ChapterPDF Available

Industrial Applications for Shape Memory Alloys

Authors:

Abstract

The high energy density of shape memory alloy actuators in combination with their self-sensing ability and their unique form factors allow for the design of miniaturized and compact but yet powerful actuator-sensor-systems. These properties as well as their noise and emission free operation make them attractive actuator solutions for industrial applications. Specifically in the fields of material handling, soft robotics and continuum robotics, there have been several developments of SMA based grippers, end-effectors and robotic structures.
https://www.sciencedirect.com/science/article/pii/B9780128035818117230
Paul Motzki, Stefan Seelecke,
Industrial Applications for Shape Memory Alloys,
Reference Module in Materials Science and Materials Engineering,
Elsevier,
2019,
,
ISBN 9780128035818,
https://doi.org/10.1016/B978-0-12-803581-8.11723-0.
(http://www.sciencedirect.com/science/article/pii/B9780128035818117230)
Encyclopedia of Smart Materials
INDUSTRIAL APPLICATIONS FOR SHAPE MEMORY ALLOYS
Author and Co-author Contact Information
Paul Motzki
Saarland University c/o ZeMA gGmbH
Eschberger Weg 46
66121 Saarbrücken, Germany
p.motzki@zema.de
+49 681 857 87 545
Stefan Seelecke
Saarland University c/o ZeMA gGmbH
Eschberger Weg 46
66121 Saarbrücken, Germany
Stefan.seelecke@imsl.uni-saarland.de
+49 681 302 71341
Abstract
The high energy density of shape memory alloy actuators in combination with their
self-sensing ability and their unique form factors allow for the design of
miniaturized and compact but yet powerful actuator-sensor-systems. These
properties as well as their noise and emission free operation make them attractive
actuator solutions for industrial applications. Specifically in the fields of material
handling, soft robotics and continuum robotics, there have been several
developments of SMA based grippers, end-effectors and robotic structures.
Keywords
SMA, shape memory alloy, actuator, soft robotics, continuum robotics, gripper,
end-effector
Body text
Introduction
The current industrial revolution describes the digitalization of manufacturing
processes and includes automation, cyber-physical systems, the Internet of Things
and further technologies for assembly assistance like human-robot-cooperation,
with the goal of flexible and adaptive “smart factories”. At the same time there is a
widespread demand for more functionality in products in the fields of consumer
electronics, household and domestic appliances, biomedicine, automotive and
aerospace to generate added-value in comparison to competitors. For the
realization of novel or improved industrial applications and products, engineers
start looking into the field of smart materials to replace state-of-the-art actuators in
order to design new multifunctional actuator-sensor-systems (Hollerbach, Hunter,
and Ballantyne 1992a; 1992b). These smart or active materials like shape memory
alloys (SMAs) can change their properties in response to external fields, which can
either be exploited to produce movement and force for actuation or for quantitative
sensor measurements (Funakubo 1987; Janocha 2007). Specifically SMAs can offer
significant savings in weight and construction space because of their high energy
and power densities and oftentimes generate very energy-efficient solutions in
comparison to electric, electromagnetic, hydraulic or pneumatic actuators and
operate without noise and emissions (Langbein and Czechowicz 2013). Even
further, completely new actuator designs and implementation become possible due
to SMAs’ specific and flexible form factors and current topics of interest such as
predictive maintenance and condition monitoring can be addressed by using their
sensing abilities. In addition, this “self-sensing” may be utilized in position control
algorithms without the use of external sensors (Furst, Crews, and Seelecke 2013;
Lambert, Gurley, and Beale 2017).
Shape memory alloy technology has been known for decades and the
manufacturing processes of SMA material have become very reliable. Especially
nickel-titanium (NiTi, Nitinol) is nowadays produced with consistent properties
such as phase transformation temperature and transformation strain. SMA
components are commercially available (SAES Getters 2017; Dynalloy Inc. 2017) at
reasonable cost and commercial applications have already proven their
functionality and durability in operation.
Shape Memory Alloy Applications
Thermal shape memory alloys remember their original geometry and even after
large deformations they are able to return to their memorized state. This ability is
described the thermal shape memory effect (SME) which is based on a reversible
phase transformation in the material. This phase transformation between a high
temperature phase (austenite) and a low temperature phase (martensite) is induced
by temperature and mechanical stress. For the design of actuators, typically nickel-
titanium (NiTi, nitinol) wires are used, but also other geometries like springs have
been established. The activation of these SMA wires in actuator applications is
realized by electric current in the wire (Joule heating) and commonly a biasing
element like a mechanical spring is used to bring the actuator back to its
deactivated state.
Typical phase transition temperatures of nitinol wires are 70 °C or 90 °C and
maximum strokes of 4-5 % of the SMA wire length are obtained. Actuator forces
strongly depend on the SMA wire diameter which ranges from 25 µm up to 500 µm.
A 500 µm SMA wire for example is able to create a force of 50 N (at a material
stress of 250 MPa). The downside of thicker wires is their smaller surface area in
relation to their volume and thus their longer cooling times. These are directly
related to maximum actuator frequencies. Thin 25 µm SMA can reach cyclic
activation frequencies of up to 20-30 Hz. One time activations are possible in the
range of several milliseconds, independently of the SMA wire diameter (Motzki et al.
2018).
Because of their benefits regarding construction space, weight, energy-efficiency
and the possibility of completely new intelligent actuator design, SMA actuators
have found their way into a number of applications. The company Actuator
Solutions GmbH (ASG) is a specialist for mass production of SMA actuator
components. Their focus is on the areas of miniaturized optical camera systems
and pneumatic and fluidic valves. Their fully automated production lines produce
more than 10 million SMA actuators per year (Actuator Solutions GmbH and
GmbH 2018). Besides this exceptional example, SMA actuators have also become
very popular in the fields of industrial-, automotive-(Williams and Elahinia 2008;
Zimmerman et al. 2009), biomedical- (Duerig, Stoeckel, and Johnson 2003; Lecce
and Concilio 2015), aerospace- (Hartl and Lagoudas 2007; Mohd Jani et al. 2014;
Wang and Ahn 2017) and MEMS (Kohl 2004; Fu et al. 2004) applications. The
industrial and automotive sectors are looking into SMA solutions for robotics and
work piece manipulation. Especially light-weight grippers based on SMA actuators
are of special interest. In analogy to the human body, a lot of these grippers use a
humanoid finger design, in which SMA wires are used like muscle fibers (Motzki,
Holz, et al. 2015).
Industrial production lines currently still contain large numbers of pneumatically
driven actuators. These actuators show high energy consumption and have thus
become relevant cost factors for companies. The need for more cost-efficient but
also smarter and electrified solutions has led to several developments mainly in the
field of gripping and material handling. One popular example is the company FFZ
Glashütte, which has been selling SMA based industrial grippers for several years
now (Glashuette 2019).
SMA Vacuum Gripper
In material handling and assembly industry, vacuum grippers are widely used for
the handling of plane work pieces such as windshields, car doors or body parts.
These vacuum gripper systems use the Venturi effect for vacuum generation, so
they need a continuous supply of energy from a central compressed air system.
These compressed air systems have become dominating cost factors in production.
SMA based systems neglect the need for pressured air, which results in significant
savings in energy and thus expenses. Additionally, further advantages of the SMA
technology including lowered noise level, unaffected air quality and lowered system
costs, weight and dimensions, as well as the self-sensing can be exploited.
