Low-Power Bed / Seat Occupancy Sensor Based on EMFi - Foundations on Natural and Artificial Computation - page 503

Table 1. EMFIT transducer properties

Property Symbol Value Unit Tolerance

-40to50 ◦ C Storage Temp. Ts

-20to50 ◦ C

Operation Temp. Tr

µm

Thickness D70

pC/N ± 5%

Sensitivity Sq 25-250

N/cm 2

Op. Range P > 100

Table 2. EMFIT transducer characterization: Capacity Vs Static Force

Force EMFi 1 EMFi 2 Force EMFi 1 EMFi 2

Test 1 Test 1 Test 2 Test 1 Test 1 Test 2

[ pF/cm 2 ][ pF/cm 2 ][ pF/cm 2 ] [ pF/cm 2 ][ pF/cm 2 ][ pF/cm 2 ] [ kg ] [ kg ]

0 65-68 61-62 60 25 86-87 67-69

5 70-73 62-63 30 87-89 68-70

10 75-77 64-66 35 90-92 69-71

15 79-81 65-66 40 93-94 70-71 73

20 81-83 67-69 70 45 95-96 70-72

50 96-97 72-73

2.3 EMFi Transducer Properties and Features

Electromechanical Film (EMFi) is a thin, cellular, biaxially oriented polypropy-
lene film. High sensitivity, light weight and relatively low cost are the main
advantages on EMFi. This transducer can be modelled as a charge source de-
pendant of dynamical forces exerted on its surface as follows [9] [8]: Δq = k s ΔF ,
where k s is the transducer sensibility. Main properties of the commercial model
from manufacturer Emfit Ltd. [10] are shown at Table 1.
On the other hand, the transducer is a capacitor with variable capacity de-
pending on the force exerted on its surface. Characterizing the transducer in
this way is not present in the literature. We have conducted some basic tests,
using a hydraulic press with digital pressure meter, and a capacimeter, in order
to obtain a first characterization. The tests were conducted on samples from the
S-Series EMFIT manufacturer in its circular format of 1 cm radius, see Figure 1.
The procedure is as follows: (1) A sample of EMFi is connected to the digital
capacimeter terminals. (2) The sample is introduced into the hydraulic press
and the weight is set to 0 kg, with the sample slightly trapped by it. (3) Finally,
the pressure exerted by the press is increased in sections of 5 kg to reach 50 kg,
registering the capacity offered by the transducer at each step. This test has
been made with two different samples of the transducer. Besides, a second test
has been made over one of the samples. The selected force range matches with
the weight that would be exerted on every single leg of the bed or seat [0-50] kg,
considering 4 as the standard number of legs and 200 kg as the maximum weight
of a person. Table 2 shows the results of the tests. Every row contains the capac-
itance measures for every sample/test at a specific force. The capacitance has
been noted as a range of registered values, since the reading was not completely
stable. An approximately logarithmic relation has been observed.