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SMA Wire Bundles - Mechanical and Electrical Concepts

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Commercially available SMA actuated devices usually integrate only a single wire, which dimensions are determined by the force necessary to achieve a desired actuation. Usually, if higher forces are requested, a wire with a bigger diameter is chosen since bigger diameters correspond to higher forces, but, on the other hand, also to a smaller surface-per-volume ratio, which limits system dynamics. The use of bundle of small diameter SMA wires enables to achieve high forces depending on the number of wires in the bundle ensuring a high surface-per-volume ratio and thus a high bundle reactivity.
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ISBN 978-3-933339-30-0 © MESSE BREMEN
514
Oral Contributions
A1 Piezoactuators I
Technical Papers No. Page
P 37
SMA Wire Bundles – Mechanical and Electrical
Concepts
R. Britz, F. Welsch, S.-M. Kirsch
Universität des Saarlandes, Saarbrücken, Germany
F. Simone, Zentrum für Mechatronik und Automatisierungstechnik gGmbH, Saarbrücken, Germany
M. Schmidt, P. Motzki, S. Seelecke
Universität des Saarlandes, Saarbrücken, Germany, and Zentrum für Mechatronik und
Automatisierungstechnik gGmbH, Saarbrücken, Germany
Abstract:
Commercially available SMA actuated devices usually integrate only a single wire, which dimensions are
determined by the force necessary to achieve a desired actuation. Usually, if higher forces are requested, a wire
with a bigger diameter is chosen since bigger diameters correspond to higher forces, but, on the other hand, also
to a smaller surface-per-volume ratio, which limits system dynamics. The use of bundle of small diameter SMA
wires enables to achieve high forces depending on the number of wires in the bundle ensuring a high surface-
per-volume ratio and thus a high bundle reactivity.
Keywords: Shape Memory Alloys, Bundle, Mechanical Characterization, Electrical Characterization, Ni-Ti
Introduction
Shape Memory Alloys (SMAs) are known for bio
medical applications like stents [1], their capabilities
in seismic applications [2], elastocaloric cooling [3
5] and their high potential as lightweight integrated
actuators [6]. Their small size, associated to their
high energy density, make them suited to be
embedded in compact devices [1,2]. SMA based
actuator systems are already established in
automotive applications and consumer electronics.
Typical actuators are single Ni-Ti based SMA wires
with a diameter between 0.025 mm and 0.5 mm
[3,4]. These materials can be activated in a
controlled manner by joule heating. SMAs undergoe
a phase transformations when exposed to heat [3,7].
The cold wire elongates, when a tensile stress is
applied (Fig. 1). Joule heating leads to a temperature
increase of the wire, which results in a contraction.
The length change is based on a first order phase
transformation from a martensitic lattice structure to
a shorter austenitic lattice [11]. The length change
can be correlated to a change in electrical resistance
of the wire. The wire shows a linear correlation
between length and resistance change. This property
is often referred to as “self-sensing” [8,9].
SMA actuator systems always consist of an active
SMA element and a biasing mechanism like a spring
[14]. Most systems use a single SMA wire and in
order to adapt the wire force to the system
requirements a thicker SMA wire is chosen. This
design concept shows several disadvantages. The
system dynamics significantly decrease with
increasing wire diameter because they are mostly
influenced by the heat dissipation of the wire upon
cooling which decreases with increasing wire
diameter.
The use of multiple SMA wires in parallel allows for
a force increase without a decreasing dynamic
performance, as shown in a bistable SMA vacuum
suction cup [15]. An additional advantage is the
redundant system design, increasing its reliability.
However, design and mounting process of SMA
bundles are challenging since both, energy density
and electrical isolation between the wires are
required to ensure high system performance and
prevent electrical shortcuts. In addition, systematic
winding procedures which allows to mount each
wire under the same stress conditions have to be
developed. Another challenge is the wire clamping
system since classical mechanisms are inappropriate
for small size wires and welding-based procedures
for these materials are still focus of current research
[16].
Another field of research for SMA bundles is related
to the electrical connection of the wire sections,
which can either be in series or in parallel. Both
configurations show different advantages. A precise
control of the wire current is an advantage of a serial
connection strategy. A parallel connection shows the
advantage that the electrical isolation of the wires
can be omitted and even if one wire breaks the
system is still able to operate.
ACTUATOR 2018: 16th International Conference on New Actuators, Bremen, Germany, 25–27 June 2018
515
Oral Contributions
A1 Piezoactuators I
Technical Papers No. Page
© MESSE BREMEN ISBN 978-3-933339-30-0
SMA Wire Bundles – Mechanical and Electrical
Concepts
R. Britz, F. Welsch, S.-M. Kirsch
Universität des Saarlandes, Saarbrücken, Germany
F. Simone, Zentrum für Mechatronik und Automatisierungstechnik gGmbH, Saarbrücken, Germany
M. Schmidt, P. Motzki, S. Seelecke
Universität des Saarlandes, Saarbrücken, Germany, and Zentrum für Mechatronik und
Automatisierungstechnik gGmbH, Saarbrücken, Germany
Abstract:
Commercially available SMA actuated devices usually integrate only a single wire, which dimensions are
determined by the force necessary to achieve a desired actuation. Usually, if higher forces are requested, a wire
with a bigger diameter is chosen since bigger diameters correspond to higher forces, but, on the other hand, also
to a smaller surface-per-volume ratio, which limits system dynamics. The use of bundle of small diameter SMA
wires enables to achieve high forces depending on the number of wires in the bundle ensuring a high surface-
per-volume ratio and thus a high bundle reactivity.
