Introduction
The circuit is
drawn for measurement of acceleration from
?1000 mg until + 1000 mg. It can be placed in kantra'n the car and be
supplied from the sheath of electric lighter. The circuit includes one
indicative LED and a screen LCD.
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The
shematic diagram of the circuit.
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The
connections for ADXL202.
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Description
The measurement of
acceleration becomes with the use of IC from Analog Devices, the ADXL202. The particular
element contains 2 autonomous vertical between them sensors, for the
measurement in two axes. For the particular application becomes use of only it's
one. Acceleration meter measures dynamic or static acceleration from ?2g until 2g (where g is the acceleration of
gravity) and when this is equal to zero, exit is a signal PWM, with
Duty Cycle 50%. As long as the acceleration increases, increases also the Duty
Cycle, with biggest response the 75% (in the 2g), while for acceleration ?2g, the Duty Cycle is 25%. As conclusion
from more, has that for change 1g, is altered also the Duty Cycle of exit at 12,5%.
The measurement of
Duty Cycle of response of acceleration meter becomes with the help of Timer1
(16 bit). The period of signal of expense, has been fixed in the 1,12 msec. The
frequency of oscillation of microcontroller is in the 3,6864 MHz and this means
that it's each circle of access lasts 0,27 * 10 -6 sec. Using timer1, in frequency of
measurement ck/1, the biggest feasible counted time is the 65536 * 0.27 * 10 -3
msec = 17.778 msec, which is
bigger than period (1,12 msec). So, any price Duty Cycle it is possible to be
measured.
Before the beginning
of measurements, it precedes a process of accelerator?s initializing. At this
process it is doing 16 measurements, and then is found their mean. In
short time interval, it is considered that the acceleration was not altered
also thus the result, us gives the point of null acceleration. In other words
at this process becomes approach of value Duty Cycle, that corresponds in null
acceleration. From now on, after each measurement, it will be removed from Duty
Cycle, the null acceleration.
At the placement
of appliance, the completed ADXL202 should be always parallel with the surface
of ground and the pins 1 and 14 they see front part of car. The
LCD is connected at the K2 connector and the ADXL202 is connected at the K1
connector (K3 is connected with K1).
Function
With the shooting
of switch of starter, the circuit is supplied. In the beginning, and as long as
LED is power-off, becomes initializing of meter. Acceleration meter is will be
always placed so as to it measures the acceleration at the address of movement
of vehicle. As long as however well and it is placed this, will always exist a
component vertical in the movement and with time to the centre of ground. This
component is the acceleration of gravity and which is always removed from each
measurement. As soon as the LED turns on (in minimal time), then the initializing
it has become. From now on the meter measures the acceleration and him presents
in the screen. When is presented the symbol "-", it means that it is
deceleration.
In the above
department of screen is presented the clue of total time interval, that your car
is in operation. The operation of this clock is based on vibrations that come
in the pin External Interrupt 0, from exterior circuit based in the timer
LM555.
In the future it
is is published new circuit, with the himself characteristically, that will use
the second exit acceleration meter, for the measurement of centripetal
acceleration, at the movement of car.
Michael
Karaoglanidis
Published
at 06.10.2004 by Serasidis Vasilis Http://www.serasidis.gr
