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