Design and fabrication of a long-life Stirling cycle cooler for space application. Phase 3: Prototype model

A second-generation, Stirling-cycle cryocooler (cryogenic refrigerator) for space applications, with a cooling capacity of 5 watts at 65 K, was recently completed. The refrigerator, called the Prototype Model, was designed with a goal of 5 year life with no degradation in cooling performance. The free displacer and free piston of the refrigerator are driven directly by moving-magnet linear motors with the moving elements supported by active magnetic bearings. The use of clearance seals and the absence of outgassing material in the working volume of the refrigerator enable long-life operation with no deterioration in performance. Fiber-optic sensors detect the radial position of the shafts and provide a control signal for the magnetic bearings. The frequency, phase, stroke, and offset of the compressor and expander are controlled by signals from precision linear position sensors (LVDTs). The vibration generated by the compressor and expander is cancelled by an active counter balance which also uses a moving-magnet linear motor and magnetic bearings. The driving signal for the counter balance is derived from the compressor and expander position sensors which have wide bandwidth for suppression of harmonic vibrations. The efficiency of the three active members, which operate in a resonant mode, is enhanced by a magnetic spring in the expander and by gas springs in the compressor and counterbalance. The cooling was achieved with a total motor input power of 139 watts. The magnetic-bearing stiffness was significantly increased from the first-generation cooler to accommodate shuttle launch vibrations

LHC Collimators Low Level Control System

The low level control system (LLCS) of the LHC collimators is responsible for accurate synchronization of 500 axes of motion at microsecond level. Stepping motors are used in open loop ensuring a high level of repeatability of the position. In addition, a position survey system based on Resolver and LVDT sensors and operating at approximately 100 Hz, verifies in real-time the position of each axis with some tens of micrometers accuracy with respect to the expected position. The LLCS is characterized by several challenging requirements such as high reliability, redundancy, strict timing constraints and compactness of the low level hardware because of the limited space available in the racks underground. The National Instruments PXI platform has been proposed and evaluated as real-time low level hardware. In this paper the architecture of the LHC collimators LLCS is presented. The solution adopted for implementing motion control and positioning sensors reading on the PXI platform are detailed

Design and Development of Intelligent Sensors

In this project, we make an extensive study of Intelligent Sensors and devise methods for analyzing them through various proposed algorithms broadly classified into Direct and Inverse Modeling. Also we look at the analysis of Blind Equalization in any sensor. A regular sensor is a device which simply measures a signal and converts it into another signal which can be read by an observer and an instrument. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and gives a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement. If the sensor is not ideal, several types of deviations can occur which render the sensor results inaccurate. On the other hand, an intelligent sensor takes some predefined action when it senses the appropriate input (light, heat, sound, motion, touch, etc.).A sensor is intelligent when it is capable of correcting errors occurred during measurement both at the input and output ends. It generally processes the signal by means of suitable methods implemented in the device before communicating it. As we discussed an ideal sensor should have linear relationship with the measures quantity. But since in practice there are several factors which introduce non-linearity in a system, we need intelligent sensors. This particular project concentrates on the compensation of difficulties faced due to the non-linear response characteristics of a capacitive pressure sensor (CPS).It studies the design of an intelligent CPS using direct and inverse modeling switched-capacitor circuit(SCC) converts the change in capacitance of the pressure-sensor into an equivalent voltage output . The effect of change in environmental conditions on the CPS and subsequently on the output of the SCC is such that it makes the output non-linear in nature. Especially change in ambient temperature causes response characteristics of the CPS to become highly nonlinear, and complex signal processing may be required to obtain correct results. The performance of the control system depends on the performance of the sensing element. It is observed that many sensors exhibit nonlinear input-output characteristics. Due to such nonlinearities direct digital readout is not possible. As a result we are forced to employ the sensors only in the linear region of their characteristics. In other words their usable range gets restricted due to the presence of nonlinearity. If a sensor is used for full range of its nonlinear characteristics, accuracy of measurement is severely affected. Similar effect is also observed in case of LVDT. The nonlinearity present is usually time-varying and unpredictable as it depends on many uncertain factors. Nonlinearity also creeps in due to change in environmental conditions such as temperature and humidity. In addition ageing of the sensors also introduces nonlinearity. The proposed scheme incorporates intelligence into the sensor. We use many algorithms and ANN models to make the sensor ‘intelligent’. Also there is an analysis of the Blind Deconvolution Techniques that maybe used for Channel Estimation. As it is a relatively new field of work, the challenges are huge but opportunities are many as well. We try to make sensors more intelligent as they would allow a varied application of them in industry, academic and domestic environments

