The novel frequency adaptation scheme is based on the adaptive Fourier analyzer suggested by Nagy. The frequency adaptation method was elaborated with a view to realizing a detector connectivity check on an FPGA in a new beam loss monitoring (BLM) system, currently being developed for beam setup and machine protection of the particle accelerators at the European Organisation for Nuclear Research (CERN). The paper summarizes the Fourier analyzer to the extent relevant to this work and the basic principle of the related frequency adaptation methods. It then outlines the suggested new scheme, presents practical considerations for implementing it and underpins it with an example and the corresponding operational experience. K : Control and monitor systems online; Data processing methods; Detector control systems (detector and experiment monitoring and slow-control systems, architecture, hardware, algorithms, databases); Digital signal processing (DSP)

This paper reports on a six-axis vibration isolator for space applications. It is divided into three parts. The first part recalls the principles of active isolation and summarizes the main theoretical results for multiple-axis decentralized control based on force feedback. The second part discusses the technology and describes the evolution of the design over the five years of this project. The third part is devoted to the identification of the transmissibility matrix and the performance evaluation. Zero-gravity tests in parabolic flight are reported. The isolator is proved efficient in a frequency band between 5 Hz and 400 Hz, with a maximum attenuation of-40 dB between 50 Hz and 200 Hz.

This paper reexamines the classical problem of active and passive damping of a piezoelectric truss. The active damping strategy is the so-called IFF (Integral Force Feedback) which has guaranteed stability; both voltage control and charge (current) control implementations are examined; they are compared to resistive shunting. It is shown that in all three cases, the closed-loop eigenvalues follow a root-locus; closed form analytical formulas are given for the poles and zeros and the maximum modal damping. It is shown that the performances are controlled by two parameters: the modal fraction of strain energy ν i in the active strut and the electromechanical coupling factor k. The paper also examines the damping via inductive shunting and the enhancement of the electromechanical coupling factor by shunting a synthetic negative capacitance. In the second part, a numerical example is examined and the analytical formulae are compared with predictions based on more elaborate models, including a full FE representation of the truss, the transducer, the electrical network and the controller. The limitations of the analytical formulae are pointed out.

This paper describes a passive 6-axis vibration isolation system for space applications. The system consists of a Stewart platform with cubic architecture; each leg is equipped with an electromagnetic transducer connected to a RL circuit. The system behaves like a relaxation isolator and its transmissibility exhibits an asymptotic decay rate of −40 dB/decade. The performances are very similar to that of an active isolator based on a sky-hook controller.

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This paper briefly describes three projects developed at the Active Structures Laboratory of ULB, in connection with precision mechanics. The first one considers the integration of piezoelectric fibers in composite structures for space applications; the second consists of an active piezoelectric bimorph mirror for adaptive optics, and the third considers the vibration isolation of sensitive payloads in spacecraft.

The infrared instruments and most of the detectors have to be operated at cryogenics temperatures. Today, this is generally achieved using mechanical coolers. Compared to traditional nitrogen systems, these coolers, which large implementation started 15 years ago, have the advantage of reducing considerably the operation effort at the observatories. Depending of the technology, these coolers are all generating a level of vibration which in most of the cases is not compatible with the extremely high stability requirement of the large size telescope. This paper described different ways which have been used at ESO to reduce the vibration caused by the large IR instruments. We show how we reached the goal to have the cryogenic instruments so quiet that they do not affect the operation of the interferometry mode of the VLT. The last section of the paper reports on a unique system based on a counter vibration principle. INTRODUCTIONWhen the VLT and the VLT instrument program were launched it was known that vibration will be an issue. The VLT intreferometry program was already defined but its schedule was well behind the completion of the first instruments. If it was clear that vibration could disturb the operation of the VLT-I it was not easy to predict the exact impact. It was also not possible to define clear specifications on the acceptable level of vibration. The instrumentation teams took the necessary care to reduce as much as possible the vibration but without real guide lines. The real amplitude of the problem came out when the VLT-I started operation, then it was time to envisage some drastic corrective actions. This paper described some of them. HISTORIC, BACKGROUNDThe first mechanical cryo-cooler was introduced at ESO in 1990 to cool the detector of IRSPEC a near infrared spectrograph. This instrument, which was already in operation since 1983 on our 3.6 m telescope, was cooled with liquid nitrogen. In 1990 while transferring IRSPEC to the NTT we also upgrade its detector from a linear array to a first 32 x 32 pixel square array. It was also the occasion to upgrade the detector cooling from pumped liquid nitrogen to a Closed Cycle Cooler. The aim of this change was mainly to suppress the rather heavy operation that was the refilling of the detector cooling tank. Finally this turned out to be a very successful experience.In a few years the philosophy to build cryogenic instrument changed completely and CCC were already adopted as the normal standard way to cool cryogenic instruments. In 1992, while starting the VLT program a survey were done in order to select "the" standard cryo-cooler for the range 65K/15K. The Leybold/Oerlikon machine was selected at this time for its simplicity and compactness. Thanks to its pneumatic drive (the displacer is free flying pressed driven by the helium) this cooler offer a cooling power 20% higher than a mechanically driven cooler. On this machine the maintenance which is extremely easy can even be done (with some cares) at cryogenic temperature. For all these attractive reas...