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SMA Antagonistic-Micro-Wire Bundle: First Measurement Results

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Shape memory alloys (SMA) are used as an attractive technology in the field of actuators and sensors due to their versatile geometry, lightweight, high energy density, and low cost. The thermal activation principle, however, makes SMA application generally suitable for low-frequency (few Hz) regimes. In this work, a novel SMA-based antagonistic actuation system and its manufacturing process are presented for the first time. The main feature of the novel actuator concept is the possibility of being operated at frequencies up to at least 20 Hz. It spares the usual complex and time-consuming manufacturing of such a system. First parameter studies of a rotary actuation system are performed. The relationship existing between the pulse energy, frequency, and the resulting rotation angle is investigated through an extensive experimental campaign.
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SMA ANTAGONISTIC-MICRO-WIRE BUNDLE: FIRST MEASUREMENT RESULTS
Susanne-Marie Kirsch1, Felix Welsch1, Domenico Bevilacqua2, David Naso2, Stefan Seelecke1, Gianluca
Rizzello1, Paul Motzki1
1Department of Systems Engineering, Department of Materials Science and Engineering, Saarland University,
66123 Saarbruecken, Germany, susanne-marie.kirsch@imsl.uni-saarland.de
2Department of Electrical and Information Engineering, Polytechnic University of Bari, Bari, Italy,
david.naso@poliba.it
ABSTRACT
Shape memory alloys (SMA) are used as an attractive
technology in the field of actuators and sensors due to their
versatile geometry, lightweight, high energy density, and low
cost. The thermal activation principle, however, makes SMA
application generally suitable for low-frequency (few Hz)
regimes. In this work, a novel SMA-based antagonistic actuation
system and its manufacturing process are presented for the first
time. The main feature of the novel actuator concept is the
possibility of being operated at frequencies up to at least 20 Hz.
It spares the usual complex and time-consuming manufacturing
of such a system. First parameter studies of a rotary actuation
system are performed. The relationship existing between the
pulse energy, frequency, and the resulting rotation angle is
investigated through an extensive experimental campaign.
Keywords: Shape memory alloy, Antagonistic system,
Cyclic actuation frequency, High actuation frequency
1. INTRODUCTION
In the field of actuation systems, SMAs represent an
attractive technology because of their versatile geometry, low
weight, high energy density, and low cost [1], [2]. In recent
years, SMA-based systems have been successfully applied in
many technical fields, such as aerospace and automotive
industry, medicine and building technology, and more recently
in the area of environmentally-friendly cooling [3][6]. The
desired application defines the design criteria of the SMA
actuator system during development. Due to the thermal
actuation, SMA applications are currently mostly considered for
low-frequency ranges below 10 Hz. Higher actuation
frequencies can be achieved by replacing one thick SMA
element with several thinner elements. In this way, thanks to the
improved surface-to-volume ratio, it is possible to enhance the
heat exchange between SMA and environment and thus the
actuation frequency. This concept is well known in the literature
[7][10].
Different criteria need to be considered when designing
SMA actuator systems, such as installation space, actuation
frequency, force, stroke, and choice between continuous or
discrete switching behavior. For repeated actuation cycles, an
SMA actuator system needs a restoring mechanism, such as a
spring or a second SMA element. When using a spring, the
restoring time depends strongly on external conditions, such as
ambient temperature and airflow, which impact the SMA cooling
behavior in a significant way. In general, the designer needs to
decide whether to favor either frequency or stroke. When one is
interested in achieving high performance in terms of both stroke
and frequency, together with the possibility of a bi-directional
actuation, an antagonistic system is usually preferred. In this type
of system, two SMA elements interact with each other in a
protagonist-antagonist configuration. In addition to the improved
dynamic performance, the use of two SMA elements enables
intermediate positions to be held passively, thus resulting in an
energy-saving feature.
The efficient and reliable functioning of such a system
requires the precise combination of both SMA elements in a
defined stress and strain state. Currently, these antagonistic
systems are assembled with relatively high effort. A novel
bundle concept is needed to replace the complex, time-
consuming, and error-prone manufacturing process.
In this contribution, an antagonistic micro-wire bundle and
its associated manufacturing process are presented for the first
time.
2. CONCEPT AND PRODUCTION METHOD
The novel concept separates a simple SMA arrangement
into two identical sections, which then build the antagonistic
framework. [11] Figure 1 shows a bundle of micro SMA wires,
which is divided at the separation point in elements 1 and 2 to
form the antagonistic structure. A straight layout enables a lateral
actuation motion of the separation point. A rotational movement
can be realized by designing the separation point as a pulley-like
structure.
Proceedings of the ASME 2020 Conference on Smart Materials,
Adaptive Structures and Intelligent Systems
SMASIS2020
September 15, 2020, Virtual, Online
SMASIS2020-2261
1
Copyright © 2020 ASME
Attendee Read-Only Copy
FIGURE 1: SCHEMATIC VIEW OF THE NOVEL
ROTARY ANTAGONISTIC SMA SYSTEM. [11].
The actuator generates a rotary motion by interacting with
the lever arm of a pulley, as shown in Figure 2.
FIGURE 2: SCHEMATIC VIEW OF THE PULLEY-LIKE
SEPARATION POINT.
With the effective lever arm , the maximum SMA
strain  and the free SMA element length , the maximum
rotation angle can be calculated with ((1).
   
