applied

sciences
Article
3D Printed Robotic Hand with Piezoresistive Touch Capability
Gonçalo Fonseca 1,2 , João Nunes-Pereira 1, * and Abílio P. Silva 1, *

1 Centre for Mechanical and Aerospace Science and Technologies (C-MAST), Universidade da Beira Interior,
Rua Marquês d’Ávila e Bolama, 6201-001 Covilhã, Portugal
2 BEDEV Lda, Bioengeneering Development, Ubimedical, 6200-284 Covilhã, Portugal
* Correspondence: j.nunespereira@ubi.pt (J.N.-P.); abilio@ubi.pt (A.P.S.)

Abstract: This work proposes the design of a low-cost sensory glove system that complements
the operation of a 3D-printed mechanical hand prosthesis, providing it with the ability to detect
touch, locate it and even measure the intensity of associated forces. Firstly, the production of the
prosthetic model was performed using 3D printing, which allowed for quick and cheap production
of a robotic hand with the implementation of a mechanical system that allows controlled movements
with high performance and with the possibility of easily replacing each piece individually. Secondly,
we performed the construction and instrumentation of a complementary sensory mimicry add-on
system, focusing on the ability to sense touch as the primary target. Using piezoresistive sensors
attached to the palm of the glove, a multi-sensor system was developed that was able to locate and
quantify forces exerted on the glove. This system showed promising results and could be used as
a springboard to develop a more complex and multifunctional system in the future.

Keywords: 3D printing; prosthetic hand; sensory system; piezoresistive sensors

1. Introduction
The development of prosthetic systems that not only prioritize superior mechanical
functionality, such as more complete movement of the mechanical hand with a wider range
of finger movement and a greater degree of freedom, but also focus on the sensory aspect
Citation: Fonseca, G.; Nunes-Pereira, is still very limited and in dire need of further research. Focusing on the above aspects,
J.; Silva, A.P. 3D Printed Robotic this work proposes the design of a low-cost sensory glove system that complements the
Hand with Piezoresistive Touch functionality of a 3D-printed mechanical robotic hand [1–3].
Capability. Appl. Sci. 2023, 13, 8002. The choice of 3D printing as a method for manufacturing prostheses allowed for
https://doi.org/10.3390/ a more controlled production process with cost reduction, maintaining the quality of each
app13148002 individual part and allowing for a wider range of choices in terms of the types of raw
Academic Editor: Laura Cercenelli
materials used [4–7]. The raw materials were selected according to the requirements of
parts and the type of function the parts would perform, for example, since the robotic
Received: 7 June 2023 hand has a greater need for resistance regarding mechanical strength than the forearm,
Revised: 26 June 2023 these two structures were made with different polymers. Polylactic acid (PLA) was the
Accepted: 5 July 2023 main material used for the hand parts due to its superior mechanical properties after
Published: 8 July 2023 printing: a tensile yield strength of ≈51 MPa, a tensile modulus of ≈2.3 GPa, a flexural
strength of ≈83 MPa and a flexural modulus of ≈3.1 GPa [8]. As the forearm is in direct
contact with the user’s body and is not subject to the same mechanical stress as the hand,
polyethylene terephthalate (PETG) was chosen for its manufacture, as it is biocompatible
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
and can also withstand some mechanical stress: ≈47 MPa tensile yield strength, ≈1.5 GPa
This article is an open access article
tensile modulus and 5.1% elongation at yield point [9].
distributed under the terms and
For the sensory glove system, it was decided to separate the prosthetic and sensory
conditions of the Creative Commons systems, as many patients who tried sensory prototypes could not fully adapt to them. Fur-
Attribution (CC BY) license (https:// thermore, only one type of somatosensory sensation (touch) was studied and/or produced,
creativecommons.org/licenses/by/ as the addition of other parameters, such as temperature and proprioception, would result
4.0/). in greater complexity without adequately addressing each parameter individually. As

Appl. Sci. 2023, 13, 8002. https://doi.org/10.3390/app13148002 https://www.mdpi.com/journal/applsci

Appl. Sci. 2023, 13, 8002 2 of 17

an example of a multi-layered system, A. Polishchuk et al. [10] developed a multi-sensory

glove where, in addition to temperature and humidity, touch measurements using piezore-
sistive sensors were considered to deepen the information about objects held by the sensory
system. The data produced by this system were more complete because it simultaneously
reduced the sensing area and evenly distributed the sensors, focusing on very small areas of
the palm rather than the main touch areas, and also placed the temperature and humidity
sensors in more central areas of the palm of the prosthetic hand, excluding the fingers.
While the use of sensors offers a wider range of applications in health monitoring and
human–machine interaction, it also has some drawbacks. The type of mounting has a major
impact on quality and aesthetics, and the connections between the processing units and the
sensors lead to complications if the wire paths are not well designed [11,12]. However, the
use of sensors allows systems to achieve high performance, functionality, lower cost and
greater accessibility compared to fiber-based solutions. Although these types of solutions
are easier to integrate and address electrically, they suffer from limited performance and,
more importantly, are more difficult to access (in terms of market and cost terms) and
replace if damaged [11,12].
The type of sensor used and the method of measurement are key issues. Several
investigations have used piezoresistive and piezoelectric effects, or both, as the basis for
operation. Detection systems based on the piezoelectric effect use mechanical deformation
to produce an electrical signal that varies proportionally with the amount of strain [13].
Typically, these types of systems are used in the design of systems wherein the objective is
to detect and locate active forces rather than measure them quantitatively, as piezoelectric
sensors are less accurate than piezoresistive ones. Well-known examples of this type of
system are applied in the mapping of foot pressure in footwear, which can reveal gait
patterns and weight distribution of patients undergoing rehabilitation to improve their
movement recovery, and professional athletes, who use the system to improve training and
performance [3,14].
By choosing to separate mechanical and sensory systems, the piezoelectric sensors
are not implemented in the prosthetic hand, but rather in a multi-layer material that could
cover the hand as a whole or be applied individually to its surface. This leads to long-term
complications in experimental protocols, as the movement of the hand to grasp objects
causes deformation of the piezoelectric sensors, generating false interaction signals. In
this sense, the use of the piezoresistive effect has been exploited and studied as a means
of generating touch by introducing piezoresistive sensors in specific areas of the sensory
glove [10,13,14]. This type of system allowed for more adequate data collection and was
a low-cost and high-functioning solution capable of quantifying and locating interactions
between the surface of the prosthetic hand and objects. Piezoresistive sensing allows further
integration of the sensing element into the robotic hand, for example, through conductive
polymers [15] or composites [16].
This work proposes the design of a low-cost sensory glove system that complements
the functioning of a 3D-printed mechanical hand prosthesis. The first objective is the
production of the robotic hand model by 3D printing that is fast, low-cost and capable of
acting by a mechanical movement control system. Then, we performed instrumentation
of a glove capable of “feeling”, locating and quantifying external forces. We believe this
work to be of great interest not only scientifically but also socially, paving the way for
the design and construction of hybrid systems that are coupled to the patient’s body and
communicate with it, just as the body communicates with the system. In addition, this work
was motivated and inspired by the development of a personalized solution for a member
of our academy who, for financial reasons, has not had access to this type of technology.

