Lifting
the dome into place.
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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. |
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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. |
