3–289
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
INTRODUCTION
Figure 1 shows the overall system architecture chosen for
this application. This system serves as a building block, from
which more advanced systems can be developed. Enhanced
accuracy, resolution, and additional features can be
integrated in a more complex design.
There are some preliminary concerns regarding the
measurement of barometric pressure which directly affect the
design considerations for this system. Barometric pressure
refers to the air pressure existing at any point within the earth’s
atmosphere. This pressure can be measured as an absolute
pressure, (with reference to absolute vacuum) or can be
referenced to some other value or scale. The meteorology and
avionics industries traditionally measure the absolute
pressure, and then reference it to a sea level pressure value.
This complicated process is used in generating maps of
weather systems. The atmospheric pressure at any altitude
varies due to changing weather conditions over time.
Therefore, it can be difficult to determine the significance of a
particular
pressure
measurement
information. However, once the pressure at a particular
location and elevation is determined, the pressure can be
calculated at any other altitude. Mathematically, atmospheric
pressure is exponentially related to altitude. This particular
system is designed to track variations in barometric pressure
once it is calibrated to a known pressure reference at a given
altitude.
For simplification, the standard atmospheric pressure at
sea level is assumed to be 29.9 in–Hg. “Standard” barometric
pressure is measured at particular altitude at the average
weather conditions for that altitude over time. The system
described in this text is specified to accurately measure
barometric pressure variations up to altitudes of 15,000 ft. This
altitude corresponds to a standard pressure of approximately
15.0 in–Hg. As a result of changing weather conditions, the
standard pressure at a given altitude can fluctuate
approximately
±
1 in–Hg. in either direction. Table 1 indicates
standard barometric pressures at several altitudes of interest.
without
additional
MC68HC11E9
MICRO–
CONTROLLER
4–DIGIT LCD
& MC145453
DISPLAY DRIVER
SIGNAL
COND.
AMPLIFIER
MPX2100AP
PRESSURE
SENSOR
DATA
CLOCK SYNCH
Figure 2. Barometer System Block Diagram
Table 1. Altitude versus Pressure Data
Altitude (Ft.)
Pressure (in–Hg)
0
29.92
500
29.38
1,000
28.85
6,000
23.97
10,000
20.57
15,000
16.86
SYSTEM OVERVIEW
In order to measure and display the correct barometric
pressure, this system must perform several tasks. The
measurement strategy is outlined below in Figure 2. First,
pressure is applied to the sensor. This produces a proportional
differential output voltage in the millivolt range. This signal
must then be amplified and level–shifted to a single–ended,
microcontroller (MCU) compatible level (0.5 – 4.5 V) by a
signal conditioning circuit. The MCU will then sample the
voltage at the analog–to–digital converter (A/D) channel input,
convert the digital measurement value to inches of mercury,
and then display the correct pressure via the LCD interface.
This process is repeated continuously.
There are several significant performance features
implemented into this system design. First, the system will
digitally display barometric pressure in inches of mercury, with
a resolution of approximately one–tenth of an inch of mercury.
In order to allow for operation over a wide altitude range (0 –
15,000 ft.), the system is designed to display barometric
pressures ranging from 30.5 in–Hg. to a minimum of 15.0
in–Hg. The display will read “l(fā)o” if the pressure measured is
below 30.5 in–Hg. These pressures allow for the system to
operate with the desired resolution in the range from sea–level
to approximately 15,000 ft. An overview of these features is
shown in Table 2.
Table 2. System Features Overview
Display Units
in–Hg
Resolution
0.1 in–Hg.
System Range
15.0 – 30.5 in–Hg.
Altitude Range
0 – 15,000 ft.
DESIGN OVERVIEW
The following sections are included to detail the system
design. The overall system will be described by considering
the subsystems depicted in the system block diagram, Figure
2. The design of each subsystem and its function in the overall
system will be presented.
F
Freescale Semiconductor, Inc.
n
.