Kevin M. Burke

The VISTA IR camera

The VISTA IR Camera has now completed its detailed design phase and is on schedule for delivery to ESO’s Cerro Paranal Observatory in 2006. The camera consists of 16 Raytheon VIRGO 2048x2048 HgCdTe arrays in a sparse focal plane sampling a 1.65 degree field of view. A 1.4m diameter filter wheel provides slots for 7 distinct science filters, each comprising 16 individual filter panes. The camera also provides autoguiding and curvature sensing information for the VISTA telescope, and relies on tight tolerancing to meet the demanding requirements of the f/1 telescope design. The VISTA IR camera is unusual in that it contains no cold pupil-stop, but rather relies on a series of nested cold baffles to constrain the light reaching the focal plane to the science beam. In this paper we present a complete overview of the status of the final IR Camera design, its interaction with the VISTA telescope, and a summary of the predicted performance of the system.

Partial discharge analysis of prestretched and unstretched acrylic elastomers for Dielectric Elastomer Actuators (DEA)

Partial discharges (PD) occur in solid insulating materials when the insulating material is partially bridged by an electrical discharge in response to an applied voltage stress. PDs typically occur at localized points of high field stresses or at voids and other inhomogeneities within the insulator. The applied fields effect on the frequency of occurrence and intensity of PDs can be used to assess the electrical breakdown strength and aging characteristics of insulating materials. PD testing is therefore a promising characterization method to understand the insulating properties of the elastomers and geometries commonly used in DEAs. Prestretched (~100% and ~230% biaxial) and unstretched acrylic elastomers (3M VHB tapes) with solid metal electrodes have been tested. We have found the number and intensity of PDs increase with applied field, and that a significant number of PDs are detected before any actuation was visibly observed, implying that the fields required for actuation will cause material aging and degradation over time. Most interestingly, the number of PDs steadily increase as the applied voltage increases up to a sufficiently high voltage, where the PDs suddenly cease. Since internal voids can cause PDs, this may indicate that the Maxwell stress minimized the thickness of or eliminated these voids, which could explain how prestretching improves performance.

Electric Explosion of Aluminum Metallized Film

Two types of capacitor-grade aluminum metallized polypropylene film were studied to investigate the factors that affect specific action integral and energy dissipation in the electrical explosion of a film, a phenomenon that occurs when a film is exposed to a high-density current pulse on the order of 107 - 108 A/cm2. The factors studied include film cross-sectional area, film length, film sheet resistance, and discharge circuit inductance. All film samples were subjected to a 2.5-kV capacitive discharge. The basic principles of the exploding-film phenomenon and the detailed effects of the stated factors are discussed.

Temporal Analysis of Exploding Film Burst Phenomenon

A technique to determine the time of burst for exploding aluminum metallized films has been studied. An experimental approach has been taken to determine the burst time relative to the discharge current, voltage, resistance, and power waveforms. A microphone transducer was employed to detect the time of burst from the sound produced by the bursting of the film and was compared to the calculated burst times. A theoretical discussion on the energy needed to melt and vaporize the film is also presented and compared to the experimental results.

Anticipating electrical breakdown in dielectric elastomer actuators

The output strain of a dielectric elastomer actuator is directly proportional to the square of its applied electric field. However, since the likelihood of electric breakdown is elevated with an increased applied field, the maximum operating electric field of the dielectric elastomer is significantly derated in systems employing these actuators so that failure due to breakdown remains unlikely even as the material ages. In an effort to ascertain the dielectric strength so that stronger electric fields can be applied, partial discharge testing is used to assess the health of the actuator by detecting the charge that is released when localized instances of breakdown partially bridge the insulator. Pre-stretched and unstretched samples of VHB4910 tape were submerged in dielectric oil to remove external sources of partial discharges during testing, and the partial discharge patterns were recorded just before failure of the dielectric sample.

The mechanical and thermal design and analysis of the VISTA infrared camera

The infrared camera for the Visible and Infrared Survey Telescope for Astronomy (VISTA) sets many technical challenges for mechanical and thermal design. The flexion between optical subsystems must be minimised to maintain alignment in various camera orientations and meet performance requirements. Thermally induced stresses, atmospheric pressure and earthquake loads place high demands on structural components, some of which must also thermally isolate the cold (~70 K) detectors and optics. The success of the design hinges on the optimisation of heat flow to minimise thermal loads on the detectors whilst holding external temperatures very close to ambient to reduce misting and convective disturbances in the field of view. This paper describes the mechanical and thermal components of the design and discusses the analyses in detail.

Aspects of concurrent design during the VISTA IR camera detailed design phase

As detailed instrument design progresses, judgements have to be made as to what changes to allow and when models such as thermal, stray-light and mechanical structure analysis have to be re-run. Starting from a well-founded preliminary design, and using good engineering design when incorporating changes, the design detailing and re-run of the models should bring no surprises. Nevertheless there are issues for maintaining the design and model configuration to a reasonably concurrent level. Using modern modeling software packages and foresight in setting up the models the process is made efficient, but at the same time the level of detail and number of cases now needed for instrument reviews is also large in order to minimise risks. We describe examples from the detailed instrument design of the VISTA IR Camera to illustrate these aspects and outline the design and analysis methods used.

Partial discharge monitoring in dielectric elastomer actuators

Dielectric elastomer actuators represent an interesting use of dielectric materials for two reasons. First, being a soft material that is designed to exert mechanical strains, the aging and dielectric breakdown mechanisms are often the result of mechanical phenomena less common in other high voltage insulating systems that use rigid dielectrics selected solely for their electrical properties. Secondly, these systems are rather unique in that realtime insulation health monitoring can provide meaningful insights to extend the working life of the system. Like most high voltage systems, dielectric breakdown results in the irreversible and catastrophic failure of the system. However, since the system can be used at lower operating voltages, albeit at a lower effectiveness, if the operating voltage can be lowered as the dielectric shows signs of aging, the useful life of the system can be extended. Partial discharge testing has been used to assess the health of the polymer films used in these systems, and the procedures for real-time monitoring of partial discharges are explored in this paper.

Examination of Initial Strike and Restrike Modes in Exploding Metallized Films

The discharge modes of a capacitive exploding metallized film setup are discerned. Long exposure still images of the plasma formation during the initial strike and restrike are presented.

Partial Discharge Characteristics of Low Kilovolt Class Capacitors

A set of advanced metallized polymer film capacitors was characterized as to their partial discharge (PD) behavior. The sample set of these capacitors was each at various stages of capacitor lifetime. The stages of lifetime were new/unused, moderate/midlife, and heavy use/end of life. The onset of partial discharges was initiated via the application of dc voltage not exceeding the design rating of 5 kVdc. The onset of PDs in the polymer material was measured by a partial discharge analyzer (PDA). The PDA was capable of displaying the apparent charge of the PDs down to less than 5 pC. Several of the capacitors under test exhibited characteristics of the phenomenon called graceful aging. Graceful aging being a process undertaken by dielectrics when the intrinsic defects already present within the material are cleared away by partial discharges. This process can reduce the original capacitance by up to a few percent. The loss of capacitance however was compensated for by the increased tolerance to a high electric field stress. This artificial aging by inducing partial discharges and characterizing the behavior may be of benefit to the capacitor industry as an accurate quality control to predict lifetime and performance of capacitors.