(
(1)
To realize high actuation frequencies, each SMA element
consists of numerous thin wires. This concept uses a single SMA
wire, which is multiple times winded around the clamping pins
on both sides, as shown in Figure 3. After the winding process,
a third clamp is mounted precisely in the middle of the newly
formed bundle to create elements 1 and 2.
FIGURE 3: SCHEMATIC VIEW OF THE NOVEL
PRODUCTION MECHANISM FOR A MICROWIRE
ANTAGONISTIC SYSTEM.
During the winding process, the SMA wire tension and
elongation, and thus the phase composition, are controlled to
desired values. The SMA wire sections in the bundle are
individually connected to the three clamping points to maintain
the defined phase composition. This procedure strongly reduces
the manufacturing and assembly work, as well as increases
reproducibility and reliability, compared to conventional
processes using numerous individual wire pieces.
In addition, the lifetime and the performance of the system
are increased since all the SMA wire sections are subjected to
the same strain state.
Further, the integration of mechanical clamping and
electrical connection significantly reduces the required space.
3. ELECTRICAL CONTROL
The individual SMA wires in each element are electrically
connected in parallel by using a conductive clamping
mechanism. Therefore, a common positive or negative potential
in the middle can be used for both actuator elements.
The motion of the antagonistic system is controlled by a
variable electrical signal generating Joule heating. In the case of
the presented system, a rectangle pulse signal is used. Figure 4
introduces a number of parameters which define the control
signals, i.e., cycle time , power-on time , power-off time ,
time shift , and power value
 during the on-time.
FIGURE 4: SCHEMATIC VIEW OF THE PULSE SIGNAL
FOR THE ANTAGONISTIC ACTUATION SYSTEM.
Signal 1 and Signal 2 are used to control the first and second
actuator elements, respectively. These two signals are coupled
by the time shift . The break time , in which the SMA
element cools down before the other element will be actuated, is
calculated with ((2).
    
In the case of Signal 1 and Signal 2 are similar,  can be
calculated with ((3).

SMAs
Separation
point

SMAs
1st element 2nd element
SMA clamp
Pulley
 

Signal 1
Signal 2



2
Copyright © 2020 ASME
The cycle frequency is computed from the period time
with ((4).
 
(
(4)
Finally,  can be converted into the duty cycle  with
((5).
 