SMA Vacuum Gripper: Version 1 - Original Concept
In the original idea (Motzki, Kunze, et al. 2015) and the first prototype of an SMA
based suction cup an SMA wire is used to actively deform a flat membrane and
thus create a vacuum between said membrane and a plane surface (Fig. 1, left-
hand side). The developed vacuum gripper consists of a casing, a mechanical
spring, one or more SMA wires and a deformable and flexible membrane. The SMA
wires (2) are connected to a casing (1) and the center of the suction membrane (4).
The SMA wires pull up the center of the membrane when they get activated and a
vacuum is created between the membrane and the work piece. A spring (3)
elongates the SMA wires and returns the membrane to its original flat state, when
the SMA wires are not activated. The CAD model of the vacuum gripper prototype is
shown in Figure 2 (left-hand side). The manufactured and assembled prototype is
displayed in Figure 2 (right-hand side).
<Figure 1 near here>
<Figure 2 near here>
This first prototype shows several drawbacks: After gripping and while holding a
work piece, the SMA wire is constantly activated and energy is lost as heat to the
environment. The actuation frequency of this prototype is also very limited because
the SMA wire takes several seconds to cool down.
SMA Vacuum Gripper: Version 2 - Energy-optimized Concept
The improved prototype attacks these disadvantages by using a different kinematic
concept in combination with a thinner SMA wire diameter. The improved kinematic
concept is displayed in Fig. 1 (right-hand side). In the suction cup’s initial state, the
membrane is already deformed by the mechanical spring. Activation of the SMA
wire forces the membrane into a flat configuration ready to grip a work piece. In
this concept, the SMA wire is only activated for a short pulse before gripping and
again to release a work piece and no energy is lost during the holding phase. By
winding the SMA wire between the upper and lower fixture multiple times, the
forces of all mechanically parallel SMA wire sections add up and a smaller SMA
wire diameter can be chosen, which improves the dynamic behavior of the actuator
(Motzki et al. 2016).
The improved prototype design of the SMA suction cup is shown in Fig. 3 (top). The
flexible membrane is connected to the lower fixation, which is also used for
mechanical connection via a screw thread to a gripping mechanism like a robotic
end-effector. The membrane is also connected to the upper fixation with a threaded
rod. The two nuts in the upper fixation are used to adjust the membrane’s relative
position to guarantee a flat state of the membrane when the SMA wires are
activated. The compression of the spring between the upper and the lower fixation
is adjusted with two adjustment screws. The mechanically parallel configuration of
the SMA wire is realized by winding of a single SMA wire between the upper and
the lower fixation.
<Figure 3 near here>
To guarantee reproducibility in the actuator performance, the SMA wire length is
measured in its austenitic, near load-free state in a special measuring setup. The
SMA wire is then inserted into a PTFE tubing for electrical and thermal insulation
and crimped with ring terminals on both ends. After the SMA wire has been
prepared, the suction cup can be assembled and adjusted (Fig. 3, bottom). After
assembly, the actuator is activated for 50 cycles and adjusted again. This “training”
is necessary because the remnant strain in SMA wires changes over the first few
cycles and thus the equilibrium points of the forces shift. The assembly starts with
the lower fixation (step 1) on which the linear spring (step 2). After that, the
threaded rod is connected to the membrane and the upper fixation is connected to
the opposing end of the rod (step 3). In step 4, the spring is manually compressed
and held in a compressed state by a mounting aid, so that the SMA wire can be
winded in a load-free state. The mounting aid is then removed (step 5) and the SMA
wire is loaded and stretched.
After this basic assembly, all components have to be adjusted to ensure
functionality of the actuator. For this purpose, the SMA wire is activated which
causes a contraction of the SMA wire and a compression of the spring (step 6). In
this state, the membrane has to be brought into a flat configuration with the help
of the threaded rod and the adjustment nuts (step 7). Finally, the linear spring is
brought to the required compression via its adjustment screws (step 8). The
assembled prototype is displayed in Fig 4.
<Figure 4 near here>
SMA Vacuum Gripper: Version 3 - Energy- and speed-optimized Concept
Feasibility of an SMA actuated gripping system has been shown by these two
prototypes but the design concept of the SMA actuator can be further improved in
multiple ways. In the presented concepts the SMA wire works against a linear
spring and the SMA wire is directly connected to the membrane. This requires a
large design space to transform the limited maximum strain of the SMA wire into a
large membrane deformation. The actuation principle of an SMA wire working
against a spring also results in continuous heating of the SMA while holding the
payload or increases the gripping time in the reverse configuration.
For an optimized gripping process, the development of a new concept is needed.
Therefore, a novel actuation concept is developed, based on the patent of a special
bistable mechanism (Motzki and Seelecke 2016). To develop a fast and energy
efficient actuation mechanism a bistable element can be used to replace the linear
bias spring. The bistable element is able to hold the payload in two stable positions.
Switching between the positions is managed by two antagonistic SMA wire
configurations. This concept only needs thermal energy for position toggle in form
of short pulses and generates time spans for the SMA wire cooling without
increasing the cycle time of the actuator system. Using the bistable mechanism as
a transmission element to convert the high energy density of the SMA wires into
large strokes, the design space of the actuator can be reduced. The bistable
actuator mechanism enables the design of a high performance actuation system
with high stroke, high activation frequency and small design space. A schematic
view of the concept of the bistable SMA vacuum suction cup is shown in Figure 5
(top) (Welsch et al. 2018).
The actuator mechanism again consists of a flexible deformable membrane, which
is connected to a bistable spring. On the ends of the bistable spring small levers
are attached. While changing the angle of the levers the bistable element switches
from the lower stable state (Figure 5, top left) to the upper stable state (Figure 5,
top right). To actively switch between these two states, two antagonistic SMA wires
are used. The orthogonal orientation of the levers related to the spring allows the
parallel arrangement of the SMA wires to minimize the construction space.
Activation of the bottom SMA wire lets the bistable mechanism switch from the
lower to the upper position (Figure 5, top right). After the position toggle of the
bistable element the energy is switched off and the SMA wire cools down. When the
membrane is sealed by the gripped object and deflected by the bistable element, a
vacuum is generated inside the cavity between the membrane and the work piece.
Release of the payload requires the activation of the top SMA wire as shown in red
in Figure 5 (top left). The actuation of the SMA wire causes a rotational movement
of the levers thus changes the state of the bistable spring, which directly deforms
the membrane. The energy-free holding of the two positions leads to an energy
efficient, cycle time reduced and fail-safe actuation mechanism.
<Figure 5 near here>
The design space is minimized by fitting the geometry of the actuation mechanism
to the rotational geometry of the membrane. The upper side of the mechanical
framework of the actuation mechanism is presented in Figure 5 (bottom).
The membrane is directly connected to the bistable spring element. The resulting
pressure difference depends on the membrane area and its deflection. A high
pressure difference results in a high deformation force of the membrane which has
to be provided by the bistable element. Maximizing the deformation force by a given
area and minimizing the manufacturing effort, the leaf spring is designed in a
crosswise configuration with four clamping points. The force and the deflection of
the leaf spring center depends on its geometry and its pre-tension. The radial
reaction forces due to the pre-tension are absorbed by a force transmission ring.
