ALTITUDE APPLICATION PAGE
The altitude measurement is an invaluable reading for a pilot. The pilot
must know at all times what the altitude of their aircraft is. We felt
that we could not build a telemetry system for an airplane without this
This was not an easy circuit to design and build. The Omron sensor which
we used provided an 86mV output, this was not much to work with. The output
of the sensor goes to an instrumentation amplifier with a gain of approximately
40 V/V. The output from the instrumentation amplifier then goes to a difference
amplifier. The second voltage on the difference amplifier is a reference
voltage which is set sithin 150 mV of the instrumentation amplifier output.
The output from the difference amplifer is then sent to a final amplifier
stage, where the output is set to 5 volts at sea level. As the altitude
goes up, the voltage output from the circuit will decrease.
The instrumentation amplifier is the first stage of the altitude sensor.
The output from the pressure sensor varies from 86 to 83 mV for a 100 ft
change in height. The instrumentation amplifier provides an initial gain
for the pressure sensor output. The gain of the instrumentation amplifier
was computed using the equation:
A1 = (1 + (2*R3)/R4)*(R1/R3).
In this case it was a gain of 45. This voltage was then sent to the
second stage, the difference amplifier.
The second stage let the voltage from the first stage to be shifted to
almost zero. A voltage reference was created using the LM334. It provides
a constant current to the resistor R5. Rref then sets the voltage at the
node Vref. This voltage was set to 150 mV. As the voltage from the instrumentation
amplifier falls, the voltage from the difference amplifier also falls.
The third and final stage of the altitude sensor board is a non-inverting
amlifier, denoted A2. This takes the 150mV output from the difference amplifier,
D1, and amplifies it to 5 Volts. As the output from the sensor falls, the
voltage from the third stage will fall to zero. The resistors for the third
stage were selected to give a gain big enough to boost 0.15V to 5 volts.
The gain of the a non-inverting amlifier is:
A2 = (R0 + R2)/R2.
This gain was tuned using the trimpot Ro.
We used a water column to calibrate this sensor. We hooked up an apparatus
which would lower the pressure at the port of the sensor. The sensor was
calibrated for ground level first, the pressure of the water column was
set zero. Then the output from the difference amplifier was set to 150mV,
and the output from the third stage was set to 5V. The pressure that the
sensor saw was then set to a water column height of 13 inches, this approximately
corresponded to 1000ft. The voltage was recorded and then the water level
was lowered and the voltage was measured again. The relationship of the
water heigh to voltage can be seen in the plot below.
When all was complete, the resolution of the sensor was fine. The sensor
output changed by 39mV per 10ft.
Design Considerations and Problems:
The biggest problem and consideration of this sensor was the small change
in the output of the pressure sensor. The output only changed 3 mV for
100ft. This possed a large problem. The multistaged amplifier and voltage
reference circuit worked as designed. By using the instrumentation amplifier
the initial output voltage was scaled so that it could be used. Then by
using the reference voltage and difference amplifer, fed to the final gain,
the resolution was large enough to be read.
The only problem was faced when the gain of the first stage needed to be
large enough to sense a change at the output. The orchestration of the
three stages also took some time to iron out. If this were to be reproduced,
the only advice that can be given, is to take your time.
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AFTS Project Group