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.

This paper describes a passive six-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 skyhook controller.

I N ACTIVE vibration control of flexible structures with a singleinput/single-output (SISO) collocated (dual) actuator/sensor pair, the transmission zeros exhibit the well-known property of interlacing, that is, the poles and zeros alternate along the imaginary axis (strictly if the structure is undamped, slightly in the left-half plane for a lightly damped structure). This property is the origin of the guaranteed stability of the so-called low authority control (LAC) strategies for active damping [1,2]. Besides, the transmission zeros coincide with the poles (natural frequencies) of a modified system which depends on the sensor configuration. For a displacement (velocity) sensor, the modified system is the constrained system where the degree of freedom (DOF) alongwhich the control operates is blocked [3,4]. For a force sensor, the modified system is obtained by removing the contribution of the active member to the global stiffness matrix of the structure [2]. As the gain increases, the closedloop poles start from the open-loop poles and those which remain at finite distance move on loops going asymptotically to the transmission zeros. For displacement sensors, the extension of the SISO result to multi-input/multi-output (MIMO) collocated pairs has already been discussed in the literature [5] and, based on the fact that transmission zeros give identically zero output response, it has been inferred that, for MIMO systems, the transmission zeros are the eigenvalues of an associated constrained modes problem. To the authors’ knowledge, however, no formal proof is available. This Note provides such a proof for an undamped structure; the two cases (force actuator, displacement sensor and displacement actuator, force sensor) are discussed separately. II. Force Actuator, Displacement Sensor

This paper describes a passive vibration isolation system with isolation properties approaching that of the active "sky-hook" isolator. It is made of an electromagnetic (or moving-coil) transducer connected to an electric circuit, and its transmissibility exhibits an asymptotic decay rate of -40dB/decade. The effects of an inductive-resistive (R - L) electric circuit are studied in detail; it is shown that such systems have an equivalent mechanical representation, the so-called "relaxation" isolator. Resonant (R - L - C) circuits are also studied and single-axis experimental results are presented. Finally, the construction of a multi-axis isolator based on the same principle is introduced.

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.

This paper analyzes in detail the enhancement of piezoelectric stack transducers by means of the well known 'negative' capacitive shunting. The stability is thoroughly studied: starting from the electrical admittance curve of the transducer, a method is introduced that quantifies the stability margins of the shunted structure. Two different implementations (series vs parallel) are investigated, and the lack of robustness of the parallel one is demonstrated.Next, this technique is experimentally applied on a truss structure. Its performances are compared with those of passive shunt circuits and with those of an active control law, the so-called Integral Force Feedback or IFF. As expected, the damping introduced by the negative capacitance shunt is larger than the damping obtained with the passive shunts; it remains, however, one order of magnitude smaller than that obtained with the IFF.

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 5 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 and 400Hz, with a maximum attenuation of -40dB between 50 and 200Hz.

This paper investigates the so-called "negative capacitance" method for the damping of a piezoelectric truss structure. This method mainly consists of shunting the electrodes of a piezoelectric actuator with an electronic circuit simulating the behavior of a negative capacitance. First, different classical methods for the damping of a truss structure are re-examined. They are compared with each other and key parameters are highlighted. Then, the negative capacitance method is introduced using the same formulation. It is shown that a piezoelectric transducer acting in parallel with a negative capacitance can be viewed as an equivalent transducer with enhanced electromechanical coupling factor. Two different implementations are introduced; their stability is examined and conditions under which they can be implemented are highlighted. Finally, experimental results are presented.

This paper reports on a six-axis vibration isolator for space applications. 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.