Instrumentation & Measurement Magazine 24-5 - 29

couple of optoelectronic components manufactured by OSRAM:
an infrared LED (code SFH480) with a typical peak
wavelength of 880 nm and a silicon NPN phototransistor
(code SFH3010) with a maximum peak sensitivity at 860 nm.
The collector current was directly translated into a voltage signal
by using a simple resistor. A matrix of 4x4 taxels with a
spatial resolution of 2 mm was implemented. The 16 voltage
signals, representing the tactile map, were digitalized with
an Analog-to-Digital (A/D) converter chip (code AD7490, 12bit,
16 channels) with an SPI interface. The voltage supply of
all sensor components was 3.3 V with a maximum power consumption
of 200 mW. The deformable layer was made of black
silicone to avoid ambient light disturbances and cross talk
among nearby taxels. Only the surface in front of the optoelectronic
devices was white in order to increase the sensitivity.
Finite Element analysis has been used to optimize the design
of deformable layer geometry. The realized layer had a square
base with dimensions equal to 11.4 × 11.4 mm, and the top was
a section of a sphere with a radius of 11.4 mm. The silicone was
a hardness of 9 Shore A, resulting into a force measurement
range equal to [0,4] N. The main application was the integration
into an anthropomorphic robotic hand.
A first objective for the evolution of the proposed tactile
sensing technology was the realization of a spatial extended
version, useful for an application in which an artificial skin
is required. During the SAPHARI European project, an extended
and conformable prototype was developed and tested
for physical human-robot interaction [7]. To this aim a modular
structure was selected, where the single module comprised
a 2 × 2 matrix of taxels and a silicone cup, and it was realized
with the same approach, components and materials used in
[6]. However, in order to realize a wide and conformable sensing
patch, 36 of these modules were realized on a single flexible
PCB with as many silicone cups, and they have been suitably
assembled (as a 6 × 6 matrix of modules) to obtain an artificial
skin patch with a total of 144 taxels and an active surface
of about 47 × 47 mm2
. The acquisition of a high number of taxels,
without the need for as many A/D channels, requested
the use of an external MicroController Unit (MCU) with a
specific " scanning control " strategy. The scanning strategy
provided the following phases: to connect the sensing modules
in groups which share 4 A/D channels; to switch on and
off the sensing modules by using the digital I/O of the microcontroller,
with a cyclic pattern; and the cyclic pattern ensured
that at a fixed time instant, only one taxel for each group was
turned on, while the others, sharing the same A/D, were
turned off. The strategy worked well since the switched off
photodetectors behave as an open circuit, without influencing
the A/D conversion, and MCU digital I/O were able to drive
the sensing modules, since the LEDs worked with a forward
current of about 1 mA and a voltage supply of 3.3 V.
More recently a new version of tactile sensor has been optimized
to be integrated in a commercial parallel gripper for
robotic systems [8] within the REFILLS European project. This
version comprises three layers: an optoelectronic PCB, a rigid
mechanical grid, and a deformable layer. Several improvements
August 2021
with respect to previous versions have been introduced. First,
the LED/phototransistor couple has been substituted by a
unique optoelectronic component: the NJL5908AR photoreflector,
manufactured by New Japan Radio. This device,
constituted by an infrared LED (with a peak wavelength at 920
nm) and a phototransistor (with a peak wavelength at 880 nm),
guarantees a higher sensitivity to light reflection, and it allows
a high reduction of taxel mechanical uncertainties related to the
relative orientation among the separate optoelectronic devices
used in previous versions. Consequently, the taxel behaviors became
more uniform and with a deeper range of measurement,
by increasing the overall sensor performance. Additionally, the
sensitive area has been increased, to manipulate a larger number
of objects, by increasing the number of taxels and the spatial
distance among them. In detail, the PCB integrates a 5 × 5 matrix
of taxels, with a spatial distance of 3.55 mm, resulting in
a sensitive area equal to 21.3 × 21.3 mm2
. The digitalization of
taxel signals has been implemented by using a couple of the
same 12-bit A/D converters with SPI interface that were previously
used. Moreover, a microcontroller section (based on a
PIC16F1824, manufactured by Microchip Technology) has been
added to the PCB design, with the objective to realize a fully integrated
sensorized finger, with a programmable device, which
allows the sensor interrogation by using a standard serial interface,
typically available in most commercial grippers. The input
stage of the PCB is completed with a standard low-noise voltage
regulator (input voltage up to 12 V from the gripper, output voltage
3.3 V for the PCB). The deformable layer is very similar to
the previous version in terms of materials and design but simply
scaled to new taxel matrix. As mentioned, when objects come
in contact with the deformable layer, they produce vertical deformations
of the cell white surfaces. These distance variations
correspond to variations of the reflected light and consequently
of the measured voltage signals. Since the NJL5908AR photoreflector
has a non-monotonic distance-voltage characteristic,
the rigid grid, introduced in this version, has become fundamental
so that the reflecting surfaces never reach distances that
fall into the non-monotonic area of the optomechanical characteristic
of the components. The grid presents holes suitably
designed to house rigid pins, which are bonded to grid via a cyanoacrylate
glue, and then the assembled grid is bonded to the
deformable layer. The rigid pins are used for the mechanical assembly
with the optoelectronic layer. The PCB presents suitable
holes used for the alignment and the connection of the mechanical
structure constituted by the deformable layer and the rigid
grid (by soldering the pins on the PCB). The obtained sensor can
be integrated into a case suitably designed to be used as a sensorized
finger for a parallel gripper. The assembled sensor can
be interrogated via a standard serial interface with a maximum
sampling rate equal to 500 Hz.
A similar version has been developed within the WIRES
experiment implemented during the ECHORD++ European
project. During this experiment, for the first time, a tactile sensor
with a flat surface and a very small thickness has been
realized for the shape recognition of deformable objects (i.e., cables)
and their manipulation in narrow space [9]. The sensing
IEEE Instrumentation & Measurement Magazine
29
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