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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Prepared by: Bill Lucas
Industrial Technology Center
INTRODUCTION
This application note describes a Digital Boat Speedometer
concept which uses a monolithic, temperature compensated
silicon pressure sensor, analog signal–conditioning circuitry,
microcontroller hardware/software and a liquid crystal display.
This sensing system converts water head pressure to boat
speed. This speedometer design using a 30 psi pressure
sensor (Motorola P/N: MPXM2202GS) yields a speed range
of 5 mph to 45 mph. Calibration of the system is performed
using data programmed into the microcontroller’s internal
memory.
A key advantage in all Motorola pressure sensors is the
patented X–ducer
, a single piezoresistive implant that
replaces the traditional Wheatstone bridge configuration used
by competitors. In addition to the X–ducer, Motorola integrates
on–chip all necessary temperature compensation, eliminat-
ing the need for separate substrates/hybrids. This state–of–
the–art technology yields superior performance and
reliability. Motorola pressure sensors are offered in several
different port configurations to allow measurement of
absolute, differential and gauge pressure. Motorola offers
three pressure sensor types: uncompensated, temperature
compensated and calibrated or fully signal conditioned.
WATER PRESSURE TO BOAT SPEED
CONVERSION
A typical analog boat speedometer employs a pitot tube, a
calibrated pressure gauge/speedometer and a hose to
connect the two. The pitot tube, located at the boat transom,
provides the pressure signal corresponding to boat speed.
This pressure signal is transmitted to the gauge via the hose.
Boat speed is related to the water pressure at the pitot tube as
described by the following equation:
P
e * (V22g)
where:
V = speed
P = pressure at pitot tube
e = specific weight of media
g = gravitational acceleration
For example, to calculate P in lb/in2 for an ocean application
use:
V = speed in mph
e = 63.99 lbs/ft3 at 60
°
F, seawater
(e will be smaller for fresh water)
g = 32 ft/sec2
15 mph = 22 ft/sec
1 ft2 = 144 in2
(63.99[lb ft3]
144[in2ft2]) (V2[mph]2
(22 15)2[(ft sec) mph]2
P
2 (32.2)[ft sec2])
P[PSI]
V
8.208
2
For example, if the boat is cruising at 30 mph, the impact
pressure on the pitot tube is:
(30 8.208)2
P
13.36 psi.
DIGITAL BOAT SPEEDOMETER DESCRIPTION
AND OPERATION
The MPXM2202GS senses the impact water pressure
against the pitot tube and outputs a proportional differential
voltage signal. This differential voltage signal is then fed (via
an analog switch and gain circuitry) to a single slope
analog–to–digital converter (A/D) which is external to the
microcontroller. The A/D circuit can complete two separate
conversions as well as a reference conversion simultaneous-
ly. This A/D utilizes the microcontroller’s internal timers as
counters and software to properly manipulate the data. The
analog switch provides a way to flip the sensor outputs after
an A/D conversion step, which is necessary to null out the
offset effects of the op–amps. This is accomplished by
performing an analog conversion, reversing the sensor’s
differential output signal, performing another analog conver-
sion, summing the two readings, then dividing this sum by two.
Any op–amp offset present will be the same polarity
regardless of the sensor output polarity, thus the op–amp
offset can be mathematically nulled out. The digital
representation of any analog signal is ratiometric to the
reference voltages of the A/D converter. Also, the sensor’s
output is ratiometric to its excitation voltage. Therefore, if both
the sensor and A/D reference voltages are connected to the
same unregulated supply, the variations in sensor output will
be nullified, and system accuracy will be maintained (i.e.,
systems in which both the A/D converter’s digital value — due
to variations in the A/D’s reference voltages — and sensor’s
output voltage are ratiometric to the supply voltage so that a
voltage regulator is not necessary).
SEMICONDUCTOR APPLICATION NOTE
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Freescale Semiconductor, Inc.
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