The ends of the leaf spring are attached to a clamping mechanism which is pivot-
mounted to enable the state toggle (up and down). The clamping mechanism is
symmetrically mounted in the ring with a rotational joint. Additionally, the clamp
provides the mounting point for the lever of the upper and lower SMA wire. The
lever is needed for the transformation of the linear SMA force in a rotary movement
to toggle the state of the bistable element.
The lateral arrangement of the SMA elements to the bistable element around its
circumference is very design space efficient. The required linear force of the SMA
element depends on the rotary movement and the length of the lever. This length is
limited by the maximum strain of the SMA wire and depends on circumference of
the SMA. For reducing the cooling time and using the design space efficiently the
SMA element consists of bundles of thinner wires (200 µm diameter) to optimize the
surface to volume ratio. The ends of the SMA elements are attached to the
connection bar, which at the same time represents the port to the electronics. The
actuator mechanism and the membrane as well as the electronics are integrated in
a closed housing to meet the requirements for harsh industrial environments as
shown in Figure 6 (top).
<Figure 6 near here>
The actuation mechanism is mounted in the housing and the membrane is
connected to the bistable element in its center and to the housing at its outer
frame. The microcontroller electronics is plugged to the actuation mechanism and
manages the joule heating of the SMA wires by measuring the electrical energy.
When applying an electrical energy to the pre-strained SMA wire, the wire heats up
and contracts due to the phase transformation from martensite to austenite. This
transformation also results in change of the electrical resistance. The integrated
electronics is able to correlate this resistance change to the actual state of the
bistable spring which leads to a position feedback of the membrane. This allows an
optimized electrical energy deposition in the SMA wire by turning-off the heating
directly after the position toggle of the membrane. Thereby a successful position
toggle is ensured independent of the ambient temperature and the real supply
voltage. The new position of the membrane is then held by the bistable spring
without the use of additional energy. This control concept leads to a reliable
gripping system with fast actuation times. The device is supplied by an industrial
power supply of 24 V and can be controlled by a standard digital output for
gripping (rising edge) and releasing (falling edge). The status feedback is given to a
standard digital or analog input.
Additionally, the integrated resistance measurement 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 RGB-LEDs.
Figure 6 (bottom) shows the first prototype of the SMA actuated bistable vacuum
suction cup. A standard 3/8” thread is used to mount the suction cup on an
industrial work holder like a robotic end-effector. The control unit is connected by
an industrial M5 connector. Figure 7 (top) shows the components of the suction
cup. To reduce the assembly effort the housing can be taken apart. The membrane
is produced by a silicone injection molding process.
To handle the occurring forces, the actuation mechanism is a combination of
aluminum parts like the force transmission ring and the clamps. The leaf spring is
laser-cut and consists of spring steel. The SMA wires are electrically isolated
mounted on the levers which consist of reinforced epoxy material.
<Figure 7 near here>
The presented new prototype of an innovative SMA actuated bistable vacuum
gripper enables an energy efficient, fail-safe, noiseless gripping system without the
need for compressed air. The integrated resistance measurement on the
microcontroller electronics allows, due to the self-sensing effect of the SMA, a
sensorless condition monitoring and energy efficient activation control. The
integrated RGB-LEDs visualize the gripping state. The sealed, compact and fully
integrated design allows the positioning and transport of different workpieces in
mobile and stationary applications (Figure 7, bottom). In industrial environments,
these suction cups are mounted on robotic end-effectors, which usually are tailored
to the specific work piece geometries.
SMA Adaptable End-effector
The development of novel material handling systems and industrial grippers has
gained considerable relevance. Because of low production costs in Asia, industrial
manufacturing and assembly has to become more efficient in western countries to
maintain international competitiveness. One goal within these efforts aims at
making factories and assembly lines more flexible and adaptive, to react to market
changes and demands with simple and fast reconfiguration processes. Therefore,
also handling systems are required to perform more complicated and complex
manipulation tasks (Kessens and Desai 2016; Firouzeh and Paik 2017; Saliba,
Vella Zarb, and Borg 2010).
For the handling and manipulation of plane workpieces, so-called end-effectors or
suction spiders are used (Figure 8 a). These systems are attached on the top of
industrial robots and consist of custom fitted suction cup arrangements or
alternative grippers, so that one end-effector is only used for one specific workpiece
geometry (Destaco 2017). If an assembly robot is going to be used for different
workpiece geometries, for example in several different production lines, usually the
whole end-effector on the robot is replaced by another one that is again specifically
designed for the new workpiece shape. The necessary stop of production during
this exchange process can be very cost intensive. Therefore, making an end-effector
reconfigurable increases flexibility in production and prevents costly down times
because gripper systems have the ability to be adapted automatically to various
workpiece geometries. Shape memory alloy actuators are the technology of choice
for this application because actuator weight and construction space in these end-
effectors are very limited. High weights like in electric motors or drives on the top of
robots result in critical mass moments of inertia during movement and have to be
avoided. Therefore, using conventional drives (electric, electro-magnetic,
pneumatic) for the reconfiguration process of an end-effector is not a viable option.
Additionally, the “self-sensing” abilities of SMA wires add more functionality and
intelligence, which can be utilized for possible condition monitoring tasks and
position control.
The basic prototype design and functionality of the SMA end-effector is displayed in
Figure 8 (Motzki et al. 2019). The whole end-effector has an outer diameter of 315
mm. The modular design allows the partial assembly and test of a quarter piece
before bringing all four quarters together to a complete prototype. Each of the four
gripping arms is able to perform a 90 ° rotation in-plane and a 30 ° tilting motion
out-of-plane for the handling of workpieces with curved surfaces. Each gripping
arm is controlled independently, which allows various end-effector gripping
configurations for the handling of a variety of different workpiece shapes (Figure 8
c). The end-effector components of this prototype are machined out of aluminum to
carry a static load of 10 kg.
<Figure 8 near here>
A basic overview of the integration of the SMA wire actuators and the mechanical
tension springs as their bias mechanism is given in Figure 9. The upper part (a)
gives an overview of the SMA wire guiding for both actuators in each arm that
create two degrees of freedom (DOF) and the attachment of the SMA wires and the
mechanical springs. The SMA-spring configuration of the first DOF is illustrated in
(b). The SMA wire is guided past the arm’s pivot point to the top of the arm and
back. A contraction of the SMA wire causes a rotational moment and the tension
spring is elongated during the rotation of the gripper arm. A similar configuration is
used for the second DOF (Figure 9 c). Additionally, the tilting actuator is combined
with a locking mechanism that allows concrete energy-free rest positions at the
rotational positions of 10 ° and 20 ° (Figure 9 d). The rotating tilting arm pushes a
snapping wedge back against a mechanical pressure spring. Two teeth define the
energy-free rest positions. To return to the initial position, the tilting arm is fully
rotated to the 30 ° position. From this position, the snapping wedge is guided
around the tilting arm teeth on its way back to the initial position.
The power supply connection is the center of the end-effector. At the top of the
tilting arms, any desired gripping system such as SMA vacuum grippers can be
added.
<Figure 9 near here>
The assembly of the SMA actuators includes SMA wire attachment (crimping,
clamping, gluing), electrical connection and insulation of the SMA wires, thermal
management and applying the correct pre-strain and pre-stress.