Keywords: Shape Memory Alloys, Bundle, Mechanical Characterization, Electrical Characterization, Ni-Ti
Introduction
Shape Memory Alloys (SMAs) are known for bio
medical applications like stents [1], their capabilities
in seismic applications [2], elastocaloric cooling [3
5] and their high potential as lightweight integrated
actuators [6]. Their small size, associated to their
high energy density, make them suited to be
embedded in compact devices [1,2]. SMA based
actuator systems are already established in
automotive applications and consumer electronics.
Typical actuators are single Ni-Ti based SMA wires
with a diameter between 0.025 mm and 0.5 mm
[3,4]. These materials can be activated in a
controlled manner by joule heating. SMAs undergoe
a phase transformations when exposed to heat [3,7].
The cold wire elongates, when a tensile stress is
applied (Fig. 1). Joule heating leads to a temperature
increase of the wire, which results in a contraction.
The length change is based on a first order phase
transformation from a martensitic lattice structure to
a shorter austenitic lattice [11]. The length change
can be correlated to a change in electrical resistance
of the wire. The wire shows a linear correlation
between length and resistance change. This property
is often referred to as “self-sensing” [8,9].
SMA actuator systems always consist of an active
SMA element and a biasing mechanism like a spring
[14]. Most systems use a single SMA wire and in
order to adapt the wire force to the system
requirements a thicker SMA wire is chosen. This
design concept shows several disadvantages. The
system dynamics significantly decrease with
increasing wire diameter because they are mostly
influenced by the heat dissipation of the wire upon
cooling which decreases with increasing wire
diameter.
The use of multiple SMA wires in parallel allows for
a force increase without a decreasing dynamic
performance, as shown in a bistable SMA vacuum
suction cup [15]. An additional advantage is the
redundant system design, increasing its reliability.
However, design and mounting process of SMA
bundles are challenging since both, energy density
and electrical isolation between the wires are
required to ensure high system performance and
prevent electrical shortcuts. In addition, systematic
winding procedures which allows to mount each
wire under the same stress conditions have to be
developed. Another challenge is the wire clamping
system since classical mechanisms are inappropriate
for small size wires and welding-based procedures
for these materials are still focus of current research
[16].
Another field of research for SMA bundles is related
to the electrical connection of the wire sections,
which can either be in series or in parallel. Both
configurations show different advantages. A precise
control of the wire current is an advantage of a serial
connection strategy. A parallel connection shows the
advantage that the electrical isolation of the wires
can be omitted and even if one wire breaks the
system is still able to operate.
ACTUATOR 2018, MESSE BREMEN 2/4
Guidelines for Authors, May 2017
Figure 1: SMA actuation principle
In this paper, first experiments with SMA bundles in
electrical parallel configuration is presented. A test
bench to study bundles of SMA wires activated
under different power and load conditions has been
developed. The goal of this work is to investigate the
actuator performance of SMA bundles in electrical
and mechanical parallel configuration.
SMA electrical connection strategy
There are two possibilities to connect a bundle of
multiple SMA wires (Fig. 2). The serial connection
shows advantages in electrical activation. A
0.25 mm wire with a length of 100 mm has a
resistance of approximately 1.85 Ω and needs a
current of 1050 mA to be activated in approx.
1 s [10].
Figure 2: SMA electrical connection strategies
(left: serial; right: parallel)
To generate this current flow through the wire, a
voltage of 1.94 V is required. A serial connection of
5 SMA wires with a length of 100 mm leads to a
resistance increase by a factor of 5, applying the
same current the voltage increases also 5 times to
9.7 V. Power supplies with higher voltage and a
relatively low current output have good commercial
availability. In contrast, in the parallel connection, a
current of 5.25 A is required. The resistance of the
bundle is 5 times lower and therefore the voltage
remains at 1.94 V. In both cases, the equivalent
amount of energy is applied. However, the parallel
connection of multiple SMA wires results in a high
maximum current which might be challenging for
standard power supplies. This negative electrical
aspect of the parallel connection is compensated by
the improved mechanical properties. The system is
mechanically redundant and the failure of one wire
does not result into a breakdown of the system. The
defect can easily be detected by a resistance
measurement and the actuator bundle can be
replaced during a planned maintenance. A single
broken wire in a serial connected bundle would lead
to a system breakdown, because it is no longer
possible to drive a current trough the remaining
wires. A change of the bundle has to take place
immediately, whereas the parallel bundle could still
work for a particular time. Also the mechanical
construction of clamps are influenced by the chosen
electrical connection. In the case of a parallel bundle,
the mechanical design requirements are lower,
because no electrical isolation is needed.
Test Rig
The characterization of the SMA bundle requires the
development of a special test rig. Fig. 3 shows the
schematic design of the test rig. A load cell is
attached to measure the generated force of an SMA
bundle. A linear drive in force control mode loads
the SMA bundle to a defined pre-stress level. A
control algorithm allows to hold this pre-stress level
during activation of the SMA bundle or holding the
position in position control mode to measure the
blocking force. In both cases, the voltage over the
bundle and the current is measured via an NI
CompactRIO data acquisition system. In addition,
the system is equipped with an IR camera which
enables the measurement of the temperature
distribution of the bundles.
Figure 3: Schematic Test Rig
The temperature difference between the single wires
in the bundle indicates the homogeneity of the joule
heating which is correlated to the current flow in
ACTUATOR 2018: 16th International Conference on New Actuators, Bremen, Germany, 25–27 June 2018
ISBN 978-3-933339-30-0 © MESSE BREMEN
516
Oral Contributions
A1 Piezoactuators I
Technical Papers No. Page
ACTUATOR 2018, MESSE BREMEN 4/4
Guidelines for Authors, May 2017
Figure 6: Temperature distribution of SMA bundles, the
wires are placed in two rows, only the front row is monitored
by the IR camera.
