Ubaldo Tiberi

Energy-efficient sampling of networked control systems over IEEE 802.15.4 wireless networks

Self-triggered sampling is an attractive paradigm for closed-loop control over energy-constrained wireless sensor networks (WSNs) because it may give substantial communication savings. The understanding of the performance of self-triggered control systems when the feedback loops are closed over IEEE 802.15.4 WSNs is of major interest, since the communication standard IEEE 802.15.4 is the de-facto reference protocol for energy-efficient WSNs. In this paper, a new approach to control several processes over a shared IEEE 802.15.4 network by self-triggered sampling is proposed. It is shown that the sampling time of the processes, the protocol parameters, and the scheduling of the transmissions must be jointly selected to achieve a good performance of the closed-loop system and an energy-efficient utilization of the network. The challenging part of the proposed analysis is ensuring globally uniformly ultimately boundedness of the controlled processes while providing efficient scheduling of the process state transmissions. Such a scheduling is difficult when asynchronous multiple control loops share the network, because transmissions over IEEE 802.15.4 are allowed only at certain time slots. The proposed approach establishes that the joint design of self-triggered samplers and the network protocol (1) ensures globally uniformly ultimately boundedness of each control loop, (2) reduces the number of sensor transmissions, and (3) increases the sleep time of the transmitting nodes. A new dynamic scheduling problem is proposed for the joint control of each process and network protocol adaptation. An algorithm is derived, which adapts the network parameters according to the self-triggered sampler of every control loop. Numerical examples illustrate the analysis and show the benefits of the approach. It is concluded that self-triggered control strategies over WSNs ensure desired control performance, reduce the network utilization, and reduce energy consumption only if the protocol parameters are appropriately regulated.

On event-based PI control of first-order processes

In this paper the design of an event-based proportional-integral (PI) control scheme for stable first-order processes is considered. A novel triggering mechanism which decides the transmission instants based on an estimate of the PI control signal is proposed. This mechanism addresses some side-effects that have been discovered in previous event-triggered PI proposals, which trigger on the process output. In the proposed scheme, the classic PI controller is further replaced with PIDPLUS, a promising version of PI controller for networked control systems. Although PIDPLUS has been introduced to deal with packet losses and time delays, and, to the best of our knowledge, a stability analysis of the closed-loop system where such a controller is used has never been performed, here the performance of such a controller in an event-based fashion are analyzed, and a stability analysis is further provided. The proposed event-based scheme ensures set-point tracking and disturbance rejection as in classic time-periodic implementations of PI controller, while greatly reducing the number of sensor transmissions. The theoretical results are validated by simulations, where the benefits in using PIDPLUS in combination with the proposed PI event-based triggering rule are shown.

Adaptive self-triggered control over IEEE 802.15.4 networks

The communication protocol IEEE 802.15.4 is becoming pervasive for low power and low data rate wireless sensor networks (WSNs) applications, including control and automation. Nevertheless, there is not yet any adequate study about control systems networked by this protocol. In this paper, the stability of IEEE 802.15.4 networked control systems (NCSs) is addressed. While in recent works fundamental results are developed for networks that are abstracted only in terms of packet loss and time delays, here the constraints imposed by the protocol to the feedback channel and the network energy consumption are explicitly considered. A general analysis for linear systems with parameter uncertainty and external bounded disturbances with control loops closed over IEEE 802.15.4 networks is proposed. To reduce the number of transmissions and thus save energy, a self-triggered control strategy is used. A sufficient stability condition is given as function of both the protocol and control parameters. A decentralized algorithm to adapt jointly the self-triggered control and the protocol parameters is proposed. It is concluded that stability is not always guaranteed unless protocol parameters are appropriately tuned, and that event-triggered control strategies may be difficult to use with the current version of IEEE 802.15.4.

A Simple Self-Triggered Sampler for Nonlinear Systems

Self-triggered control is a promising aperiodic control paradigm that allows the design of resource-efficient control schemes. Several results were recently developed for self-triggered control of linear systems, but self-triggered control for nonlinear systems is still not well understood. Existing techniques are mainly limited to homogeneous or polynomial closed-loop systems that are input-to-state stable with respect to measurement errors. These assumptions obviously limit the class of applicable cases. In this paper, a new simple self-triggered controller, based on the assumption of local asymptotic stability of the closed-loop system, is presented. The controller ensures local ultimate uniform boundedness of the state trajectories in the sense that the self-triggered controller guarantees the trajectories to be confined into an arbitrary small region. In the case with measurement time delays, it is shown how the size of the region depends on the maximum delay. The effectiveness of the proposed method is illustrated by simulations.

