■ FIGURE 1.
Schematic of
flight recorder.
The revised PICAXE code is listed in Figure 3.
Relative Altitude
Recordings of vertical acceleration can be integrated
to yield a velocity profile and, by a second integration,
an estimate of peak altitude. Unfortunately, such derived
altitude information becomes less accurate as a slowing
rocket's flight path departs from the vertical, and may not
therefore provide a reliable means of detecting apogee.
Although attempts at determining absolute altitude using
a calibrated pressure sensor can often be frustrated by
errors caused by instrumentation bay venting and air
turbulence around the model's airframe, these need not
present a problem when detecting the point of minimum
pressure at apogee. Therefore, a low cost (~ $20)
Honeywell 24PPC is used to measure the rocket's relative
altitude. This basic pressure sensor employs a 5K bridge
that has a typical output sensitivity of 15 mV/psi when
powered from 10 volts. (See the sidebar “An Introduction
to Pressure Sensing.”)
As model rockets rarely reach beyond a kilometer in
altitude, the typical output range of the chosen pressure
sensor will only be around 7 mV. Unfortunately, this low
level signal is inconveniently positioned at the maximum
(near sea level) end of its total pressure range and must
therefore be offset before amplification. The simplest way
to introduce this offset is by adding resistance (VR1 + R6)
between one side of the bridge and ground. Although this
marginally degrades the sensor’s specified sensitivity and
linearity, it is a perfectly acceptable (low component
count) solution for this application.
The differential output signal from the pressure sensor
is amplified, by a MAX 4194 single supply instrumentation
amplifier before being presented to a 10 bit ADC channel.
The amplifier output may swing to within about 30 mV of
the ground and supply rails when operating with an REF
input of 1.5 volts. The MAX 4194 implements a classic
three amplifier topology that enables gain to be set by a
single resistor (R3) between pin 1 and pin 8. The gain (G)
of the instrumentation can be determined by G = 1 +
50K/R3, where R3 is the gain setting resistor. In order to
accommodate the normal variations in atmospheric
pressure, plus the specified spread in sensor
characteristics, the amplifier gain was set at a rather
modest figure of 228 and VR1 adjusted to give an output
offset of 600 mV. This provides adequate measurement
resolution over the expected altitude range without the
risk of exceeding the three-volt ADC input limit.
Roll Rate
A photoelectric device can be used to measure the
rate of roll (rotation about the rocket's vertical axis) by
detecting the direction of the sun through a narrow
vertical slot in the side of the instrumentation bay. The
detector circuit outlined in Figure 2 employs a general-purpose yellow LED that generates a maximum change in
January 2009 35
