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Networked Embedded Control Systems: from Modelling to Implementation

Electronic Proceedings in Theoretical Computer Science, 2013

Discrete abstractions of continuous and hybrid systems have been the topic of intensive study in the last twenty years from both the control systems and the computer science communities, see e.g. [10]. While physical world processes are often described by differential equations, digital controllers and software and hardware at the implementation layer are usually modeled through discrete/symbolic processes. These mathematical models heterogeneity has posed during the years interesting and challenging theoretical problems that must be addressed in order to ensure the formal correctness of control algorithms in the presence of non-idealities at the implementation layer. From the synergistic collaboration of researchers in the control systems and computer science communities a novel and sound approach has recently emerged, which is termed "Correct-by-Design Embedded Control Software". This research line can be roughly described as a three-step process, as shown in Figure 1, and detailed hereafter:

Symbolic Abstractions of Networked Control Systems

IEEE Transactions on Control of Network Systems, 2017

The last decade has witnessed significant attention on networked control systems (NCS) due to their ubiquitous presence in industrial applications, and, in the particular case of wireless NCS, because of their architectural flexibility and low installation and maintenance costs. In wireless NCS the communication between sensors, controllers, and actuators is supported by a communication channel that is likely to introduce variable communication delays, packet losses, limited bandwidth, and other practical non-idealities leading to numerous technical challenges. Although stability properties of NCS have been investigated extensively in the literature, results for NCS under more complex and general objectives, and in particular results dealing with verification or controller synthesis for logical specifications, are much more limited. This work investigates how to address such complex objectives by constructively deriving symbolic models of NCS, while encompassing the mentioned network non-idealities. The obtained abstracted (symbolic) models can then be employed to synthesize hybrid controllers enforcing rich logical specifications over the concrete NCS models. Examples of such general specifications include properties expressed as formulae in linear temporal logic (LTL) or as automata on infinite strings. We thus provide a general synthesis framework that can be flexibly adapted to a number of NCS setups. We illustrate the effectiveness of the results over some case studies.

Symbolic Control Design of Nonlinear Networked Control Systems

Networked Control Systems (NCS) are distributed systems where plants, sensors, actuators and controllers communicate over shared networks. Non-ideal behaviors of the communication network include variable sampling/transmission intervals and communication delays, packet losses, communication constraints and quantization errors. NCS have been the object of intensive study in the last few years. However, due to the inherent complexity of NCS, current literature focuses on only a subset of these non-idealities and mostly considers stability and stabilizability problems. Recent technology advances indeed demand that different and more complex control objectives are considered. In this paper we present first a general model of NCS, including all the non-idealities of the communication network; then, we propose a symbolic model approach to the control design with objectives expressed in terms of non-deterministic transition systems. The presented results are based on recent advances in symbolic control design of hybrid and continuous control systems. An example in the context of robot motion planning with remote control is included, showing the effectiveness of the approach taken. network so that it was possible to obtain reasonable performance by aggregating subsystems that were locally designed and optimized. However the growth of complexity of the physical systems to control, together with the continuous increase in functions that these systems must perform, requires today to adopt a unified design approach where different disciplines (e.g. control systems engineering, computer science, software engineering and communication engineering) should come together to reach new levels of performance. The heterogeneity of the subsystems that are to be connected in a NCS make the control of these systems a hard but challenging task. NCS have been the focus of much recent research in the control community: Murray et al. in presented control over networks as one of the important future directions for control. Following [2], the most important non-idealities considered in the study of NCS are: (i) variable sampling/transmission intervals; (ii) variable communication delays; (iii) packet dropouts caused by the unreliability of the network; (iv) communication constraints (scheduling protocols) managing the possibly simultaneous transmissions over the shared channel; (v) quantization errors in the digital transmission with finite bandwidth. There are two approaches to manage such non-idealities: the deterministic approach, which assumes worst-case (deterministic) bounds on the aforementioned imperfections, and the stochastic approach, which provides a stochastic description of the non-ideal communication network. We focus our attention on the deterministic methods, which can be further distinguished according to the modeling assumptions and the controller synthesis for NCS: a) the discrete-time approach (see e.g. [3], [4]) considers discrete-time controllers and plants; b) the sampled-data approach (see e.g. [5], [6]) assumes discrete-time controllers and continuous-time (sampled-data) plants; c) the continuous-time (emulation) approach (see e.g. [7], [8]) focuses on continuous-time controllers and continuous-time (sampled-data) plants. In the deterministic approach, results obtained during the past few years are mostly about stability and stabilizability problems, see e.g. [9, 2, 10], with results that depend on the method considered and the assumptions on the non-ideal communication infrastructure. In addition, current approaches in the literature take into account only a subset of these non-idealities. As reviewed in [2], for example, [11] studies imperfections of type (i), (iv), (v), [3], [12], [6] consider simultaneously (i), (ii), (iii), [8] focuses on (i), (iii), (iv), while [5] manages (ii), (iii) and (v). Three types of non-idealities, namely (i), (ii), (iv), are considered for example in [13], [14], [7]. In [15], the five non-idealities are dealt with but small delay and other restrictive assumptions are considered. Finally, novel results in the stability analysis of NCS can be found in [16], [17], [18], [19].