The SMA wires are mechanically attached with screws by clamping the wire
between washers (Figure 10). This clamping method allows the exchange of the
SMA wires in this prototype. The electrical connection wires are directly soldered to
brass washers and nuts (Figure 10, lower part) in a way that the electrical and
mechanical connections are always separated from each other. This also helps the
assembly process or the exchange of a broken SMA wire. To ensure electrical
insulation of the aluminum structure, PTFE tubing is used to guide the SMA wire
where it touches the structure (Figure 10, upper part). PTFE is also suitable for
thermal insulation, so the aluminum structure does not act as a heat sink for the
SMA wire.
The clamping screws are electrically insulated by little 3D-printed plastic tubes
(Figure 10, lower part). The SMA wire of the first DOF is wrapped around the top of
the gripping arm twice to produce a higher total SMA force. That allows the use of
thinner SMA wire diameters and thus better actuator dynamics because of faster
cooling times. The total displacement is not affected by the SMA wire wrapping,
since only the effective SMA wire length (Figure 10, upper part) is relevant for
estimating the SMA stroke. A special “tension device” (Figure 10, lower part) is
designed for two reasons: It guides the SMA wire at this end of the arm and at the
same time can be used to control the pretension in the SMA wire.
To guarantee the functionality and repeatability, it is crucial to integrate the SMA
wires with the correct pre-strain and pretension. In this case, the SMA wire length
is first measured in its unloaded, activated austenitic state, because this is the only
reproducible reference length. After cooling and returning to its martensitic state,
the wire is stretched for 4 % of the measured austenitic length before integrating it
into the end-effector. The fine-tuning of the pretension in the application is realized
with this tension device via two adjustment screws.
In an industrial production application, for example in automotive industry, the
automatic adaption of the SMA end-effector can lead to more flexibility and cost
savings in production lines. If more than one car model is produced in the same
production line, end-effectors have to be exchanged because of sometimes only
slightly different work piece geometries. This exchange process can take up to 15
minutes and more and the resulting downtime is very costly. The reconfiguration
process in the SMA end-effector, in comparison to the exchange of a whole custom
fitted end-effector, can be realized in under 10 seconds opposed to several minutes.
Even faster actuation times can be realized with bundles of SMA wires with a
smaller diameter. The total cross-sectional area has to remain the same to handle
the necessary forces, but at the same time the cooling speed of the SMA wires
increases because of a larger surface area.
<Figure 10 near here>
The reconfiguration process always takes place without additional external loads.
Once a workpiece is gripped, the end-effector structure has to carry the workpiece’s
weight, but the SMA actuator is not exposed to additional stress. The self-sensing
of the SMA wires in this application is used for the position control of the end-
effector arms. The electrical resistance of the SMA wires is used in a feedback loop,
so that no additional sensors are needed for this closed loop control strategy. The
complete gripping system consisting of an industrial robot, the SMA end-effector
with four integrated SMA vacuum grippers is shown in Fig. 11.
<Figure 11 near here>
SMA Continuum Robots
Continuum robots are inspired by biological trunks, snakes and tentacles. Unlike
conventional robot manipulators, there are no rigid structures or joints.
Advantageous is the ease of miniaturization combined with high dexterity, since
limiting components such as bearings or gears can be omitted. Most currently used
actuation elements in continuum robots require a large drive unit with electric
motors or similar mechanisms. Contrarily, shape memory alloys can be integrated
into the actual robot. The actuation is realized by applying current to the wires,
which eliminates the need of an additional outside drive unit. In industrial
applications these robots are becoming relevant for delicate manipulation tasks as
well as inspection systems like industrial endoscopes.
In contrast to conventional manipulators, continuum robots have infinite joints.
Linking parts like bearings or gears can be omitted, which simplifies the
construction of miniaturized robots. Most continuum robots compose of only one
core, varying from superelastic NiTi rods to complex structures. Different actuation
methods have been evaluated for these so-called single backbone robots. Extrinsic
actuation strategies like tendon/cable driven mechanisms as well as concentric-
tube robots need quite large transmission mechanisms to realize axial rotations
and translations of the continuum robot. Hydraulic, pneumatic or shape memory
actuation elements belong to intrinsic actuation methods, where the actuator
element itself is integrated into the moving structure. Using shape memory alloy
wires as actuators allows for the construction of small continuum robots without
the need for large drive units, but only an electrical circuit to apply a current to the
SMA wire.
Different SMA actuated continuum robots have been built to cover various
applications (Goergen et al. 2019). The robot on the left in Fig. 12 consists of a
superelastic NiTiCo rod covered by a PTFE tube as core and restoring element and
an SMA actuator wire attached to this core. The maximum bending angle is 180 °.
By using only one SMA actuator wire with a diameter of 200 µm, this continuum
robot is restricted to bending in one direction. An outer PTFE tube fixes the
actuator wire and leads to an outer diameter of 1 mm (Table 1, column 1). This
specific robot covers low force applications that require a high range of motion and
large deformations. The continuum robot on the right (Figure 12) with a length of
220 mm and an outer diameter of 60 mm consists of a 2.5 mm diameter spring
steel wire as core and actuator wires with a diameter of 500 µm (Table 1,
column 3). The continuous arrangement of SMA wires in a circular ring allows this
prototype to bend in every direction. This actuator can be seen as one element of a
larger robot, which is used to carry higher loads. The robot in the middle (Figure
12) shows the prototype of an endoscopic robot for inspection tasks. Its outer
diameter is 3.6 mm (Table 1, column 2). Three SMA wires with a diameter of 50 µm
are arranged inside of the 3D printed structure, which functions as core and
restoring force, on an annulus with 120 ° offset each.
<Figure 12 near here>
The theoretical design allows the calculation of the bending angle of the robotic
structure at a given length and contraction of the SMA wires. The core of the
continuum robot is shown as a black line, representing the neutral fiber of the
actual core structure (Fig. 13). The blue and red line, respectively, represent the
SMA wires in defined states. Blue indicates a cold wire, without any current being
applied, while the red line stands for a heated and thus activated SMA wire, with
current flowing. In the initial situation (Figure 13, top left), the wire is installed cold
and pre-stretched with 2 %. The core and the SMA wire are fixed at the lower end.
Heating the SMA wire above its austenite transformation temperature leads to
bending of the continuum robot (Figure 13, top right). Here r describes the center
of the osculating circle, x the distance of the neutral fiber of the core element to the
SMA wire and α the bending angle of the core.
The SMA wires need to be mounted in a defined condition. Both, pre-tensioning
and pre-stretching of the wires are of decisive importance. In the given
configuration (Figure 13, bottom), an SMA wire reaches its maximum length when
the other two wires are activated and contract at maximum. Assuming a maximum
elongation of the SMA wires of 4 %, an optimum operating point is obtained with
2 % pre-stretching of the wires.
The following table gives an overview of the most important characteristics of the
three SMA driven continuum robots mentioned at the beginning (Figure 12).
This overview of different SMA driven continuum robots shows the versatility SMA
technology brings to the broad field of continuum and soft robotics, and
demonstrates the potential of SMA actuators for large number of different
industrial applications.
<Figure 13 near here>
<Table 1 near here>
References
Actuator Solutions GmbH, and Actuator Solutions GmbH. 2018. Actuator
Solutions SMA Products. 2018. http://www.actuatorsolutions.de/products/.
Destaco. 2017. Lightweight End Effector Tooling. 2017.
http://www.destaco.com/lightweight-tooling.html.
Duerig, T, Dieter Stoeckel, and D. Johnson. 2003. SMA: Smart Materials for
Medical Applications. Proceedings of SPIE.
https://doi.org/10.1117/12.508666.
