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Sergio Callegari

Assistant professor

Department of Electrical, Electronic, and Information Engineering "Guglielmo Marconi"

Academic discipline: ING-INF/01 Electronics


  1. Nonlinear circuits and systems
    • analysis and desing of nonlinear circuits (and particularly of discrete time ones) for the synthesis of signals with pre-assigned statistical features
  2. Sensors and signal processing for fluid dynamics appliacations
    • novel mehodologies for the estimation of flight attitude angles and air speed
    • indirect approaches for the measurement of air data, exploiting "on-skin" measurements
    • sensor networks
    • capacitive sensors
  3. Pulse codings for actuation, signal synthesis, audio amplification and power conversion
    • computationally efficient codings delivering energy friendly power management, low distortion and low electromagnetic emission levels
  4. True-Random Number Generators
    • design of generators for true-random sequences based on chaotic dynamics and on the reuse of ADC analog modules
  5. Oscillation Based Test e Complex Oscillation Based Test:
    • forcing of oscillatory regimes and specifically of complex ones for the validation of analog and mixed mode circuits

The research of Sergio Callegari is aimed at various aspects electronics and microelectronics and involves two lines of core activities: the first related to the analysis and optimization Non-linear systems, the second to sensors.

1 Analysis, synthesis, optimization of non-linear systems

Since 1996, Sergio Callegari deals with non-linear circuits and systems. Recently, the activity is particularly focused on complex dynamic circuits and on the exploitation of non-linear features for the optimization of engineering systems.

1.1 Pulse modulations and switching systems

This research line aims to develop innovative encoders for pulse-based signals characterized by binary or anyway discrete levels, with particular reference to waveform synthesis, to actuation, and to audio amplification.

Traditionally, modulations have mostly been used in engineering as a smart means to exploit transmission media. However, in more general terms, they constitute a whole paradigm for the representation of information where the informative content is distributed over time (and possibly over space) into signals (or signal vectors). In particular, pulse modulation (such as width, position, density and frequency modulations — PWM, PPM, PDM and PFM respectively) exploit this property to allow a signal characterized by discrete values (and as such processable by digital means) to exhibit analog properties, such as the ability to be directly processed by filters and continuous physical plants.

This property is used with success in actuation. For instance, switched regulators and amplifiers take advantage of pulse modulations to control the operation of a power bridge that in turns administers in a discontinuous and virtually lossless way the energy delivery to a load, be it an electric car, a loudspeaker or any other electrical apparatus. However, the possibilities offered by pulse modulations are not generally exploited in full. In many cases, the ability to meet power regulation targets is a side effect of the properties of a standard modulator and not the result of a deliberate coding strategy.

The objective of this research is to develop new generations of encoders, similar in usage to traditional ones and mostly compatible with them (so as to promote rapid acceptance), but based on radically different operating principles. Rather than using standard modulations, the encoders will be explicitly designed to enhance specific performance indexes thanks to a wide application of optimization techniques and to the identification of key mathematical properties. In other terms, once an information content to be represented is assigned together with a list of constraints and a series of merit factors, the final decision on how to encode the information will be produced through an explicit attempt to optimize the merit factors themselves.

This research line has recently given rise to the “OpIMA” project (Optimized Impulsive Modulations for Actuation), which is funded under the Strategic Projects programme at the University of Bologna and coordinated by the writer. The project obtained excellent marks when considered for funding. For more information, see

1.2 Non linear circuits characterized by complex dynamics

Chaotic systems exhibit a borderline dynamic behaviour sharing purely-deterministic and random-like features. Consequently, they can either be modeled by classical means (as systems of differential or difference equations, etc.) or in a statistical way, like stochastic processes. It should be noted that despite the theoretical possibility of using classical, deterministic models, features such as aperiodic behavior and sensitivity to initial conditions make it practically impossible to predict their future behavior. In this sense chaotic systems are best assimilated to random-like sources.

Since long ago, this apparent paradox has made chaotic systems a subject of speculation and interest. However, the study of engineering applications of chaos is only very recent and similarly recent is the development of analysis and synthesis methodologies for electronic circuits capable of exhibiting complex behavior. In fact, at least initially, the engineering approach to chaos was very much limited to the identification of strange behaviors and the set up of means to avoid them. Only in recent times, the possibility of fruitful applications was recognized, finally boosting research in the explicit design of exploitable chaotic models.

Currently, the areas where chaos is applied range from neural networks to the design of associative memories, from spread spectrum telecommunication systems to cryptography, from watermarking to the reduction of electromagnetic interference, from simulation to the synthesis of keys for secure authentication, and so on. The research carried on by Sergio Callegari directly regards some of these fields, while maintaining a strong view towards implementation aspects.

The basis of his scientific activity consists in the usage of mathematical tools derived from statistics and geared at the simultaneous observation of a plurality of system trajectories. This approach, that only recently received recognition in engineering, enables a quantitative understanding of some chaotic systems (particularly of discrete-time ones) as well as a deeper characterization of their properties in comparison to what can be achieved by observing individual orbits. Clearly, the interest is directed both at the improvement of the available tools and at their practical exploitation. In this regard, a significant choice is to focus on those problems for which classical solutions already exist, so that the advantages and disadvantages deriving from the exploitation of chaos based techniques can be objectively assessed. Particularly important are those applications where the merit factors are already traditionally expressed in probabilistic terms (for example, spread spectrum communications, electromagnetic interference reduction, and so on).

