Yerang Hur

Hierarchical modeling and analysis of embedded systems

This paper describes the modeling language CHARON for modular design of interacting hybrid systems. The language allows specification of architectural as well as behavioral hierarchy and discrete as well as continuous activities. The modular structure of the language is not merely syntactic, but is exploited by analysis tools and is supported by a formal semantics with an accompanying compositional theory of refinement. We illustrate the benefits of CHARON in the design of embedded control software using examples from automated highways concerning vehicle coordination.

Modular Specification of Hybrid Systems in CHARON

We propose a language, called Charon, for modular speci fication of interacting hybrid systems. For hierarchical description of the system architecture, Charon supports building complex agents via the operations of instantiation, hiding, and parallel composition. For hierarchical description of the behavior of atomic components, Charon supports building complex modes via the operations of instantiation, scoping, and encapsulation. Features such as weak preemption, history retention, and externally defined Java functions, facilitate the description of complex discrete behavior. Continuous behavior can be specified using differential as well as algebraic constraints, and invariants restricting the flow spaces, all of which can be declared at various levels of the hierarchy. The modular structure of the language is not merely syntactic, but can be exploited during analysis. We illustrate this aspect by presenting a scheme for modular simulation in which each mode can be compiled solely based on the locally declared information to execute its discrete and continuous updates, and furthermore, submodes can integrate at a finer time scale than the enclosing modes.

A Framework and Architecture for Multi-Robot Coordination

In this paper, we present a framework and the software architecture for the deployment of multiple autonomous robots in an unstructured and unknown environment, with applications ranging from scouting and reconnaissance, to search and rescue, to manipulation tasks, to cooperative localization and mapping, and formation control. Our software framework allows a modular and hierarchical approach to programming deliberative and reactive behaviors in autonomous operation. Formal definitions for sequential composition, hierarchical composition, and parallel composition allow the bottom-up development of complex software systems. We demonstrate the algorithms and software on an experimental testbed that involves a group of carlike robots, each using a single omnidirectional camera as a sensor without explicit use of odometry.

Hierarchical Hybrid Modeling of Embedded Systems

This paper describes the modeling language CHARON for modular design of interacting hybrid systems. The language allows specification of architectural as well as behavioral hierarchy, and discrete as well as continuous activities. The modular structure of the language is not merely syntactic, but is exploited by analysis tools, and is supported by a formal semantics with an accompanying compositional theory of refinement. We illustrate the benefits of CHARON in design of embedded control software using examples from automated highways concerning vehicle coordination.

Distributed simulation of multi-agent hybrid systems

Systems such as coordinating robot systems, automobiles, aircraft, and chemical process control systems can be modeled as interacting hybrid systems, where hybrid systems are finite state machines with continuous dynamics. The language CHARON and its simulator have been developed to model and analyze interacting hybrid systems as communicating agents. Simulations are widely used for the analyses of hybrid systems. The simulation of a complex system is, however usually very slow. This paper proposes four algorithms for distributed simulations of hybrid systems. The idea behind distributed simulations is to achieve a speedup by utilizing multiple computing resources. The agents of a modeled system are distributed over multiple processors to simulate the agents more efficiently. Since the state of the agent is affected by the input from other agents, they synchronize to update their local states. The challenge here is how to reduce the agent synchronization overhead. We present two approaches for resolving the problem: conservative and optimistic approaches. For the optimistic approach, we present three different algorithms for distributed simulations of hybrid systems, and compare them.

Sound Code Generation from Communicating Hybrid Models

Precise translation from hybrid models to code is difficult because models are defined in the continuous-time domain whereas code executes on digital computers in a discrete fashion. Traditional approach is to associate the model with a sampling rate before code generation, and rely on an approximate algorithm that computes the next state numerically. Depending on the choice of the sampling rate and the algorithm, the behavior of the code may vary significantly due to numerical errors, but the discrepancy has been addressed informally, making the analysis results at the model level less meaningful for implementation. Formal relationship between the model and the code becomes even more unclear when components of the code execute concurrently. In this paper, we propose a formal framework that addresses the issue of soundness of concurrent programs generated from communicating hybrid models. The motivation is that concurrent programs executing in different rates may cause an erroneous transition when transition conditions are evaluated using values from different time instances. The essence of our technique is to refine the model by tightening transition conditions according to the maximum errors due to different sampling rates. We claim that the generated code has a trace of discrete transitions that is equivalent to one of the traces observable from the model, and that the values of variables are bounded. Our framework demonstrates how hybrid models defined in the continuous time domain are translated into discretized models with or without consideration of errors due to asynchronous sampling, and finally into executable code with real-time scheduling.

Generating Reliable Code from Hybrid-Systems Models

Hybrid systems have emerged as an appropriate formalism to model embedded systems as they capture the theme of continuous dynamics with discrete control. Under this paradigm, distributed embedded systems can be modeled as a network of communicating hybrid automata. Several techniques for code generation from these models have also been proposed and commercially implemented. Providing formal guarantees of the generated code with respect to the model, however, has turned out to be a hard problem. While the model is set in continuous time with concurrent execution and instantaneous switching, the code running on an inherently discrete platform, can be affected by the sampling interval, round-off errors, and communication delays between the sensor, controller, and actuators. Consequently, semantic differences between the model and its code can arise with potentially different system behavior. This paper proposes a criterion for faithful implementation of the hybrid-systems model with a focus on its switching semantics. We discuss different techniques to ensure a faithful implementation of the model, and test the feasibility of our concepts by implementing a model heater system. In this heater case study, we successfully eliminate all fault transitions and, thereby, generate code with correct behavior complying with the specification.

Modeling distributed autonomous robots using CHARON: formation control case study

We present the modeling and analysis of distributed autonomous robots using the specification language for hybrid systems, called CHARON. Coordination between distributed autonomous robots has attracted researchers of embedded and hybrid systems, since there has been increasing demand for multiple robots working together in a dynamically changing or unknown environment to carry out missions such as search and rescue, cooperative localization, and scouting and reconnaissance. To maximize the capability of performing tasks collaboratively as a team, formation control is one of crucial parts in developing distributed autonomous robots. In this paper formation control of a team of robots is modeled using CHARON and the model is analyzed using simulation with assertion checking capability of the CHARON toolset.

Ensuring Sound Numerical Simulation of Hybrid Automata

A hybrid system is a dynamical system in which states can be changed continuously and discretely. Simulation based on numerical methods is the widely used technique for analyzing complicated hybrid systems. Numerical simulation of hybrid systems, however, is subject to two types of numerical errors: truncation error and round-off error. The effect of such errors can make an impossible transition step to become possible during simulation, and thus, to generate a simulation behavior that is not allowed by the model. The possibility of an incorrect simulation behavior reduces con.dence in simulation-based analysis since it is impossible to know whether a particular simulation trace is allowed by the model or not. To address this problem, we define the notion of Instrumented Hybrid Automata (IHA), which considers the effect of accumulated numerical errors on discrete transition steps. We then show how to convert Hybrid Automata (HA) to IHA and prove that every simulation behavior of IHA preserves the discrete transition steps of some behavior in HA; that is, simulation of IHA is sound with respect to HA.