Dynalloy Inc. 2017. Technical Characteristics of Flexinol Actuator Wires. 2017.
http://www.dynalloy.com/pdfs/TCF1140.pdf.
Firouzeh, A, and J Paik. 2017. Grasp Mode and Compliance Control of an
Underactuated Origami Gripper Using Adjustable Stiffness Joints.
IEEE/ASME Transactions on Mechatronics 22 (5): 216573.
https://doi.org/10.1109/TMECH.2017.2732827.
Fu, Yongqing, Hejun Du, Weimin Huang, Sam Zhang, and Min Hu. 2004. TiNi-
Based Thin Films in MEMS Applications: A Review. Sensors and Actuators A:
Physical 112 (23): 395408. https://doi.org/10.1016/j.sna.2004.02.019.
Funakubo, Hiroyasu. 1987. Shape Memory Alloys. Edited by Dimitris C. Lagoudas.
Shape Memory Alloys - Modeling and Engineering Applications. Vol. 1.
Amsterdam: Gordon and Breach Science Publ. https://doi.org/10.1007/978-
0-387-47685-8.
Furst, Stephen J, John H Crews, and Stefan Seelecke. 2013. Stress, Strain, and
Resistance Behavior of Two Opposing Shape Memory Alloy Actuator Wires for
Resistance-Based Self-Sensing Applications. Journal of Intelligent Material
Systems and Structures 24 (16). https://doi.org/10.1177/1045389X13486715.
Glashuette, FFZ. 2019. FFZ Glashuette - Greiftechnik. 2019. http://ffz-
glashuette.com/greiftechnik/.
Goergen, Yannik, Romol Chadda, Rouven Britz, Dominik Scholtes, Nataliya Koev,
Paul Motzki, Roland Werthschützky, Mario Kupnik, and Stefan Seelecke. 2019.
SHAPE MEMORY ALLOYS IN CONTINUUM AND SOFT ROBOTIC
APPLICATIONS. In ASME 2019 Conference on Smart Materials, Adaptive
Structures and Intelligent Systems - SMASIS19. Louisville.
Hartl, D J, and D C Lagoudas. 2007. Aerospace Applications of Shape Memory
Alloys. Proceedings of the Institution of Mechanical Engineers, Part G: Journal of
Aerospace Engineering 221 (4): 53552.
https://doi.org/10.1243/09544100JAERO211.
Hollerbach, J M, I Hunter, and J Ballantyne. 1992a. A Comparative Analysis of
Actuator Technologies for Robotics. In The Robotics Review 2, 299342.
Hollerbach, J M, I W Hunter, and J A Ballantyne. 1992b. Comparative Analysis of
Actuator Technologies for Robotics. The Robotics Review 2, 299342.
Janocha, Hartmut. 2007. Adaptronics and Smart Structures. Edited by Hartmut
Janocha. 2. Berlin Heidelberg: Springer Verlag.
https://doi.org/10.1017/CBO9781107415324.004.
Kessens, C C, and J P Desai. 2016. Versatile Passive Grasping for Manipulation.
IEEE/ASME Transactions on Mechatronics 21 (3): 12931302.
https://doi.org/10.1109/TMECH.2016.2520306.
Kohl, Manfred. 2004. Shape Memory Microactuators. Berlin Heidelberg: Springer
Verlag.
https://books.google.com/books?hl=de&lr=&id=v_qZN7JaatcC&pgis=1.
Lambert, Tyler Ross, Austin Gurley, and David Beale. 2017. SMA Actuator
Material Model with Self-Sensing and Sliding-Mode Control; Experiment and
Multibody Dynamics Model. Smart Materials and Structures 26 (3): 35004.
http://stacks.iop.org/0964-1726/26/i=3/a=035004.
Langbein, S, and A Czechowicz. 2013. Leitfaden Zur Entwicklung von
Formgedächtnisaktoren. In Konstruktionspraxis Formgedächtnistechnik, 188
204. https://doi.org/10.1007/978-3-8348-2343-4_13.
Lecce, Leonardo, and Antonio Concilio. 2015. Shape Memory Alloy Engineering: For
Aerospace, Structural and Biomedical Applications. Edited by Leonardo Lecce
and Antonio Concilio. Oxford: Elsevier.
Mohd Jani, Jaronie, Martin Leary, Aleksandar Subic, and Mark A. Gibson. 2014. A
Review of Shape Memory Alloy Research, Applications and Opportunities.
Materials and Design. https://doi.org/10.1016/j.matdes.2013.11.084.
Motzki, Paul, Tom Gorges, Mirco Kappel, Marvin Schmidt, Gianluca Rizzello, and
Stefan Seelecke. 2018. High-Speed and High-Efficiency Shape Memory Alloy
Actuation. Smart Materials and Structures 27 (7): 075047.
https://doi.org/10.1088/1361-665X/aac9e1.
Motzki, Paul, Benedikt Holz, Filomena Simone, and Stefan Seelecke. 2015.
Formgedächtnislegierungen in Applikationen Der Greif- Und
Handhabungstechnologie - Shape Memory Alloys in Applications of Gripping-
and Material-Handling-Technology. In Fachtagung Mechatronik 2015, 5560.
Dortmund, Germany: VDI/VDE.
https://doi.org/http://dx.doi.org/10.17877/DE290R-7388.
Motzki, Paul, Frank Khelfa, Lukas Zimmer, Marvin Schmidt, and Stefan Seelecke.
2019. Design and Validation of a Reconfigurable Robotic End-Effector Based
on Shape Memory Alloys. IEEE/ASME Transactions on Mechatronics 24 (1):
293303. https://doi.org/10.1109/TMECH.2019.2891348.
Motzki, Paul, J Kunze, A York, and S Seelecke. 2016. Energy-Efficient SMA
Vacuum Gripper System. In Actuator 16 - 15th International Conference on
New Actuators, edited by Hubert Borgmann, 52629. Bremen: MESSE
BREMEN. https://doi.org/10.13140/RG.2.2.25486.97609.
Motzki, Paul, Julian Kunze, Benedikt Holz, Alexander York, and Stefan Seelecke.
2015. Adaptive and Energy Efficient SMA-Based Handling Systems. In SPIE -
Active and Passive Smart Structures and Integrated Systems 2015, edited by
Wei-Hsin Liao, 9431:943116. San Diego, USA: SPIE.
https://doi.org/10.1117/12.2083580.
Motzki, Paul, and Stefan Seelecke. 2016. BISTABLE ACTUATOR DEVICE HAVING
A SHAPE MEMORY ELEMENT. WO 2017/194591 A1, issued 2016.
https://depatisnet.dpma.de/DepatisNet/depatisnet?action=bibdat&docid=WO
002017194591A1.
SAES Getters. 2017. SmartFlex Brochure. 2017.
https://www.saesgetters.com/sites/default/files/SmartFlex Brochure_2.pdf.
Saliba, Michael A., Andrew Vella Zarb, and Jonathan C. Borg. 2010. A Modular,
Reconfigurable End Effector for the Plastics Industry. Assembly Automation 30
(2): 14754. https://doi.org/10.1108/01445151011029781.
Wang, Wei, and Sung-Hoon Ahn. 2017. Shape Memory Alloy-Based Soft Gripper
with Variable Stiffness for Compliant and Effective Grasping. Soft Robotics 4
(4): 37989. https://doi.org/10.1089/soro.2016.0081.
