In a hard real-time system, it is assumed that no deadline is missed, whereas, in a soft or firm real-time system, deadlines can be missed, although this usually happens in a nonpredictable way. However, most hard real-time systems could miss some deadlines provided that it happens in a known and predictable way. Also, adding predictability on the pattern of missed deadlines for soft and firm real-time systems is desirable, for instance, to guarantee levels of quality of service. We introduce the concept of weakly hard real-time systems to model real-time systems that can tolerate a clearly specified degree of missed deadlines. For this purpose, we define four temporal constraints based on determining a maximum number of deadlines that can be missed during a window of time (a given number of invocations). This paper provides the theoretical analysis of the properties and relationships of these constraints. It also shows the exact conditions under which a constraint is harder to satisfy than another constraint. Finally, results on fixed priority scheduling and response-time schedulability tests for a wide range of process models are presented.

Weakly hard real-time systems

A real-time application is typically composed of a number of cooperating activities that must execute within specific time intervals. Since there are usually more activities to be executed than there are processors on which to execute them, several activities must share a single processor. Necessarily, satisfying the activities' timing constraints is a prime concern in making the scheduling decisions for that processor.
Unfortunately, the activities are not independent. Rather, they share data and devices, observe concurrency constraints on code execution, and send signals to one another. These interactions can be modeled as contention for shared resources that must be used by one activity at a time. An activity awaiting access to a resource currently held by another activity is said to depend on that activity, and a dependency relationship is said to exist between them. Dependency relationships may encompass both precedence constraints and resource conflicts.
No algorithm solves the problem of scheduling activities with dynamic dependency relationships in a way that is suitable for all real-time systems. This thesis provides an algorithm, called scDASA, that is effective for scheduling the class of real-time systems known as supervisory control systems.
Simulation experiments that account for the time required to make scheduling decisions demonstrate that scDASA provides equivalent or superior performance to other scheduling algorithms of interest under a wide range of conditions for parameterized, synthetic workloads. scDASA performs particularly well during overloads, when it is impossible to complete all of the activities.
This research makes a number of contributions to the field of computer science, including: a formal model for analyzing scheduling algorithms; the scDASA scheduling algorithm, which integrates resource management with standard scheduling functions; results that demonstrate the efficacy of scDASA in a variety of situations; and a simulator. In addition, this work may improve the current practices employed in designing and constructing supervisory control systems by encouraging the use of modern software engineering methodologies and reducing the amount of tuning that is required to produce systems that meet their real-time constraints--while providing improved scheduling, graceful degradation, and more freedom in modifying the system over time.

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Scheduling dependent real-time activities

This paper introduces condition/event (C/E) systems as a class of continuous-time discrete event dynamic systems (DEDS) with two types of discrete-valued input and output signals:condition signals andevent signals. In applications such as discrete control, C/E systems provide an intuitive continuous-time modeling framework amenable to block diagram representation. In this paper we consider C/E systems with discrete state realizations, and study the relationship between continuous-time C/E systems and untimed models of their sequential inputoutput behavior called C/E languages. We show that C/E systems with discrete state realizations are necessarilytime-change invariant (Theorem 3.1), which means the ensemble of admissible continuous-time input-output behaviors is completely characterized by the C/E language for the system (Theorem 4.1). It is also shown that deterministic C/E systems with discrete state realizations are necessarily discrete-time (clocked) systems (Corollary 3.1), and that finite discrete state realizations exist for a C/E system only if its related C/E language has a finite state generator (Theorem 4.2). Finally, we develop equivalent discrete-state realizations for C/E systems resulting from cascade and feedback interconnections. The paper concludes with a discussion of several directions for future research.

On condition/event systems with discrete state realizations

The manufacturing environment has changed dramatically. In the past, manufacturing systems were sufficiently simple, in most cases, to permit the use of intuitive and informal solutions in the development of supervisory control systems. The increasing level of automation, integration, and flexibility encountered in automated manufacturing systems renders formal approaches to the supervisory control system development a necessity, The controlled-automata based approach to supervisory control development considered in this work is one such approach. It offers important advantages over other approaches. It guarantees that: i) the resulting controlled behaviors do not contradict the behavioral specifications and are nonblocking and ii) the controlled behaviors are maximally permissive within the behavioral specifications. The work presented illustrates the applicability of the method to the supervisory control of industrial automated manufacturing systems. The paper provides an overview of the automated manufacturing field, and introduces supervisory-control-system development. The main concepts are illustrated through an example of a small manufacturing cell. The development of a supervisory control system for an experimental manufacturing cell is presented subsequently. Both centralized and modular supervision are considered. The automata models for the complete cell are provided together with the corresponding supervisors. The implementation of the resulting supervisory control system only required the use of "off-the-shelf" industrial control systems.

The real-time supervisory control of an experimental manufacturing cell

There are increasing calls to employ conventional magnetic testers, such as the Epstein frame and single sheet tester, for the accurate measurements of magnetic properties of soft magnetic materials under fully controlled nonsinusoidal flux density waveforms. This paper presents a computerized automated adaptive digital feedback system built for that purpose. We present several examples of the ability of the system to control an arbitrary shape of the flux density waveforms over the frequency range of the data acquisition system (0.5 Hz to 2 kHz) and for peak flux density up to 90% of saturation. The control algorithm is capable of magnetizing magnetic material under controlled sinusoidal, triangular, trapezoidal, and pulsewidth-modulated magnetizing conditions as well as other arbitrary waveforms that do not contain dc components. We provide a full description of the adaptive digital feedback technique together with measurements showing the B-H loops for several magnetic materials under various controlled excitation conditions.

Leo Motus

Papers

Real-time computer control: An introduction: Stuart Bennett☆