NON UNIFORM GEOMETRY BEND SENSORS EXPLOITED
FOR BIOMEDICAL SYSTEMS
Giovanni Saggio, Stefano Bocchetti, Carlo Alberto Pinto, Giuseppe Latessa and Giancarlo Orengo
Dept. of Electronic Engineering, University of Rome “Tor Vergata”, Rome, Italy
Keywords: Bend sensors, Human postures, Data glove, Linearization.
Abstract: In biomedical systems the bend sensors have been increasingly used stands their interesting properties
useful to measure human joint static and dynamic postures. These commercially available sensors are
usually made of a polyester film printed on with a special carbon ink. The film acts as a support while the
ink’s resistance value changes with bending dues to an applied external force. The substrate film material is
usually made by Kapton and/or Mylar for their properties, stands the fact that substrate must be able to bend
repeatedly without failure for the sensor to work. In spite of their interesting properties the commercial bend
sensors have a resistance vs. bent angle characteristic which is not actually ideal as a linear function, to
measure human postures, would be. So we introduce here a novel solution useful to linearize the sensor
response.
1 INTRODUCTION
Commercial bend sensors are usually made of a few
micrometer tick resistive material deposited onto a
thicker plastic insulating substrate. The overall
thickness is anyway negligible compared to the total
largeness and lengthiness, giving to the sensor a
rectangular geometry, with one side somewhat larger
than the other.
Figure 1: Unbent sensor (a) top (b) lateral view and bent
sensor (c) with the sensible part elongated.
These devices can be adopted as sensors when
placed on human joints with the larger side bent
according to the joints.
From a characterization point of view, the model
which takes into account the mechanical aspect of
the sensor predicts a linear behavior of the electric
resistive variation with the bending angle (Saggio et
al., 2009). Even the Ohm’s law, 
, with
resistivity, l length and S section, suggests that
when the lengthiness l of the resistive sensor
material increases due to bending (see Fig. 1),
supposing a constant value of , it must correspond
a linear increase of the value R.
Nevertheless an electrical characterization of the
sensors furnishes non linear characteristics.
2 SENSOR
CHARACTERIZATION
We measured the characteristic of several
commercial bend sensors thanks to an home made
set-up previously described (Saggio et al., 2009;
Orengo et al., 2009) and as a result we selected
sensors provided by Flexpoint Inc. In particular, we
investigated the 2 inches long Flexpoint non
encapsulated sensors, polyester encapsulated sensors
and polyimide encapsulated sensors.
The results of our measurements, reported in Fig.
2, demonstrated the non linear mentioned
characteristic. In particular the resistance variation is
greater for non encapsulated sensors stands their
higher flexibility.
389
Saggio G., Bocchetti S., Pinto C., Latessa G. and Orengo G..
NON UNIFORM GEOMETRY BEND SENSORS EXPLOITED FOR BIOMEDICAL SYSTEMS .
DOI: 10.5220/0003121203890392
In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing (BIOSIGNALS-2011), pages 389-392
ISBN: 978-989-8425-35-5
Copyright
c
2011 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 2: Resistance variation vs bending angle for three
different Flexpoint sensors.
These imply that the resistive material must be non
isotropic and must present non uniformity variation
when bent. This hypothesis can be demonstrated by
the profilometer measure we performed on the
shorter side of a Flexpoint bend sensor and reported
in Fig. 3.
Figure 3: Profilometer characteristic of the sensor by
Flexpoint. The two profiles are of the sensible film and the
contact film respectively.
3 LINEARITY VS NON
LINEARITY
The non linearity in the sensor’s characteristic is the
cause of some important drawbacks such as the time
consuming calibration and the more complexity in
designing both the conditioning electronics and the
algorithms to analyze the recorded data.
So to reduce these drawbacks an interesting
method to increase sensor linearity has been
previously proposed (Gentner & Classen, 2009).
Differing from that, here we propose a novel
approach to solve the same problem in a simpler
way.
4 SENSOR EXPLOITATION
These kind of sensors are usually (but not solely)
adopted for realizing the so called data glove, i.e. a
wearable system which is capable to measure all the
static and dynamic postures of the human hand (Di
Pietro et al., 2008). So, in order to measure finger
joint positions and movements, as a usual way of
proceeding, the sensors are commonly inserted in a
closed sleeve on top of a Lycra glove in
correspondence of each finger joints (Simone et al.,
2007). Differing from that, we adopted each sensor
in a open pocket a bit wider but a bit shorter than the
sensor itself (see Fig. 4). The pocket’s open end
allows free sliding movements for the sensor. Only
the sensor tip having the two electric terminals
lodged is stitched with the pocket. All the system is
then housed sewn on the Lycra glove in
correspondence to a finger joint. Let’s indicate this
as the 1F (1 end Fixed) configuration. With joint
bending, this configuration does not bent always the
same part of the sensor, because of a translation of
the sensor itself (due also to skin and glove
elongation), as depicted in Fig. 5 where is
represented our measurement bench with a hinge
applied to simulate the human finger joint.
(a)
(b)
Figure 4: (a) Data glove, 1F configuration (b)
magnification of a detail.
0 3000 6000
2
5
20
15
10
5
μ
m
μ
m
BIOSIGNALS 2011 - International Conference on Bio-inspired Systems and Signal Processing
390
So, since the section of the sensor bent depends
on the bending angle, we can calculate exactly
which section is concerned every time.
Stand these points our idea was to change the
regular (rectangular) geometry of the sensor cutting
some part of it, so to increase or decrease its
resistance value, obtaining a linearization of the
previously reported non linear behavior (Fig. 2). In
particular our necessity was to increase the sensor
resistance value especially near the stitched sensor
tip, so the cut was done in a triangular shape with a
greater amount near the contacts (in correspondence
of the electrical terminals), matching the low
bending angle values (as represented in Fig. 6).
Figure 5: The sensor section which is bent changes @
every bending angle.
The validity of the idea was proved with ad hoc
measures. Several sensors, differing from their
triangular cut part in the shorter cathetus b) with
respect to a) in Fig. 6, were characterized.
Figure 6: (a) Regular (uncut) geometry sensor, (b)
changed (cut) geometry sensor.
Different amount of cuts were tried, and the results
for some of them, in particular for the non
encapsulated ones (which present greater non
linearity), are reported in Fig. 7.
Figure 7: Resistance variation vs. bending angle measured
for 3 different amount of cut.
As it can be noticed the linearization was increased
with the amount of cut, leading to a very good
behavior for a b) cathetus value of 1/6 with respect
to the uncut sensor. Indeed our measurements
demonstrated how a really interesting linearization
of the sensor resistance variation vs. bending angle
can be obtained with a sensor not rectangular shape
differing with respect to the common commercial
sensors.
5 CONCLUSIONS
The linearization of the bend sensor’s characteristic
leads to undeniable advantages. So here we
demonstrated how a linearization can be obtained for
such sensors especially when they are exploited for
measure human joint static and dynamic postures.
The linear characteristic was obtained with a
novel method operating few changes on the sensors
geometry and the measures demonstrated really
interesting results.
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