Smithsonian Astrophysical Observatory
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As a research institute of the Smithsonian Institution, the Smithsonian Astrophysical Observatory (SAO) is headquartered in Cambridge, MA, where it is joined with the Harvard College Observatory (HCO) to form the Harvard-Smithsonian Center for Astrophysics (CfA). More than 300 scientists at the CfA are engaged in a broad program of research in astronomy, astrophysics, earth and space sciences, and science education.

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SMA Antenna Servo Control Board

The receiver lab staff has designed and programmed a Digital Servo Control Board (SCB) for the velocity servo of the SMA antennas, complete with a Palm Pilot hand-paddle control. For more details, click on the photo of the SCB on the right to go to the servo page. The SCB receives velocity commands from a position loop running on the real-time antenna computer (LynxOS) and closed about the high-resolution encoder. The following link contains an overview of the SCB and detailed information about the servo bandwidths. The quality of this new servo control system as a whole can be judged by the tracking and slewing performance of the antenna measured on Mauna Kea. This plot (PDF version) shows the 0.3 arcsec RMS sidereal-rate tracking achieved on antenna 4 in comparison with the old servo on the other antennas. Antennas 4, 5, 7 and 8 presently still use 22-bit encoders (0.308 arcsec/bit). The newer antennas (1,2,3,6) are outfitted with Heidenhain RCN823 23-bit encoders (0.154 arcsec/bit). The source acquisition time (PDF version) is also much reduced with the new servo. Here is a detailed plot of the acquisition time of the new servo as a function of slew angle. Here is a plot (PDF version) of six antennas tracking in strong winds on August 23, 2003 (x-axis = minutes, y-axis = arc seconds). Finally, the new servo allows a fast-switching mode (PDF version) for the SMA even against significant winds (25 mph). If you have a PDA running PalmOS, you can download a gMovie of the antenna motion that created the previous plot. The total power dissipated by the SCB power supplies is 70 watts (0.6A at 120VAC). Servo control board
Photo of the SMA digital servo control board. Click the image to enlarge.

Last modified: Wed Oct 24 15:35:23 2007