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.
Return to the receiver lab homepage.
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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).
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Photo of the SMA digital servo control board. Click
the image to enlarge.
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