Welsch, Felix, Susanne-Marie Kirsch, Paul Motzki, Marvin Schmidt, and Stefan
Seelecke. 2018. Vacuum Gripper System Based on Bistable SMA Actuation.
In ASME 2018 Conference on Smart Materials, Adaptive Structures and
Intelligent Systems, V001T04A014. https://doi.org/10.1115/SMASIS2018-
7980.
Williams, E., and M. H. Elahinia. 2008. An Automotive SMA Mirror Actuator:
Modeling, Design, and Experimental Evaluation. Journal of Intelligent Material
Systems and Structures 19 (12): 142534.
https://doi.org/10.1177/1045389X07087328.
Zimmerman, E., V. Muntean, T. Melz, Bj Seipel, and Th Koch. 2009. Novel Pre-
Crash-Actuator-System Based on SMA for Enhancing Side Impact Safety. In
Advanced Microsystems for Automotive Applications 2009: Smart Systems for
Safety, Sustainability, and Comfort. https://doi.org/10.1007/978-3-642-
00745-3_4.
... Shape memory alloys (SMA) are well known for their superelasticity as well as their actuation capabilities and are widely used in bio-medical applications and specific actuator products [1][2][3]. The high energy density of Nickel-Titanium (NiTi) based SMAs [4,5] allows for the development of high force industrial actuator applications, typically used in the shape of wires [6,7]. The shape memory effect describes a reversible phase transformation between the monoclinic low-temperature phase martensite and the cubic space-centered high-temperature phase austenite [8,9]. ...
Conference Paper
Full-text available
Today, Nickel-Titanium (NiTi)-based shape memory alloy (SMA) wires are already used in a wide range of commercial actuator applications. The combination of their unique properties, such as their high energy density as well as their biocompatibility provides a broad range of applications. The former in particular allows for designing light-weight actuator systems with high forces using small installation spaces. In systems like emergency brakes or switch disconnectors, which require high forces as well as high actuation speed, the high-power capability of NiTi actuators is exploited. The presented work discusses the development of a giant power catapult demonstrator, that combines the high-speed and high-force capability of SMA wires. To illustrate the vast force, speed, and power potential of SMA wires, a bowling ball is launched from its resting position into the air using SMA wire bundles. For demonstration purposes a target altitude for the bowling ball of 500 mm is chosen, requiring a theoretical actuation force of F ≅ 920 N. To be able to launch the bowling ball to the desired height, an instantaneous energy release with an overall peak-power of P ≅ 0.5 MW is required.
... Conventional electric and nonelectric actuators such as electro-magnets or pneumatics are increasingly replaced by more energy-efficient, compact, light-weight or versatile drive systems based on smart materials like shape memory alloys (SMA) or electroactive polymers (EAP) [1], [2]. SMA wires are known for their high work capability in relation to their mass and construction space and are already used in various commercially available products, for example in industry [3], the automotive and the aerospace sectors [4], [5]. ...
Conference Paper
Full-text available
For repeated actuation in shape memory alloy (SMA) actuators, a restoring force is needed to return to the initial starting position after activation. Therefore, SMA wires are often coupled with mechanical springs, which lengthen said wires again after activation through heating and resulting contraction. In more advanced SMA actuation systems a second SMA wire is used as an actively controllable restoring element instead of passively working spring forces. A disadvantage of these antagonistic SMA actuator systems is that after activation of the first SMA wire, the return movement cannot be carried out immediately by the antagonistic partner. This delay caused by the first SMA wire's cooling time leads to longer cycle times. To compensate for this disadvantage, a decoupled antagonistic SMA actuator has been developed. This enables the actuator to move back to its initial position immediately, regardless of the state of the antagonistic SMA wire. This work deals with the construction as well as the control of two rotatory decoupled antagonistic SMA actuators. The first actuator enables a 90 ° rotational movement through 2 mm of SMA wire stroke via a gear drive. The second actuator contains a bistable element to enable two energy-free switching positions. This bistable element serves as output device of the actuator and an output stroke of 8 mm is realized by an SMA wire stroke of 1.9 mm.
... , a bistable mechanism is used to reach a high efficiency. Bistable SMA actuator system arrangements have proven to create energy-efficiency in industrial applications [8], [9] The developed concept features a spring-loaded toggle lever mechanism, which is switched by antagonistic SMA wires. It is depicted in figure 1. ...
Conference Paper
Full-text available
In this paper, the development process of a shape memory alloy (SMA) driven pinch valve is proposed. The features of SMA actuator wires like small installation space, small weight, and the high energy density, allow for designing compact systems with high force outputs. A functional prototype is presented, allowing for closing forces of up to 250 N and 3 mm of stroke, which enables pinching hoses up to a diameter of 5 mm. It is driven by four mechanically parallel 500 µm diameter wires to open the valve and two 300 µm diameter to close the valve and cut off the medium flow inside of the hose. This SMA driven pinch valve features a spring-loaded toggle mechanism. It leads to stable and energy free position holding in both, opened and closed state of the valve, compared to commonly used valves, which allow for only one energy free position. As the system can be built without using any ferromagnetic materials the valve is predestined to be used in Magnetic Resonance Imaging (MRI). Due to the higher energy efficiency and weight reduction in comparison to commonly used solenoid pinch valves, they can be a substitute in a variety of applications.
... SMA is generally available in the form of wires and coils/springs. SMA wires are used as orthodontic wires [4], to actuate gloves for supporting physiotherapy in orthopedic treatments [5,6], in minimally invasive surgeries [7][8][9][10], in civil engineering applications [11][12][13][14], as vacuum grippers in material handling and assembly industries [15,16], in assistive and rehabilitation devices [17], or to actuate continuum robots [18,19], etc. Whereas, SMA coils as actuators are preferred over wires where a large stroke is required [20]. Some applications of coil-type SMA actuator are in endoscopy [21][22][23][24], micro-robotic fish-fin [25], miniature soft robots [26], artificial muscles [27], etc. ...
Article
This paper presents a mathematical model of a shape memory alloy (SMA) spring actuator in an antagonistic configuration to determine the work generation potential. The mathematical model incorporates the heat transfer equations, constitutive model of the SMA material, and phase kinetics equations as well as loading history. The simulation results incorporating loading history and arbitrary loading are reported for the first time for an antagonistic SMA spring actuator. The model is implemented in MATLAB using the implicit solver method. Performance of antagonistic SMA spring actuator is investigated for its force generation capability and displacement, considering its material properties, geometrical parameters, and applied input voltage. At least three investigations are carried out for each parametric variation. Simulation results are found to be qualitatively in agreement with the published literature. Higher force generation capability of antagonistic SMA spring pair is reported under arbitrary thermo-mechanical loading conditions. It is concluded that the geometrical parameters of SMA spring such as the number of turns, diameter of spring wire, and spring index significantly affect the performance of the antagonistic spring pair than the material parameters.