2. Three-Dimensional Printing of the Prosthesis

At the manufacturing level, the production method was critical to maximizing func-
tionality and cost. Utilizing 3D printing allowed the choice of different types of material
depending on the specific purpose of each part of the robotic hand. The chosen prosthetic
Appl. Sci. 2023, 13, 8002 3 of 17

model was taken from an open-source research platform, which allowed the needs of each
part to be quickly identified and the model to be divided into sections, with the hand and
forearm being separate parts. All 3D prints were made on a Prusa I3 MK3S+ printer with
a building volume of 25 × 21 × 21 cm (XYZ) and a standard nozzle size of 0.4 mm. The
printer itself is connected and programmed through its own software called Prusa Slicer,
which allows the user to fully control every aspect of the production phase, allowing control
in production speed, axis speed movement, layer weight, positioning on the feeding bed,
percentage in support use, etc.
For the robotic hand, the raw material chosen had to be able to withstand considerable
mechanical stress (tensile and flexural strength), since the fingers, the middle of the hand
and the palm would be subjected to mechanical forces generated by servomotors during
the instrumentation of the prosthesis. There were several materials that could have been
used, such as polycarbonate (PC) and polyamide (PA) (nylon), but PLA was chosen as
the raw material for the hand parts due to its cost effectiveness and long-term production
characteristics. It provided the required tensile and flexural strength for each part, while
allowing production time to be reduced. In contrast, the other polymers mentioned would
provide greater tensile strength, but would increase production time and would be difficult
to handle in the 3D printer, causing problems such as clogged nozzles, among others [17,18].
Each part was printed separately (Figure 1), allowing more accurate production cus-
tomization and cost savings. Each part was monitored for size, geometric position on the print
bed, infill percentage, layer weight, support percentage, ironing, surface quality, print speed
and nozzle diameter. These parameters affect the production time, especially the geometric
Appl. Sci. 2023, 13, x FOR PEER REVIEW 4 of 19
position, as it can change the amount of support material and can change the layer deposition
during printing, affecting the strength of the part and the orientation of the layer.

Figure 1. Representation of each of the parts of the prosthetic hand: (A) the core piece of the hand; (B) all the phalanges of each finger;
(C) the attachments to the core Figure 1. Representation
of the hand: on the left, theof each of the
attachment parts
of the of thewhich
thumb, prosthetic
allowshand: (A) the
it to move morecore piece
freely; ofon
and thethe
hand;
right, the completion of the palm;
(B) (D) the attachments
all the phalanges of between the different
each finger; phalanges,
(C) the whichtoallow
attachments the fingers
the core of the to move.on the left, the
hand:
attachment of the thumb, which allows it to move more freely; and on the right, the completion of the
Depending
palm; (D) on the size
the attachments of the
between part,
the the infill
different percentage
phalanges, whichcan be changed,
allow asto
the fingers parts with
move.
larger dimensions tend to reduce the infill percentage, compared to smaller parts. This
parameter also has a significant effect on production time. Another aspect taken into
account was the aesthetics of the robotic hand, i.e., the surface quality of the printed parts,
which plays a fundamental role not only for the prosthesis user but also for the efficiency
of the sensory system, since the surface of the palm and associated irregularities influence
Appl. Sci. 2023, 13, 8002 4 of 17

Depending on the size of the part, the infill percentage can be changed, as parts with
larger dimensions tend to reduce the infill percentage, compared to smaller parts. This
parameter also has a significant effect on production time. Another aspect taken into
account was the aesthetics of the robotic hand, i.e., the surface quality of the printed parts,
which plays a fundamental role not only for the prosthesis user but also for the efficiency
of the sensory system, since the surface of the palm and associated irregularities influence
the magnitude of the touch felt by the sensors [18,19].
As mentioned above, each part was printed using specific polymeric 3D printing
materials, which varied from part to part depending on the end use. Figure 1A–C shows
the parts produced using PLA as the primary material. The only parts wherein PLA was not
used are shown in Figure 1D. These specific parts were used for the attachments between
the prosthetic fingers (Figure 1B), allowing them to move against each other, making the
general movement of the prosthetic hand similar to the movement of a human hand. In this
sense, the commercial polyurethane elastomer “FLEX” was chosen for this purpose because
of its excellent flexibility and toughness, which allowed the parts to be moved very easily
without damaging the material. However, due to the flexibility of its filament compared to
PLA, this type of polymer presents some difficulties in terms of print management, so the
temperature of the print bed and the printer as well the nozzle must be constant, without
failures or inconsistencies during the printing process. The print bed temperature was
85 ◦ C and the nozzle temperature was 250 ◦ C. Variations in these temperatures result in the
parts not sticking to the print bed and the nozzle feed channel clogging up, interrupting
the flow of filament. The production of the forearm parts followed the same methodology
as above, taking into account the same parameters, focusing on the final quality of the part,
reducing the production time and, thus, the final cost. Table 1 shows a summary of the
manufacturing conditions of the various parts, including percentage of material used in
perimeter, internal infill and printing time.
Table 1. Manufacture conditions of the different 3D-printed parts of the prosthetic hand.

Part Internal Infill Perimeter External Support Printing Time

Material (%) (%) Perimeter (%) (%) (minutes)
Arm parts:
Forearm base 45.8 17.2 30.6 - 548
Lower part of the forearm PETG 43.5 19.3 30.0 - 522
Forearm coverage 20.4 16.0 24.7 25.1 314
Hand 35.6 10.5 18.0 21.1 715
Palm cover PLA 20.4 16.0 24.7 25.1 314
Thumb 20.4 16.0 24.7 25.1 314
Articulations parts between
hand and fingers FLEX 13.5 47.1 32.0 - 173
Articulations parts between
phalanges
Proximal phalanges
Medium phalanges PLA 32 14.2 24.6 9.6 601
Distal phalanges

As the forearm is the only part of the robotic hand in direct contact with the end user,
PETG polymer was chosen because it is biocompatible, ensuring greater compatibility
with the end user. In addition, the material had to be able to be modified, if necessary,
for instrumentation of the robotic hand without losing its structural integrity or being
damaged in any way, and PETG offered the possibility of machining the part easily and
without problems after printing. Its production settings, unlike those of FLEX, make it, like
PLA, easier to use with few complications in the long-term production phase.
To aid decision making in the 3D printing process, the 3D rendering program evaluated
the 3D models as changes were made, based on the production/printing data, through the
various parameters mentioned above, thus helping to evaluate the best course of action.
Figure 2 shows the final result of the 3D printing of the entire prosthetic limb, together with
details of the inner part of the hand.
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Figure 2. (A) Side view of the printed prosthetic limb; (B) view of the inside of the prosthetic hand