(
(5)
4. REALIZATION
Figure 5 illustrates the manufacturing process of the first
realized bundle. The SMA wire consists of a conventional 90°C
actuator material with a diameter of 25 µm. In Figure 5a, the
spool of the winding machine, with two collet chucks on both
ends, is holding the winding pins in position. The positioning
stage in the middle supporting the center clamp is placed after
the winding process. Figure 5b/d show microscopic images of
the grooved winding pins with 10 windings of the SMA wire.
Figure 5c presents the mounted center clamping with a total of
20 wires between the pins. The wires are clamped by a friction-
based connection consisting of a V-groove combined with a
press-fitted pin.
FIGURE 5: MANUFACTURING PROCESS OF THE
NOVEL ANTAGONISTIC SMA ACTUATOR.
After the manufacturing process, the bundle is mounted to
its mechanical application. The mechanical framework,
consisting of two carbon tubes connected by a rotational joint
and the mounted actuator, is shown in Figure 6. The diameter of
the pulley is chosen to 3 mm. By considering a maximum strain
of 2.5 % and the active austenite element length of 55 mm, the
reachable rotation angle is calculated with ((1) to approximately
53°.
FIGURE 6: MECHANICAL FRAMEWORK TO TEST THE
NOVEL ROTARY ANTAGONISTIC SMA SYSTEM.
4.1. Experimental Setup
The presented realized antagonistic actuation system is
tested in a laboratory test setup, as shown in Figure 7. The
actuator is powered by a laboratory power supply (Hameg 4030),
which is modulated by a waveform generator (Agilent 33522A).
The motion angle of the actuator is captured by an optical camera
(iDS UI-3360CP-M-GL) with a framerate of 350 fps. In the
performed experiments, the waveform generator specifies the
current for each SMA element, generating the heating power
    . The currents and , as well as the voltages
and , are acquired via an oscilloscope (Agilent DSO-X
2004A).
FIGURE 7: LABORATORY TEST SETUP FOR THE
MICRO-WIRE ANTAGONISTIC ACTUATION SYSTEM.
4.2. First results
In the presented parameter sweep, both elements of the
antagonistic system are controlled symmetrically, using equal
but time-shifted signals with frequency , duty cycle , and
current . Table 1 presents the parameters used in the parameter
sweep.
a)
b) c) d)
20x 25µm-SMAs 1st element
2nd element
SMA clamp
Pulley = 3 mm
= 55 mm
3 carbon tube
= 9 mm
= 6 mm
Oscilloscope
Waveform-
generator
Power-
supply
Optical
camera
Actuator
Current
probe
3
Copyright © 2020 ASME
TABLE 1: PARAMETER SWEEP
Number
Frequency
(Hz)
Current
(A)
Duty cycle
(%)
Rotation
angle (°)
1 *
10
1.50
25
51
2 *
15
1.50
25
42
3 *
20
1.50
25
31
4 *
20
1.75
25
46
5 *
20
2.00
25
21
* conducted with cooling air (5 m/s)
The parameter sweep is conducted by changing the
parameters manually. Figure 8 shows the maximum deflection in
both directions by presenting overlapped pictures from
experiment 1. A nearly symmetrical motion range can be
observed.
FIGURE 8: OVERLAYED PICTURES AT MAXIMUM
POSITIONS DURING EXPERIMENT 1.
Experiments 1 to 3 are conducted without airflow, a current
of 1.5 A and a duty cycle of 25 %. The resulting rotation angle
decreases from 51° to 31° with increasing actuation frequency
from 10 Hz to 20 Hz, due to the reduced cooling time of the
antagonistic sections. Based on this result, in experiments 4 and
5 the heat transfer for faster cooling and the heating power are
increased. The homogeneous air crossflow of approx. 5 m/s is
provided by a radial fan (ebmpapst RL65-21/14H).
Experiment 4 with a current of 1.75 A shows a higher
rotation angle of 46° compared to experiment 3, due to the
enhanced cooling. Experiment 5 shows that despite the
increasing current of 2 A, a lower rotation angle is achieved. This
is because the SMAs are overheated far above their
transformation temperature. As a result, a higher cooling time is
also required. The adopted break time turns out to be insufficient
to cool down one section before the other section is activated.
These experiments demonstrate that a continuous actuation
with a frequency of 20 Hz and large movement is feasible when
using an antagonistic SMA actuator system based on thin wires.
4.3. Application
The first application for the antagonistic actuation system is
the integration into a flapping micro air vehicle like a bat, as
presented in Figure 9. The left-hand side shows the concept as
CAD model, on the right-hand side is the realized wing with two
actuator systems is presented. Currently one half of the bat model
is assembled to evaluate and improve the kinematic behavior.
FIGURE 9: SCHEMATIC VIEW AND PHOTOGRAPHY
OF THE BAT MODEL.
The bat model consists of a 3D-printed body and a skeleton,
based on carbon fiber tubes with 3D-printed joints. The wings
exhibit two degrees of freedom with movable elbow and
shoulder joint, as depicted in Figure 10a. The chosen two degrees
of freedom per wing permit to sufficiently approximate the
natural motion of real bats [12], [13]. The membrane in Figure
10b consists of a carbon printed 50   , which is
clipped to the skeleton with 3D-printed connectors after the
assembly of the actuator systems.
FIGURE 10: BAT SKELETON WITH JOINTS,
MEMBRANE MOUNTS, AND SCREENPRINTED
SILICONE MEMBRANE.
Figure 11 shows the integrated antagonistic actuators with
the electrical wiring in the bat model. In future work, the
