Josep M. Fuertes

Jitter compensation for real-time control systems

In this paper, we first identify the potential violations of control assumptions inherent in standard real-time scheduling approaches (because of the presence of jitters) that causes, degradation in control performance and may even lead to instability. We then develop practical approaches founded on control theory to deal with these violations. Our approach is based on the notion of compensations wherein controller parameters are adjusted at runtime for the presence of jitters. Through time and memory overhead analysis, and by elaborating on the implementation details, we characterize when offline and on-line compensations are feasible. Our experimental results confirm that our approach does compensate for the degraded control performance when EDF and FPS algorithms are used for scheduling the control tasks. Our compensation approach provides us another advantage that leads to better schedulability of control tasks. This derives from the potential to derive more flexible timing constraints, beyond periods and deadlines necessary to apply EDF and FPS. Overall, our approach provides guarantees offline that the control system will be stable at runtime-if temporal requirements are met at runtime-even when actual execution patterns are not known beforehand. With our approach, we can address the problems due to (a) sampling jitters, (b) varying delays between sampling and actuation, or (c) both-not addressable using traditional EDF and FPS based scheduling, or by previous real-time and control integration approaches.

Managing quality-of-control in network-based control systems by controller and message scheduling co-design

In network-based control systems (NCSs), plant sensor-controller-actuator nodes in closed-loop operation drive principal network traffic. The quality-of-control (QoC) in an NCS, i.e., the performance delivered by each closed-loop operation, depends not only on the controller design but also on the message scheduling strategy. In this paper, we show that the co-design of adaptive controllers and feedback scheduling policies allows for the optimization of the overall QoC. First, we discuss the limitations of standard discrete-time control models for controllers of control loops that are closed over communication networks. Afterwards, we describe an approach to adaptive controllers for NCS that: 1) overcomes some of the previous restrictions by online adapting the control decisions according to the dynamics of both the application and executing platform and 2) offers capabilities for dynamic management of QoC through message scheduling.

Optimal state feedback based resource allocation for resource-constrained control tasks

In many application areas, including control systems, careful management of system resources is key to providing the best application performance. Most traditional resource management techniques for real-time systems with multiple control loops are based on open-loop strategies that statically allocate a constant CPU share to each controller, independent of their current resource needs. This provides average control performance with minimal overhead but in general fails to provide the best performance possible within the available resources. We show that by using feedback to dynamically allocate resources to controllers as a function of the current state of their controlled systems, control performance can be significantly improved. We present an optimal resource allocation policy that maximizes control performance within the available resources and provide experimental results showing that the optimal policy 1) significantly increases control performance compared to traditional control system implementations (by more than 20% in our experiments), 2) maximizes control performance over other feedback-based policies, 3) saves resources when perturbations occur infrequently, and 4) incurs negligible overhead.

Improving quality-of-control using flexible timing constraints: metric and scheduling

Closed-loop control systems are dynamic systems subject to perturbations. One of the main concerns of the control is to design controllers to correct or limit the deviation that transient perturbations cause in the controlled system response. The smaller and shorter the deviation, the better the achieved performance. However, such controllers have been traditionally implemented using fixed timing constraints (periods and deadlines). This precludes controllers to execute dynamically, accordingly to the system dynamics, which may lead to sub-optimal implementations: although higher execution rates may be preferable when reacting to perturbations in order to minimize the response deviations, they imply wastage of resources when the system is in equilibrium. In this paper we argue and demonstrate that the responsibility of maximizing the performance of closed-loop systems relies on both the controller designer and the scheduler. We show that the dynamic optimization of the quality of the controlled system response calls for (a) flexible control task timing constraints that deliver effective control performance; flexible constraints allow us to achieve faster reaction by adaptively choosing the controller sampling rate and completion time upon transient perturbations, (b) a quality-of-control (QoC) metric; it associates with each control task timing a quantitative value expressing control performance (in terms of the closed-loop system error), and (c) new scheduling approaches; their goal is to quickly react to perturbations by dynamically scheduling tasks based on the chosen control task execution parameters to maximize the QoC. This combination offers the possibility of taking scheduling decisions based on the control information for each control task invocation, rather than using fixed timing constraints with constant periods and deadlines.

