Télécommunications optiques spatiales

Les télécommunications optiques spatiales sont une catégorie de télécommunications spatiales reposant sur l'utilisation de laser pour la transmission des données. Cette technique permet d'accroitre considérablement le débit par rapport aux liaisons radio tout en réduisant la puissance électrique nécessaire. L'utilisation du laser dans ce contexte se heurte toutefois à la nécessité d'un pointage extrêmement précis depuis un support se déplaçant éventuellement à grande vitesse par rapport au récepteur et lorsque ce dernier se trouve au sol à des problèmes de transparence de l'atmosphère. Les télécommunications optiques spatiales font l'objet de nombreuses expérimentations depuis le début des années 2000. Le recours au laser vise à répondre à différents contextes : croissance du volume des données transmises par les instruments embarqués sur les satellites de plus en plus puissants, éloignement de l'engin spatial de la Terre, exigences croissantes des consommateurs. Les applications étudi

Compact fast-steering tip/tilt laser scanner for high power material processing applications

Moustapha Hafez

The common configuration for deflecting laser beams in two dimensions is based on two single-axis galvanometric scanners placed perpendicular to each other. Over recent years, these fast scanners have proved to be both very reliable and very accurate. They are widely available in the marketplace and have wide-ranging technical specifications. However, such a configuration is rather bulky and can not satisfy the needs of industry which seeks compact solutions for an easy integration of these scanning heads into their production lines. In addition, two separate mirrors leads to optical aberrations when these scanning heads are used in conjunction with scanning lenses, which are specially designed for a single entrance pupil. Therefore, a compact fast-steering tip/tilt laser scanner with a single mirror would act as a key component in diverse applications, e.g. in material processing, astronomy, intersatellite laser communications, imaging systems, bio-medical and ophthalmologic applications. The compact two-axis tilt laser scanner presented in this work can perform a variety of functions such as image tracking, beam stabilization and alignment, line-of-sight pointing, and scene scanning. Designing such a scanner required a multi-disciplinary approach involving optics, mechanics, electromagnetics, sensors and control theories. The scanner is composed of one single mirror with a large active area and has a single point of rotation for the two axes of tilt. Moreover, the center of gravity coincides with the center of rotation to reach optimum dynamic behavior. The scanner's compact dimensions allow an easy integration in the various optical systems. Furthermore, the mirror is easily interchangeable which gives greater flexibility in the choice of the laser source. Another major advantage of the proposed design is its ability to be scaled down to miniaturized versions. The scaling depends mainly on the active surface of the mirror, which is generally specified by the application. The larger the mirror, the higher its inertia and the larger are the actuators in order to guarantee high performance. Two pairs of push-pull linear electromagnetic actuators are used to drive the mirror of the scanner. The mirror suspension is based on a sliding bearing composed of a cone-ball contact with optimized friction and wear behavior. A magnetic pre-load force is used to hold the mirror and to give sufficient rotational stiffness. Furthermore, a position transducer based on a photodetector and a miniature laser pen is integrated in the module and is used for closed loop feedback positioning. The mirror can be tilted around both axes by ± 61mrad (± 3.5°) with an accuracy better than 50μrad. Moreover, a differential resolution in the order of 2.5μrad, and settling times for maximum tip and tilt deflections of 10ms and 14ms respectively are achieved. The overall volume of the scanner is 30 x 40 x 50 mm3, and its total weight does not exceed 90g. Design, simulations and experimental investigations were carried out in this work in order to optimize the different components of the tip/tilt scanner. These components are: the motors, the bearing system, the position transducer and the feedback control loop. An analytical model that allows an optimization of the geometrical parameters for an electromagnetic actuator based on rectangular coils and magnets is proposed. This model proved to be very reliable and can be extended to the design of a variety of electromagnetic driving systems. In addition, an extensive analysis of the beam path distortions generated when a single mirror is pivoted around two axes is covered. Furthermore, correcting equations used to compensate for the systematic errors resulting from this configuration are proposed. The tip/tilt scanner is suited to a variety of applications in industrial material laser processing. These applications are not just restricted to conventional processes like welding, cutting, drilling, and marking, but also to new technologies such as micro-manipulation of parts by laser. In this new approach, bending and shrinkage of the material result due to laser irradiation, allowing fast adjustment and positioning of micro-mechanical and opto-mechanical components with sub-micron accuracy. Furthermore, local annealing of shape memory alloys, leading to stiffness variation in monolithic parts such as micro-grippers, represents a very innovative application in smart materials. On the other hand, the scanner is also a key component in bio-medical and ophthalmologic applications. The first measurements carried out with the scanner for optical tomography of the retina appeared very promising. Furthermore, an optical tweezers set-up has been realized using the scanner, allowing easy and flexible manipulation of laser beam trapped cells and molecules.