Conclusion and Perspectives
Fist experimental results of SMA bundles in
mechanical and electrical parallel configuration has
been presented. The measurements show the
behaviour and the performance of SMA bundles in
electrical parallel connection. The results show that
with a parallel connection of the wires in a bundle,
the same stroke can be achieved as with a single
wire. The thermographic investigations show a
homogeneous temperature distribution in the bundle,
which turns out to be an important factor in the
functionality of SMA bundles. In order to determine
the amount of electrical energy for activation, the
thermal boundary conditions of the bundle has to be
taken into account also.
In a next step, a comparison between SMA bundles
in serial and parallel connection should be done. In
addition, the dynamic performance of the system has
to be investigated, the potential increase of the
operation frequency is the biggest advantage of the
bundle concept in comparison to single wire systems.
In Addition, an automatically winding procedure
will be developed to bundle the wire in a defined
and controlled manner under constant pre-load.
References
[1] J. Mohd Jani, M. Leary, A. Subic, and M. A.
Gibson, “A review of shape memory alloy
research, applications and opportunities,”
Mater. Des., vol. 56, 2014.
[2] M. Dolce and D. Cardone, “Mechanical
behaviour of shape memory alloys for seismic
applications 2. Austenite NiTi wires subjected
to tension,” Int. J. Mech. Sci., vol. 43, no. 11,
pp. 26572677, 2001.
[3] M. Schmidt, S.-M. Kirsch, S. Seelecke, and A.
Schütze, “Elastocaloric cooling: From
fundamental thermodynamics to solid state air
conditioning,” Sci. Technol. Built Environ.,
vol. 22, no. 5, pp. 475488, 2016.
[4] M. Schmidt, A. Schütze, and S. Seelecke,
“Scientific test setup for investigation of shape
memory alloy based elastocaloric cooling
processes,” Int. J. Refrig., vol. 54, pp. 8897,
2015.
[5] S. Qian et al., “A review of elastocaloric
cooling: Materials, cycles and system
integrations,” Int. J. Refrig., vol. 64, pp. 119,
2016.
[6] P. Motzki and S. Seelecke, “Systematic Design
of an SMA-based Actuator System for
Reconfigurable Robotic End Effectors.”
[7] H. Janocha, Unkonventionelle Aktoren - Eine
Einführung. Oldenburg Verlag, München,
2010.
[8] J. M. Hollerbach, I. Hunter, and J. Ballantyne,
“A comparative analysis of actuator
technologies for robotics,” The Robotics
Review, vol. 2. 1991.
[9] SAES Group, “SmartFlex ®,” SmartFlex
brochure. [Online]. Available:
www.saesgetters.com.
[10] Dynalloy Inc., “Technical characteristics of
flexinol actuator wires,” Dynalloy Inc., 2016.
[Online]. Available:
http://www.dynalloy.com/pdfs/TCF1140.pdf.
[11] J. Van Humbeeck, M. Chandrasekaran, and L.
Delaey, “Shape memory alloys: materials in
action,” Endeavour, vol. 15, no. 4, 1991.
[12] N. Lewis, A. York, and S. Seelecke,
“Experimental characterization of self-sensing
SMA actuators under controlled convective
cooling,” Smart Mater. Struct., vol. 22, no. 9,
2013.
[13] F. Simone, G. Rizzello, and S. Seelecke,
“Metal muscles and nerves - A self-sensing
SMA-actuated hand concept,” Smart Mater.
Struct., vol. 26, no. 9, 2017.
[14] M. Kohl, “Entwicklung von Mikroaktoren aus
Formgedächtnislegierungen,” no. April, 2002.
[15] S.-M. Kirsch, F. Welsch, M. Schmidt, P.
Motzki, and S. Seelecke, “Bistable SMA
Vacuum Suction Cup,” 2018.
[16] D. Scholtes, R. Zäh, M. Schmidt, P. Motzki, B.
Faupel, and S. Seelecke, “Resistance Welding
of NiTi Actuator Wires.”
ACTUATOR 2018, MESSE BREMEN 3/4
Guidelines for Authors, May 2017
each wire and the differences in wire resistance. Fig.
4 shows the final test rig including the SMA bundle.
Figure 4: Final Test Rig
Constant Force Measurement
The actuator performance of the SMA wire bundles
can be investigated by experiments at constant force.
During the experiment, the force is kept constant
while the applied joule heating is oscillating. This
results into a contraction of the wire with increasing
joule heating. The wires elongate again upon a
reduction of the joule heating. The IR measurements
show a homogeneous temperature distribution in the
SMA bundle (Fig. 6). Because the temperature
distribution correlates with the resistance of a single
wire, the measurement is an indicator for the current
distribution in the bundle. Varying the number of
wires shows the dependency of the current
distribution to the number of wires which influences
the performance of the SMA actuator system. In this
experiment SMA bundles with a wire diameter of
0.25 mm and a length of 100 mm are used. The
number of wires is decreased from 28 to 4 in
decrements of 4. Each bundle is loaded to a force
equivalent to a pre-stress of 200 MPa per wire
(Fig. 5: Red: 28 wires; Dark blue: 4 wires). A
current ramp with a rising and falling time of 90 s is
applied to activate the wires (Fig. 5, Current). The
peak of each current triangle corresponds to a
current of 0.39 A per wire to prevent overheating.
