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Issue
98, September 1998
Smart
Rockets - Data Acquisition in Model Rocketry
by
Tom Consi & Jim Bales
Start
• Introduction to Model
Rocketry
• Rocket Science 101
• Control & Data Logging •The
Accelerometer •
Software • Power
• System Construction •
Launch Control Box
• Results
• Future Developments
• Software
& Sources
THE ACCELEROMETER
We chose Analog Devices’
ADXL50 accelerometer, which requires a regulated +5-VDC
supply and has an on-chip amplifier. The gain of this
inverting final stage can be set by external resistors,
which enables us to trade off range for sensitivity.
We selected the ADXL50’s
external resistors to condition the sensor output to have
a zero-g bias level of about 2.5 V. Since the maximum
acceleration is expected to be 14 g, we selected the gain
resistors R4–R6 to create an inverting amplifier
with a range of ?20 g.
An internal voltage reference
makes the output insensitive to variations in the supply
voltage. The output of the accelerometer connects directly
to the PIC’s 8-bit ADC (pin RA0), giving a resolution
of approximately 0.2 g.
One potential source of error
is fluctuations in the regulated power supply, which serves
as the reference voltage (VREF) for the PIC’s ADC.
A better design would be to use a voltage reference chip
for VREF, although that would, of course, consume precious
circuit board space.
Another potential source
of error is misalignment of the sensor with respect to
the rocket’s thrust axis. This error is small: one
minus the cosine of the misalignment. So, for a misalignment
of 5?, the error is less than 0.5%.
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