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pointing error when tiedowns lose tension

27may04

Background:

The platform is run in an unbalanced configuration. As the dome moves up and down in za, the entire platform tilts. The elevation rails have been shimmed so that this weight imbalance causes the horns to tilt onto the paraxial surface. This should be repeatable as long as the weight in the dome and the temperature remains constant.
If the main cables are the same length, then temperature changes should make the entire platform move up and down without any extra tilting. We use the tiedown cables to compensate for the motion of the platform due to temperature. The distomats measure the average height of the platform every 2 minutes. If the average platform height is more than .04 inches from the nominal position, the tiedowns will pull (or release) equally on all 3 corners to bring the average height back to the nominal position.

Losing tension in a tiedown cable.

The tiedown cables can lose tension as the dome passes in front of them. This occurs more often at high zas and high temperatures. The program controlling the tiedowns does not use the td tensions to monitor this condition. It continues to control the average height of the platform by moving all 3 tiedowns the same amount. With tension lost in one tiedown this operation will tilt the platform and cause a pointing error.
On 27may04 the source 3C48 (J0137+331) was used during a calibration run with sbw. The source was tracked from 9:00 AST to 10:30 AST. The temperature was about 84 F. The plots show the pointing error caused by the tension loss in tiedown 4 (.ps) (.pdf) (az=122.87 degrees).

Fig 1 top: This is the tension in the tiedowns during the run. The tiedowns are color coded: green is td 12, Blue is td 4, and purple is td 8. There are two cables at each tiedown block. The tensions in the 2 cables at each td should be the same. Differences are caused by calibration errors of the load cells in each cable. The gradual motion of the tensions is caused by the dome and azimuth arm moving while tracking the source. At 10.2 hours, td 4 (the blue lines) stop changing. These tiedown cables have gone slack (the 5 and 7 kip readings are calibration errors).
Fig 1 second: This plots the pointing error versus hour. The black is the azimuth error and the red is the zenith angle error. When td 4 loses tension (10.2 hours), the azimuth and zenith angle errors start increasing.
Fig 1 third: The azimuth angle during the track. Td 4 lost tension at an azimuth of 156 degrees. The azimuth of td 4 is 122.87 degrees.
Fig 1 bottom: The zenith angle during the track. Td 4 lost tension at a zenith angle of 16.4 degrees.
Fig 2 top: The platform temperature during the track.
Fig 2 bottom: The plot shows the (Average platform height - nominal platform height) in inches. At 10.2 hours, td 4 went slack and the platform began to drop. At 10.6 hours we lost the reading from one of the distomats. When this happens the control program switches to use the last recorded platform height and temperature. Every two minutes it reads the temperature and computes the delta temp from the last good reading. The delta temp is used to compute how far to move to get back to the nominal position (using .2 platform inches/deg F and 1.73 td inches/platform inch). This causes a jump in the tiedowns and since the platform was not at the nominal position when the last reading was taken.

    Losing tension in a tiedown causes the average height of the platform to be too low. A more serious problem is that the tiedown control algorithm will cause the platform to become tilted. The tilt is a pointing error that can be much more serious than the gain loss from being out of focus. For 3C48 the pointing error was 90 asecs (one beam at 3500 Mhz).
    This problem is occurring more frequently as we continue to add weight to the dome. Adding 4000 lbs to the dome should cause the platform to tip by .4 inches at za=20 degrees. This will move the tiedowns by .7 inches.
    We need to find a reliable way to monitor the loss of tension in a cable so that the control algorithm does not make things worse.

Some other pages:

The platform corner height versus dome position (at fixed average platform height) shows that the platform corners move by +/- 6 inches just from the dome motion (this was before the alfa upgrade).
The tiedown position versus platform position and temperature for 2002.

processing: x101/model/misc/tdLoseTension_may04.pro

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