3. Robotic Hand Instrumentation

Figure 2. (A) Side view ofprinted
the printed prosthetic limb;
Figure 2. (A) Side view of the prosthetic limb; (B) view(B)
of view of the
the inside of inside of the prosthetic
the prosthetic hand. han
3.1. Description of the Materials and Hardware
3.3.
Robotic
RoboticHand
Hand Instrumentation
Instrumentation
In the instrumentation of the prosthetic hand, two main factors were considered: cos
3.1. Description of the Materials and Hardware
reduction and the
3.1. Description most
of the functional
Materials system possible. In this sense, the choice betwee
and Hardware
In the instrumentation of the prosthetic hand, two main factors were considered:
different hardware systems
In the instrumentation was fundamental
of the prosthetic hand,to achieving
twoInmain both objectives,
factors were as was co
considered: th
cost reduction and the most functional system possible. this sense, the choice be-
design
reduction
tween of the
differentand circuit
hardware and
the most its assembly.
functional
systems For example,
system
was fundamental possible. aInsimpler
to achieving this approach
bothsense, aswas
was taken
the choice
objectives, betweeb
replacing
different
the design of the design
hardware and
the circuitsystems manufacture
was fundamental
and its assembly. of a custom
For example, printed
to achieving circuit board
both objectives,
a simpler approach (PCB)
was taken with
as was th a
Arduino
by replacing
design UNO,
of the adesign
the system
circuit and
andthatitsallows
manufacture theofsame
assembly. aFor functionality
custom
example,printed as theboard
circuit
a simpler PCB, with the
(PCB)
approach advantag
with
was taken b
of
an being
Arduino off-the-shelf
UNO, a system and
thateasily
allows accessible.
the same functionality as the
replacing the design and manufacture of a custom printed circuit board (PCB) with aPCB, with the advantage
of beingTooff-the-shelf and easilymovement
Arduino allow
UNO, mechanical
a system thataccessible.
allows the of samethefunctionality
prosthetic hand, as the two
PCB,Zhiting
with the270° 25 k
advantag
To allow mechanical
servomotors were movement
placed on theof the
mainprosthetic
part hand,
of the two
forearm, 270◦ 25
Zhiting each kg servomo- to th
perpendicular
of being off-the-shelf and easily accessible.
tors were placed on
corresponding the main
wires part oftothe
attached theforearm, each fingers
prosthetic perpendicular
(Figure to the corresponding
3).two
In this way,270°
the wire
To allow
wires attached to mechanical
the prostheticmovement
fingers (Figure of the
3). prosthetic
In this way,hand,the wires Zhiting
are kept in 25 k
are kept
servomotorsin a straight
were position
placed on from
the the
main fingers
part to
of the
the motors,
forearm, allowing
each more direct
perpendicular contro
to th
a straight position from the fingers to the motors, allowing more direct control and faster
and faster
corresponding
response response
when thewires when
motorattached the
applies forcemotor
to the applies
to prosthetic force
the wires, so to
fingers the
that the wires,
(Figure so
3). In
prosthetic that the
this way,
fingers prostheti
work the wir
fingers
are kept
immediately work
inanda immediately
straight
withoutposition
delay.andfromwithout delay. to the motors, allowing more direct contr
the fingers
and faster response when the motor applies force to the wires, so that the prosthet
fingers work immediately and without delay.

Figure
Figure 3. 3. Representation
Representation of:placement
of: (A) (A) placement of the servomotors
of the servomotors on the mainon
partthe main
of the part taking
forearm, of the forearm
into account the height between the motor head and the main part (x); (B) alignment between the betwee
taking into account the height between the motor head and the main part (x); (B) alignment
the servomotors
servomotors and theand thethat
wires wires thatthe
control control
fingerthe finger movements
movements (y). (y).
Figure 3. Representation of: (A) placement of the servomotors on the main part of the forearm
taking into account the height between the motor head and the main part (x); (B) alignment betwe
In order to and
the servomotors improve the that
the wires functionality
control theof the robotic
finger hand(y).
movements and provide a greater rang
of motion for the thumb, the control of the movements of this finger was separated from
the rest of the to
In order hand, with the
improve onefunctionality
servomotor offorthe
therobotic
function of the
hand andindex, middle,
provide ringrang
a greater an
of motion for the thumb, the control of the movements of this finger was separated fro
 Appl. Sci. 2023, 13, 8002 6 of 17

Appl. Sci. 2023, 13, x FOR PEER REVIEW 7 of 1

In order to improve the functionality of the robotic hand and provide a greater range
of motion for the thumb, the control of the movements of this finger was separated from the
rest of fingers,
little the hand,andwith one servomotor
a second servomotorfor the function
for of theonly.
the thumb index, middle,
Two ring and little
potentiometers were use
fingers, and a second servomotor for the thumb only. Two potentiometers
to control the servomotors, each controlling a specific servomotor. This allowed mor were used to
control
precise,thedynamic
servomotors,
andeach controlling
real-time a specific
control of the servomotor. This
force exerted byallowed
each ofmore precise,
the servomotors o
dynamic and real-time control of the force exerted by each of the servomotors on the
the prosthetic fingers, which in turn allowed greater accuracy in the subsequen
prosthetic fingers, which in turn allowed greater accuracy in the subsequent measurement
ofmeasurement of thebyforces
the forces detected detected
the sensory by the
system. The sensory system.
final result of the The final result of th
instrumentation
instrumentation
assembly is shown inassembly
Figure 4.is shown in Figure 4.

Figure
Figure 4. 4. Illustration
Illustration of anofinstrumented
an instrumented robotic
robotic hand: (A)hand: (A) servomotors
servomotors (A1 and A2) (A1 and A2)
and power and powe
cable
cable (F); (B) potentiometers (B1 and B2), Arduino UNO (C), capacitor (D)
(F); (B) potentiometers (B1 and B2), Arduino UNO (C), capacitor (D) and breadboard (E).and breadboard (E).