A control approach to bandwidth management in networked control systems

Bandwidth allocation techniques for control loops closed over communication networks are based on static strategies that ensure average control performance at the expense of permanently occupying the available bandwidth. We present a dynamic approach to bandwidth management in networked control systems that allows control loops to consume band-width according to the dynamics of the controlled process while attempting to optimize overall control performance. By augmenting the original state-space representation of each controlled process with a new state variable that describes the network dynamics, 1) the allocation of bandwidth lo control loops can he done locally at run-time according to the state of each controlled process without causing overload situations and 2) control laws can be designed to account for the variations in the assigned bandwidth preventing the unexpected control performance degradation and even destabilization that would otherwise occur. Experimental data shows that this approach improves control performance with respect to the static strategy and uses less bandwidth.

Control loop performance analysis over networked control systems

In this paper we discuss the suitability of standard discrete-time control models for the analysis and design of control loops implemented over networked control systems. In fact, depending on the specific control loop topology, different control approaches may apply. We distinguish between control approaches that offer synchronous and asynchronous actuation. Through simulation results we show that the performance of control loops closed over communication networks can be improved using approaches that support asynchronous actuation. This leads us to conclude that there is a need for more control approaches relying on the asynchronous actuation paradigm.

Self-triggered networked control systems: An experimental case study

A self-triggered controller is characterized, in general, by a non-periodic sequence of job activations. And each job execution, apart from performing sampling, control algorithm computation and actuation, calculates the next job activation time as a function of the plant state. This paper describes the implementation of self-triggered controllers in networked control systems (NCS). The implementation corroborates that self-triggered control can be used for minimizing bandwidth utilization while providing similar control performance than periodic controllers.

An integrated approach to real-time distributed control systems over fieldbuses

The increasing application of flexible and powerful real-time distributed control systems is presently characterizing the industrial automation field. Such systems involve three main disciplines: control systems, real-time systems, and communication systems. Control systems, due their stringent timing constraints, demand real-time computing technology. In addition, communication systems are needed for the data messaging between field devices. We propose an integrated approach to the design and implementation of such systems. We show that by a separate control design and its posterior distributed implementation, the system performance may suffer degradation. That is, when control loops are closed over communication networks, timing problems, as communication induced varying delays, can appear, decreasing the control system performance, and even leading the system to instability. However, we show that by an adequate integrated approach, that takes advantage of control theory, real-time communication properties, an adequate timing analysis, and an appropriate distribution of the control functions, the system performance increases dramatically.

Design of an Embedded Control System Laboratory Experiment

This paper presents a prototype laboratory experiment to be integrated in the education of embedded control system engineers. The experiment, a real-time control of a dynamical system, is designed to drive students to a deeper understanding and integration of the diverse theoretical concepts that often come from different disciplines such as real-time systems and control systems. Rather than proposing the experiment for a particular course within an embedded system engineering curriculum, this paper describes how the experiment can be tailored to the needs and diverse background of both undergraduate and graduate students education.

Performing Flexible Control on Low-Cost Microcontrollers Using a Minimal Real-Time Kernel

In recent years, approaches to control performance and resource optimization for embedded control systems have been receiving increased attention. Most of them focus on theory, whereas practical aspects are omitted. Theoretical advances demand flexible real-time kernel support for multitasking and preemption, thus requiring more sophisticated and expensive software/hardware solutions. On the other hand, embedded control systems often have cost constraints related with mass production and strong industrial competition, thus demanding low-cost solutions. In this paper, it is shown that these conflicting demands can be softened and that a compromise solution can be reached. We advocate that recent research results on optimal resource management for control tasks can be implemented on simple multitasking preemptive real-time kernels targeting low-cost microprocessors, which can be easily built in-house and tailored to actual application needs. The experimental evaluation shows that significant control performance improvement can be achieved without increasing hardware costs.