The Amazing Frisbee
Figure 4. Serial connection details.
ly imbalanced Frisbee just after the
throw. The radial accelerometer also
shows a -0.5 g offset, due to the fact
that the accelerometer isn't quite
dead-center, and so it records a constant offset due to the centripetal
acceleration, proportional to the
square of the spin rate. The radial signal has a spin-modulated component
about this mean, due to the
accelerometer axis alternately being
along and against the drag force
(actually, the disc flies with a slight
nose-up "angle of attack", so what is
sensed here is drag force and a bit of
lift that is inclined backwards). The
period of this oscillation directly indicates the spin rate of the Frisbee,
which a careful study shows to be
decreasing from about 6. 5 to about
5. 5 revolutions per second.
Interpreting these results in quantitative aerodynamic terms is, of
course, an involved business, but just
looking at the graphs gives you a
good idea of what is going on.
Analyzing data like this might make a
good science fair or other project.
more sophisticated circuitry
with an analog-to-digital converter, as PWM
outputs are not
typical for such
also try all kinds of variants on a
circuit like this — photodiodes to
measure the position of the sun and
hence the "wobble" on the Frisbee
or pressure sensors to measure the
suction that causes the Frisbee's lift.
Maybe infrared or ultrasonic rangers
could tell you the altitude as a function of time. Happy flying! NV
Figure 5. Flight results (see text). Diamonds show the
wobble-modulated axial (lift) accelerometer, with a solid line
showing smoothed data. Crosses are the spin-modulated
radial (drag) accelerometer.Accelerations go out-of-range
for the throw at the beginning and the impact at the end.
ABOUT THE AUTHOR
Dr. Ralph Lorenz is a planetary scientist at
the University of Arizona, where his studies
focus on Saturn's moon, Titan. He enjoys
building small sensor systems and learning
about the physical world with them. Visit his
website at www.lpl.arizona.edu/~rlorenz
You could use this sort of circuit
on a radio-controlled airplane, car, or
boat. A similar circuit could be used
for a model rocket, although the g-loads will be much larger than the ±2
g range of the ADXL202, so a different accelerometer would need to be
used — the ADXL210 is similar to the
202, but with a 10 g range. For higher accelerations, you may need to use
Circle #126 on the Reader Service Card.