KPO

Observatory
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The Dome (Pukerua Bay)
An observatory dome presents some interesting challenges to an automation system. There are issues like rotation, position encoding, shutter control, shutter sensing, power transfer, failsafe handling and locking to deal with. This dome was not designed with automation in mind and the retrofit of many of these features is still outstanding.

Pretty much everything here except the fibreglass shell has been changed for the rebuild at Titahi Bay.

Night view of KPO
Lifting the dome into place.
The dome is of fibreglass construction and 2.4 metres in diameter. The shutter is split equally in half and each half hinges open as seen in the picture.

The fibreglass mould was originally designed for the shell of an orange drink stand in the shape of an orange. There appears to be a dent in the dome which is, in fact, the dimple of the orange!

The dome rides on a wheel system made from 24 skateboard wheels. 8 provide a rolling surface, 8 prevent lateral movement and another 8 prevent it from lifting - a realistic possibility in a strong wind!
Skateboard wheels provide the dome transport. This shows a main support wheel and a lateral support wheel. A hold-down wheel is not visible but rides atop the white rim.

View of the dome drive.
The drive mechanism is a friction drive. The undersurface of dome's running ring is painted with non-slip paint and a wheel pinched from a wheelbarrow is inflated in place to drive with. The hold-down runners are, of course, essential to make this work.

The motor visible in the picture is a 24V DC motor driving through a gearbox and chain drive to the wheel shaft. The dome rotates 360° in about a minute.

Power and sensor transfer block.
Power is transferred from the building up to the dome in one position only - a position we call its 'park position'. At this position a sprung pickup makes contact with a guiding U-channel and when stopped in this position, the motors can be powered and shutter sensors can be read.

Also seen in the above picture is a 6-bit gray-scale azimuth encoder which provides absolute positional resolution of 120mm (5.6 degrees). The detector of this is visible at the left end on another sprung truck. It uses 6 retroreflective infra-red detectors to read the position. This was a nice theory but we ditched it due to reliability problems.

Shutter motor.
The shutters are opened and closed with smaller 24V motors (apparently truck windscreen wiper motors) pulling nylon coated steel rope.

At the bottom of this picture is something that looks like a railway track. Ok, it is a railway track. This was another variation on the azimuth encoding scheme. It is divided into 64 electrical segments on one rail with a fixed resistance between each one. A constant 5mA current is fed into one rail via a truck and the voltage is measured on the other rail. Although this sounds ridiculously low-tech was the most reliable method we'd tried based  on physical encoding.