Self-triggered Control of Multiple Loops over IEEE 802.15.4 Networks

Given the communication savings offered by self-triggered sampling, it is becoming an essential paradigm for closed-loop control over energy-constrained wireless sensor networks (WSNs). The understanding of the performance of self-triggered control systems when the feedback loops are closed over IEEE 802.15.4 WSNs is of major interest, since the communication standard IEEE 802.15.4 is the de-facto the reference protocol for energy-efficient WSNs. In this paper, a new approach to control several processes over a shared IEEE 802.15.4 network by self-triggered sampling is proposed. It is shown that the sampling time of the processes, the protocol parameters, and the scheduling of the transmissions must be jointly selected to ensure stability of the processes and energy efficiency of the network. The challenging part of the proposed analysis is ensuring stability and making an energy efficient scheduling of the state transmissions. These transmissions over IEEE 802.15.4 are allowed only at certain time slots, which are difficult to schedule when multiple control loops share the network. The approach establishes that the joint design of self-triggered samplers and the network protocol 1) ensures the stability of each loop, 2) increases the network capacity, 3) reduces the number of transmissions of the nodes, and 4) increases the sleep time of the nodes. A new dynamic scheduling problem is proposed to control each process, adapt the protocol parameters, and reduce the energy consumption. An algorithm is then derived, which adapts to any choice of the self-triggered samplers of every control loop. Numerical examples illustrate the analysis and show the benefits of the new approach.

Beyond Herding Cats: Aligning Quantitative Technology Evaluation in Large-Scale Research Projects

A large-scale research project involving many research and industry organizations working on a common goal should be an ideal basis for profound technology evaluations. The possibility for industrial case studies in multiple settings ought to enable reliable quantitative assessment of the performance of new technologies in various real-world settings. However, due to diverse challenges, such as internal agendas, implicit constraints, and unaligned objectives, leveraging this potential goes beyond the usual challenge of cat-herding in such projects. Based on our experience from coordinating technology evaluations in several research projects, this paper sketches the typical issues and outlines an approach for dealing with them. Although new in its composition, this approach brings together principles and techniques perceived to have been useful in earlier projects (e.g., cross-organizational alignment, abstract measures, and internal baselining). Moreover, as we are currently applying the approach in a large research project, this paper presents first insights into its applicability and usefulness.

Dead-band self-triggered PI control for processes with dead-time

Current implementations of digital controllers assume that sensing, control and actuation are performed in a periodic fashion. In classic control schemes, where sensors and controllers are directly connected, periodicity does not provide particular drawbacks, but, in the case of wireless sensor networks, such a choice may be questionable. One of the driving constraints in the design of wireless sensor networks is represented by its energy efficiency, and it has been shown that the main cause of energy consumption is due to the radio activities of the sensor nodes. By using periodic implementations, the sensor nodes are enforced to keep on transmitting measurements to the controller even if it is not really needed, thus wasting energy. To cope with these problems, self-triggered control was recently introduced. This technique aims at reducing the conservativeness of periodic implementations providing an adaption of the inter-sampling intervals based on the current output of the system. Existing work on self-triggered control considers linear systems controlled by state feedback controllers under the assumption of small time-delays. In this paper the problem of designing a self-triggered control scheme that applies to first-order processes with large dead-times controlled by PI controllers is addressed. Moreover, the proposed self-triggered scheme is robust with respect to set-point changes and external disturbances, which are typical in process industry. The results are validated by simulations.

Wireless Ventilation Control for Large-Scale Systems: the Mining Industrial Case

Mining ventilation is an interesting example of a large scale system with high environmental impact where advanced control strategies can bring major improvements. Indeed, one of the first objectives of modern mining industry is to fulfill environmental specifications [1] during the ore extraction and crushing, by optimizing the energy consumption or the production of polluting agents. The mine electric consumption was 4 % of total industrial electric demand in the US in 1994 (6 % in 2007 in South Africa) and 90 % of it was related to motor system energy [2]. Another interesting figure is given in [3] where it is estimated that the savings associated with global control strategies for fluid systems (pumps, fans and compressors) represent approximately 20 % of the total manufacturing motor system energy savings. This motivates the development of new control strategies for large scale aerodynamic processes based on appropriate automation and a global consideration of the system. More specifically, the challenge in this work is focused on the mining ventilation since as much as 50 % or more of the energy consumed by the mining process may go into the ventilation (including heating the air). It is clear that investigating automatic control solutions and minimizing the amount of pumped air to save energy consumption (proportional to the cube of airflow quantity [4]) is of great environmental and industrial interest.