The development of a detector system for mint object spectroscopy on the Isaac Newton telescope

The work reported in this thesis describes the development of the CCD instrumentation for the Faint Object Spectrograph on the 2.5m Isaac Newton Telescope at the Observatorio del Roque de los Muchachos, more commonly known as the La Palma Observatory. The Faint Object Spectrograph is a highly efficient, fixed-format CCD spectrograph aimed at low resolution spectrophotometry (15-20 A FWHM) over a wide spectral range (400-1050 nm). Its high throughput, compared with that of more conventional spectrographs, is due to the small number of optical surfaces, and the minimum vignetting which results from, locating the CCD inside the spectrograph camera. A CCD camera system is described which was developed primarily to test and commission the Faint Object Spectrograph, but also to assess the characteristics of the GEC P8603 CCD used In the spectrograph, and optimize its performance for this application. The use of CCDs in astronomy is now commonplace but there still remains some uncertainty as to which aspects of their performance need to be most critically assessed when choosing a device for a particular application. It is argued that it is important to consider not only the obvious characteristics such as quantum efficiency, spectral coverage, readout noise and geometrical format, but also, and particularly at astronomically relevant low-light levels, the consequences of the more subtle properties such as charge transfer efficiency, threshold effects and chip defects. The CCD detector in the Faint Object Spectrograph is located inside the spectrograph camera and needs to be positioned to high accuracy within the optical path. A microprocessor system is described which enables the CCD detector to be aligned remotely from the observer's control console. Finally, the commissioning of the Faint Object Spectrograph on the Isaac Newton Telescope is described, and some of the first results obtained during commissioning are presented in order to illustrate its potential in the field of faint object spectroscopy

