arXiv subjects

Zhiyu Liu

Publications and source records attributed to Zhiyu Liu.

41 records · Page 3Linked to original sources

arXiv2017

Communication-aware Motion Planning for Multi-agent Systems from Signal Temporal Logic Specifications

We propose a mathematical framework for synthesizing motion plans for multi-agent systems that fulfill complex, high-level and formal local specifications in the presence of inter-agent communication. The proposed synthesis framework consists of desired motion specifications in temporal logic (STL) formulas and a local motion controller that ensures the underlying agent not only to accomplish the local specifications but also to avoid collisions with other agents or possible obstacles, while maintaining an optimized communication quality of service (QoS) among the agents. Utilizing a Gaussian fading model for wireless communication channels, the framework synthesizes the desired motion controller by solving a joint optimization problem on motion planning and wireless communication, in which both the STL specifications and the wireless communication conditions are encoded as mixed integer-linear constraints on the variables of the agents' dynamical states and communication channel status. The overall framework is demonstrated by a case study of communication-aware multi-robot motion planning and the effectiveness of the framework is validated by simulation results.

eess.SY↗

arXiv2017

Distributed Communication-aware Motion Planning for Multi-agent Systems from STL and SpaTeL Specifications

In future intelligent transportation systems, networked vehicles coordinate with each other to achieve safe operations based on an assumption that communications among vehicles and infrastructure are reliable. Traditional methods usually deal with the design of control systems and communication networks in a separated manner. However, control and communication systems are tightly coupled as the motions of vehicles will affect the overall communication quality. Hence, we are motivated to study the co-design of both control and communication systems. In particular, we propose a control theoretical framework for distributed motion planning for multi-agent systems which satisfies complex and high-level spatial and temporal specifications while accounting for communication quality at the same time. Towards this end, desired motion specifications and communication performances are formulated as signal temporal logic (STL) and spatial-temporal logic (SpaTeL) formulas, respectively. The specifications are encoded as constraints on system and environment state variables of mixed integer linear programs (MILP), and upon which control strategies satisfying both STL and SpaTeL specifications are generated for each agent by employing a distributed model predictive control (MPC) framework. Effectiveness of the proposed framework is validated by a simulation of distributed communication-aware motion planning for multi-agent systems.

cs.MA↗

arXiv2017

Well-Structured Futures and Cache Locality

In fork-join parallelism, a sequential program is split into a directed acyclic graph of tasks linked by directed dependency edges, and the tasks are executed, possibly in parallel, in an order consistent with their dependencies. A popular and effective way to extend fork-join parallelism is to allow threads to create futures. A thread creates a future to hold the results of a computation, which may or may not be executed in parallel. That result is returned when some thread touches that future, blocking if necessary until the result is ready. Recent research has shown that while futures can, of course, enhance parallelism in a structured way, they can have a deleterious effect on cache locality. In the worst case, futures can incur $Ω(P T_\infty + t T_\infty)$ deviations, which implies $Ω(C P T_\infty + C t T_\infty)$ additional cache misses, where $C$ is the number of cache lines, $P$ is the number of processors, $t$ is the number of touches, and $T_\infty$ is the \emph{computation span}. Since cache locality has a large impact on software performance on modern multicores, this result is troubling. In this paper, however, we show that if futures are used in a simple, disciplined way, then the situation is much better: if each future is touched only once, either by the thread that created it, or by a thread to which the future has been passed from the thread that created it, then parallel executions with work stealing can incur at most $O(C P T^2_\infty)$ additional cache misses, a substantial improvement. This structured use of futures is characteristic of many (but not all) parallel applications.

cs.DC↗

arXiv2015

On instability and mixing on the UK Continental Shelf

The stability of stratified flows at locations in the Clyde, Irish and Celtic Seas on the UK Continental Shelf is examined. Flows are averaged over periods of 12-30 min in each hour, corresponding to the times taken to obtain reliable estimates of the rate of dissipation of turbulent kinetic energy per unit mass, $\varepsilon$. The Taylor-Goldstein equation is solved to find the maximum growth rate of small disturbances to these averaged flows, and the critical gradient Richardson number, $\textrm{Ri}_\textrm{c}$. The proportion of unstable periods where the minimum gradient Richardson number, $\textrm{Ri}_{\textrm{min}}$, is less than $\textrm{Ri}_\textrm{c}$ is about 35%. Cases are found in which $\textrm{Ri}_\textrm{c} <$ 0.25; 37% of the flows with $\textrm{Ri}_{\textrm{min}}<$ 0.25 are stable, and $\textrm{Ri}_\textrm{c}<$ 0.24 in 68% of the periods where $\textrm{Ri}_{\textrm{min}}<$ 0.25. Marginal conditions with 0.8 $<$ Ri$_{\textrm{min}}$$/$Ri$_\textrm{c}$ $<$ 1.2 occur in 30% of the periods examined. The mean dissipation rate at the level where the fastest growing disturbance has its maximum amplitude is examined to assess whether the turbulence there is isotropic and how it relates to the Wave-Turbulence boundary. It is concluded that there is a background level of dissipation that is augmented by instability; instability of the averaged flow does not account for all the turbulence observed in mid-water. The effects of a horizontal separation of the measurements of shear and buoyancy are considered. The available data do not support the hypothesis that the turbulent flows observed on the UK shelf adjust rapidly to conditions that are close to being marginal, or that flows in a particular location and period of time in one sea have stability characteristics that are very similar to those in another.

physics.ao-ph↗

arXiv2014

Global geometry and topology of spacelike stationary surfaces in the 4-dimensional Lorentz space

For spacelike stationary (i.e. zero mean curvature) surfaces in 4-dimensional Lorentz space one can naturally introduce two Gauss maps and Weierstrass representation. In this paper we investigate their global geometry systematically. The total Gaussian curvature is related with the surface topology as well as the indices of the so-called good singular ends by a generalized Jorge-Meeks formula. On the other hand, as shown by a family of counter-examples to Osserman's theorem, finite total curvature no longer implies that Gauss maps extend to the ends. Interesting examples include the generalization of the classical catenoids, helicoids, the Enneper surface, and Jorge-Meeks' k-noids. Any of them could be embedded in the 4-dimensional Lorentz space, showing a sharp contrast with the case of 3-dimensional Euclidean space.

math.DG↗