José Alberto Fonseca

The FTT-CAN protocol: why and how

The requirement for flexible operation is becoming increasingly important in modern industrial systems. This requirement has to be supported at all system levels, including the field level in process industry, as well as the cell and machine control levels in manufacturing industry, where fieldbus-based communication systems are commonly found. Furthermore, typical applications at these levels require both time- and event-triggered communication services, in most cases under stringent timing constraints, to convey state data in the former case and alarms and management data in the latter. However, neither the requirement for flexible operation under guaranteed timeliness nor for joint support of time and event-triggered traffic are efficiently fulfilled by most of existing fieldbus systems. This paper presents a new protocol, flexible time-triggered communication on controller area network, which fulfills both requirements: it supports time-triggered communication in a flexible way as well as being an efficient combination of both time- and event-triggered traffic with temporal isolation. These types of traffic are handled by two complementary subsystems, the synchronous and the asynchronous messaging systems, respectively. The paper includes a justification for the new protocol as well as its description and worst case temporal analysis for both subsystems. This analysis shows the capability of the protocol to convey real-time traffic of either type.

VTP-CSMA: A Virtual Token Passing Approach for Real-Time Communication in IEEE 802.11 Wireless Networks

Currently, there is a trend towards the implementation of industrial communication systems using wireless networks. However, keeping up with the timing constraints of real-time traffic in wireless environments is a hard task. The main reason is that real-time devices must share the same communication medium with timing unconstrained devices. The VTP-CSMA architecture has been proposed to deal with this problem. It considers an unified wireless system in one frequency band, where the communication bandwidth is shared by real-time and non-real-time communicating devices. The proposed architecture is based on a virtual token passing (VTP) procedure that circulates a virtual token among real-time devices. This virtual token is complemented by an underlying traffic separation mechanism that prioritizes the real-time traffic over the non-real-time traffic. This is one of the most innovative aspects of the proposed architecture, as most part of real-time communication approaches are not able to handle timing unconstrained traffic sharing the same communication medium. A ring management procedure for the VTP-CSMA architecture is also proposed, allowing real-time stations to adequately join/leave the virtual ring.

The FTT-CAN protocol for flexibility in safety-critical systems

A new communication protocol for distributed embedded systems attempts to find a compromise between the often-opposing goals of system flexibility and safety.

Schedulability analysis of real-time traffic in WorldFIP networks: an integrated approach

The WorldFIP protocol is one of the profiles that constitute the European fieldbus standard EN-50170. It is particularly well suited to be used in distributed computer-controlled systems where a set of process variables must be shared among network devices. To cope with the real-time requirements of such systems, the protocol provides communication services based on the exchange of periodic and aperiodic identified variables. The periodic exchanges have the highest priority and are executed at run time according to a cyclic schedule. Therefore, the respective schedulability can be determined at pre-run-time when building the schedule table. Concerning the aperiodic exchanges, the situation is different since their priority is lower and they are bandied according to a first-come-first-served policy. In this paper, a response-time-based schedulability analysis for the real-time traffic is presented. Such analysis considers both types of traffic in an integrated way, according to their priorities. Furthermore, a fixed-priorities-based policy is also used to schedule the periodic traffic. The proposed analysis represents an improvement relative to previous work and it can be evaluated online as part of a traffic online admission control. This feature is of particular importance when a planning scheduler is used, instead of the typical offline static scheduler, to allow online changes to the set of periodic process variables.

A Flexible Time-Triggered Communication System Based on the Controller Area Network: Experimental Results

Controller Area Networks are normally used in event-triggered communication systems. These are known for not supporting composability in respect with the system temporal behavior. When using a time-triggered communication paradigm, such composability is achieved. However, common time-triggered systems rely on static message scheduling which compromises system flexibility. This paper proposes the use of centralized scheduling together with a planning scheduler to achieve flexible time-triggered communication on a CAN system. A model is deduced to allow the prediction of message production delays for a particular implementation using low processing power microcontrollers (Philips 80C592). Experimental results shown confirm the validity of the model. These results are also in agreement with a model for the scheduler execution time presented by the authors in previous works concerning the use of the planning scheduler in FIP-like networks.

