Camilo Lozoya

The One-Shot Task Model for Robust Real-Time Embedded Control Systems

Embedded control systems are often implemented in small microprocessors enabled with real-time technology. In this context, control laws are often designed according to discrete-time control systems theory and implemented as hard real-time periodic tasks. Standard discrete-time control theory mandates to periodically sample (input) and actuate (output). Depending on how input/output (I/O) operations are performed within the hard real-time periodic task, different control task models can be distinguished. However, existing task models present important drawbacks. They generate task executions prone to violate the periodic control demands, a problem known as sampling and latency jitter, or they impose synchronized I/O operations at each task job execution that produce a constant but artificially long I/O latency. In this paper, the one-shot task model for implementing control systems in embedded multitasking hard real-time platforms is presented. The novel control task model is built upon control theoretical results that indicate that standard control laws can be implemented considering only periodic actuation. Taking advantage of this property, the one-shot task model is shown to remove endemic problems for real-time control systems such as sampling and latency jitters. In addition, it can minimize the harmful effects that long I/O latencies have on control performance. Extensive simulations and real experiments show the feasibility and effectiveness of the novel task model, compared to previous real-time and/or control-based solutions.

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.

Control Performance Evaluation of Selected Methods of Feedback Scheduling of Real-time Control Tasks ⋆

Feedback scheduling (FS) often refers to the problem of sampling period selection for real-time control tasks that compete for limited computing resources such as processor, network, or battery power. Its goal is to optimize the aggregated control performance achieved by all tasks by efficiently using the scarce resources. In this paperrepresentative existing FS methods are selected, their main features are identified, and a simple control performance evaluation is performed. The latter shows that a) jitters in job executions hide the true performance that can be achieved by the analyzed FS methods, b) after completely removing the degrading effects of jitters, the performance of each FS method dramatically changes, and c) the relative benefit provided by each method depends on the type of perturbations affecting the plants.

Analysis and design of networked control loops with synchronization at the actuation instants

Varying time delays prevent successful operation of control loops closed over communication networks, i.e. networked control system (NCS). To mitigate the negative effects of delays, existing research mainly focuses on deriving control approaches/solutions built upon the assumption that the operation of control loops is synchronized at the sampling instants. In this paper we propose an approach to design networked control systems with synchronization at the actuation instants. The immediate benefit is that the harmful effects that delays within control loops have in stability/performance are removed. However, the application of the proposed solution requires accurate clock synchronization among the network nodes. This requirement is discussed and analyzed for the specific case of NCS based on the controller area network (CAN).

On the Timing of Discrete Events in Event-Driven Control Systems

This paper presents an analysis method to determine offline at what intervals have to be taken the samples for various types of event-driven control systems.

Preliminary approach to Lyapunov sampling in CAN-based networked control systems

This paper presents a preliminary approach to networked control systems (NCS) that relies on an event-driven control method based on Lyapunov sampling. The goal is to study and develop approaches for NCS capable of offering controllers with low bandwidth demands. The paper starts by presenting the theoretical framework required for applying Lyapunov sampling to a set of closed-loop systems that share a serial bus line. The discussion permits identifying which requirements must be fulfilled in order to guarantee overall stability. In addition, the feasibility of the theoretical framework in terms of implementation requires to address problems caused by the implicit distributed architecture of the NCS. In particular, an efficient implementation approach is described for the CAN network. Simulation results illustrate the operation and benefits of the presented approach.

Minimizing control cost in resource-constrained control systems: From feedback scheduling to event-driven control

This paper evaluates approaches aimed at minimizing aggregated control cost of a set of controllers that concurrently execute sharing limited computing resources. The evaluation focuses on feedback scheduling and event-driven control methods. The performance results drive the analysis to explore self-triggered controllers in the context of minimizing control cost when given a fixed amount of computing resources. This leads to the formulation of an optimization problem, that for given example, is numerically solved. The solution helps understanding the behavior of self-triggered controllers.

Embedding Kalman techniques in the one-shot task model when non-uniform samples are corrupted by noise

The performance of several closed-loop systems whose controllers concurrently execute in a multitasking real-time system may be deteriorated due to timing uncertainties in taskséxecutions, problem known as scheduling jitters. Recently, the one-shot task model, that combines irregular sampling, a predictor observer, and strictly periodic actuation, was presented in order to remove the negative effects of jitters. However, its successful application required noise-free samples. In this paper we extend the one-shot task model to the case of noisy measurements. In particular, we embed a Kalman filter into the model taking into account that the available measurements are not periodic. This poses the problem of adapting the standard discrete-time Kalman filter to the case under study, and decide when to apply the prediction and the correction phase. Two different strategies are presented, and their control performance and computation demand are analyzed through real experiments.

Implementation analysis of GPRS communication for precision agriculture

The implementation of precision agriculture relies on the acquisition of large amounts of spatially distributed data from a remote area. Specifically closed-loop irrigation systems requires the installation of a large group of sensor nodes distributed over a crop area, in order to measure soil moisture and climatic variables. Wireless sensor network (WSN) based on Zigbee protocol represents a popular communication solution for precision agriculture applications. However due the short range of Zigbee based nodes, a complementary long range communication network is required to integrate the WSN into Internet for global access. Due its popularity in the personal mobile communication domain, GPRS (general packet radio service) represents an adequate technology to provide Internet access for a group of devices that comprises a sensor network. This paper analyzes different approaches on how to implement GPRS communication as a gateway between Zigbee WSN and Internet for precision agriculture, considering the specific needs required by closed-loop irrigation systems.