The Green Bank Telescope is a 100-m aperture single-dish radio telescope. For high-frequency observations (above 100 GHz), it needs a tracking error below 1.5 arc sec rms. The present system has a tracking error of 1 arc sec rms for very low wind speeds of ≤1 m/s, which increases well above 1.5 arc sec for wind speeds above 4 m/s. Hence, improvements in the servo control system are needed to achieve pointing accuracy goals for high-frequency observations. As a first step toward this goal, it is necessary to evaluate the dynamic response of the present servo system and the telescope, which forms a large flexible structure. We derive the model of the telescope dynamics using finite element analysis data. This model is further tuned and validated using system identification experiments performed on the telescope. A reduced model is developed for controller design by using modes with the highest Hankel singular value for frequencies up to 2 Hz. We quantify the uncertainty in azimuth axis dynamics with a change in elevation angle by varying the zeros of the model. We discuss the effects of transient response, wind disturbances, and azimuth track joint disturbances on telescope tracking performance.
We report the past two years of collaboration between the different actors on the ALMA nutator. Building on previous developments, the nutator has seen changes in much of the design. A high-modulus carbon fiber structure has been added on the back of the mirror in order to transfer the voice coils forces with less deformation, thus reducing delay problems due to flexing. The controller is now an off-the-shelf National Instrument NI-cRIO, and the amplifier a class D servo drive from Advanced Motion Controls, with high peak power able to drive the coils at 300 Volts DC. The stow mechanism has been totally redesigned to improve on the repeatability and precision of the stow position, which is also the reference for the 26 bits Heidenhain encoders. This also improves on the accuracy of the stow position with wind loading. Finally, the software, written largely with National Instrument's LabView, has been developed. We will discuss these changes and the preliminary performance achieved to date.
In millimeter wavelength telescope design and construction, there have been a number of mysterious failures of simple
CFRF-metal joints. Telescope designers have not had satisfactory interpretations of these failures. In this paper, factors
which may influence the failure of joints are discussed. These include stress concentration, material creep, joint fatigue,
reasons related to chemical process and manufacture process. Extrapolation formulas for material creep, joint fatigue,
and differential thermal stresses are derived in this paper. Detailed chemical and manufacturing factors are also discussed.
All these issues are the causes of a number of early failures under a loading which is significantly lower than the strength
of adhesives used. For ensuring reliability of a precision instrument structure joint, the designer should have a thorough
understanding of all these factors.
The NRAO Green Bank Telescope (GBT), located in Green Bank, West Virginia, is supported by 16 steel wheels which
rest upon a composite steel and concrete Azimuth Track, 210 feet (64 meters) in diameter. From the start of observing in
February 2001, the Azimuth Track design presented an operational problem for NRAO. By the spring of 2001, slippage
of the top plate on the base plate was causing hold-down bolt failures. In July 2002, wear between the top and base
plates (fretting) had become evident around the entire track circumference. NRAO engineers took immediate action to
reduce both the track slippage and wear problems. But in January 2003, cracks were discovered in two adjacent top
plates; by 2006 the top plates were cracking at a rate of almost one a month - an alarming rate given the design service
life of 20 years. This paper will summarize the engineering analysis efforts that were subsequently conducted to assess
the root cause of the GBT track degradation problem. We will also discuss a trial modification section that was installed
in June 2004. Finally, we will discuss the design solution that was developed to remedy the track performance problem.
The azimuth track of the Green Bank Telescope did not perform as designed. Relative movement of components was
noted during construction; in addition, fretting of the base plate and wear plate faying surfaces, fatigue cracking of the
wear plates, fatigue failure of wear plate fasteners, and deterioration of the cementitous grout layer occurred at a rapid
pace during the first few years of operation. After extensive failure analysis, a new system of components was designed
and fabricated, and installation of the components was performed during 2007 (Symmes, Anderson, and Egan,
"Improving the service life of the 100m Green Bank Telescope azimuth track", SPIE 7012-121). The highlights and
lessons learned during the fabrication and installation phases are described herein. This information will benefit any
organization performing a similar replacement, and may be helpful in new installations as well.