The maximum stroke of the bundles is about 4.5 %
which is the maximum actuation stroke of the SMA
wires. The conspicuous lower stroke of the bundle
with 4 and 8 wires is based on a lower maximum
temperature (Fig. 6). The lower temperature increase
results into a reduced amount of transformed
material. Although the current per wire is equivalent,
there is a lower indirect heating by the neighbour
wires. The influence of the indirect heating is also
shown in Fig. 6 (upper part). A temperature gradient
from the outer wires to the inner wires can be
observed. The temperature distribution is related to
an improved heat exchange with the environment of
the outer wires. The reduced amount of transformed
material can also be observed in the voltage signal
(Fig. 5, Voltage). The phase transformation
corresponds to a resistance change, which influences
the voltage drop over the wires, clearly visible in the
bundle of 28 wires. In contrast, the bundle of 4 wires
only shows a small change in the voltage signal.
The investigations showed a homogeneous
temperature distribution in all bundles, except for
the outer wires, independently of the number of
wires per bundle. The worse performance of the
bundle with 8 and 4 wires is related to the higher
thermal losses to the environment and can be
compensated by a higher current. For almost all
bundle configurations, the same stroke can be
achieved and the behaviour is equivalent to one
single wire with higher possible loads.
Figure 5: Measurement data with constant force
(Red: 28 wires; Dark blue: 4 wires).
ACTUATOR 2018: 16th International Conference on New Actuators, Bremen, Germany, 25–27 June 2018
517
Oral Contributions
A1 Piezoactuators I
Technical Papers No. Page
© MESSE BREMEN ISBN 978-3-933339-30-0
ACTUATOR 2018, MESSE BREMEN 4/4
Guidelines for Authors, May 2017
Figure 6: Temperature distribution of SMA bundles, the
wires are placed in two rows, only the front row is monitored
by the IR camera.
Conclusion and Perspectives
Fist experimental results of SMA bundles in
mechanical and electrical parallel configuration has
been presented. The measurements show the
behaviour and the performance of SMA bundles in
electrical parallel connection. The results show that
with a parallel connection of the wires in a bundle,
the same stroke can be achieved as with a single
wire. The thermographic investigations show a
homogeneous temperature distribution in the bundle,
which turns out to be an important factor in the
functionality of SMA bundles. In order to determine
the amount of electrical energy for activation, the
thermal boundary conditions of the bundle has to be
taken into account also.
In a next step, a comparison between SMA bundles
in serial and parallel connection should be done. In
addition, the dynamic performance of the system has
to be investigated, the potential increase of the
operation frequency is the biggest advantage of the
bundle concept in comparison to single wire systems.
In Addition, an automatically winding procedure
will be developed to bundle the wire in a defined
and controlled manner under constant pre-load.
References
[1] J. Mohd Jani, M. Leary, A. Subic, and M. A.
Gibson, “A review of shape memory alloy
research, applications and opportunities,”
Mater. Des., vol. 56, 2014.
[2] M. Dolce and D. Cardone, “Mechanical
behaviour of shape memory alloys for seismic
applications 2. Austenite NiTi wires subjected
to tension,” Int. J. Mech. Sci., vol. 43, no. 11,
pp. 26572677, 2001.
[3] M. Schmidt, S.-M. Kirsch, S. Seelecke, and A.
Schütze, “Elastocaloric cooling: From
fundamental thermodynamics to solid state air
conditioning,” Sci. Technol. Built Environ.,
vol. 22, no. 5, pp. 475488, 2016.
[4] M. Schmidt, A. Schütze, and S. Seelecke,
“Scientific test setup for investigation of shape
memory alloy based elastocaloric cooling
processes,” Int. J. Refrig., vol. 54, pp. 8897,
2015.
[5] S. Qian et al., “A review of elastocaloric
cooling: Materials, cycles and system
integrations,” Int. J. Refrig., vol. 64, pp. 119,
2016.
[6] P. Motzki and S. Seelecke, “Systematic Design
of an SMA-based Actuator System for
Reconfigurable Robotic End Effectors.”
[7] H. Janocha, Unkonventionelle Aktoren - Eine
Einführung. Oldenburg Verlag, München,
2010.
[8] J. M. Hollerbach, I. Hunter, and J. Ballantyne,
“A comparative analysis of actuator
technologies for robotics,” The Robotics
Review, vol. 2. 1991.
[9] SAES Group, “SmartFlex ®,” SmartFlex
brochure. [Online]. Available:
www.saesgetters.com.
[10] Dynalloy Inc., “Technical characteristics of
flexinol actuator wires,” Dynalloy Inc., 2016.
[Online]. Available:
http://www.dynalloy.com/pdfs/TCF1140.pdf.
[11] J. Van Humbeeck, M. Chandrasekaran, and L.
Delaey, “Shape memory alloys: materials in
action,” Endeavour, vol. 15, no. 4, 1991.
[12] N. Lewis, A. York, and S. Seelecke,
“Experimental characterization of self-sensing
SMA actuators under controlled convective
cooling,” Smart Mater. Struct., vol. 22, no. 9,
2013.
[13] F. Simone, G. Rizzello, and S. Seelecke,
“Metal muscles and nerves - A self-sensing
SMA-actuated hand concept,” Smart Mater.
Struct., vol. 26, no. 9, 2017.
[14] M. Kohl, “Entwicklung von Mikroaktoren aus
Formgedächtnislegierungen,” no. April, 2002.
[15] S.-M. Kirsch, F. Welsch, M. Schmidt, P.