3.2. Software/Routine Code Description

3.2. Software/Routine Code Description
The entire programming and control of the mechanical prosthetic system also followed
The entire programming and control of the mechanical prosthetic system als
a simplified approach, focusing on controlling the rotation of the servomotors and control-
followed
ling the force a applied
simplifiedto theapproach,
wires that focusing on controlling
allow the prosthetic fingersthe rotation
to move. Theof the servomotor
rotation of
the motors was controlled by potentiometers, each connected to a specific servomotor. In to move
and controlling the force applied to the wires that allow the prosthetic fingers
The
the rotation servomotor
following, of the motors M1,was controlled
responsible for by
thepotentiometers,
movement of theeach index,connected to a specifi
middle, ring
and little fingers,
servomotor. In was connected to
the following, potentiometer
servomotor M1,A,responsible
and servomotor M2,movement
for the responsibleof forthe index
the movement of the thumb, was connected to potentiometer B.
middle, ring and little fingers, was connected to potentiometer A, and servomotor M2
As the focus
responsible for of this
the research isof
movement the
theimplementation of a sensorytosystem
thumb, was connected in the robotic
potentiometer B.
hand, there was no need to extract specific results or values from the servomotors, only
As the focus of this research is the implementation of a sensory system in the roboti
to control the movement of the prosthetic fingers as much as possible. To do this, it
hand, there was no need to extract specific results or values from the servomotors, only t
was necessary to implement a signal filter to refine the control of the motors when the
control the movement
potentiometers of the
were activated, prosthetic
thus moving the fingers as muchThis
servomotors. as filter
possible. To do by
was applied this, it wa
necessarythe
connecting toanalogue
implementpin ofathesignal
Arduino filter
UNOtotorefine the control
a data array. Each set of the was
of data motors
storedwhen th
inpotentiometers
its own array, one were
for activated,
each motor.thusWhen moving the servomotors.
the potentiometer This filter
was activated, was
there wereapplied b
connecting the analogue pin of the Arduino UNO to a data array. Each set of data wa
variations in the electrical resistance, and each variation corresponded to a certain amount
ofstored
force produced
in its own byarray,
the servomotor.
one for each These variations
motor. When were
thethe results storedwas
potentiometer in the array.
activated, ther
As these variations occurred, errors began to accumulate due to constant
were variations in the electrical resistance, and each variation corresponded to a certaisignal interference
and delays in the exchange of information between the servomotor, the potentiometer and
amount of force produced by the servomotor. These variations were the results stored i
the Arduino system, leading to inaccurate control of the servomotor and, consequently, the
the array. As these variations occurred, errors began to accumulate due to constant signa
prosthetic fingers. The filter continuously calculated the average between the previous
interference
signal stored in and delays
the array andinthethe exchange
signal being read,of thus
information
reducing thebetween
overall the servomotor,
signal errors th
bypotentiometer and the Arduino
refining the servomotor control. system, leading to inaccurate control of the servomoto
and, consequently, the prosthetic fingers. The filter continuously calculated the averag
4.between
Robotic Hand Sensingsignal stored in the array and the signal being read, thus reducin
the previous
4.1. Description of
the overall signal the errors
Materials/Hardware
by refining the servomotor control.
Following the same methodology as the instrumentation, the sensory system was
implemented with aSensing
4. Robotic Hand focus on cost reduction while maintaining the performance of the
system. It was also important that the sensory system was removable and independent
of4.1.
theDescription of the Materials/Hardware
limb instrumentation, so that the end user could decide whether to include it or
Following the same methodology as the instrumentation, the sensory system wa
implemented with a focus on cost reduction while maintaining the performance of th
system. It was also important that the sensory system was removable and independent o
the limb instrumentation, so that the end user could decide whether to include it or mak
 glove in certain key areas of interest (Figure 5), allowing more accurate detection
prosthetic hand’s interactions with objects and the measurement of forces and
locations.
Appl. Sci. 2023, 13, 8002 7 of 17
Appl. Sci. 2023, 13, x FOR PEER REVIEW 8 of 19

make the robotic hand even more economical. A breathable polyester-based glove with a
glove in certain
polyurethane key areas
coating on the of interest
palm (Figurewas
and fingertips 5), used
allowing
as the more accurate
basis for detection
the sensory system.of the
The proposed
prosthetic system
hand’s consisted of
interactions sixteen
with piezoresistive
objects and the sensors attached of
measurement to the glove
forces in their
and
certain
locations.key areas of interest (Figure 5), allowing more accurate detection of the prosthetic
hand’s interactions with objects and the measurement of forces and their locations.

Figure 5. Representation of: (A) prosthetic hand; (B) locations where the sixteen piezore
sensors were placed.

The piezoresistive sensors used were force resistive sensors (FRS) from the 400 s
model FRS402, shown in Figure 6, a device based on a thick polytetrafluoroethylene
Figure
Figure5.5.Representation
polymer Representation
film that exhibits of: (A)a prosthetic
of: (A) prosthetic
decreasehand; hand;(B)(B) locations
locations
in electrical where
where
resistance the sixteen
the sixteen
when piezoresistive
piezoresistive
a force is applied
sensors
sensorswere
wereplaced.
placed.
active surface. It has a circular detection area of 1.27 cm, an overall length of 6.032
The
and aThewidthpiezoresistive
of 1.905 cm. sensorsThisused wereofforce
type resistive
sensor hassensors (FRS)sensitivity
a suitable from the 400 range
series, (100 g
piezoresistive sensors used were force resistive sensors
model FRS402, shown in Figure 6, a device based on a thick polytetrafluoroethylene (PTF)
(FRS) from the 400 series,
kg)
modeland measuring
FRS402, shown range
in Figure(12.7 mm)
6, a device [20].
based Its morphology
on a thick made it easy
polytetrafluoroethylene to install
polymer film that exhibits a decrease in electrical resistance when a force is applied to its (PTF)
glove,
activeas
polymer the that
film
surface.strip
It hasfixed
exhibits ina the
a circular measurement
decrease
detection in of 1.27area
electrical
area allowed
resistance
cm, an a simplified
when
overall lengtha of
force isconnection
6.0325 applied
cm and to itsbet
the sensor
active surface.
a width and
of 1.905 the aThis
It has
cm. Arduino.
circular The
type ofdetection
sensor FSR402
area
has sensor
of hasanbeen
1.27sensitivity
a suitable cm, used
overall
range in
length
(100 similar
kg) resear
of106.0325
g to cm
anda measuring
width of range
1.905 (12.7
cm. mm)
This [20].
type ofIts morphology
sensor has a made it
suitable easy
measure forces in a variety of applications [21,22]. According to [10], this type of10
and to install
sensitivity in
rangethe glove,
(100 g to sen
as and
kg) the strip
suitable fixed in range
measuring
for detecting the pressure
measurement
(12.7 mm) area
[20].allowed
differences Its on aprosthetic
simplifiedmade
morphology connection
it easybetween
surfaces. the in the
to install
sensor
glove, asand
the the Arduino.
strip fixed inThe
theFSR402 sensor has
measurement been
area used inasimilar
allowed research
simplified to measure
connection between
forces in a variety of applications [21,22]. According to [10], this type of sensor is suitable
the sensor and the Arduino. The FSR402 sensor has been used in similar research to
for detecting pressure differences on prosthetic surfaces.
measure forces in a variety of applications [21,22]. According to [10], this type of sensor is
suitable for detecting pressure differences on prosthetic surfaces.

Figure 6. Illustration of the FRS402 piezoresistive sensor used.

