A high precision, dual drive system has been designed and developed for the Wide Field Upgrade to the Hobby-Eberly
Telescope* at McDonald Observatory in support of the Hobby-Eberly Telescope Dark Energy Experiment. Analysis,
design and controls details will be of interest to designers of large scale, high precision robotic motion devices. The drive
system positions the 19,000 kg star tracker to a precision of less than 5 microns along its 4-meter travel. While
positioning requirements remain essentially equal to the existing HET, tracker mass increases by a factor greater than 5.
The 10.5-meter long tracker is driven at each end by planetary roller screws, each having two distinct drive sources
dictated by the desired operation: one slowly rotates the screw when tracking celestial objects and the second rotates the
nut for rapid displacements. Key results of the roller screw rotordynamics analysis are presented. A description of the
complex bearing arrangement providing required degrees of freedom as well as the impact of a detailed Failure Modes
and Effects Analysis addressing necessary safety systems is also presented. Finite element analysis results demonstrate
how mechanical springs increase the telescope's natural frequency response by 22 percent. The critical analysis and
resulting design is provided.
To enable the Hobby-Eberly Telescope Wide Field Upgrade, the University of Texas Center for Electromechanics and
McDonald Observatory are developing a precision tracker system - a 15,000 kg robot to position a 3,100 kg payload
within 10 microns of a desired dynamic track. Performance requirements to meet science needs and safety requirements
that emerged from detailed Failure Modes and Effects Analysis resulted in a system of 14 precision controlled actuators
and 100 additional analog and digital devices (primarily sensors and safety limit switches). This level of system
complexity and emphasis on fail-safe operation is typical of large modern telescopes and numerous industrial
applications. Due to this complexity, demanding accuracy requirements, and stringent safety requirements, a highly
versatile and easily configurable centralized control system that easily links with modeling and simulation tools during
the hardware and software design process was deemed essential. The Matlab/Simulink simulation environment, coupled
with dSPACE controller hardware, was selected for controls development and realization. The dSPACE real-time
operating system collects sensor information; motor commands are transmitted over a PROFIBUS network to servo
amplifiers and drive motor status is received over the same network. Custom designed position feedback loops,
supplemented by feed forward force commands for enhanced performance, and algorithms to accommodate self-locking
gearboxes (for safety), reside in dSPACE. To interface the dSPACE controller directly to absolute Heidenhain sensors
with EnDat 2.2 protocol, a custom communication board was developed. This paper covers details of software and
hardware, design choices and analysis, and supporting simulations (primarily Simulink).
The Wide Field Upgrade presents a five-fold increase in mass for the Hobby-Eberly Telescope's* tracker system. The design of the Hobby-Eberly Telescope places the Prime Focus Instrument Package (PFIP) at a thirty-five degree angle from horizontal. The PFIP and its associated hardware have historically been positioned along this uphill axis (referred to as the telescope's Y-axis) by a single screw-type actuator. Several factors, including increased payload mass and design for minimal light obscuration, have led to the design of a new and novel configuration for the Y-axis screw-drive as part of the tracker system upgrade. Typical screw-drive designs in this load and travel class (approximately 50 kilonewtons traveling a distance of 4 meters) utilize a stationary screw with the payload translating with the moving nut component. The new configuration employs a stationary nut and translating roller screw affixed to the moving payload, resulting in a unique drive system design. Additionally, a second cable-actuated servo drive (adapted from a system currently in use on the Southern African Large Telescope) will operate in tandem with the screw-drive in order to significantly improve telescope safety through the presence of redundant load-bearing systems. Details of the mechanical design, analysis, and topology of each servo drive system are presented in this paper, along with discussion of the issues such a configuration presents in the areas of controls, operational and failure modes, and positioning accuracy. Findings and results from investigations of alternative telescope safety systems, including deformable crash barriers, are also included.
The Hobby-Eberly Telescope (HET) is an innovative large telescope of 9.2 meter aperture, located in West Texas at the
McDonald Observatory (MDO). The HET operates with a fixed segmented primary and has a tracker which moves the
four-mirror corrector and prime focus instrument package to track the sidereal and non-sidereal motions of objects. A
major upgrade of the HET is in progress that will increase the pupil size to 10 meters and the field of view to 22' by
replacing the corrector, tracker and prime focus instrument package. In addition to supporting the existing suite of
instruments, this wide field upgrade will feed a revolutionary new integral field spectrograph called VIRUS, in support
of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). This paper discusses the current status of this
upgrade.
