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Active Surface - Introduction

• Active Surface Operating Modes - Summary
• Active Surface Operating Modes - Details
• Active Surface Acceptable Operation - Summary
• Active Surface Acceptable Operation - Details and Call-out Arrangements

Active Surface Operating Modes - Summary

• Prime Focus: Active Surface Power Supplies Off; Active Surface Manager Off.
• Gregorian L, S-band: Active Surface Power Supplies On, Active Surface Manager Zero Offsets selected, Active Surfce Manager FE model not selected.
• Gregorian C-band and above: Active Surface Power Supplies On, Active Surface Manager Zero Offsets Selected, Active Surface FE model selected.
• PTCS tests: As requested.

Active Surface Operating Modes - Details

The active surface has no specific command to "datum" the actuators. When the VxWorks ActiveSurfaceServo system is rebooted, it reads a configuration file which contains the photogrammetry corrections, and applies these to the surface. Further adjustments (such as FE corrections) are applied on top of the zeropoints. Thus, the way to return to the photogrammetry zeropoints is simply to turn off all other corrections.

The actuators will not adopt any specific default position when the motor power supplies are turned off - they simply remain at their current position. So it is important that the FE and any other corrections are removed via the Active Surface Manager before the active surface hardware is powered down.

Given the above, the complete active surface system (Active Surface Manager, VxWorks ActiveSurfaceServo software, and the active surface hardware) has four main operating modes, as follows:

• Completely Disabled / Prime Focus Operation To prevent any possible concern over low-frequency, RFI the active surface system may be completely powered off. This is the default mode for prime focus operation. If the power is already off, this is also an acceptable observing mode for low frequency gregorian operation (L and S band). See the Power Supplies page for instructions for powering down the system.

• Photogrammetry Zeropoints Only / Low Frequency Gregorian Operation At L and S-band (below 4GHz), the residual gravitational corrections are un-necessary, and so to avoid wear and tear on the actuators, the FE corrections should be left turned off. Since the process of powering the hardware on and off is rather laborious, the power supplies should normally be left turned on. However, if they are already off, and no higher-frequency operation is immiment, the system may be left powered off.

• FE corrections / Normal Gregorian Operation At C-band and above (above 4GHz) the FE corrections will improve the performance of the antenna, and so these should be enabled. Obviously, this requires the hardware to be turned on.

• Additional Corretions / PTCS Operation We are currently exploring the use of making additional adjustments to the surface, for example parameterized as a set of Zernike polynomals, to further improve the performance of the antenna. These additional, non-standard modes are currently only used during PTCS testing.

Active Surface Acceptable Operation - Summary

There should be no more than 100 disabled actuators, of which no more than 50 should be within one contiguous region of the surface.

Active Surface Acceptable Operation - Details and Call-out Arrangements

The performance of the GBT antenna depends on a host of variables, and the performance of the Active Surface is only only component of that. It is therefore difficult to derive performance specifications from first principles. The following are therefore somewhat arbitrary but nevertheless reasonable guidlines for when the Active Surface is within acceptable operation.

• We will strive to have no more than 1% (or ~20) of the actuators out of service at any one time.
• Up to 5% (or ~100) of the actuators can be out of service before this starts to become a significant contributor to the overall surface error.
• Random actuator errors are more benign than having systematic deformations of the surface. A failure of a single H-Drive or LVDT module affects groups of 16 adjacent actuators; groups of up to three H-Drive/LVDT drive failures (up to 48 adjacent actuators) is acceptable, but not more than this.

-- RichardPrestage - 19 Aug 2004