Motzki, and S. Seelecke, “Bistable SMA
Vacuum Suction Cup,” 2018.
[16] D. Scholtes, R. Zäh, M. Schmidt, P. Motzki, B.
Faupel, and S. Seelecke, “Resistance Welding
of NiTi Actuator Wires.”
ACTUATOR 2018, MESSE BREMEN 3/4
Guidelines for Authors, May 2017
each wire and the differences in wire resistance. Fig.
4 shows the final test rig including the SMA bundle.
Figure 4: Final Test Rig
Constant Force Measurement
The actuator performance of the SMA wire bundles
can be investigated by experiments at constant force.
During the experiment, the force is kept constant
while the applied joule heating is oscillating. This
results into a contraction of the wire with increasing
joule heating. The wires elongate again upon a
reduction of the joule heating. The IR measurements
show a homogeneous temperature distribution in the
SMA bundle (Fig. 6). Because the temperature
distribution correlates with the resistance of a single
wire, the measurement is an indicator for the current
distribution in the bundle. Varying the number of
wires shows the dependency of the current
distribution to the number of wires which influences
the performance of the SMA actuator system. In this
experiment SMA bundles with a wire diameter of
0.25 mm and a length of 100 mm are used. The
number of wires is decreased from 28 to 4 in
decrements of 4. Each bundle is loaded to a force
equivalent to a pre-stress of 200 MPa per wire
(Fig. 5: Red: 28 wires; Dark blue: 4 wires). A
current ramp with a rising and falling time of 90 s is
applied to activate the wires (Fig. 5, Current). The
peak of each current triangle corresponds to a
current of 0.39 A per wire to prevent overheating.
The maximum stroke of the bundles is about 4.5 %
which is the maximum actuation stroke of the SMA
wires. The conspicuous lower stroke of the bundle
with 4 and 8 wires is based on a lower maximum
temperature (Fig. 6). The lower temperature increase
results into a reduced amount of transformed
material. Although the current per wire is equivalent,
there is a lower indirect heating by the neighbour
wires. The influence of the indirect heating is also
shown in Fig. 6 (upper part). A temperature gradient
from the outer wires to the inner wires can be
observed. The temperature distribution is related to
an improved heat exchange with the environment of
the outer wires. The reduced amount of transformed
material can also be observed in the voltage signal
(Fig. 5, Voltage). The phase transformation
corresponds to a resistance change, which influences
the voltage drop over the wires, clearly visible in the
bundle of 28 wires. In contrast, the bundle of 4 wires
only shows a small change in the voltage signal.
The investigations showed a homogeneous
temperature distribution in all bundles, except for
the outer wires, independently of the number of
wires per bundle. The worse performance of the
bundle with 8 and 4 wires is related to the higher
thermal losses to the environment and can be
compensated by a higher current. For almost all
bundle configurations, the same stroke can be
achieved and the behaviour is equivalent to one
single wire with higher possible loads.
Figure 5: Measurement data with constant force
(Red: 28 wires; Dark blue: 4 wires).
ACTUATOR 2018: 16th International Conference on New Actuators, Bremen, Germany, 25–27 June 2018
... To optimize the actuation mechanism a bundle of four 200 µm wires is used instead of a thick actuator. The resulting improved surface-to-volume ratio leads to faster cooling and increases the maximum possible actuation frequency [11]. Each bundle is attached to the connection bar via screws ( Figure 1C), to keep it in place. ...
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Pneumatic Systems, especially those working with compressed air, come with several drawbacks like poor energy efficiency, high level of emission and limited digitalization amongst others. Therefore, there are efforts in industry to replace pneumatics with purely electrical systems. A promising approach in gripping and handling technology is the shape memory alloy (SMA)-based vacuum suction cup, which was first presented at the 2018 SMASIS conference [1]. The working principle relies on an antagonistic SMA-based actuator system in combination with a bistable spring and a silicone membrane. This paper presents the structure and further development of the vacuum suction cup, whose vacuum generation is independent of a temporary or stable airflow. The focus lies on the improvement of the existing mechanics to a more maintenance-friendly design on route to a commercial product. For the implementation new SMA-bundles are created and the wire guides are adjusted accordingly. Besides, several experiments and analyses are carried out to validate the behavior of the mechanics with the new setup using the self-sensing effect. These investigations focus on the effects of varying bundle lengths, lever arms, and damaged bundles on performance. It is shown that the new design approach offers easier handling and robustness in the event of an actuator break.
... Mounted on the screw shaft (9), a screw slider (8) transforms rotation into linear displacement, which is connected to the primary clamp (7) located at the bottom of the eighteen NiTi wires, following a similar clamping design as predecessor prototypes, as shown in Figure 1B. 12,25 The top side of the NiTi wires is connected to the second clamp (3), which is mounted to a rail (1) and rail slider (2) assembly so that the horizontal movement of the two clamps can be synchronized using the two vertical linear bearings. A hot-side radiator as a heat sink (5) is fixed on one side of the NiTi wires with a cooling fan on top of it to reject the heat to the ambient. ...