Hybrid Modeling and Verification of Embedded Control Systems

IFAC Proceedings Volumes, 1997

Contemporary process control includes continuous time PID controllers for actuation, and at a higher level, supervisory control mechanisms to select appropriate control algorithms for the different modes of system operation in an attempt to achieve optimal or near-optimal control. To model and analyze such control phenomena that include discrete and continuous components requires the use of hybrid modeling techniques. This paper presents a hybrid modeling paradigm that identifies and captures the phenomena specific to hybrid system models, and specifies its execution semantics based on the principles of invariance of 8tate and temporal evolution of state. The modeling methodology is applied to analysis of control behavior in dynamic physical systems, and model verification based on divergence of time demonstrates possible applications in design tasks.

Design of Symbolic Controllers for Networked Control Systems

Towards a Unified Theory for the Control of CPS A Symbolic Approach

2014

Cyber--Physical Systems (CPS) are attracting particular attention in the research and industrial communities given their impact on the future of Internet of Things, Systems of Systems, wearable electronics, brain--machine interaction and swarm systems, and other fields that require the integration of different, complementary competences. We focus on the role of control and in particular of Networked Control Systems (NCS) [1] and embedded control software synthesis [2] in building the foundations of CPS. Particular emphasis is given in NCS to the non--idealities affecting communication among plants and controllers. The relevant ones are quantization errors, variable sampling/transmission intervals, time--varying delay in delivering messages through the network, limited bandwidth, packet dropouts, and scheduling protocols. Given the generality of the NCS model, results for the analysis and control design are difficult to obtain. Researchers, thus, typically focus only on subsets of these non--idealities. Results available in the literature mostly concern stability and stabilizability issues [1]. However, emerging requirements for CPS address different and perhaps more complex control objectives, for example: obstacle avoidance, synchronization specifications, enforcement of limit cycles and oscillatory behaviour. The computer science community has developed methodologies for the control design of discrete systems with complex logic specifications. These techniques are general enough to address issues arising from CPS applications. However, this methodology cannot be directly applied to NCS, which include continuous dynamics. The focus of this article is an approach to deal with this class of systems by applying "correct--by--design embedded control software synthesis". Central to this approach is the construction of symbolic models, which are abstract descriptions of continuous systems where a symbol corresponds to an "aggregate" of continuous states. Once a symbolic model is constructed, which is equivalent to or approximates the original continuous dynamics, then the techniques developed for discrete models can be applied. Several classes of dynamical and control systems admit symbolic models, see for example [2] and the references therein.

Hybrid control of networked embedded systems

2005

Abstract Hybrid systems that involve the interaction of continuous and discrete dynamics have been an active area of research for a number of years. In this paper, we start by briefly surveying the main theoretical control problems that have been treated in the hybrid systems setting and classify them into stabilization, optimal control and language specification problems.

SENSE: Abstraction-Based Synthesis of Networked Control Systems

Electronic Proceedings in Theoretical Computer Science, 2018

While many studies and tools target the basic stabilizability problem of networked control systems (NCS), nowadays modern systems require more sophisticated objectives such as those expressed as formulae in linear temporal logic or as automata on infinite strings. One general technique to achieve this is based on so-called symbolic models, where complex systems are approximated by finite abstractions, and then, correct-by-construction controllers are automatically synthesized for them. We present tool SENSE for the construction of finite abstractions for NCS and the automated synthesis of controllers. Constructed controllers enforce complex specifications over plants in NCS by taking into account several non-idealities of the communication channels. Given a symbolic model of the plant and network parameters, SENSE can efficiently construct a symbolic model of the NCS, by employing operations on binary decision diagrams (BDDs). Then, it synthesizes symbolic controllers satisfying a class of specifications. It has interfaces for the simulation and the visualization of the resulting closed-loop systems using OMNeT++ and MATLAB. Additionally, SENSE can generate ready-to-implement VHDL/Verilog or C/C++ codes from the synthesized controllers.

Invisible formal methods for embedded control systems

Proceedings of the IEEE, 2003

Embedded control systems typically comprise continuous control laws combined with discrete mode logic. These systems are modeled using a hybrid automaton formalism, which is obtained by combining the discrete transition system formalism with continuous dynamical systems. This paper develops automated analysis techniques for asserting correctness of hybrid system designs. Our approach is based on symbolic representation of the state space of the system using mathematical formulas in an appropriate logic. Such formulas are manipulated using symbolic theorem proving techniques.

A symbolic approach to the design of nonlinear networked control systems

Proceedings of the 15th ACM international conference on Hybrid Systems: Computation and Control - HSCC '12, 2012

Networked control systems (NCS) are spatially distributed systems where communication among plants, sensors, actuators and controllers occurs in a shared communication network. NCS have been studied for the last ten years and important research results have been obtained. These results are in the area of stability and stabilizability. However, while important, these results must be complemented in different areas to be able to design effective NCS. In this paper we approach the control design of NCS using symbolic (finite) models. Symbolic models are abstract descriptions of continuous systems where one symbol corresponds to an "aggregate" of continuous states. We consider a fairly general multiple-loop network architecture where plants communicate with digital controllers through a shared, non-ideal, communication network characterized by variable sampling and transmission intervals, variable communication delays, quantization errors, packet losses and limited bandwidth. We first derive a procedure to obtain symbolic models that are proven to approximate NCS in the sense of alternating approximate bisimulation. We then use these symbolic models to design symbolic controllers that realize specifications expressed in terms of automata on infinite strings. An example is provided where we address the control design of a pair of nonlinear control systems sharing a common communication network. The closed-loop NCS obtained is validated through the OMNeT++ network simulation framework. [FP7/2007[FP7/ -2013 under grant agreement n. 257462 HYCON2 Network of excellence.

This work is licensed under the Creative Commons Attribution License. Networked Embedded Control Systems: from Modelling to Implementation (original) (raw)

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