Due to the large number of sensors used in the assembly of the sensory glove, the
Figure 6. Illustration
processing ofprovide
unit had to the FRS402 piezoresistive
a large sensor used.
number of simultaneous analogue read inputs. This
led to the choice of the Arduino Mega2560, which has a total of sixteen analogue inputs.
Due
The to the of
design large number
the glove’s of sensors used
instrumentation again in
tookthe assemblyapproach,
a simplified of the sensory
using glov
Figure
the 6. Illustration
Arduino Mega2560of the
(5 FRS402
V) as thepiezoresistive
power supplysensor used. unlike the mechanical system
controller,
processing unit had to provide a large number of simultaneous analogue read inputs
ledinstrumentation
to the choicewhich of thewas powered directly, reducing signal interference to the sensors.
Arduino Mega2560, which has a total of sixteen analogue inp
Following the manufacturer’s of
Due to the large number sensorsguidelines,
assembly used in the assembly
each of the
sensor was sensoryusing
assembled glove, the
The
processing design
unit had
a current divider of
as tothe glove’s
provide
shown instrumentation
a large
in Figure again took a simplified
7. number of simultaneous analogue read inputs. approach,
This
the Arduino
led to Mega2560
the choice (5 V) asMega2560,
of the Arduino the powerwhich
supplyhascontroller, unlikeanalogue
a total of sixteen the mechanical
inputs. sy
instrumentation
The design ofwhich was powered
the glove’s directly,
instrumentation reducing
again took asignal interference
simplified approach,tousing
the se
Following
the Arduino the manufacturer’s
Mega2560 (5 V) as the assembly guidelines,
power supply each
controller, sensor
unlike was assembled
the mechanical systemus
current divider which
instrumentation as shown
was in Figuredirectly,
powered 7. reducing signal interference to the sensors.
Following the manufacturer’s assembly guidelines, each sensor was assembled using a
current divider as shown in Figure 7.
Appl. Sci.
Appl. Sci.2023, 13,x8002
2023,13, FOR PEER REVIEW 9 89ofof 17
Appl. Sci. 2023, 13, x FOR PEER REVIEW of1919

Figure
Figure 7. 7.
(A) The
(A) electric
The circuit
electric used
circuit inin
used each FSR402
each piezoresistive
FSR402 sensor;
piezoresistive (B)
sensor; final
(B) assembly
final ofof
assembly the
the
Figure 7. (A) The electric circuit used in each FSR402 piezoresistive sensor; (B) final assembly of the
circuit for the sensor glove.
circuit for the sensor glove.
circuit for the sensor glove.
4.2.
4.2.Description
4.2. Descriptionofofofthe
Description theMaterials/Hardware
the Materials/Hardware
Materials/Hardware
Due
Due to the
Duetotothe large
thelarge
large number
number
number ofof
of sensors
sensors
sensors used,
used,
used, the
the theprogramming
programming
programming of ofofthe
the thesystem
system waswas
system wascare-
care-
carefully
fully
fullyconsidered,
considered, as theasArduino
considered, asthetheArduino
Arduino
Mega2560Mega2560
Mega2560
only hadonly
only
onehad
hadoneoneanalogue-to-digital
analogue-to-digital
analogue-to-digital converter
converter converter
(ADC),
(ADC),
(ADC),
which which
which
allowed allowed
allowed
only oneonlyonlyone
readingonereadingone and
reading
and and
input one
oneinput
each timeeach
input each
the time
time
system the
the
was system
used.was
system was
To used.
used.
overcome ToTo
overcome
overcome
this this limitation,
thisalimitation,
limitation, simple toggle a simple
a simple toggle
functiontoggle function
wasfunction was
implemented implemented
was implemented to
to make the sensors make the
to makework sensors
the sensors
one at
worktimeone
awork one
by atata atime
selecting timethebybyselecting
selecting
number thatthethenumber
number
identifies that
eachthatidentifies
identifies
sensor each
and its each sensor
sensor
specific and
andits(Figure
position itsspecific
specific
8).
position
Inposition (Figure
this way,(Figure 8).8).InInthis
the difference way,
this way,the
between thedifference
differencebetween
simultaneous between simultaneous
simultaneous
or individual ororindividual
functionality individual func-
func-
of the sensors
tionality
did ofofthe
not have
tionality a sensors
themajor
sensors did
didnot
impact onhave
not the
have a amajor
final impact
impactononthe
results.
major thefinal
finalresults.
results.

Figure
Figure8.
Figure 8.8.(A)
(A)Identification
(A) Identificationof
Identification ofofeach
eachsensor
each sensorby
sensor bynumber
by number(from
number (from11 1to
(from toto16),
16),allowing
16), allowingmeasurement
allowing measurementand
measurement and
and
identification of the
identificationofofthe
identification specific
thespecific area
specificarea where
areawhere interactions
whereinteractions occurred;
interactionsoccurred; (B)
occurred;(B) the
(B)the glove
theglove sensor
glovesensor system
sensorsystemmounted.
system mounted.
mounted.

5.
5.5.Functionality
FunctionalityTests
Functionality Tests
Tests
The
The five
five tests
The five tests described
described
described below
below
belowwere
were
werecarried
carriedout
carried outtoto
outassess thethe
toassess
assess functionality
thefunctionalityof the
functionality robotic
ofofthe
thero-
ro-
hand.
botic These
hand. tests
These were
tests designed
were to
designed validate
to not
validate only
not the
only functionality
the functionality
botic hand. These tests were designed to validate not only the functionality of the mechan- of the
of mechanical
the mechan-
system,
ical
ical but but
system,
system, also
but that
also
also of
that the
thatofof sensory
the system,
sensory
the sensorysystem,in order
system, inin to establish
order
order toto a correlation
establish
establish a correlationbetween
a correlation between the
between
application
thetheapplicationof force,
application its measurement
ofofforce,
force, itsitsmeasurement
measurement andandlocation.
andlocation. The
location. Thetests
The were
tests were
tests divided
were dividedaccording
divided according
accordingto
daily
to activities
todaily
dailyactivitiesasas
activities aas
basis
a abasisforfor
basis formulation:
forformulation:
formulation:
-- - First
First Test:
FirstTest: Grab
Grabaaaplastic
Test:Grab plastic cup;
plasticcup; cup;
-- - Second
Second Test: Grab the same
Second Test: Grab the sameplastic
Test: Grab the same plastic cup
plasticcup with
cupwith
with150150 mL
150mLmLofofofwater;
water;
water;
-- - Third
Third Test:
Test: Grab
Grab an
an empty
empty plastic
plastic cup
cup with
with larger
larger
Third Test: Grab an empty plastic cup with larger dimensions; dimensions;
dimensions;
- - Fourth
FourthTest:
Test:Grab
Grabananegg; egg;
Appl. Sci. 2023, 13, 8002 9 of 17

Appl. Sci. 2023, 13, x FOR PEER REVIEW 10 of 19

- Fourth Test: Grab an egg;
- Fifth Test: Give a person a handshake.

-
The tests were carried out with objects of different weights, sizes and sensitivities.
Fifth Test: Give a person a handshake.
As this research is proposing the development of a sensory system, it also made sense to
The tests were carried out with objects of different weights, sizes and sensitivities. As
include a more human test that simulates human interaction, such as a simple handshake.
this research is proposing the development of a sensory system, it also made sense to in-
clude a more human test that simulates human interaction, such as a simple handshake.
5.1. First Test—Grab a Plastic Cup
For this
5.1. First experiment,
Test—Grab a Plasticthe
Cupprotocol design was quite simple and effective, allowing the
same For
steps
thistoexperiment,
be carriedthe
outprotocol
in all the other
design experiments.
was quite simple The steps were
and effective, as follows:
allowing the
1.samePlace
steps the
to beempty
carriedcup
outin
in the
all the
openother experiments.
prosthetic hand;The steps were as follows:
2.1. Move theempty
Place the thumb cupfinger
in thefirst;
open prosthetic hand;
3.2. Move
Move thethethumb
index,finger
middle,first;ring and little fingers at the same time;
4.3. Before
Move the index, the
moving middle, ringfurther,
fingers and little fingers
check at the
that the same
cup istime;
well positioned in the palm of
4. the
Before moving the
prosthetic fingers further, check that the cup is well positioned in the palm
hand;
5. of the prosthetic
Continue to movehand;the fingers, increasing the force applied to the cup;
5.
6. Continue to move
Lift the robotic handthe fingers,
with the increasing
cup; the force applied to the cup;
6. Lift the robotic hand with the cup;
7. If the cup falls, return to step 1;
7. If the cup falls, return to step 1;
8. If the cup is firm in the hand, take measurements and check the results.
8. If the cup is firm in the hand, take measurements and check the results.
The
The test isshown
test is shownininFigure
Figure 9 and
9 and thethe results
results are shown
are shown in Figure
in Figure 10 and10 and2.Table 2.
Table