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Elastocaloric cooling is regarded as one of the most promising cutting-edge alternatives to conventional vapor compression refrigeration systems. This technology is based on the temperature change of materials when being subjected to uniaxial stress, which has been observed in polymers, alloys, and ceramics. However, the existing elastocaloric prototypes have a bottleneck problem of an excessive mass ratio between the actuator and the solid-state refrigerant. To overcome this challenge, this study proposes an elastocaloric refrigerator using a single actuator with an inclined angle to produce a vertical tensile force to Ni-Ti shape-memory wires and a lateral motion to translate the Ni-Ti wires between the hot and cold sides. The refrigerator can achieve a 90% improvement in the mass ratio between the solid-state refrigerant and actuator compared to the currently best-reported elastocaloric cooling prototype. The Ni-Ti wires exhibit an adiabatic temperature change of 6.6 K during unloading at the strain of 4.8%. The proposed refrigerator can achieve a 9.2 K temperature span when the heat source and sink are insulated from the ambient and has the cooling power of up to 3.1 W under the zero-temperature-span condition. By using thinner Ni-Ti wires or Ni-Ti plates, the developed elastocaloric refrigerator could be a starting point to promote applications of this technology in the future.
... The dynamics of an SMA actuator depends on the cooling time of the wire, which is directly related to the ratio of surface to cross sectional area of the wire. To reach high actuation frequencies of up to 35 Hz small diameter wires have to be used and can be bundled to achieve high force outputs [8], [9]. In recent research SMA driven pick-and-place systems, robotic applications and medical devices have been developed [10]- [13]. ...
Conference Paper
Within industrial manufacturing, most processing steps are accompanied by transporting and positioning of workpieces. The active interfaces between handling system and workpiece are industrial grippers, which often are driven by pneumatics. On the way to better energy efficiency and digitalization, companies are looking for new actuation technologies with more sensor integration and higher efficiencies. Commonly used actuators like solenoids and electric engines are in many cases too heavy and large for direct integration into the gripping system. Shape memory alloy (SMA) actuators are suited to overcome those drawbacks of conventional actuation systems, because of their high energy density. Additionally, they feature self-sensing abilities that lead to sensor-less monitoring and control of the actuator element. Another drawback of conventional grippers is their design, which is based on moving parts with linear guides and bearings. These parts are prone to wear, especially in abrasive environments. This can be improved by a compliant gripper design that is based on flexure hinges and thus dispenses with joints, bearings and guides. In the presented work, the development process of a functional prototype for a compliant gripper driven by a bistable SMA actuator for industrial applications is outlined. The focus lies on the development of the compliant kinematics, where first results of FEM simulation are discussed. As a result, a working gripper-prototype which is manufactured with modern 3D-printing technologies is introduced.
... Besides for elastocaloric applications, SMA wire bundles are also used in the field of actuation [18]. The elastocaloric bundle arrangement was developed by Kirsch et. ...
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As an eco-friendly alternative, elastocaloric cooling promises to become an alternative to the prevalent vapor compression-based process. In the past years, many elastocaloric cooling devices have been developed. One of the most promising ones by Kirsch et al., which utilizes thin NiTiCo shape memory alloy (SMA) wires in bundle arrangements to perform continuous elastocaloric air cooling. So far, the air-cooling behavior of single wires has been analyzed and intensive parameter studies on different wire diameters and airflow rates have been performed. Based on these experiments, this work aims at analyzing how the bundle arrangement influences the air temperature changes ΔTair as well as the achievable cooling power. In addition, the air conditioning performance of the wire bundle is compared to the performance achieved by single SMA wires to estimate the potential impact of the bundle arrangement on the thermal power. The results show that the influence of the wire bundle on ΔTair differs from the 0.2 mm single wire indicated by a worse heat transfer from the wire bundle to air similar to a single wire with a larger diameter (e.g. 0.5 mm), presumably caused by the mutual interaction of the wires. In terms of cooling power, a 30x0.2 mm wire bundle leads to an increase by a factor of 12.5-20 compared to a single 0.2 mm wire.
... The dynamics of an SMA actuator depends on the cooling time of the wire, which is directly related to the ratio of surface to cross sectional area of the wire. Because of this, bundling many thin wires to one actuator system results in similar force output like one equivalent wire with a large diameter, but can reach actuation frequencies of up to 35 Hz [14], [15]. To create energy efficient systems driven by SMAs, the heat transfer from the SMA wire to its environment must be minimized. ...
Conference Paper
Full-text available
Within industrial manufacturing most processing steps are accompanied by transporting and positioning of workpieces. The active interfaces between handling system and workpiece are industrial grippers, which often are driven by pneumatics, especially in small scale areas. On the way to higher energy efficiency and digital factories, companies are looking for new actuation technologies with more sensor integration and better efficiencies. Commonly used actuators like solenoids and electric engines are in many cases too heavy and large for direct integration into the gripping system. Due to their high energy density shape memory alloys (SMA) are suited to overcome those drawbacks of conventional actuators. Additionally, they feature self-sensing abilities that lead to sensor-less monitoring and control of the actuation system. Another drawback of conventional grippers is their design, which is based on moving parts with linear guides and bearings. These parts are prone to wear, especially in abrasive environments. This can be overcome by a compliant gripper design that is based on flexure hinges and thus dispenses with joints, bearings and guides. In the presented work, the development process of a functional prototype for a compliant gripper driven by a bistable SMA actuation unit for industrial applications is outlined. The focus lies on the development of the SMA actuator, while the first design approach for the compliant gripper mechanism with solid state joints is proposed. The result is a working gripper-prototype which is mainly made of 3D-printed parts. First results of validation experiments are discussed.
... To achieve a dynamic range of 1 Hz and faster at still air, which is requested for many applications, wire diameters of 100 µm and less are required. However, in order to combine high dynamics with high force output, several wires can be bundled to one actuator module [13]. ...
