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.
KEYWORDS: Computer programming, Telescopes, Calibration, Error analysis, Astronomical telescopes, Systems modeling, Detection and tracking algorithms, Servomechanisms, Analog electronics, Radio telescopes
Various forms of measurement errors limit telescope tracking performance in practice. A new method for identifying the correcting coefficients for encoder interpolation error is developed. The algorithm corrects the encoder measurement by identifying a harmonic model of the system and using that model to compute the necessary correction parameters. The approach improves upon others by explicitly modeling the unknown dynamics of the structure and controller and by not requiring a separate system identification to be performed. Experience gained from pin-pointing the source of encoder error on the Green Bank Radio Telescope (GBT) is presented. Several tell-tale indicators of encoder error are discussed. Experimental data from the telescope, tested with two different encoders, are presented. Demonstration of the identification methodology on the GBT as well as details of its implementation are discussed. A root mean square tracking error reduction from 0.68 arc seconds to 0.21 arc sec was achieved by changing encoders and was further reduced to 0.10 arc sec with the calibration algorithm. In particular, the ubiquity of this error source is shown and how, by careful correction, it is possible to go beyond the advertised accuracy of an encoder.
There is a need to integrate tactile sensing into robotic manipulators performing tasks in space environments, including those used to repair satellites. Integration can be achieved by embedding specialized tactile sensors. Reliable and consistent signal interpretation can be obtained by ensuring that sensors with a suitable sensing mechanism are selected based on operational demands, and that materials used within the sensors do not change structurally under vacuum and expected applied pressures, and between temperatures of -80°C to +120°C. The sensors must be able to withstand space environmental conditions and remain adequately sensitive throughout their operating life. Additionally, it is necessary to integrate the sensors into the target system with minimum disturbance while remaining responsive to applied loads. Previous work has been completed to characterize sensors within the selected temperature and pressure ranges. The current work builds on this investigation by embedding these sensors in different geometries and testing the response measured among varying configurations. Embedding material selection was aided by using a dynamic mechanical analyzer (DMA) to determine stress/strain behavior for adhesives and compliant layers used to keep the sensors in place and distribute stresses evenly. Electromechanical characterization of the embedded sensor packages was conducted by using the DMA in tandem with an inductance-capacitance-resistance (LCR) meter. Methods for embedding the sensor packages were developed with the aid of finite element analysis and physical testing to account for specific geometrical constraints. Embedded sensor prototypes were tested within representative models of potential embedding locations to compare final embedded sensor performance.
This paper outlines an anti-cogging methodology and summarizes the current state of motor cogging cancellation on the Green Bank Telescope (GBT). An iterative, model-based algorithm is developed for finding the anticogging signal which yields rapid convergence. This method fills a gap in present methodologies in that it can serve as a drop-in cogging solution which operates in the presence of unknown structural dynamics as well as with an existing feedback controller. The algorithm is described and demonstrated on a 40 HP DC brushed motor test bed and also on the GBT’s elevation axis motors. Results and implementation experience from deploying the algorithm on a motor test bed and on the GBT are discussed.
To conduct astronomical observations during windy days and increase the time available for exploration with the Green Bank Telescope (GBT) it is necessary to reduce the sensitivity of the telescope structure to wind forces. A promising approach is to design an advanced robust control system for wind induced vibration attenuation. As a first step it is necessary to (1) model analytically the structure and the servo system of the telescope and (2) validate the model through systems identification experiments. This paper presents the results of the identification experiments of the structure and the servo system along with the subsequent interval analysis.
We present the design, commissioning, and initial results of the Green Bank Earth Station (GBES), a RadioAstron data downlink station located at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. The GBES uses the modernized and refurbished NRAO 140ft telescope. Antenna optics were refurbished with new motors and drives fitted to the secondary mirror positioning system, and the deformable subreflector was refurbished with a new digital controller and new actuators. A new monitor and control system was developed for the 140ft and is based on that of the Green Bank Telescope (GBT), allowing satellite tracking via a simple scheduling block. Tools were developed to automate antenna pointing during tracking. Data from the antenna control systems and logs are retained and delivered with the science and telemetry data for processing at the Astro Space Center (ASC) of the Lebedev Physical Institute (LPI) of the Russian Academy of Sciences and the mission control centre, Lavochkin Association.
The National Radio Astronomy Observatory (NRAO) in Green Bank was charged with replacing and enhancing
the original control system on the NRAO 43-Meter (43m) telescope, for a minimum amount of labor, time
and materials. The original 1960's vintage design required continuous operator presence for monitoring and
control of the telescope. A fully automated, unattended operation was desired, along with better tracking
performance at high speeds and reduced maintenance costs. We responded with a design based on proven
industrial control technology, RTAI/Linux computers, and hardware and software adapted from the GBT and
other NRAO telescopes. Commercial off-the-shelf software packages were also used in the system.
We describe the overall design of the system and the decision process that led to the adoption of the various
pieces of hardware and software, including the tradeoffs made between buying and building systems, and
allocation of telescope functions between subsystems.