independent control of both degrees of freedom per wing is
investigated.
FIGURE 11: DETAIL PHOTOGRAPHY OF
INTEGRATED ANTAGONISTIC SMA ACTUATORS.
120 mm
CAD model
500 mm
Realization
a) b)
Membrane mounts
Elbow actuatorShoulder actuator
Shoulder joint
Elbow joint
4
Copyright © 2020 ASME
5. CONCLUSION
The first experimental results of a novel integrated SMA
based antagonistic actuation system are presented in this paper.
The measurements show the behavior and the performance of
cycle actuation at high frequencies. The results demonstrate that
the mechanical concept is able to perform actuation frequencies
up to 20 Hz with high actuation movements comparable to the
stationary motion output.
The compact and integrated design of the antagonistic
bundle allows reducing the installation space requirement, as
well as the implementation effort for the electrical connection.
The manufacturing method and structure of the system allow for
variable customization, thus making the resulting actuators
adaptable to several application scenarios. In this way,
manufacturing and assembly costs can be significantly reduced.
In future research, the dynamic performance of the system
will be investigated at different airflow rates and pulse width.
Based on the conducted analysis, we aim at further increasing
the driving frequency and the resulting rotation angle.
Additionally, a microcontroller-based electronic will be
developed to drive the antagonistic actuation system
autonomously. Finally, motion control of the two joins will be
implemented, and first motion tests will be performed.
ACKNOWLEDGMENT
The authors gratefully acknowledge the support of Prof.
Dr.-Ing. habil. Hartmut Janocha.
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... Consequently, design parameters are the structure geometry and also the dimension of the SMA wire. For example, in their work, Kirsch et al. (2020) shown an actuation system consisted of an antagonist wire element of 0.025 mm diameter and 550 mm length and a pulley of 3 mm in diameter. The SMAs are heated using the Joule effect and cooled using natural air convection, resulting in a rotation stroke of 31°with an actuation frequency of 10 Hz. ...
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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.
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Solid state cooling is an environment‐friendly, no global warming potential alternative to vapor compression‐based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. This paper presents an overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices. The paper particularly highlights current work performed within the DFG Priority Program SPP 1599 "Ferroic Cooling", which is focused on the development and realization of a continuously operating elastocaloric cooling device. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, first CAD models of the discussed solid state air cooling device are presented.
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The main objective of the BATMAV project is the development of a biologically-inspired Micro Aerial Vehicle (MAV) with flexible and foldable wings for flapping flight. While flapping flight in MAV has been previously studied and a number of models were realized they usually had unfoldable wings actuated with DC motors and mechanical transmission to provide the flapping motion, a system that brings the disadvantage of a heavy flight platform. This phase of the BATMAV project presents a flight platform that features bat-inspired wings with a number of flexible joints to allow mimicking the kinematics of the real mammalian flyer. The bat was chosen after an extensive analysis of the flight parameters of small birds, bats and large insects characterized by a superior maneuverability and wind gust rejection. Morphological and aerodynamic parameters were collected from existing literature and compared concluding that bat wing present a suitable platform that can be actuated efficiently using artificial muscles. Due to their wing camber variation, the bat species can operate effectively at a large rage of speeds and allow remarkably maneuverable and agile flight. Bat skeleton measurements were taken and modeled in SolidWorks to accurately reproduce bones and body via rapid prototyping machines. Much attention was paid specifically to achieving the comparable strength, elasticity, and range of motion of a naturally occurring bat. Therefore, a desktop model was designed, fabricated and assembled in order to study and optimize the effect of various flapping patterns on thrust and lift forces. As a whole, the BATMAV project consists of four major stages of development: the current phase — design and fabrication of the skeletal structure of the flight platform, selection and testing different materials for the design of a compliant bat-like membrane, analysis of the kinematics and kinetics of bat flight in order to design a biomechanical muscle system for actuation, and design of the electrical control architecture to coordinate the platform flight.
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Contin o s Op rating Elastocaloric Cooling Device based on Shape Continuously Operating Elastocaloric Cooling Device based on Shape Memory Alloys: Development and R a ization
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