Analog Devices car-app Application Notes

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ONE TECHNOLOG Y WAY • P. O . BO X 9106 • NORWOOD, MASSACHUSETTS 02062-9106 • 781/329-4700

USING THE ADXL202 ACCELEROMETER AS A MULTIFUNCTION SENSOR

(TILT , VIBRATION AND SHOCK) IN CAR ALARMS

by Harvey Weinberg and Christophe Lemaire

By using an intelligent algor ithm, the ADXL202 (±2 g dual axis accelerometer) can serve as a low
cost, multifunction sensor for vehicle security systems, capable of acting simultaneously as a
shock/vibration detector as well as a tilt sensor (to detect towing or jacking up of the car). The
accelerometer’s output is passed through two parallel filters. A bandpass filter to extract
shock/vibration information, and a low pass filter to extract tilt information. This application note
describes the basics of such an implementation.

INTRODUCTI O N

The ADXL202 is a low cost, low power, com plete
dual axis accelerometer with a measurement

g

range of ±2

. The ADXL202 outputs analog and
digital si gnals proportional to acceleration in each
of t h e s en si ti v e axes ( s ee Figu r e 1 ) .

+2.7V TO 5.25V

C

DEMOD

DEMOD

X

DD

R

FILT

32kΩ

R

FILT

32kΩ

Y

FILT

VDDV

X SENSOR

C

DC

OSCILLATOR

Y SENSOR

X

FILT

ADXL202

MODULATOR

C

Y

DUTY

CYCLE

(DCM)

T2COM

SELF TEST

R

SET

X OUT

Y OUT

C
O
U

P

N
T
E
R

Figure 1. ADXL202 Block Diagram

Currently automotive security systems use
shock/vibration sensors to detect collision
or forced intrusion into the car. Typically, these
sensors are based on magneto-inductive sensing.
Sensors of this type generally have adequate
sensitivity, but fall short in other areas. Often
a fair am ount of signal conditi oning and trim ming
is required between the shock sensor and
microcontroller due to variations in magnetic
material and Hall effect sensor sensitivity
and their frequency response is fairly
unpredictable due to inconsistency in mounting.
In addition such sensors have no response

to gravity-induced acceleration, so they are
incapable of sensing inclination (a static
acceleration). Tilt sensing is the most di rect way
of detecting if a vehicle is being jacked up, about
to be towed, or being loaded onto a flatbed truck.
Some of the most common methods of car theft
today.

The ADXL202 is a true accelerometer, easily
capable of shock/vibration sensing with virtually
no external signal conditioning ci rcuit ry. Since the
ADXL202 is also sensitive to static (gravitat ional)
acceleration, tilt sensing is also possible. Tilt
sensing requires a very low noise floor which
usually necessitates restricting the bandwidth of
the accelerometer, while shock/vibration sensing
requires wide bandwidth. These conflicting
requirements may be met using clever design
techniques.

PRINCIPLE OF O PERATIO N

The ADXL202 is set up to acquire acceleration from
0 to 200 Hz (the maximum frequency of interest).
Figure 2 shows a block diagram of the system. The
accelerometer’s output is fed into two filters; a low
pass filter with a cor ner frequency at 12.5 Hz used to
lower the noise floor sufficiently for accurate tilt
sensing, and a band pass filter t o m i nim i ze the noise
in the shock/vibration pass band of int erest. The l ow
pass filtered (tilt ) output then goes to a differentiator
(described in the Tilt Sensing section) where the
determination is made as to whether the
accelerometer actually sensed tilt or some other
event such as noise or temperature drift. Then an
auto-zero block performs f urther signal processing to
reject temperature drift. The band pass filtered
output goes to an integrator (described in the

8/27/98 Rev. A

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ADXL202

200 Hz Low
Pass Filtered

Digital Output

12.5 Hz L ow P ass
Filter (Samples

Averaging)

Differentiator

Auto-Zero

Recalibration

Tilt Alarm

Low Cost

Analog Output

10 Hz High Pass

Analog Filter

Figure 2. Shock and Tilt Sensing Using the ADXL202

Shock Sensing section) that m easures vibrational
energy over a small period of time (40 ms).
A decision as to whether or not to set off the
alarm may then be made by the microcontroller.
Most of these tasks are most easily im plemented
in the digital domain and require very little
computati onal power.

Since the two measurements (shock/vibrati on and
tilt) are basically exclusive and only share
a common sensor, their respective signal
processing tasks will be described separately.

TILT SENSING

FUNDAMENTALS

The alarm system must detect a change in tilt
slow enough to be the result of the vehicle being
towed or jacked up, but must be immune
to temperature changes and movement due
to passing vehicles or wind. Note that the
ADXL202 is most sensitive to tilt when its
sensitive axes are perpendicular to the force of
gravity, i. e., parallel to t he earth’s surface. Figure
3 shows that the change in projection of a 1
gravity-induced acceleration vector on the axis of
sensitivity of the accelerometer will be more
significant if the axis is tilted 10 degrees from the
horizontal than if it is tilted by the same amount
from the vertical.

Axis of
Sensitivity

1 g

Axis of
Sensitivity

Figure 3. Tilt Sensitivity

Analog to

Digital

Converter

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Integrator

Shock Alarm

However, the car may not always be level when
the alarm is activated, and while the zero
offset can be recalibrated for any initial
inclination, effectively the farther from the
horizontal the axes of sensitivity are, the less
sensitive the system will be to tilt (see ADXL202
datasheet, page 9). In most cases, this should
not be of great concern, since the sensitivi ty only

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declines by about 2.5 m

per degree of tilt when
inclination goes from zero (horizontal) to thirty
degrees of tilt. Nevertheless, installation
guidelines should recommend that the tilt sensing
module containi ng the accelerometer be mounted
such that the axes of sensitivity be as level as
possible.

IMPLEMENTATION

In general we are interested in knowing if
the inclination of the car has changed more than
±5 degrees from it’s inclination when initially
parked. When the car is turned off, a
measurement of the car’s inclination is made.
If the car’s inclinat ion is changed by more than ±5
degrees, an alarm is tri ggered. Alternatively, the
rate of change of tilt may be evaluated and if
its absolute value is above 0.2 degrees per
second for several seconds the alarm may be
triggered.

Each technique has certain advantages. The
former algori thm is better at false alarm r ejection
due to jostling of the car, while the rate of change
algorithm may be set up to react more quickly.
Algorithms using a combination of both
techniques may be used as well. It is left
to the reader to decide which technique is best
for their application. While all of the concepts
presented here are valid for both algorithms,
for consistency this application note will describe
the former (absolut e incli nation) algor ithm .

For the purpose of the following discussion,
we will assume a less than perfect tilt sensitivity

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for the accelerometer of 15 m
or 75 m

g

for 5 degrees. The ADXL202 will be

per degree of tilt ,

set up to have a bandwidth of 200 Hz so that
vibration may be detected. A 200 Hz bandwidth
will result in a noise floor of:

g