are crucial for the growing field of precision medicine as they can enable point-of-care diagnosis, continuous health monitoring and closed-loop drug delivery. Fluorescence (FL) sensing is known to be one of the most reliable, sensitive, and widely adopted sensing modality for many biomarkers [1], [3]. However, detecting the weak FL signal requires complex optical setups, especially narrowband optical filters to block the strong excitation (EX) light. Prior efforts [1] to miniaturize and implement FL sensing in CMOS technologies have been limited to on-chip high-pass filters using dense vertical waveguide arrays. More importantly, the reported wavelength range of 800nm in [1] is not compatible with most of the commonly used fluorescent proteins that work with living cells [4]. Luminescence is another mechanism for detecting biomarkers that does not require an EX source and optical filtering [2]. However, there are limited number of luminescence proteins, and it is not feasible to use them in a closed-loop system since they can interfere with optogenetic control integration [5].

Barrier-Based Test Synthesis for Safety-Critical Systems Subject to Timed Reach-Avoid Specifications

Abstract: —We propose an adversarial, time-varying test- synthesis procedure for safety-critical systems without requiring specific knowledge of the underlying controller steering the sys- tem. From a broader test and evaluation context, determination of difficult tests of system behavior is important as these tests would elucidate problematic system phenomena before these mistakes can engender problematic outcomes, e.g. loss of human life in autonomous cars, costly failures for airplane systems, etc . Our approach builds on existing, simulation-based work in the test and evaluation literature by offering a controller-agnostic test-synthesis procedure that provides a series of benchmark tests with which to determine controller reliability. To achieve this, our approach codifies the system objective as a timed reach-avoid specification. Then, by coupling control barrier functions with this class of specifications, we construct an instantaneous difficulty metric whose minimizer corresponds to the most difficult test at that system state. We use this instantaneous difficulty metric in a game-theoretic fashion, to produce an adversarial, time-varying test-synthesis procedure that does not require specific knowledge of the system’s controller, but can still provably identify realizable and maximally difficult tests of system behavior. Finally, we develop this test-synthesis procedure for both continuous and discrete-time systems and showcase our test-synthesis procedure on simulated and hardware examples.