Manuel Mazo

Decentralized Event-Triggered Control Over Wireless Sensor/Actuator Networks

Event-triggered control has been recently proposed as an alternative to the more traditional periodic execution of control tasks. In a typical event-triggered implementation, the control signals are kept constant until the violation of a condition on the state of the plant triggers the recomputation of the control signals. The possibility of reducing the number of recomputations, and thus of transmissions, while guaranteeing desired levels of control performance, makes event-triggered control very appealing in the context of sensor/actuator networks. In particular, by reducing the network traffic we also reduce the energy expenditures of battery powered wireless sensor nodes. In this paper we present a decentralized event-triggered implementation, over sensor/actuator networks, of centralized nonlinear controllers.

Brief paper: An ISS self-triggered implementation of linear controllers

Nowadays control systems are mostly implemented on digital platforms and, increasingly, over shared communication networks. Reducing resources (processor utilization, network bandwidth, etc.) in such implementations increases the potential to run more applications on the same hardware. We present a self-triggered implementation of linear controllers that reduces the amount of controller updates necessary to retain stability of the closed-loop system. Furthermore, we show that the proposed self-triggered implementation is robust against additive disturbances and provide explicit guarantees of performance. The proposed technique exhibits an inherent trade-off between computation and potential savings on actuation.

On event-triggered and self-triggered control over sensor/actuator networks

Event-triggered and self-triggered control have been recently proposed as an alternative to the more traditional periodic execution of control tasks. The possibility of reducing the number of executions while guaranteeing desired levels of performance makes event-triggered and self-triggered control very appealing in the context of sensor/actuator networks. In this setting, reducing the number of times that a feedback control law is executed implies a reduction in transmissions and thus a reduction in energy expenditures. In this paper we introduce two novel distributed implementations of event-triggered and self-triggered policies over sensor/actuator networks and discuss their performance in terms of energy expenditure.

Symbolic Models for Nonlinear Control Systems Without Stability Assumptions

Finite-state models of control systems were proposed by several researchers as a convenient mechanism to synthesize controllers enforcing complex specifications. Most techniques for the construction of such symbolic models have two main drawbacks: either they can only be applied to restrictive classes of systems, or they require the exact computation of reachable sets. In this paper, we propose a new abstraction technique that is applicable to any nonlinear sampled-data control system as long as we are only interested in its behavior in a compact set. Moreover, the exact computation of reachable sets is not required. The effectiveness of the proposed results is illustrated by synthesizing a controller to steer a vehicle.

PESSOA: a tool for embedded controller synthesis

In this paper we present Pessoa, a tool for the synthesis of correct-by-design embedded control software Pessoa relies on recent results on approximate abstractions of control systems to reduce the synthesis of control software to the synthesis of reactive controllers for finite-state models We describe the capabilities of Pessoa and illustrate them through an example.

System Architectures, Protocols and Algorithms for Aperiodic Wireless Control Systems

Wide deployment of wireless sensor and actuator networks in cyber-physical systems requires systematic design tools to enable dynamic tradeoff of network resources and control performance. In this paper, we consider three recently proposed aperiodic control algorithms which have the potential to address this problem. By showing how these controllers can be implemented over the IEEE 802.15.4 standard, a practical wireless control system architecture with guaranteed closed-loop performance is detailed. Event-based predictive and hybrid sensor and actuator communication schemes are compared with respect to their capabilities and implementation complexity. A two double-tank laboratory experimental setup, mimicking some typical industrial process control loops, is used to demonstrate the applicability of the proposed approach. Experimental results show how the sensor communication adapts to the changing demands of the control loops and the network resources, allowing for lower energy consumption and efficient bandwidth utilization.

Input-to-state stability of self-triggered control systems

Event-triggered and self-triggered control have recently been proposed as an alternative to periodic implementations of feedback control laws over sensor/actuator networks. In event-triggered control, each sensing node continuously monitors the plant in order to determine if fresh information should be transmitted and if the feedback control law should be recomputed. In general, event-triggered control substantially reduces the number of exchanged messages when compared with periodic implementations. However, such energy savings must be contrasted with the energy required to perform local computations. In self-triggered control, computation of the feedback control law is followed by the computation of the next time instant at which fresh information should be sensed and transmitted. Since this time instant is computed as a function of the current state and plant dynamics, it is still much larger than the sampling period used in periodic implementations. Moreover, no energy is spent in local computations at the sensors. However, the plant operates in open-loop between updates of the feedback control law and robustness is a natural concern. We analyze the robustness to disturbances of a self-triggered implementation recently introduced by the authors for linear control systems. We show that such implementation is exponentially input-to-state stable with respect to disturbances.

Asynchronous decentralized event-triggered control

In this paper we propose an approach to the implementation of controllers with decentralized strategies triggering controller updates. We consider set-ups with a central node in charge of the computation of the control commands, and a set of not co-located sensors providing measurements to the controller node. The solution we propose does not require measurements from the sensors to be synchronized in time. The sensors in our proposal provide measurements in an aperiodic way triggered by local conditions. Furthermore, in the proposed implementation (most of) the communication between nodes requires only the exchange of one bit of information (per controller update), which could aid in reducing transmission delays and as a secondary effect result in fewer transmissions being triggered.

Decentralized event-triggered control with asynchronous updates

We propose taking event-triggered control actions to implement decentralized control over wireless sensor/actuator networks without requiring synchronized measurement updates. In comparison with the existing results on event-triggered decentralized control, the proposed implementation does not rely on weak coupling between subsystems, nor does it assume the synchronization of local clocks or the existence of a central broadcasting node, and is applicable to nonlinear systems. In addition, higher energy efficiency at the sensors is expected because of the great reduction of the listening times of the sensors. We prove that with asynchronous measurement updates, the event-triggered control actions can guarantee semiglobal practical stability for the sensor/actuator system of interest. We also show that the time between any two consecutive transmissions of measurements at each sensor is bounded from below by a positive constant. Furthermore, asymptotic stability can be achieved when more complicated triggering conditions are introduced. The theoretical analysis is validated by simulations.

Self-triggered control over wireless sensor and actuator networks

Energy and communication bandwidth are scarce resources in wireless sensor and actuator networks. Recent research efforts considered the control of physical processes over such resource limited networks. Most of the existing literature addressing this topic is dedicated to periodically sampled control loops and scheduled communication, because it simplifies the analysis and the implementation. We propose instead an aperiodic network transmission scheme that reduces the number of transmission instances for the sensor and control nodes, thereby reducing energy consumption and increasing network lifetime, without sacrificing control performance. As an added benefit, we show the possibility of dynamically allocating the network bandwidth based on the physical system state and the available resources. In order to allow timely, reliable, and energy efficient communication, we propose a new co-design framework for the wireless medium access control, compatible with the IEEE 802.15.4 standard. Furthermore, we validate our approach in a real wireless networked control implementation.