Appl. Sci. 2023, 13, x FOR PEER REVIEW 11 of 19

Figure 9. Illustration of the first experiment, grab an empty plastic cup. (A) general view; (B) top
Figure 9. Illustration of the first experiment, grab an empty plastic cup. (A) general view; (B) top view.
view.

Figure
Figure10.
10.Representation
Representationof the activated
of the sensors
activated (blue(blue
sensors circles) that detected
circles) interactions
that detected in the in
interactions 1stthe 1st test.
test.

Table 2. Results obtained by the measurements taken from the 1st test.

Sensor Analog Reading Voltage (mv) FSR Resistance (Ω) Conductance (S) Force (N)
Appl. Sci. 2023, 13, 8002 10 of 17

Table 2. Results obtained by the measurements taken from the 1st test.

Sensor Analog Reading Voltage (mv) FSR Resistance (Ω) Conductance (S) Force (N)
A(1) 163 796 52,814 18 0
A(2) 0 0 0 0 NP *
A(3) 0 0 0 0 NP *
A(4) 0 0 0 0 NP *
A(5) 701 3431 4573 218 2
A(6) 918 4491 1133 882 10
A(7) 478 2336 11,404 87 1
A(8) 0 0 0 0 NP *
A(9) 558 2727 8335 119 1
A(10) 776 3792 3185 313 3
A(11) 0 0 0 0 NP *
A(12) 510 2492 10,064 99 1
A(13) 770 3763 3287 304 3
A(14) 0 0 0 0 NP *
A(15) 0 0 0 0 NP *
A(16) 0 0 0 0 NP *
* NP—No pressure.

Data analysis showed that there were more interactions in the middle, ring and little
fingers. These interactions are explained by the nature of the movement of the prosthetic
hand in relation to the plastic cup, causing these three fingers to support the cup. It is
logical that the interactions with the plastic cup would be felt where the cup interacted
most with the hand, as these fingers supported the cup to prevent it from falling. There
were eight active sensors and a maximum force of 10 N was recorded on sensor A(6), at the
tip of the middle finger. The average force recorded from the set of all interactions between
Appl. Sci. 2023, 13, x FOR PEER REVIEW 12 of 19
the glove and the object was 2.63 N.

5.2. Second Test—Grab the Same Plastic Cup with 150 mL of Water
The second test used the same cup as the first but added 150 mL of water to increase
The second test used the same cup as the first but added 150 mL of water to increase
the weight lifted by the hand. The protocol used was the same as in the first test, which
the weight lifted by the hand. The protocol used was the same as in the first test, which
made it possible to compare the results and observe differences in the interactions be-
made it possible to compare the results and observe differences in the interactions between
tween objects of the same shape but different weights. The results are shown in Figure 11
objects of the same shape but different weights. The results are shown in Figure 11 and
and Table 3.
Table 3.

Figure
Figure11.
11.Representation
Representationofofthe
theactivated
activatedsensors
sensors(blue
(bluecircles)
circles)ininthe
the2nd
2ndtest.
test.
The results of the second test are in line with expectations and confirm an increase
Table 3. Results obtained by the measurements taken from the 2nd test.
in the number of activated sensors, mainly in the areas of the middle, ring, little and
Sensor thumb fingers,
Analog Reading due to
Voltage a greater
(mv) FSRnumber of interactions
Resistance between(S)
(Ω) Conductance the cup Force
and the
(N)robotic
A(1) 194 948 42,742 23 0
A(2) 589 2878 7373 135 1
A(3) 0 0 0 0 NP *
A(4) 0 0 0 0 NP *
 Appl. Sci. 2023, 13, 8002 11 of 17

hand caused by the increased weight of the cup. The value of the measured forces also
increased compared to the results of the first test. The increased weight of the cup led to
an increase in the total forces detected along the sensory system and in the active sensors,
which recorded nine interactions. However, a greater distribution of forces was observed
and the maximum value, also in sensor A(6), reached 6 N. The average force was 2.22 N,
representing a decrease of ≈15% compared to the first test. This decrease can be explained
by the increase in the number of active sensors registering very low interactions (≈0 N)
which resulted in a greater distribution of the interactions felt along the sensory glove.
Table 3. Results obtained by the measurements taken from the 2nd test.

Sensor Analog Reading Voltage (mv) FSR Resistance (Ω) Conductance (S) Force (N)
A(1) 194 948 42,742 23 0
A(2) 589 2878 7373 135 1
A(3) 0 0 0 0 NP *
A(4) 0 0 0 0 NP *
Appl. Sci. 2023, 13, x FOR PEER REVIEW 13 of 1
A(5) 793 3875 2903 344 4
A(6) 853 4169 1993 501 6
A(7) 724 3538 4132 242 3
A(8) 0 0 0 0 NP *
recorded
A(9) nine159interactions. However,
777 a greater
54,350 distribution of18 forces was 0observed an
A(10) 695 3396 4723 211 2
the maximum 0value, also in sensor
A(11) 0 A(6), reached
0 6 N. The average
0 force was
NP * 2.22 N, rep
89
A(12)
resenting a decrease of ≈15%434 105,207
compared to the 9
first test. This decrease can be0 explained b
A(13) 813 3973 3584 386 4
the increase in 0the number of active
A(14) 0 sensors registering
0 very low0 interactions
NP *(≈0 N) whic
0 0 0
A(15)
resulted
A(16)
in a greater
0 distribution
0 of the interactions
0 felt along 00the sensory NP *
glove.
NP *
* NP—No pressure.
5.3. Third Test—Grab an Empty Plastic Cup with Larger Dimensions
5.3. Third Test—Grab an Empty Plastic Cup with Larger Dimensions
As in the previous tests, the same protocol was followed, with the exception of th
cupAs in the
used, previous
which hadtests, the same
a larger protocol
diameter was followed,
(Figure 12). Thewith the exception
increase of the
in cup size cupthe diffe
and
used, which had a larger diameter (Figure 12). The increase in cup size and the difference
ence in surface morphology predicted an increase in the number of activated sensors an
in surface morphology predicted an increase in the number of activated sensors and force
force values compared to the first test. The results are shown in Figure 13 and Table 4.
values compared to the first test. The results are shown in Figure 13 and Table 4.

