Nuts & Volts - February 2004

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 high-g devices.

You could 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

Other Ideas

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