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Sensor - Sun angle sensor |
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Written by Chin Fung Lai
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Sunday, 30 January 2005 |
| Sensors
Currently, there are a lot of sensors employed in the Mars rover. Main
sensors that are used on current Rovers include stereo vision sensor,
sun angle sensor, inertial sensor, angular rate sensor and wheel motion
(current) sensor. There are other sensors which are not entirely important
for navigation, e.g. thermostats for measuring the "rover body's"
temperature. We will start with what is required in a rover.
Sun angle sensor |
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Fig 1 The idea of telling position with a pole
using the sun |
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As we know, a small orientation error can cause large position error,
therefore it is very important to develop sensors that can provide the absolute
heading angle of the rover. During day time, the rover can estimate its
position with wheel motion sensors, inertial sensors and angular rate sensors.
At the end of the day, the rover will use the sun angle sensing to work
out its absolute position and orientation. The use of the sun to work out
rover's position and heading angle has been around for a long time. Most
satellites have a sun sensor on board for attitude determination and control.
The concept of this method is very simple. |
To start with, let's look at a simple example, First of
all, stick a long pole in the ground upright, and mark the end of the
shadow. Then wait for twenty minutes, mark the shadow of the pole again.
Draw a line between the two marks as shown on the diagram on the left.
The first mark is West (W) and the second mark is East (E). From this
we can work out other directions as well. We hope this model give you
an idea on how it's possible to work out your direction using the sun.
Of course sun sensor used on the rover is much more complicated, and we
will now find out more about the sun sensors that are used on the rovers.
In order to find the position and orientation of the rover, we must make
more use of the sun. If we know the time of the date, we can predict where
shadow of the pole will be. By taking a few measurements of the angle
and the length of the pole, we can work the position. The rover can get
the time from an onboard clock, which is very accurate. Now, let's go
into the detail design of the sun angle sensor employed on the rovers,
it consists of a chare-coupled device (CCD) monochrome camera, a wide
angle lens and two neutral-density filters. The camera will capture the
image of the sun, by using the neutral-density filters, the rover can
obtain the centroid of the sun. From these data, it's enough to compute
the heading angle of the rover. The result of these new technologies allows
the rover to determine its absolute heading angle to within 3 .
The two rovers currently working on Mars uses this technology, at the
end of the day, to work out its position independently and uses this information
for planning navigation and pointing the communications antenna. The next
step is to allow the rovers to observe the surrounding terrain and avoid
any hazard using cameras. |
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Last Updated ( Sunday, 23 October 2005 )
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