Initially, scientific activity was mostly devoted to those applications where chaotic sources can substitute for traditional pseudo-random ones. For example, a noise source to be used in a stochastic neuron model was developed as well as circuits capable of producing chaotic binary sequences with reduced self-correlation and good 0-1 balance. Also, circuits for the optimization of spread spectrum communication systems were proposed.

More recent applications regard the design of chaotic circuits based on general purpose field programmable devices or on standard IC building blocks.

The study regarding the synthesis and the usage of spread spectrum signals a required significant effort on theoretical matters and opened new research lines, such as the already cited optimization of pulse modulations, the generation of cryptographic keys and the testing of analog building blocks by means of chaotic excitations.

1.3 Non-conventional techniques for the generation of random sequences and keys for cryptography and authentication

In the field of Information and Communication Technology, it has recently been observed that chaotic dynamics may enable a particularly effective design of true-random number generators which are a fundamental primitive for cryptographic, authentication and information security systems.

The synthesis of random binary sequences is inherent in: algorithms such as the DSA; key generation procedures for algorithms dedicated to public/symmetric-key cryptography; RSA moduli; and in many secure communication schemes. The ability of cryptographic techniques to resist attacks based on pattern search critically depends on the quality and the unpredictability of the random number generators being adopted. As a result, generators for cryptographic applications need to meet much more stringent requirements than those for other applications.

It is generally acknowledged that the true-random generators can at best be approximated. An ideal source should be capable of producing infinitely long sequences composed of bits that are fully independent of each other, with property that restarting the source never allows an already produced sequence to be re-delivered (non-repeatability property). In practice, electronic random generators fall into two categories. On the one hand there are the so-called pseudo-RNGs, on the other the pysical-RNGs. Pseudo-RNGs are in fact deterministic algorithms capable to expand an initial seed into a long binary sequence. On the other hand, physical-RNGs are devices that use micro-cosmic phenomena that are observable in macroscopic terms and that are generally characterized as noise (eg, quantum noise, intervals in the emission of radioactive decay processes, thermal and shot noise in electronic circuits, fluctuations in the frequency of oscillators, activity pattern of activity of human operators, etc.). Clearly, pseudo-RNGs are those most distant from ideal specifications: being based on finite memory algorithms they show periodic behaviors and deliver correlated samples. For the same reason they are fully repeatable. The consequent possibility (that necessarily exists, at least potentially) to recover information on the seed from the observation of output sequences is obviously hardly desirable in applications related to information security and cryptography. However, their substantial advantage, lays in their algorithmic nature that makes them easily implementable in digital circuits and in software. Physical generators, on the other hand, are the best at approximating ideal random sources. Unfortunately, they generally require highly specialized circuits and a strong control over environmental and operational conditions. This makes them ill-adapted to embedding in electronic and information technology systems. Nevertheless, the growing importance of applications related to data security has recently pushed major companies to adopt them in replacement for pseudo-RNGs in their hardware platforms.

Obviously, it would be very desirable to introduce generators capable of combining the benefits of physical sources with the implementation ease of pseudo-RNGs. This requires inventing design strategies enabling the reuse of standard circuit blocks that are already present in a majority of electronic systems. Recently, research in this direction has produced some relatively successful designs based on the reuse of peripheral blocks from FPGAs. However, these systems are generally characterized by very low data-rates (in the order of tens kbit/s). This is invariably due to the need to refer to physical noisy phenomena on which the designer has little or no control at all.

A recent proposal is to exploit chaotic dynamics and statistical techniques for introducing a new class of random generators. An intuitive justification for this approach stems from the consideration that many of micro-cosmic phenomena used in physical generators could actually be modeled in a deterministic form and that it is simply their extreme sensitivity to initial conditions to make them unpredictable an almost random to the external observer. From this premise, it is obvious that rather than exploiting natural models that are difficult to control and handle it would be more convenient to use simpler artificial ones.

Today, it is possible to rely on non-linear discrete-time models showing sensitivity to initial conditions, capable of producing complex behaviors and at the same time relatively well understood from a mathematical point of view. A particularly interesting aspect is that their applicability to random sequence generation can be proved not just heuristically, but also formally. Thanks to an intuition from Kalman, it is possible to study and implement such models by means of Markov chains and methods from symbolic dynamics. In addition, it has recently been shown that some of these models can be implemented in hardware through the re-use of A/D converter stages. This is a crucial factor from an applicative point of view, because ADCs are among those analog blocks that have seen better and more constant refinement and investment in the last years. They are currently available as reusable cores and codified as IP blocks. The possibility to derive high performance random generators from such blocks represents a guarantee of success in terms of cost reduction and embeddability.