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As a smart material thermal shape memory alloys (SMAs) feature actuator behavior combined with self-sensing capabilities. With their high energy density and design flexibility they are predestined to be used in soft robotics and the emerging field of morphing surfaces. Such shape changing surfaces can be used for novel human-machine interaction (HMI) elements based on mode-/situation-dependent interfaces that may be applied to all kind of machines, appliances and smart home devices as well as automotive interiors. Since many of those contain textile surfaces, it is of special interest to place SMA-based actuator-sensor-elements beneath a textile cover or integrated them in the textile itself. In this study, the unique features of SMAs are used to design a system which represents an active "morphing" button. It can lower into the surface it is integrated in, pops up to be used and shows a proportional signal output depending on the pushing stroke. The system is characterized concerning haptics and sensor technology. The button consists of a TPU structure, to which two NiTi wires are attached. When activated, the SMAs contract and the structure curves upwards. The user can now push on the device to use it as a button. In the future, the use of SMA wires and for example TPU fibers enables direct integration in the production process of a possible smart and functional textile.
Article
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The attractive properties of shape memory alloys (SMA), especially their high energy density, steadily expand the range of applications in which SMA wires represent attractive alternative actuator components. Industrial applications in particular, oftentimes require large forces, which scale with the diameter of SMA actuator wires. The higher the required actuator force, the larger the total effective cross-sectional area of the SMA wires is needed. Increasing the SMA wire diameter results in worse actuator dynamics due to a decreasing surface-to-volume ratio and thus slower convective cooling. Instead of simply increasing the wire diameter, the bundling of several thin wires offers a suitable alternative to generate higher forces. This results in an increased surface-to-volume ratio and thus permits higher dynamic system performance. This paper discusses the mechanical and thermal behavior of SMA wire bundles and shows the influence of contact resistance inside the clamps, which is important to optimize for a long-lasting functionality of SMA bundles. In addition, the experiments show an indirect heating between single SMA wires inside a bundle, which leads to an energy saving capability up to 60%. Two electro-mechanical experiments show the behavior of SMA bundles in a constant force and a constant strain measurement. The experimental results show a maximum stroke of 4-4.5% and a generated maximum force of 1200 N. The results are discussed to provide an understanding of the mechanical characteristics of SMA bundles. In addition, an infrared (IR)-Camera provides an insight into the thermal behavior.
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In der industriellen Fertigung sind die meisten Bearbeitungsschritte mit dem Transportieren und Positionieren von Werkstücken verbunden. Die aktiven Schnittstellen zwischen Handhabungssystem und Werkstück sind industrielle Greifer, die vor allem im kleinteiligen Bereich oft pneumatisch angetrieben werden. Auf dem Weg zu höherer Energieeffizienz und digitalen Fabriken sind Unternehmen auf der Suche nach neuen Antriebstechnologien mit mehr Sensorintegration und besseren Wirkungsgraden. Gängige Aktoren wie Magnete und Elektromotoren sind in vielen Fällen zu schwer und groß für eine direkte Integration in das Greifsystem. Aufgrund ihrer hohen Energiedichte sind Formgedächtnislegierungen (FGL) geeignet, diese Nachteile herkömmlicher Aktuatoren zu überwinden. Zusätzlich verfügen sie über sogenannte Self-Sensing Fähigkeiten, die zu einer sensorlosen Überwachung und Steuerung des Antriebssystems führen. Ein weiterer Nachteil konventioneller Greifer ist ihr Aufbau, der auf beweglichen Teilen, die besonders in abrasiven Umgebungen schnell verschleißen. Dies kann durch Festkörpergelenke, die konventionelle Gelenke ersetzen, vermieden werden. In der vorliegenden Arbeit wird der Entwicklungsprozess eines Funktionsprototyps für einen elastischen Greifer, der von einer bistabilen FGL-Antriebseinheit angetrieben wird, für industrielle Anwendungen umrissen. Der Schwerpunkt liegt auf der Entwicklung des FGL-Antriebs, während ein erster Designansatz für den nachgiebigen Greifmechanismus mit Festkörpergelenken vorgestellt wird. Das Ergebnis ist ein funktionierender Greifer-Prototyp, der hauptsächlich aus 3D-gedruckten Teilen besteht. Erste Ergebnisse von Validierungsversuchen werden diskutiert. Abstract Within industrial manufacturing most processing steps are accompanied by transporting and positioning of workpieces. The active interfaces between handling system and workpiece are industrial grippers, which often are driven by pneumatics, especially in small scale areas. On the way to higher energy efficiency and digital factories, companies are looking for new actuation technologies with more sensor integration and better efficiencies. Commonly used actuators like solenoids and electric engines are in many cases too heavy and large for direct integration into the gripping system. Due to their high energy density shape memory alloys (SMA) are suited to overcome those drawbacks of conventional actuators. Additionally, they feature self-sensing abilities that lead to sensor-less monitoring and control of the actuation system. Another drawback of conventional grippers is their design, which is based on moving parts, are prone to wear, especially in abrasive environments. This can be overcome by flexure hinges that dispense with bearings and guides. In the presented work, the development process of a functional prototype for a compliant gripper driven by a bistable SMA actuation unit for industrial applications is outlined. The focus lies on the development of the SMA actuator, while the first design approach for the compliant gripper mechanism with solid state joints is proposed. The result is a working gripper-prototype which is mainly made of 3D-printed parts. First results of validation experiments are discussed.