EPFL2000

Cybersecurity Solutions for Active Power Distribution Networks

Teklemariam Tsegay Tesfay

An active distribution network (ADN) is an electrical-power distribution network that implements a real-time monitoring and control of the electrical resources and the grid. Effective monitoring and control is realised by deploying a large number of sensing and actuating devices and a communication network to facilitates the two-way transfer of data. The reliance of ADN operations on a large number of electronic devices and on communication networks poses a challenge in protecting the system against cyber-attacks. Identifying these challenges and commissioning appropriate solutions is of utmost importance to realize the full potential of a smart grid that seamlessly integrates distributed generation, such as renewable energy sources. As a first step, we perform a thorough threat analysis of a typical ADN. We identify potential threats against field devices, the communication infrastructure and servers at control centers. We also propose a check-list of security solutions and best practices that guarantee a distribution network's resilient operation in the presence of malicious attackers, natural disasters, and other unintended failures that could potentially lead to islanded communication zone. For the next step, we investigate the security of MPLS-TP, a technology that is mainly used for long-distance inter-domain communication in smart grid. We find that an MPLS-TP implementation in Cisco IOS has serious security vulnerabilities in two of its protocols, BFD and PSC. These two protocols control protection-switching features in MPLS-TP. In our test-bed, we demonstrate that an attacker who has physical access to the network can exploit the vulnerabilities in order to inject forged BFD or PSC messages that affect the network's availability. Third, we consider multicast source authentication for synchrophasor data communication in grid monitoring systems (GMS). Ensuring source authentication without violating the stringent real-time requirement of GMS is challenging. Through an extensive review of existing schemes, we identified a set of schemes that satisfy some desirable requirements for GMS. The identified schemes are ECDSA, TV-HORS and Incomplete- key-set. We experimentally compared these schemes using computation, communication and key management overheads as performance metrics. A tweak in ECDSA's implementation to make it use pre-generated tokens to generate signatures significantly improves the computation overhead of ECDSA, making it the preferred scheme for GMS. This finding is contrary to the generally accepted view that asymmetric cryptography is inapplicable for real-time systems. Finally, we studied a planning problem that arises when a utility wants to roll out a software patch that requires rebooting to all PMUs while maintaining system observability. The problem we address is how to find a partitioning of the set of the deployed PMUs into as few subsets as possible such that all the PMUs in one subset can be patched in one round while all the PMUs in the other subsets provide full observability. We show that the problem is NP-complete in the general case and and formulated it as binary integer linear programming (BILP) problem. We have also provided an heuristic algorithm to find an approximate solution. Furthermore, we have identified a special case of the problem where the grid is a tree and provided a polynomial-time algorithm that finds an optimal patching plan that requires only two rounds to patch the PMUs.

EPFL2017

Building Blocks for a 24 GHz Phased-Array Front-End in CMOS Technology for Smart Streetlights

Cyril Botteron, Pierre-André Farine, Gabriele Tasselli, Ban Wang

According to a recent European Union report, lighting represents a significant share of electricity costs and the goal of reducing lighting power consumption by 20% demands the coupling of light-emitting diode (LED) lights with smart sensors and communication networks. In this context, this paper proposes the integration of these three elements into a smart streetlight, incorporating a 24 GHz phased-array (Ph-A) front-end (FE). The main building blocks of this Ph-A FE integrated in a low cost 90 nm complementary metal-oxide-semiconductor (CMOS) technology are fully characterized. The selected FE’s architecture allows the implementation of transceivers as well as Doppler radar sensors functionalities. More specifically, the Ph-A technology is applied to a Doppler radar sensor in order to realize multi-lane road scanning and pedestrian detection. That way, the smart streetlight can become eco-friendly by turning on the LEDs only when necessary as well as to measure traffic parameters such as vehicle speed, type and direction. Intercommunication between the smart streetlights is based on a time-sharing mechanism that uses the same FE reconfigured as transceiver. Thanks to this functionality, the recorded traffic information can be relayed through adjacent streetlights to a control center, and control commands and warnings can be spread through the network. The system requirements are derived assuming a simplified model of the operating scenario with a typical inter-light distance of 50 m and line-of-sight between lights. The radar range is around 60 m, which allows for continuous coverage from one streetlight to the adjacent one. Meanwhile, a communication range of 140 m is derived as a fundamental requirement for reliable communication between streetlight sensors because it allows bypassing of one node in case of failure. For the developed building blocks — a low-noise amplifier, a variable-gain amplifier, a voltage-controlled oscillator and a vector modulation phase shifter — the design methodology is presented together with measurement results. The system power, consumption, noise figure and gain are estimated by means of a system analysis based on the measured data from the implemented blocks and the state of the art performances for the missing parts. It is shown that the requirements can be fulfilled with a total power consumption of around 375 mW in Doppler radar sensor mode and around 190 mW in transceiver mode. To the authors’ knowledge, this kind of integration is new and overcomes some limitations of the currently used solutions based on infrared sensors and low-throughput communications.

2014