1.4 COBT: Complex Oscillation Based Test

OBT (Oscillation Based Test) is an emerging technique for the validation of analog and mixed-mode circuits. The idea is to drive the block under test into a self-sustained oscillation mode capable to make the block faults quite evident. The approach is very attractive thanks to its extreme simplicity: first of all, it provides an excitation to the circuit under test without the burden of external signal sources; and secondly, it allows failures to be detected from measurements taken on an extremely limited number of nodes. Both things represents important advantages, particularly if one considers the increasing difficulty in accessing internal nodes in complex mixed-mode architectures. Furthermore, many functional blocks can be made suitable for OBT with only a very limited number of changes in their design.

Notwithstanding the above advantages, the OBT approach is facing much criticism. The most frequent one is that the adopted oscillation regime is almost invariably sinusoidal, so that the circuit under test gets excited by a simple tone. It has repeatedly been highlighted that such an excitement is incapable to reveal all possible faults. In literature there are techniques to overcome this problem by the sequential use of different oscillation frequencies, but this hinders other benefits of the approach.

In this context, the research activity of Sergio Callegari led to the introduction of complex OBT techniques, where the block under test is forced into a chaotic oscillation regime where the excitement is rich and able to reveal a wider range amount of faults. The first presentation of the COBT concept took place in May 2008 at the ISCAS international conference with regards to the validation of A/D converters and received a considerable amount of interest.

2 Sensors

Since 2002, Sergio Callegari has been working with the Distributed Sensor Laboratory (LYRAS) of the Advanced Research Center on Electronic Systems for Information and Communication Technologies “Ercole de Castro” (ARCES). LYRAS is established at the Second School of Engineering of the University of Bologna and its activities are focused on sensors for fluid dynamics and aerospace applications and on sensors for biological applications.

2.1 Sensors for applications in fluid-dynamics

Knowledge of normal and tangential strains over a structure immersed in a fluid is of primary importance in applications related to mechanics, aeronautics and fluid dynamics. This information can be gathered by practicing local measures on many points ideally creating a mesh on the surface under test. To this aim it is necessary to deploy a network of coordinated sensors whose raw readings are jointly processed to infer the higher level quantities such as strain gradients, lift, friction, detachment points of the boundary layer, and so on. The approach requires sensors that are small (to make local measurements), cheap (to provide large replication factors), smart (to coordinate them) and robust (to deploy them in harsh environments).

Given the above premses, the interest in defining new types of sensors sharing properties from conventional macro-sensors and MEMS micro-sensors is self evident. Similarly evident is the interest in shifting the research focus from the single transducer to sensor systems capable of managing a large number of probes.

2.2 Signal processing for data from fluid-dynamics sensor systems

All aircrafts have air-data systems used to deliver input data to the automatic flight control unit or to alert the pilot. Pieces of information that are particularly important include the air speed and the attitude angles, primarily the angle of attack and of side-slip.

Traditionally, flight parameters are read by pieces of ad-hoc instrumentation that include parts protruding outside the aircraft silhouette such as pipes, pitot tubes, wingbooms, nosebooms, vanes, flow deflectors, and so on. Sensor systems are therefore characterized by a high degree of intrusivity. Moreover, pneumatic links are often needed between the different elements of the instrumentation or between the interior and the exterior of the aircraft. This is particularly undesirable in the case of small unmanned air vehicles (UAVs). Another feature of conventional sensors is to aim at direct measurements, with a one-to-one device-to-measure relationship. Such an approach is clearly motivated by the cost of the instruments and their installation, but is not without drawbacks.

These premises justify an interest in radically different measurement approaches, strongly based on indirect measures and exploiting a large number of redundant sensors. The idea is to get all the flight parameters from a set homogeneous readings variously related to them, estimating and decoupling them by signal processing techniques.

Particularly, it would be desirable to deploy arrays of pressure or flow sensors directly placed on the very aerodynamic surfaces of the aircraft (for example on the wings) and to indirectly, simultaneously read from them both the average air speed and the angle of attack. The possibility of using measures of pressure to infer flight parameters by means of estimation techniques was experimented in the past (eg, as part of the work by Whitmore at the NASA labs). However, the experiments reported in the literature typically use conventional transducers. For example, the NASA experiments relied on orifices opened on the fuselage of the airplane and pneumatically linked to standard pressure sensors. This does not allow high levels of redundancy to be achieved. On the contrary, the distributed usage of low cost surface sensors would allow a much larger amount of information to be collected. With this, it may becomes possible to relax the accuracy requirements placed on the individual sensors and at the same time to improve the ability to decouple physical effects and the resilience to faults.

Presently, research has already produced some early prototypes of low-cost sensors produced by Printed-Circuit-Board (PCB) technologies and suitable for the above mentioned distributed applications. Moreover, by fluid dynamic simulation, the feasibility of the measurement approach has been verified. It has been shown that by increasing the level of redundancy there is a significant relaxation in the accuracy requirements placed on the individual sensing units at no loss in the overall accuracy. Research is currently focused on algorithmic aspects, model identification and management of computational load. There is also a plan for experimentally validating the approach in a wind tunnel and for checking the feasibility of techniques for fault detection and fault tolerant operation.

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