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Shape Memory Alloy (SMA) wires are a promising actuator technology, especially for designing lightweight and small systems, due to their high energy density. Their working principle is based on the fact that they contract when heated, which is typically done via an electrical current. State-of-the-art bonding technologies of SMA wires are crimping and clamping. Miniaturizing actuator systems and the complexity of the crimping process are two problematic fields for these technologies. Other possible methods are gluing, soldering and laser welding. None of these have established in the industrial field until now. To add another alternative to these joining mechanisms, the presented work focuses on resistance welding of NiTi actuator wires to stainless steel. The examined SMA wires have diameters of 25 to 100 µm. The welding parameters, the strength of the weld seam and the thermomechanical behavior of the welded samples are part of the investigation. The results show promising breaking loads of the welded samples and no obvious effect on the actuator behavior.
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Bio-inspired hand-like grippers actuated by Shape Memory Alloy (SMA) wires represent an emerging new technology with potential applications in many different fields, ranging from industrial assembly processes to biomedical applications. The inherently high energy density makes SMAs a natural choice for compact, lightweight, and silent actuator systems capable of producing a high amount of work, such as hand prostheses or robotic systems in industrial human/machine environments. In this work, a concept for a compact and versatile gripping system is developed, in which SMA wires are implemented as antagonistic muscles actuating an artificial hand with three fingers. In order to combine high gripping force with sufficient actuation speed, the muscle implementation pursues a multi-wire concept with several 0.1 mm diameter NiTi wires connected in parallel, in order to increase the surface-to-volume ratio for accelerated cooling. The paper starts with an illustration of the design concept of an individual 3-phalanx-finger, along with kinematic considerations for optimal placement of SMA wires. Three identical fingers are subsequently fabricated via 3-D printing and assembled into a hand-like gripper. The maximum displacement of each finger phalanx is measured, and an average phalanxes dynamic responsiveness is evaluated. SMA self-sensing is documented by experiments relating the wires change in resistance to the finger motion. Several finger force measurements are also performed. The versatility of the gripper is finally documented by displaying a variety of achievable grasping configurations.
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Shape memory alloys (SMAs) belong to a class of shape memory materials (SMMs), which have the ability to ‘memorise’ or retain their previous form when subjected to certain stimulus such as thermomechanical or magnetic variations. SMAs have drawn significant attention and interest in recent years in a broad range of commercial applications, due to their unique and superior properties; this commercial development has been supported by fundamental and applied research studies. This work describes the attributes of SMAs that make them ideally suited to actuators in various applications, and addresses their associated limitations to clarify the design challenges faced by SMA developers. This work provides a timely review of recent SMA research and commercial applications, with over 100 state-of-the-art patents; which are categorised against relevant commercial domains and rated according to design objectives of relevance to these domains (particularly automotive, aerospace, robotic and biomedical). Although this work presents an extensive review of SMAs, other categories of SMMs are also discussed; including a historical overview, summary of recent advances and new application opportunities.
Conference Paper
Shape memory alloy (SMA) materials represent an attractive technology in the field of actuator mechanisms for the realization of low cost and lightweight actuators and sensors. This article presents an innovative SMA actuated bistable vacuum suction cup. The sealed, compact and fully integrated design enables the positioning and transport of different components in mobile and stationary applications. The bistable actuator mechanism based on SMA wires combined with a bistable spring represent an energy efficient, noiseless gripping system without the need for compressed air. Fail-safe applications are enabled by the wattless sustained vacuum due to the bistable mechanism. Additionally, the self-sensing effect of the SMA enables a sensorless condition monitoring and energy efficient control. The gripping state is reported to the superordinate process control by the integrated electronics and visualized by the integrated LEDs.
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This paper discusses fundamental thermodynamic concepts as well as experimental investigations of elastocaloric cooling processes and presents a concept of a potential elastocaloric air conditioning device. Various cooling cycles suitable for elastocaloric cooling are introduced and the process efficiencies are determined based on a graphical approach. The graphical method is validated experimentally with a specially designed scientific test setup, which enables the measurement of mechanical and thermal process quantities. The material, a newly developed quaternary Ni-Ti-Cu-V alloy, is investigated in various thermodynamic cycles and an advanced cycle control is applied to increase the process efficiency. In addition, the influence of the thermal boundary conditions on the material and system efficiency is investigated. The results are compared with the values predicted by the graphical approach. Furthermore, a concept of a continuously operating elastocaloric air cooling device is introduced.
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Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be promising alternatives to conventional compression refrigeration. A novel solid state elastocaloric refrigeration process using the large latent heats of Shape Memory Alloys (SMA), specifically NiTi, could also hold potential in this field. This paper describes the development of a scientific test setup to investigate shape memory alloy based cooling processes. The setup allows for an independent control of the different variables, e.g. strain, strain rate etc., which influence the process functions. An integrated multi sensor system is used for synchronized measurement of mechanical and thermal quantities. First experiments demonstrate the control options of the test platform and the effect of each control variable on the elastocaloric cooling process.
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Shape memory alloy (SMA) wires are attractive for actuation systems due to their high energy density, light weight and silent operation. In addition, they feature self-sensing capabilities by relating electrical resistance measurements to strain changes. In real world applications SMAs typically operate in non-ambient air and it is imperative to understand an actuator's behavior under varying convective cooling conditions, especially for smaller diameter wires, where convective effects are amplified. This paper shows that the multi-functionality of SMA actuators can be further extended by related heating power to convective air speed. It investigates the relationship between the normalized excess power needed and corresponding airspeed under controlled, laminar airflow patterns in a small-scale wind tunnel. For each experiment, airflow through the wind tunnel, strain in the SMA wire, and power supplied to the SMA wire were controlled, while the stress and resistance of the wire were measured. The ability to understand and predict an SMA wire's behavior under various external airflows will aid in the design and understanding of future SMA actuated structures, such as micro-air vehicles, and shows that SMAs can function as self-sensing actuators and airspeed sensors.
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