Low frequency noise suppression for the development of gravitational astronomy

The existence of gravitational radiation, predicted by the General Relativity theory, was indirectly demonstrated by the observation of the orbital decay in the binary pulsar 1913+16, for which R.A. Hulse and J.H. Taylor were awarded with the Nobel Prize in 1993. From then on, the direct detection of gravitational waves became a main issue in the experimental physics, not only for the verification of the theory itself but, most important, because it can open a new "observation window" of the universe. In fact, many astronomical objects, such as neutron stars and black holes, can be directly studied only through their gravitational emission. Moreover, since its interaction with matter is intrinsically weak, the degradation of informations carried by gravitational waves is negligible, and their revelation will allow us to understand the internal structure of massive objects which emit them, and will also provide a complementary point of view to the traditional astronomy and cosmology. The direct detection must face the extreme weakness of gravitational radiation, hence very high sensitive detectors are required in order to reveal the quadrupolar effect produced by the passage of gravitational waves. The first attempts in this field were based on massive resonant bars, relying on the pioneering technique developed by J. Weber. In recent decades a more promising strategy based on interferometry was developed, providing the advantage of a wide-frequency detection-band (from few Hz to some kHz) jointly to an extreme sensitivity (the detectable strain is smaller than the size of a proton). The global network of first generation interferometric detectors, composed of Virgo, LIGO, GEO600 and TAMA300, demonstrated the feasibility of such a technique; in particular the kilometric-scale detectors Virgo and LIGO achieved a sensitivity high enough to determine the first upper limits for the gravitational emission of some known neutron stars, such as the Crab and Vela pulsars. In the next few years the upgraded version of these detectors, namely the second generation of detectors (such as Advanced Virgo and Advanced LIGO) will become operational and are expected to achieve the first direct detections of gravitational waves. However, the signal-to-noise ratio (SNR) of these first detections will be too low for precise astronomical studies of the gravitational wave sources and for complementing optical, radio and X-ray observations in the study of fundamental systems and processes in the Universe. For this reason the investigation on the design of a new, namely third, generation of detectors is already started, leading to the proposal of the European Einstein Telescope (ET). With a considerably improved sensitivity these new machines will open the era of routine gravitational wave astronomy, leading to the birth of a complete multimessenger astronomy. In particular, to enlarge the detector bandwidth in the range of 1 Hz, where interesting gravitational signals, such as those emitted by rotating neutron stars, can be detected, a further reduction of the so-called low-frequency noise, with respect to the second generation detectors, is required. In this low-frequency band the main limitation to the sensitivity of an interferometric detector arises from the thermal noise, and at lower frequencies, from the seismic and Newtonian noises. The suppression of the thermal noise will require the implementation of a cryogenic apparatus, in order to cool the test masses down to about 10 K, so that the development of position-control devices capable of cryogenic operations will be also necessary for the suspension and payload control. The seismic attenuation was already obtained in first generation detectors by means of long suspension chains of vertical and horizontal oscillators (e.g. the superattenuator of Virgo), so that a further reduction requires a smaller seismic noise at the input of the suspension system; moreover, mass density fluctuations produced by the seismic motion induce also a stochastic gravitational field (the so-called Newtonian or gravity-gradient noise) which shunts the suspension and couples directly to the mirrors of the interferometer. In order to suppress these two seismically-generated noises, third generation interferometers will be constructed in underground sites, where Rayleigh surface waves are attenuated, and the surrounding rock layers are more homogeneous and stable, reducing the density fluctuations. The feasibility of a cryogenic and underground interferometer was already tested by the Japanese prototype-detector CLIO, in the same site where is currently under construction KAGRA (formerly known as LGCT), the first full-scale interferometric detector based on these approaches. For these aspects, this second generation detector will be the forerunner of third generation interferometers such as ET, therefore a collaboration between the two scientific collaborations has been established. My experimental work is focused on the suppression of these low noise sources, so that this thesis is structured in two parallel fields of research: the seismic characterization of a potential site for the construction of the Einstein Telescope, and the development, calibration and test of a cryogenic vertical accelerometer, which can be used as a position control device, analogously to those used in the actual room-temperature superattenuator of Virgo, but also to check the vibrations introduced by the cryogenic apparatus, as I did with the measurements I performed on the cryostats of KAGRA, presented at the end of this thesis. The scheme of this thesis is subdivided in three main parts: in the first part I introduce the foundations of the gravitational astronomy, from the theory and the astrophysical sources to the experiments which will lead to the gravitational observations; in the second part I discuss the theory of low frequency noise sources and their suppression; in the third part I present the experimental work I performed in this context. Every part is composed of two chapters, structured as follows. In the first chapter I describe the derivation of gravitational waves from the Einstein's field equations, discussing their properties and the astrophysical and cosmological sources, especially those whose emission is expected at low frequencies. In the second chapter I describe the direct interferometric detection of gravitational waves and the main noise sources which limit the sensitivity, concluding with an overview of present and future detectors. In the third chapter I discuss the main features of the seismic and Newtonian noises, and the strategies necessary to suppress them, especially in third generation detectors. In the fourth chapter I discuss the theory of thermal noise, from the ideal case of the damped harmonic oscillator to the real dissipative mechanical systems and optical components of the interferometer. In the fifth chapter I present my experimental work on the long-period characterization of the Sos Enattos site in Sardinia (proposed for hosting the Einstein Telescope), from the construction and instrumentation of an underground array of sensors to the analysis of seismic and meteorological data collected in one year of observations. Finally, in the sixth chapter I present my experimental work on the development of a cryogenic vertical accelerometer, from the designing to the cryogenic calibration and tests at T=20 K. In this chapter I also present the results of the implementation of this device into the cryostats dedicated to the test masses of KAGRA, where I verified the operations of the accelerometer at T=8 K and I measured the vibrations of the inner radiation shield of the cryostats. These measurements led to a first experimental estimate of the additional vibrational noise which will be injected by the cryogenic refrigerators to the detector test masses

Vibration control with shape-memory alloys in civil engineering structures

Dissertação apresentada à Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Doutor em Engenharia CivilThe superelastic behavior exhibited by shape-memory alloys shows a vast potential for technological applications in the field of seismic hazard mitigation, for civil engineering structures. Due to this property, the material is able to totally recover from large cyclic deformations, while developing a hysteretic loop. This is translated into a high inherent damping, combined with repeatable re-centering capabilities, two fundamental features of vibration control devices. An extensive experimental program provides a valuable insight into the identification of the main variables influencing superelastic damping in Nitinol while exploring the feasibility and optimal behavior of SMAs when used in seismic vibration control. The knowledge yielded from the experimental program, together with an extensive bibliographic research, allows for the development of an efficient numerical framework for the mathematical modeling of the complex thermo-mechanical behavior of SMAs. These models couple the mechanical and kinetic transformation constitutive laws with a heat balance equation describing the convective heat problem. The seismic behavior of a superelastic restraining bridge system is successfully simulated, being one of the most promising applications regarding the use of SMAs in civil engineering structures. A small-scale physical prototype of a novel superelastic restraining device is built. The device is able to dissipate a considerable amount of energy, while minimizing a set of adverse effects, related with cyclic loading and aging effects, that hinder the dynamic performances of vibration control devices based on passive superelastic wires.Fundação Calouste Gulbenkian (bolsa de curta duração) and of Fundação para a Ciência e Tecnologia (FCT/MCTES grant SFRH/BD/37653/2007