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.

The paper, “Tracker controls development and control architecture for the Hobby-Eberly Telescope Wide Field Upgrade,” coauthored by Jason Mock and Joseph Beno (UTCEM) and Tom Rafferty and Mark Cornell (UT McDonald Observatory) and presented at the SPIE Astronomical Telescopes and Instrumentation 2010, San Diego, California, 27 June-2 July 2010, covers details of software and hardware, design choices and analysis, and supporting simulations.

For more information contact Joe Beno.hetdex bridge

HETDEX tracker showing the key elements of the X and Y drive systems

The University of Texas, Center for Electromechanics (UTCEM) is making a major upgrade to the robotic tracking system on the Hobby Eberly Telescope (HET) as part of the Wide Field Upgrade (WFU). The upgrade focuses on a seven-fold increase in payload and necessitated a complete redesign of all tracker supporting structure and motion control systems, including the tracker bridge, ten drive systems, carriage frames, a hexapod, and many other subsystems. The cost and sensitivity of the scientific payload, coupled with the tracker system mass increase, necessitated major upgrades to personnel and hardware safety systems. To optimize kinematic design of the entire tracker, UTCEM developed novel uses of constraints and drivers to interface with a commercially available CAD package (SolidWorks). For example, to optimize volume usage and minimize obscuration, the CAD software was exercised to accurately determine tracker/hexapod operational space needed to meet science requirements. To verify hexapod controller models, actuator travel requirements were graphically measured and compared to well defined equations of motion for Stewart platforms. To ensure critical hardware safety during various failure modes, UTCEM engineers developed Visual Basic drivers to interface with the CAD software and quickly tabulate distance measurements between critical pieces of optical hardware and adjacent components for thousands of possible hexapod configurations. These advances and techniques, applicable to any challenging robotic system design, are documented and new ways to use commercially available software tools to more clearly define hardware requirements and help ensure safe operation are described in the paper, "Kinematic optimization of upgrade to the Hobby Eberly Telescope through novel use of commercially available three dimensional CAD package," coauthored by Gregory Wedeking, Joseph Zierer, and John Jackson and presented at the SPIE Astronomical Telescopes and Instrumentation 2010, San Diego, California, 27 June-2 July 2010.

For more information, contact Dr. Joe Beno.

tracker control

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