Figure
Figure 12.12. Third
Third experiment
experiment withwith a larger
a larger plasticplastic cup.
cup. (A) (A) view;
lateral lateral(B)view; (B) top view.
top view.
The results showed an increase in the number of active sensors compared to the
first test, with eight sensors activated, mainly in the periphery and in the palm of the hand,
specifically sensors A(2) and A(16). This was a consequence of the increase in the diameter
of the cup, which led to an increase in the area of activity in the most central zone of the
palm. As the cup was empty and the only weight felt on the surface of the hand was the
Appl. Sci. 2023, 13, 8002 12 of 17

weight of the cup itself, the maximum force detected was relatively low, in the order of 6 N.
The average force was 1.5 N, a reduction of around 43% compared to the first test, which can
Figure
also be12.attributed
Third experiment with a number
to the greater larger plastic cup. (A)
of active lateral
sensors view;
that (B) topthe
provide view.
force distribution.

Figure
Figure13.
13.Representation
Representationof
ofthe
theactivated
activatedsensors
sensors(blue
(bluecircles)
circles)in
inthe
the3rd
3rdtest.
test.

Table 4.
Table Resultsobtained
4. Results obtainedby
by the
the measurements
measurements taken
taken from
from the
the 3rd
3rd test.
test.

Sensor Sensor
Analog Reading Analog Reading
Voltage (mv) Voltage (mv) FSR(Ω)
FSR Resistance Resistance (Ω)
ConductanceConductance
(S) (S)
Force Force (N)
(N)
A(1) 201 A(1) 201
982 982 40,916 40,916 24 24 0 0
A(2) 769 3758 3304 302 3
A(3) 0 0 0 0 NP *
A(4) 0 0 0 0 NP *
A(5) 866 4232 1814 551 6
A(6) 698 3411 4658 214 2
A(7) 0 0 0 0 NP *
A(8) 0 0 0 0 NP *
A(9) 176 860 48,139 20 0
A(10) 505 2468 10,256 97 1
A(11) 0 0 0 0 0
A(12) 623 3044 6425 155 1
A(13) 704 3440 4534 220 2
A(14) 0 0 0 0 NP *
A(15) 0 0 0 0 NP *
A(16) 41 200 24,000 4 0
* NP—No pressure.

5.4. Fourth Test—Grab an Egg

The purpose of this test was to evaluate the sensitivity of the robotic hand. In order to
grasp the egg without damaging it (Figure 14), the mechanical and sensory system had to
be well calibrated to precisely control the force exerted by the servomotor on the cables that
move the prosthetic fingers and detect small interactions and quantify small forces. For this
test, it was assumed that the number of active sensors would be smaller than in previous
tests and that the forces detected would be smaller. The results are shown in Figure 15 and
Table 5.
The results show a reduction in both the number of active sensors in the hand–object
interactions and the force value. The areas of interaction were small and very specific, with
greater activity in the middle, ring and thumb fingers. In terms of force levels, the thumb
A(2) and the middle fingertip A(6) dominated with 3 N and 4 N, respectively. The other
interactions were not significant enough to generate measurable force values, but the low
voltage and high resistance values indicate the detection of small interactions between the
 5.4. Fourth Test—Grab an Egg
The purpose of this test was to evaluate the sensitivity of the robotic hand. In orde
to grasp the egg without damaging it (Figure 14), the mechanical and sensory system ha
to be well calibrated to precisely control the force exerted by the servomotor on the cable
Appl. Sci. 2023, 13, 8002 13 of 17
that move the prosthetic fingers and detect small interactions and quantify small forces
For this test, it was assumed that the number of active sensors would be smaller than i
previous tests and that the forces detected would be smaller. The results are shown i
sensor and the object. This test also proved that the calibration of the servomotors allows
Figure 15 and Table 5.
fine and delicate movements.

Appl. Sci. 2023, 13, x FOR PEER REVIEW 15 of 19

Figure14.14.
Figure Illustration
Illustration of the
of the 4th test,
4th test, grabbing
grabbing an egg.
an egg.

Figure
Figure15.
15.Representation
Representationofofthe
theactivated
activatedsensors
sensors(blue
(bluecircles)
circles)ininthe
the4th
4thtest.
test.

Table
Table5.5.Results
Resultsobtained
obtainedby
bythe
themeasurements
measurementstaken
takenfrom
fromthe
the4th
4thtest.
test.

Sensor Analog Reading

Sensor Voltage
Analog (mv)
Reading FSR Resistance
Voltage (mv) (Ω)
FSR Conductance
Resistance (Ω) (S)
ConductanceForce
(S) (N)
Force (N)
A(1) 196 A(1) 196
957 95742,246 42,246 23 23 0 0
A(2) 757 3699 3517 284 3
A(2) 757 A(3) 3699
0 0 3517 0 284 0 3 NP *
0 0 0 0
A(3) 0 A(4) 0 0 0 NP* NP *
0 0 0 0
A(4) A(5)
0 A(6) 7990 3905 0 2804 0 356 NP* NP 4
*

A(5) 0 A(7) 1750 855 0 48,479 0 20 NP* 0

A(8) 0 0 0 0 NP *
A(6) 799 3905 2804 356 4
A(9) 147 718 59,637 16 0
A(7) 175 A(10) 855
168 82148,479 50,901 20 19 0 0
A(11) 0 0 0 0 NP *
A(8) 0 A(12) 1130 552 0 80,579 0 12 NP* 0
A(9) 147 A(13) 718
0 0 59,637 0 16 0 0 NP *
A(10) 168 A(14) 0
821 0 50,901 0 19 0 0 NP *
A(15) 0 0 0 0 NP *
A(11) 0 A(16) 00 0 0 0 0 0 NP* NP *
A(12) 113* NP—No pressure. 552 80,579 12 0
A(13) 0 0 0 0 NP*
A(14) 0 0 0 0 NP*
A(15) 0 0 0 0 NP*
A(16) 0 0 0 0 NP*
* NP—No pressure.

The results show a reduction in both the number of active sensors in the hand–object
 Appl. Sci. 2023, 13, 8002 14 of 17

Appl. Sci. 2023, 13, x FOR PEER REVIEW 16 of 1

5.5. Fifth Test—Give a Person a Handshake
Appl. Sci. 2023, 13, x FOR PEER REVIEW As the main focus of this research is the development of touch detection,16it of made
19
sense to develop a test that simulated one of the simplest human interactions, a handshake
(Figure 16). For this test, the expected result was a greater number of active sensors (Figur
(Figure 16). For this test, the expected result was a greater number of active sensors
17), as the human hand would be in contact with a superior surface area when interacting
(Figure 17), as the human hand would be in contact with a superior surface area when
with the
(Figure 16).sensory glove.
For this test, the Depending onwas
expected result the aforce exerted
greater numberbyofthe human
active hand
sensors on the sensory
(Figure
interacting with the sensory glove. Depending on the force exerted by the human hand on
17), as the human hand would be in contact with a superior surface area when 17interacting
the sensory glove and vice versa, the results of the forces detected varied. Figure 17Table
glove and vice versa, the results of the forces detected varied. Figure and and 6 show
with the sensory
representative glove. Depending
results. results. on the force exerted by the human hand on the sensory
Table 6 show representative
glove and vice versa, the results of the forces detected varied. Figure 17 and Table 6 show
representative results.

