Biomedical Engineering Reference
In-Depth Information
3-axis referential [
23
]. Due to the use of Bragg gratings, they could measure the in-
plane (or horizontal force component) in the direction of each fiber. Unfortunately,
these authors did not present any results related to the application of this sensor on
the measurement of plantar forces.
Perhaps the two most well sustained, tested and validated solutions were
proposed by Razian et al. and Faivre et al. The first developed what they called “
The
Kent in-shoe system
” since it was developed at Kent University. Each sensor con-
sisted on a layer of a piezoelectric copolymer of 500
m
thick, in contact with three
layers of double-faced printed circuit boards, constituting the electrode connections
necessary to obtain the electrical signal representing the three orthogonal force
components. These authors tested and validate their approach, by incorporating four
sensors in an insole, which in turn was used introduced in a shoe and used for gait
locomotion of a non-pathological individual [
36
]. The four locations were chosen
as those showing an intensity peak in the vertical component [
49
]. The solution
by Faivre et al. used dynamometric rings together with strain-gages in the outer
surfaces. The ring deformation is transmitted to the strain-gauge [
16
]. These authors
also validated their sensors by developing a shoe with eight holes to accommodate
the sensors. The regions to position the sensors were chosen in accordance to several
authors [
3
,
34
] as being those where the information obtained could have more
significance.
Finally, in 2008, Marques et al. [
30
] developed an in-plane sensor based on
piezoelectric devices, which is patented [
32
], whose main purpose was measuring
plantar shear force components, even though the vertical component was also
measured. This approach represents a novelty, since it's the first approach where
all the sensing elements are distributed in the same plane. The sensor consisted on
three piezoelectric ceramic elements, with in-plane polarization, distributed around
a central point, with an angular separation of 120
ı
. In the central point, a small
piezoelectric film was placed to measure the vertical force component. The complete
assembly was about 25 mm diameter and 4 mm thickness, with an average weight
of 2.5 gf. The combination of the three ceramic elements electrical signals will
provide information about
F
x
and
F
y
components, these being the medio-lateral and
the anterior-posterior components, respectively. Another output alternative was to
combine the signals to get the intensity and direction of the in-plane horizontal force.
Its resolution is about 0.3 N for the force and 10
ı
for the angular direction. Also,
it showed 10% linearity for the force and 3% for the direction angle. This sensor
was also characterized in terms of frequency and temperature, since piezoelectric
materials are also pyroelectric (i.e., output electrical signal depends on temperature),
showing a negligible dependence in both parameters. To validate this sensor, authors
designed a shoe (Fig.
3
) to accommodate the sensor in three different locations:
underneath the 1st-2nd metatarsal heads, the 3rd-5th metatarsal heads and under the
hallux. These regions are known to be those presenting higher intensities with the
exception of the heel [
50
].
The proposed integration solution presented one disadvantage. As the sensor's
main purpose was measuring the in-plane force, and since the geometry has to allow
for this type of measurement, these two factors resulted in the need of a previous
