The FIDO rover uses four systems to determine its location during surface operations.
- The rover uses wheel
odometry as its primary positioning system. This is simply
knowing how many times the wheels have rotated. The distance traveled can be calculated by
multiplying the wheel's circumference by the number of rotations. Wheel odometry also
includes knowing the angle through which the wheels have rotated when making a turn. In
this way, the rover's heading relative to it's initial direction is known.
- A sun sensor is used to orient the rover's frame of reference to an absolute frame of
reference. Since the Sun's position is a known quantity at all times, the rover's relative
headings can be translated to an absolute direction.
- On-board gyroscopes
are used to measure the rover's rate of pitch, yaw, and roll. This information is used to
determine the rover's orientation, say if it is sitting at a slant, relative to it's
beginning orientation. It is also possible to obtain position information from the
gyroscope measurements. Since the gyroscopes measure the velocity of the pitch, yaw, and
roll, integrating the measurements gives positional information (since velocity is the
first derivative of position). But, the integration over time serves to magnify any
errors. Errors are introduced into the measurements due to the gyroscopes themselves. Over
time gyroscopes 'drift'. This drift can be roughly corrected for by testing the gyroscopes
before launch and measuring the amount of drift with time, but errors will be introduced
- The accelerometers are used to determine the rover's attitude with respect to the local
gravitational field. The accelerometer measurements are used to convert from the rover's
frame of reference to that of the absolute frame of reference. And, since the hazard
camera, belly camera, and sun sensor, are rigidly mounted to the rover, their attitude
needs to be known. (Note: accelerometers are not used for determining the rover's exact
position, since the required double integration would significantly magnify errors.)
All these measurements are tied together in what is called "State
Fusion." That is the determination of the absolute position and attitude of the rover
on the surface.
A graphing calculator and a CBL can be used for exploring the accelerations that occur
around you, such as a car, an elevator, or a roller coaster. Acceleration is a change in
velocity during a time interval. This change in velocity can either be a change in speed
or a change in the direction of motion. Direction is indicated by the algebraic sign of
the acceleration: a positive sign indicates increasing positive distances from the origin,
while a negative sign indicates decreasing positive distances from the origin. For
example, while riding in a car that leaves from a stop sign, you undergo positive
acceleration. As the car slows to a stop, you experienece a negative acceleration.
- TI-83+ calculator and CBL
- Vernier Software's PHYSICS
- low-g accelerometer
- Connect and turn on the calculator and CBL. Start the PHYSICS
- Setup one LOW-G ACCELEROMETER using the stored calibration.
- Use the manual TRIGGERING option to collect data. This allows you to
disconnect the calculator from the CBL.
- Orient the arrow in the direction you plan to take data (either
horizontally or vertically) and zero the accelerometer.
- Enter a data collection rate (usually 0.5 seconds between samples is
- To begin data collection, disconnect the calculator and press the TRIGGER
button on the CBL.
- Once the CBL displays DONE, reconnect the calculator and use the RETRIEVE
- Time values (in seconds) will be stored in list #1 and accelerations (in
m/s2) will be stored in list #2. An acceleration versus time graph will be
- Linear Acceleration on a Road - Lay the accelerometer on the dashboard of a car. Zero the accelerometer with
the arrow pointed horizontally toward the front windshield and collect data for 30
seconds. Have the driver accelerate to a safe speed and then slow to a stop. Compare
different cars and manual vs automatic transmissions.
- Centripetal Acceleration around a Corner - Lay the accelerometer on the dashboard of a car. Zero the accelerometer with
the arrow point horizontally toward the driver's side window and collect data for 20
seconds. Have the driver make a wide, left-hand turn maintaining a constant rate of speed.
Compare circular turns of different radii.
- Acceleration in an Elevator - Place the accelerometer against the wall of an elevator. Zero the
accelerometer with its arrow pointing upward and collect data for the transit time of the
elevator. Compare the motion of the elevator as it moves upward and downward.
- Acceleration of the Vertical Loop on a Roller
Coaster - Secure the accelerometer inside a fanny pack
around the rider's waist. Zero the accelerometer with the arrow pointed vertically upwards
and collect data for 15 seconds. Compare the accelerations at the top, bottom, and sides
of the loop.
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