Article
Shape-memory alloys (SMAs) are preferred currently for multifarious purposes because of their exceptional properties as compared to counterparts, viz. shape-memory effect, superelasticity, corrosion resistance, bioadaptability, resistance to wear, etc. A multitude of research activities regarding the machining of SMAs has been done. The conventional machining techniques have shortcomings related to surface morphology as it generates undesirable tool wear and low accuracy of machined parts. Among the unconventional methods, electrical discharge machining (EDM) and its allied variations have created a buzz in machining of SMAs. This study is an effort to carry out an investigation of the work done by vivid researchers in machining of SMAs using die-sinking EDM and die-sinking micro-EDM. The input parameters and response features of EDM are discussed. The research focusing on nickel titanium (NiTi)-based, copper (Cu)-based and other SMAs using EDM in particular is elaborated here. The general overview of several optimization methods, viz. non-traditional methods, multi-criteria decision making methods, and statistical methods, is presented elaborately. A review of optimization with their implementation in EDM machining of SMAs by researchers is incorporated in this article. The techniques for advanced processing of SMAs and hybrid EDM methods are reviewed. The future scope of research on the current topic is suggested through this review.
Article
Full-text available
Nickel-Titanium (NiTi) based shape memory alloy (SMA) wires are already often used in industrial actuator applications. Their high energy density allows to build light-weight actuator systems with high forces using small installation spaces. Combined with the biocompatibility of NiTi, a huge field of applications can be covered by SMA actuated systems. In systems like emergency brakes or switch disconnectors, which require high forces as well as high actuation speed, the high-power capability of NiTi actuators is exploited. The presented work details the development and characterization of a giant power catapult demonstrator, that combines the high-speed and high-force capability of SMA wires. To illustrate the vast force, speed, and power potential of SMA wires, a bowling ball is launched from its resting position vertically into the air using SMA wires. For demonstration purposes a target altitude for the bowling ball of 500 mm is chosen. With the height and the overall accelerated mass given, an actuation force F ≅ 920 N is needed. The instantaneous energy release from the designed power source results in the targeted flight height and an overall peak power of P ≅ 0.5 MW.
Article
Full-text available
Active elements made of Ti50Ni45Cu5 shape memory alloy (SMA) were martensitic at room temperature (RT) after hot rolling with instant water quenching. These pristine specimens were subjected to two thermomechanical training procedures consisting of (i) free recovery shape memory effect (FR-SME) and (ii) work generating shape memory effect (WG-SME) under constant stress as well as dynamic bending and RT static tensile testing (TENS). The structural-functional changes, caused by the two training procedures as well as TENS were investigated by various experimental techniques, including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffraction (XRD), and atomic force microscopy (AFM). Fragments cut from the active regions of trained specimens or from the elongated gauges of TENS specimens were analyzed by DSC, XRD, and AFM. The DSC thermograms revealed the shift in critical transformation temperatures and a diminution in specific absorbed enthalpy as an effect of training cycles. The DMA thermograms of pristine specimens emphasized a change of storage modulus variation during heating after the application of isothermal dynamical bending at RT. The XRD patterns and AMF micrographs disclosed the different evolution of martensite plate variants as an effect of FR-SME cycling and of being elongated upon convex surfaces or compressed upon concave surfaces of bent specimens. For illustrative reasons, the evolution of unit cell parameters of B19′ martensite, as a function of the number of cycles of FR-SME training, upon concave regions was discussed. AFM micrographs emphasized wider and shallower martensite plates on the convex region as compared to the concave one. With increasing the number of FR-SME training cycles, plates’ heights decreased by 84–87%. The results suggest that FR-SME training caused marked decreases in martensite plate dimensions, which engendered a decrease in specific absorbed enthalpy during martensite reversion.
Article
Full-text available
Soft robotics is an emerging field of robotics that focuses on the design of soft machines and devices with effective human-machine interaction, high conformity, and environmental adaptability. The conventional robots made of hard materials have already achieved precision and accuracy, but they lack in reachability, adaptability, Degree of Freedom (DoF), and safe interaction. Moreover, soft robots mimic the behavior of biological creatures by mimicking their locomotive patterns. The actuation or the locomotion of the soft robots is achieved by soft actuators which are a very important part of soft robotic systems. Herein, a comprehensive review based on the evolution of six actuation methodologies is presented. Various approaches used for the design and fabrication of soft robots such as pneumatic, shape memory alloy (SMA), dielectric elastomers, chemical-reaction enforced, and pneumatic and magneto-rheological elastomers-based actuation methods reported in the last decade. Furthermore, the advancement of these approaches has been rigorously discussed in chronological order for parameters like efficiency, power requirement, frequency, and possible applications. Future challenges and directions toward the advancement in soft robotics are also discussed for achieving the remarkable performance of soft robots in a real-time environment. Furthermore, we believe, this is a complete review package for the young researchers which can help them to understand the evolution of bioinspired robotics.
Article
Full-text available
This work presents the development of an innovative shape memory alloy (SMA) actuator principle, which allows high‐speed switching cycles through the decoupling of antagonistically arranged SMA wires. Being optimized for the use at high ambient temperatures up to 65 °C, a possible application area is the active venting of injection molds where it can be used to expel air, which is trapped during the injection mold process. The patented actuator principle is based on a decoupled agonist‐antagonist SMA‐spring‐system and allows a high‐speed closing movement by a compact and lightweight design. Another innovation compared to conventional antagonistic SMA actuator systems is the integrated fail‐safe mechanism, which guarantees a defined closed state in case of power failure. Subsequently, in the motivation the need for active venting valves for injection molding is first described. Secondly, the novel actuator principle is introduced, and the development of an electronics concept is discussed. Finally, the design process, assembly, and validation of two iterations of the actuator prototype are presented. The final prototype validation measurements showcase high performance by valve strokes of 1 mm within 100 ms at ambient temperature of 65 °C. This article is protected by copyright. All rights reserved.
Conference Paper
Full-text available
Continuum robots are inspired by biological trunks, snakes and tentacles. Unlike conventional robot manipulators, there are no rigid structures or joints. Advantageous is the ease of miniaturization combined with high dexterity, since limiting components such as bearings or gears can be omitted. Most currently used actuation elements in continuum robots require a large drive unit with electric motors or similar mechanisms. Contrarily, shape memory alloys (SMAs) can be integrated into the actual robot. The actuation is realized by applying current to the wires, which eliminates the need of an additional outside drive unit. In the presented study, SMA actuator wires are used in variously scaled continuum robots. Diameters vary from 1 to 60 mm and the lengths of the SMA driven tentacles range from 75 to 220 mm. The SMAs are arranged on an annulus in a defined distance to the neutral fiber, whereby the used cores vary from superelastic NiTi rods to complex structures and also function as restoring unit. After outlining the theoretical basics for the design of an SMA actuated continuum robot, the design process is demonstrated exemplarily using a guidewire for cardiac catheterizations. Results regarding dynamics and bending angle are shown for the presented guidewire.
Patent
Full-text available
The invention relates to an actuator device (1) for providing at least two actuator positions, comprising an elastic bending element (2), which at at least one fastening point (31, 32, 34) is held such that by exerting a switching torque at the fastening point (31, 32, 34), an elastic deformation of the bending (2) leads to a change from a first actuator position into a second actuator position, and comprising at least one actuator element (41, 42) having a shape memory wire, wherein by heating, the shape memory wire generates a tractive force, and is thus coupled to a section of the bending element (2) at the fastening point (31, 32, 34), such that the tractive force causes the switching moment to be brought about at the fastening point (31, 32, 34) in order to move the bending element (2) from the first actuator position into the second actuator position.
