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transducer. This signal, after impedance conditioning, can be directly sampled
and processed by a micro-controller to get a capacity estimation; which is also
an estimation of the weight and the occupancy probability. A variant of the
schematics is also proposed in which, capacitor C and EMFi transducer swap
positions.
3.2
Implementation and Integration in the Wireless Sensor Node
The sensor node prototype incorporates a connector for an EMFi transducer. It's
terminals are wired to an analog switch that allows alternative connections to
either of the proposed circuits. It implements the described force change detector
and weight sensor. The second one offers two variants, as mentioned, and the
prototype has been built to allow testing of both, depending on the assembled
components.
The force change detector output has been adapted to generate two different
external interrupts to the micro-controller. The weight sensor output has been
driven to the analogical-to-digital converter (ADC).
The application programmed on the micro-controller (firmware) manages the
external interrupts from the force change detector, feeds the capacitor voltage
divider with a square signal of 100 kHz and, finally, samples and processes it's
output. The sequence is as follows: (1) When a force change occurs, the device
fires a weight measuring over the voltage divider. (2) It compares the result
to the previously calculated threshold and fires an event to communicate the
bed/seat occupancy. (3) The event is transmitted via radio to the PC trough
the base station. Additionally, a LED is lighted when occupancy detection is
positive.
The signal is not static so the measurement is calculated from 150 samples
from the ADC. Several algorithms have been tested to get an stable and sig-
nificant value. After the test the following one has been selected: The first 10
values are used to get a starting average value. After that, every single sample
( m n ) is added to the current average value M n = M n− 1 (1
n )+ m n n ,where c
is a pondering coecient, which is established by comparing the current sample
with the previous average as follows: c =1when m n
c
1
3
M n 1
[0 . 9 , 1 . 5]. c =
when
m n
M n 1
[0 . 5 , 0 . 9)
(1 . 5 , 2]. c =0otherwise.
4R su s
4.1 Results for the Occupancy Detection
The prototype of force changes detector has been assembled according to the
schematics shown on Figure 1 with the following values: R=1 M Ω C=220 pF.
This value for R is recommended by the manufacturer of the transducer, while
the value of C is that for a time constant of tau = RC = 220 μ s ; enough for the
signal to be detected by the microcontroller. Figure 2 shows the ouput of the
 
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