Introduction to Intelligent Transportation Systems

Transportation systems are very important in modern life; therefore, massive research efforts has been devoted to this field of study in the recent past. Effective vehicular connectivity techniques can significantly enhance efficiency of travel, reduce traffic incidents and improve safety, alleviate the impact of congestion; devising the so-called Intelligent Transportation Systems (ITS) experience. This chapter aims to provide basic concepts and background that is useful for the understanding of this book. An overview of intelligent transportation systems and their applications is presented, followed by a brief discussion of vehicular communications. The chapter also overviews the concepts related to dependability on distributed real-time systems in the scope if ITS.

Real-time communication in unconstrained shared Ethernet networks: the virtual token-passing approach

Traditional shared Ethernet networks have a non-deterministic behaviour, which impairs the support of real-time communication. In this paper, it is proposed a shared Ethernet deterministic architecture, able to interconnect sensors, controllers and actuators at the field level, allowing the coexistence of standard devices with enhanced (real-time) devices. Such solution is based on the control of the medium access right, by means of a virtual token passing procedure among enhanced stations, complemented by an underlying prioritization mechanism. Such underlying mechanism guarantees that, whenever an enhanced (real-time) station is contending for the bus access, it will be able to access the bus prior to any other station. Thus, it enables the traffic separation between standard and enhanced (real-time) stations, being able to guarantee real-time communication in unconstrained traffic environments

Analysis of a simple model for non-preemptive blocking-free scheduling

Non-preemptive scheduling is known for its lower efficiency in meeting temporal constraints when compared to preemptive scheduling. However, it is still used in certain cases such as in message scheduling over serial broadcast buses and in light multi-tasking kernels for embedded systems based on simple microprocessors. These cases are typically found in control applications requiring the periodic execution (or transmission) of a set of tasks (or messages) with low jitter. This paper refers to a simple execution strategy based on synchronised timetriggering and non-preemptive scheduling that allows to eliminate the blocking factor commonly associated to non-preemption and thus reduce activation jitter. The elimination of such blocking factor is achieved by using inserted idle-time. The paper focuses on the schedulability analysis of a generic task set executed according to the referred model. In one part, a specific response time-based analysis is presented which supports, under worst-case assumptions, a necessary and sufficient schedulability assessment. In a following part, the paper presents a general theorem that allows to adapt the existing analysis for preemptive scheduling to the referred model. In particular, this theorem allows to develop adequate utilization bounds for guaranteed schedulability, based on the well known bounds for ratemonotonic analysis.

B-Live - A Home Automation System for Disabled and Elderly People

This paper describes the architecture, operation and implementation of the B-Live home automation system. This system has been developed at Micro I/O for assisting elderly and disabled people in their homes. The paper also discusses the demonstrator deployed at the CMRRC Rovisco Pais and proposes an improved information exchange mechanism for the B-Live system.

Implementing a distributed sensing and actuation system: The CAMBADA robots case study

The use of distributed computing architectures has become commonplace in complex embedded systems with potential advantages, for example, in terms of scalability, dependability and maintainability. One particular area in which that trend can be witnessed is mobile autonomous robotics in which several sensors and actuators are interconnected by means of a control network. In this paper we address one case study concerning the CAMBADA robots that were developed at the University of Aveiro for the Robocup Middle Size League. These robots have a distributed architecture with two layers, a coordination layer responsible for the global behaviors and a distributed sensing and actuating layer that conveys internal state information and executes coordination commands. This paper focuses on the latter layer, which is based on the FTT-CAN protocol, following a network-centric approach that provides an efficient framework for the synchronization of all systems activities. We describe the computing and communication requirements, the robot architecture, the system design and implementation, and finally we provide experimental results that show advantages with respect to a non-synchronized distributed approach