MPXM2102AS datasheet(471/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1536

Digital Boat Speedometers

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:

T e* (V2 2g)

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[in2 ft2]) (V2[mph]2

(22 15)2[(ft sec) mph]2

2 (32.2)[ft sec2])

P[PSI]

8.208

For example, if the boat is cruising at 30 mph, the impact

pressure on the pitot tube is:

+ (30 8.208)2 + 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).

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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