Figure
Figure
Figure16.16.
16. Illustration
Illustration
Illustration of5th
ofofthe
the the 5th
5thtest, test,
test,giving giving
giving a person
a aperson
person a handshake.
a ahandshake.
handshake.(A) top
(A) (A)
view;
top top
(B)
view; view;
lateral
(B) (B) lateral view.
view.
lateral view.

Figure
Figure17.
17.Representation
Representationofofthe
theactivated
activatedsensors
sensors(blue
(bluecircles)
circles)ininthe
the5th
5thtest.
test.

Table InResults
6. this test, it wasbyessential
obtained to control the force
theofmeasurements applied by the servomotor so that it
Figure 17. Representation the activatedtaken from
sensors the
(blue 5th test.
circles) in the 5th test.
was not so high as to harm the subject. By setting the forces produced by the servomotor
Sensor Analog Reading
to a minimum Voltage
value,(mv) FSR showed,
the results Resistanceas (Ω) Conductance
expected, that the (S)
number of Force (N)sensors
active
A(1) Table
200was 6. Results
greater obtained
977
than in by the
any other test,measurements
41,177 activity
showing taken from
in more the 5th test. areas 0(central and
24 generalized
A(2)
Sensor 787peripheral).
Analog Reading As Voltage
the
3846forces(mv)
applied toFSRthe
3000human hand(Ω)
Resistance were333 small, the forces
Conductance (S)measured
4 were (N)
Force
A(3) 488also small, in the order
2385 of 4 N. As in the
10,964 previous test, the
91 interactions detected
1 were of
A(1) 200
low intensity and,
977
therefore, could not
41,177
result in significant force
24
values. However, the
0
low
A(4) 684 3362 4872 205 2
A(2) 787 3846 3000 333 4
A(5) 0 stress values and, 0consequently, the high 0 resistance values 0 proved that there NP*were small
A(3)
A(6) 0488
interactions between 0 2385 10,964 hand and
the surface of the0 prosthetic 91
0 the object. NP* 1
A(4)
A(7) 3684 14 3362 3,561,4284872 0 205 0 2
A(5)
A(8) 00 0 0 0 0 0 0 NP* NP*
A(6)
A(9) 1570 767 0 55,189 0 18 0 0 NP*
A(10)
A(7) 03 0 14 0 3,561,428 0 0 NP* 0
Appl. Sci. 2023, 13, 8002 15 of 17

Table 6. Results obtained by the measurements taken from the 5th test.

Sensor Analog Reading Voltage (mv) FSR Resistance (Ω) Conductance (S) Force (N)
A(1) 200 977 41,177 24 0
A(2) 787 3846 3000 333 4
A(3) 488 2385 10,964 91 1
A(4) 684 3362 4872 205 2
A(5) 0 0 0 0 NP *
A(6) 0 0 0 0 NP *
A(7) 3 14 3,561,428 0 0
A(8) 0 0 0 0 NP *
A(9) 157 767 55,189 18 0
A(10) 0 0 0 0 NP *
A(11) 0 0 0 0 NP *
A(12) 82 400 115,000 8 0
A(13) 96 469 96,609 10 0
A(14) 0 0 0 0 NP *
A(15) 7 34 1,460,588 0 0
A(16) 0 0 0 0 0
* NP—No pressure.

6. Conclusions
The attribution of somatosensory properties (touch sensation) to an upper arm pros-
thesis, making both the fabrication of the robotic hand and its instrumentation (mechanical
and sensory) as affordable as possible, was successfully achieved.
Considering the model of the prosthesis used and the patient-centered methodology,
it was possible to achieve the highest quality and functionality of the robotic hand at the
lowest possible cost. In addition, by mimicking the design of the human arm, it was possible
to provide a system with functionality close to that of the human hand, giving the thumb
a greater range of movement. In terms of aesthetics, the prosthetic fingers and palm closely
mimicked the surface characteristics of the human hand, with uneven and rounded surfaces.
This was made possible by the use of 3D printing, which allowed the customization of
each parameter of the manufacturing phase (filling, ironing, layer weight, etc.), producing
a high-quality, low-cost prototype that could be easily instrumented and modified as
needed and for which the sensory system could be easily installed and removed.
The instrumentation of the mechanical system followed a simplistic approach, using
simple off-the-shelf components that could be easily replaced if damaged without signifi-
cantly increasing the cost. The programming of the overall mechanical system focused on
refining the motor control to allow more precise hand movements and, thus, control of the
force exerted by the prosthetic hand on the objects with which it interacts.
By defining the piezoresistive sensor as the main sensing element, the assembly of
the sensing glove and its application to the prosthetic hand were straightforward. As the
Arduino has only one ADC converter, the programming followed a select-sensor approach,
making it possible to locate and measure the interactions between the sensing glove and
the specific object at a given moment or continuously, millisecond by millisecond. As with
the mechanical system, in the event of malfunction or damage, any part of the sensing
glove can be easily replaced at a low cost to the user.
The overall test results provided the expected results and predictions, allowing the
design and construction of an advanced and cost-effective prosthetic system capable of
detecting touch and measuring force which can be designed and customized for each user
without increasing the overall cost and allows easy replacement of any part in the event
of damage or excessive use. The promising results of this research make this approach an
effective tool for the production of customizable prosthetic systems with the possibility of
interaction in both directions: patient–prosthesis and prosthesis–patient.
Appl. Sci. 2023, 13, 8002 16 of 17

Author Contributions: Conceptualization, G.F., J.N.-P. and A.P.S.; methodology, G.F., J.N.-P. and
A.P.S.; software, G.F.; validation, G.F., J.N.-P. and A.P.S.; formal analysis, J.N.-P. and A.P.S.; investiga-
tion, G.F.; writing—original draft preparation, G.F. and J.N.-P.; writing—review and editing, J.N.-P.
and A.P.S.; project administration, A.P.S. and J.N.-P.; funding acquisition, A.P.S. All authors have read
and agreed to the published version of the manuscript.
Funding: This research was funded by the Portuguese Foundation for Science and Technology, I.P.
(FCT, I.P.) FCT/MCTES through national funds (PIDDAC), under the R&D Unit C-MAST/Centre for
Mechanical and Aerospace Science and Technologies (Project UIDB/00151/2020). João Nunes-Pereira
would also like to thank FCT, I.P., for the contract under the Stimulus of Scientific Employment,
Individual Support: 2022.05613.CEECIND.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Acknowledgments: The authors are grateful for the support of the Portuguese Foundation for Science
and Technology, I.P. (FCT, I.P.) FCT/MCTES through national funds (PIDDAC), under the R&D Unit
C-MAST/Centre for Mechanical and Aerospace Science and Technologies (Project UIDB/00151/2020).
João Nunes-Pereira would also like to thank FCT, I.P., for the contract under the Stimulus of Scientific
Employment, Individual Support: 2022.05613.CEECIND.
Conflicts of Interest: The authors declare no conflict of interest.

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