Article
Full-text available
Thermal shape memory alloy (SMA) actuators are known for their superior energy density (force-volume-ratio) compared to other actuation principles, allowing the construction of lightweight and compact systems. Furthermore, SMA actuators can be used as sensors, as their electrical resistance changes during activation. Using this multifunctionality, this work aims at presenting the development, fabrication and validation of an SMA driven robotic end-effector. The end-effector prototype is designed in a modular concept and consists of four independent arms with two degrees of freedom (DOF). Each arm can rotate in-plane and also tilt out-of-plane to allow gripping of various workpiece geometries. Both DOF actuator components consist of an SMA wire working against a tension spring. The tilting joint has an additional mechanism that creates two energy-free rest positions to improve energy-efficiency. The end-effector is designed to carry a maximum load of 10 kg. In a test bench for the validation of the SMA driven end-effector joints, hall sensors are used to measure the gripping arm displacement. In addition, the resistance of the SMA wires is monitored during activation. The dynamic system performance is analyzed using different activation current levels. Finally, a PI control with Hall sensor feedback is implemented to position the first DOF at arbitrary angles within its 90° rotation radius.
Conference Paper
Full-text available
This paper presents the design and the realization of an innovative SMA actuated bistable vacuum suction cup. The sealed, compact and fully integrated design enables the positioning and transport of inherent stable 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. Additionally, the self-sensing effect of the SMA enables a sensorless condition-monitoring and energy-efficient control. The mechanics consists of antagonistic SMA wires, which are laterally arranged and connected to the bistable spring via levers. The membrane is directly connected to the bistable spring. The actuation of the wires leads to a rotational movement of the levers thus changes the state of the bistable spring, which directly deforms the membrane. When the membrane is sealed connected to the workpiece, the deformation of the membrane generates a vacuum. The integrated microcontroller electronics manages the joule heating of the wires by measuring the transmitted electrical energy. By applying an electrical energy to the pre-strained SMA wire, the wire heats up and contracts due to the phase transformation from martensite to austenite. The contraction of the wire is accompanied by a significant change in electrical resistance, which enables a resistance based strain feedback. The integrated electronics is able to correlate this resistance change to the actual state of the bistable spring, which leads to a position feedback of the membrane. This allows an adequate electrical energy deposition in the SMA wire by turning-off the heating directly after the position toggle of the membrane. Thereby, a successful position toggle is ensured independent from the ambient temperature and the real supply voltage. The new position of the membrane is then held by the bistable spring without the use of additional energy. This concept leads to a reliable gripping system with fast actuation times.
Article
Full-text available
When standard voltage levels commonly adopted in industry are used to activate thermal shape memory alloy (SMA) wire actuators, they often result in very high electrical currents which may eventually damage or destroy the actuators. To improve performance of SMA wire actuators operating in industrial environments, in this paper we investigate a novel, fast and energy-efficient actuation strategy based on short pulses in the millisecond range. The use of higher voltages leads to a highly dynamic activation process, in contrast to commonly used quasi-static activation based on low voltage. A test setup is designed to examine the effects of the control parameters (i.e., supply voltage, activation pulse duration, SMA wire pre-tension and wire diameter) on the measured displacement and force output of the SMA wire. It is shown that actuation times in the range of 20 ms and strokes of more than 10 % of the SMA wire length can be reached. Additionally, energy savings of up to 80 % with respect to conventional quasi-static actuation are achieved. Possible applications for this activation method are release mechanisms, switches or safety applications.
Patent
Full-text available
The invention relates to an actuator device (1) for providing at least two actuator positions, comprising an elastic bending element (2), which at at least one fastening point (31, 32, 34) is held such that by exerting a switching torque at the fastening point (31, 32, 34), an elastic deformation of the bending (2) leads to a change from a first actuator position into a second actuator position, and comprising at least one actuator element (41, 42) having a shape memory wire, wherein by heating, the shape memory wire generates a tractive force, and is thus coupled to a section of the bending element (2) at the fastening point (31, 32, 34), such that the tractive force causes the switching moment to be brought about at the fastening point (31, 32, 34) in order to move the bending element (2) from the first actuator position into the second actuator position.
Article
Full-text available
Soft pneumatic actuators and motor-based mechanisms being concomitant with the cumbersome appendages have many challenges to making the independent robotic system with compact and lightweight configuration. Meanwhile, shape memory actuators have shown a promising alternative solution in many engineering applications ranging from artificial muscle to aerospace industry. However, one of the main limitations of such systems is their inherent softness resulting in a small actuation force, which prevents them from more effective applications. This issue can be solved by combining shape memory actuators and the mechanism of stiffness modulation. As a first, this study describes a shape memory alloy-based soft gripper composed of three identical fingers with variable stiffness for adaptive grasping in low stiffness state and effective holding in high stiffness state. Each finger with two hinges is fabricated through integrating soft composite actuator with stiffness changeable material where each hinge can approximately achieve a 55-fold changeable stiffness independently. Besides, each finger with two hinges can actively achieve multiple postures by both selectively changing the stiffness of hinges and actuating the relevant SMA wire. Based on these principles, the gripper is applicable for grasping objects with deformable shapes and varying shapes with a large range of weight where its maximum grasping force is increased to ∼10 times through integrating with the stiffness changeable mechanism. The final demonstration shows that the finger with desired shape-retained configurations enables the gripper to successfully pick up a frustum-shaped object.
Article
Every robotic gripper requires an equilibrated solution towards the grasp adaptability, precision, and load bearing capacity. A versatile soft robotic gripper requires adjustable grasp mode, for objects with different sizes and shapes, and adjustable compliance, for switching between soft mode; for small loads and delicate objects; and stiff mode; for larger loads and heavier objects. In this paper, we present the design of a tendon-driven robotic origami, robogami, gripper that provides self-adaptability and inherent softness through its redundant and under-actuated degrees of freedom (DoF). Robogami is a planar and foldable robotic platform that is scalable and customizable thanks to its unique layer-by-layer manufacturing process. The nominally 2D fabrication process allows embedding different functional layers with a high fidelity. In particular, a polymer layer with adjustable stiffness enables the independent control of the stiffness for each joint. Using this feature, we can control the input energy distribution between different joints and hence the motion of the robogami. Here, we model the behavior of a single finger; and demonstrate the compliance control of the end-effector along different directions in simulations and experiments. We also validate the gripper's task versatility in soft and stiff modes by assigning model-based joints stiffness for performing different grasp modes.
Article
Shape memory alloys (SMA) can be used to create actuators that are simple, high strength, and inexpensive. These benefits come at the cost of low electrical efficiency, moderate lifetime, and complex mechanical behavior that makes them difficult to design into new applications and products. To improve the integration of SMA actuators - in particular thin SMA wires heated by passing electric current through them - into modern mechanical applications, we have created tools for modeling SMA mechanical and thermal behavior in dynamic systems and under feedback controls. Thermo-electro-mechanical constitutive models are implemented in a multibody dynamics software where they are easily applied to an actuator emplaced in a multibody dynamic system. Mechanical behavior is modeled with 1D constitutive equations. The material state determines the electrical resistivity of the material which drives ohmic heating, while thermal cooling is based on a heat transfer analysis of thin cylinders. These models contain states which are very difficult to measure experimentally (such as crystal phase fraction) and thus provide insight into the material behavior and design that experimental results cannot offer. This thermomechanical model is used in conjunction with sliding mode control - historically difficult to simulate in numerically integrated models - to develop a working ball-on-a-beam setup in which the ball position is controlled via current passed through an SMA wire and with application of an original self-sensing method. The constitutive model is developed in the multibody dynamics software MSC ADAMS and validated through the simulation of the same system.