Citation
Abstract
The 70-m antenna servo system consists of the antenna itself and the master equatorial (ME). The two subsystems track in difierent coordinates: the antenna moves in the azimuth/elevation (AZ/EL) coordinates while the ME moves in the hour angle/declination (HA/DEC) coordinates. Di–culties in tracking appear due to the existence of keyholes. Tracking near the keyhole requires rates that exceed the imposed rate limits, and the servo errors grow to unacceptable values. The 70-m antenna’s keyhole is at EL = 90 deg, and the ME keyhole is at DEC = 90 deg, which corresponds to AZ = 0 deg and EL = 35.425 deg at Goldstone. In this article, two control modes are analyzed near the ME keyhole: the autocollimator mode, in which the ME is commanded and the antenna follows it, and the ME encoder mode, in which the antenna is commanded and the ME follows the antenna. For the autocollimator mode, the analysis showed that near the keyhole the ME causes unacceptable delays (up to 120 s). In the ME encoder mode, the slow-moving ME destabilizes the antenna near the keyhole. In order to avoid antenna lagging near the ME keyhole, the antenna controller should switch to the antenna encoder mode and the ME should be disconnected. In order to trigger the switch, the ME hour servo error is monitored. It was determined the switching should happen when the ME hour error exceeds the threshold value of 0.2 deg. The analysis showed that multiple switches occurred when transferring from one mode to another and that the switching causes jerks in the servo system. In the autocollimator mode, the command preprocessor (CPP) or low-pass filter smoothes the jerks; in the ME encoder mode, switching jerks are insignificant|the same as jerks in the autocollimator mode with the CPP.
Details
- Volume
- 42-148
- Published
- February 15, 2002
- Pages
- 1–18
- File Size
- 353.5 KB