Eduardo I. Silva

Brief paper: Architectures and coder design for networked control systems

In networked control systems (NCSs) achievable performance is limited by the communication links employed to transmit signals in the loop. In the present work, we characterise LTI coding systems which optimise performance for various NCS architectures. We study NCSs where the communication link is situated between plant output and controller, and NCSs where the communication link is located between controller and actuator. Furthermore, we present a novel NCS architecture, which is based upon the Youla parameterisation. We show that, which of these architectures gives best performance depends, inter alia, upon characteristics of a related non-networked design, plant disturbances and reference signal. A key aspect of our work, resides in the utilisation of fixed signal-to-noise ratio channel models which give rise to parsimonious designs, where channel utilisation is kept low. The results are verified with simulations utilising bit-rate limited channels.

Brief paper: Control system design subject to SNR constraints

This paper deals with networked control systems comprising LTI plants controlled over scalar additive noise channels subject to signal-to-noise ratio (SNR) constraints. We present a general framework, based upon convex optimization concepts, that can accommodate several situations of interest. Our results make explicit the fact that exploiting feedback around the channel plays a key role in reducing the minimal SNR that is compatible with stability. The results also provide a characterization of the best achievable performance subject to an SNR constraint. We apply the results to specific networked control architectures, and provide a numerical example.

Packetized Predictive Control over Erasure Channels

In digital networked control systems links between controller and plant are not transparent, but are affected by time-delays, data-dropouts and quantization. An important observation is that, in contemporary communication networks, such as those employing Ethernet, data is sent in large packets. This motivates the development of networked control schemes where signal predictions are sent as packets. In the present work we present such a strategy. We focus on a configuration where the controller output is connected to the plant input via a network which we assume is prone to transmission errors. By using methods from predictive control theory, we show how closed loop stability can be ensured directly in the design.

Control over unreliable networks affected by packet erasures and variable transmission delays

This paper describes a novel control strategy aimed at achieving good performance over an unreliable communication network affected by packet loss and variable transmission delays. The key ingredient in the method described here is to use the large data packet frame size of typical modern communication protocols to transmit control sequences which cover multiple data-dropout and delay scenarios. Stability and performance of the resultant scheme are addressed under nominal networked conditions. Simulations verify that the strategy performs exceptionally well under realistic conditions with noise and unmeasured disturbances.

Matrix converter controlled with the direct transfer function approach: analysis, modelling and simulation

Power electronics is an emerging technology. New power circuits are invented and have to be introduced into the power electronics curriculum. One of the interesting new circuits is the matrix converter (MC), and this paper analyses its working principles. A simple model is proposed to represent the power circuit, including the input filter. The power semiconductors are modelled as ideal bidirectional switches and the MC is controlled using a direct transfer function approach. The modulation strategy of the converter is explained in a complete and clear form. The commutation problem of two switches and the generation of overvoltages are clarified. The paper also includes a soft-switching commutation method that allows for a safe commutation of the switches. Finally a complete simulation scheme, using Matlab®–Simulink®, is discussed.

A Framework for Control System Design Subject to Average Data-Rate Constraints

This paper studies discrete-time control systems subject to average data-rate limits. We focus on a situation where a noisy linear system has been designed assuming transparent feedback and, due to implementation constraints, a source-coding scheme (with unity signal transfer function) has to be deployed in the feedback path. For this situation, and by focusing on a class of source-coding schemes built around entropy coded dithered quantizers, we develop a framework to deal with average data-rate constraints in a tractable manner that combines ideas from both information and control theories. As an illustration of the uses of our framework, we apply it to study the interplay between stability and average data-rates in the considered architecture. It is shown that the proposed class of coding schemes can achieve mean square stability at average data-rates that are, at most, 1.254 bits per sample away from the absolute minimum rate for stability established by Nair and Evans. This rate penalty is compensated by the simplicity of our approach.

Operating experience of shovel drives for mining applications

This paper presents an overview analysis and a discussion of field experience with different drive configurations used in shovel applications. The analysis includes different drive topologies, shovel duty cycles for A typical truck pass loading and energy regeneration capabilities. Special attention is given to the interaction between the high power shovel drives and grid harmonics injection, reactive power requirements, and voltage regulation. This paper presents operational experiences obtained in copper mine industries. The main characteristics of the newest shovels with active front end (AFE) at the line side are also included. Finally, a critical evaluation of the different topologies is presented, showing that the shovel drive operation strongly affects the behavior of the power distribution system and that the introduction of the AFE technology produces an important performance improvement.

Predictive Control of a Flying Capacitor Converter

This paper presents a predictive control strategy for single phase flying capacitor converters. The proposed method allows for the regulation of both the output current and the intermediate flying capacitor voltages, achieving transient dynamics that are superior to those in standard pulse width modulation based strategies. By means of an appropriate choice of the associated cost function, the designer is allowed to tradeoff the load current harmonic content, the capacitor voltage balancing and the total number of commutations in a simple fashion. In addition, the strategy permits the enforcement of an arbitrary output voltage spectrum.

On Optimal Perfect Reconstruction Feedback Quantizers

This paper presents novel results on perfect reconstruction feedback quantizers (PRFQs), i.e., noise-shaping, predictive and sigma-delta A/D converters whose signal transfer function is unity. Our analysis of this class of converters is based upon an additive white noise model of quantization errors. Our key result is a formula that relates the minimum achievable MSE of such converters to the signal-to-noise ratio (SNR) of the scalar quantizer embedded in the feedback loop. This result allows us to obtain analytical expressions that characterize the corresponding optimal filters. We also show that, for a fixed SNR of the scalar quantizer, the end-to-end MSE of an optimal PRFQ which uses the optimal filters (which for this case turn out to be IIR) decreases exponentially with increasing oversampling ratio. Key departures from earlier work include the fact that fed back quantization noise is explicitly taken into account and that the order of the converter filters is not a priori restricted.

Brief paper: Control of LTI plants over erasure channels

This paper studies linear time-invariant (LTI) control architectures for LTI plants when communication takes place over a scalar erasure channel. Assuming i.i.d. data dropouts, we first show that such a channel is equivalent, in a second order moment sense, to an additive white noise channel subject to an instantaneous signal-to-noise ratio constraint. This key result is then exploited in two ways: First, we use it to characterize the set of all LTI controllers that achieve mean square stability in control architectures closed over scalar erasure channels. Second, we use it to show that the optimal design of LTI controllers over scalar erasure channels can be carried out by using tools from standard quadratic optimal control theory. We finally apply our results to the dynamic output feedback control of LTI single-input single-output (SISO) plants subject to data dropouts. In this case, we are able to establish closed form necessary and sufficient conditions on the minimal successful transmission probability that allows one to design mean square stabilizing controllers.