Hexapod systems (6 legged Stewart Platforms), offer advantages in accuracy over other positioning systems and are
finding applications in numerous telescopes. However, instruments with increased sophistication for modern telescopes
continue to grow in size and required positioning accuracy. This paper details an alternative hexapod configuration and
design approach, particularly focused on relatively large, high precision hexapod systems supporting high mass
payloads. The new configuration improves accuracy, reduces actuator mass, simplifies design, and reduces system cost
but requires modest additional control algorithm sophistication.
Hexapods are finding increased use in telescope applications for positioning large payloads. Engineers from The
University of Texas at Austin have been working with engineers from ADS International to develop large, high force,
highly precise and controllable hexapod actuators for use on the Wide Field Upgrade (WFU) as part of the Hobby Eberly
Telescope Dark Energy Experiment (HETDEX). These actuators are installed in a hexapod arrangement, supporting
the 3000+ kg instrument payload which includes the Wide Field Corrector (WFC), support structure, and other
optical/electronic components. In addition to force capability, the actuators need to meet the tracking speed (pointing)
requirements for accuracy and the slewing speed (rewind) requirements, allowing as many observations in one night as
possible. The hexapod actuator stroke (retraction and extension) was very closely monitored during the design phase to
make sure all of the science requirements could be met, while minimizing the risk of damaging the WFC optical
hardware in the unlikely event of a hexapod actuator or controller failure. This paper discusses the design trade-offs
between stiffness, safety, back-drivability, accuracy, and leading to selection of the motor, high ratio worm gear, roller
screw, coupling, end mounts, and other key components.
To enable the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the McDonald Observatory (MDO) and
the Center for Electro-mechanics (CEM) at the University of Texas at Austin are developing a new HET tracker in
support of the Wide-Field Upgrade (WFU) and the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). The
precision tracker is required to maintain the position of a 3,100 kg payload within ten microns of its desired position
relative to the telescope's primary mirror. The hardware system to accomplish this has ten precision controlled
actuators. Prior to installation on the telescope, full performance verification is required of the completed tracker in
CEM's lab, without a primary mirror or the telescope's final instrument package. This requires the development of a
laboratory test stand capable of supporting the completed tracker over its full range of motion, as well as means of
measurement and methodology that can verify the accuracy of the tracker motion over full travel (4m diameter circle,
400 mm deep, with 9 degrees of tip and tilt) at a cost and schedule in keeping with the HET WFU requirements. Several
techniques have been evaluated to complete this series of tests including: photogrammetry, laser tracker, autocollimator,
and a distance measuring interferometer, with the laser tracker ultimately being identified as the most viable method.
The design of the proposed system and its implementation in the lab is presented along with the test processes, predicted
accuracy, and the basis for using the chosen method*.
The quantity and length of optical fibers required for the Hobby-Eberly Telescope* Dark Energy eXperiment
(HETDEX) create unique fiber handling challenges. For HETDEX‡, at least 33,600 fibers will transmit light from the
focal surface of the telescope to an array of spectrographs making up the Visible Integral-Field Replicable Unit
Spectrograph (VIRUS). Up to 96 Integral Field Unit (IFU) bundles, each containing 448 fibers, hang suspended from the
telescope's moving tracker located more than 15 meters above the VIRUS instruments. A specialized mechanical system
is being developed to support fiber optic assemblies onboard the telescope. The discrete behavior of 448 fibers within a
conduit is also of primary concern. A life cycle test must be conducted to study fiber behavior and measure Focal Ratio
Degradation (FRD) as a function of time. This paper focuses on the technical requirements and design of the HETDEX
fiber optic support system, the electro-mechanical test apparatus for accelerated life testing of optical fiber assemblies.
Results generated from the test will be of great interest to designers of robotic fiber handling systems for major
telescopes. There is concern that friction, localized contact, entanglement, and excessive tension will be present within
each IFU conduit and contribute to FRD. The test apparatus design utilizes six linear actuators to replicate the movement
of the telescope over 65,000 accelerated cycles, simulating five years of actual operation.
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