Design of a Compliant Industrial Gripper Driven by a Bistable Shape Memory Alloy Actuator

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 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.

Full text

DESIGN OF A COMPLIANT INDUSTRIAL GRIPPER DRIVEN BY A BISTABLE SHAPE
MEMORY ALLOY ACTUATOR
Dominik Scholtes1a, Stefan Seelecke1,2, Gianluca Rizzello2, Paul Motzki1,2
1 Center for Mechatronics and Automation Technologies (ZeMA), Saarbruecken, Germany
2 Department of Systems Engineering, Department of Materials Science and Engineering, Saarland
University, Saarbruecken Germany
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 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.
a
Contact author: dominik.scholtes@imsl.uni-saarland.de
1. MOTIVATION AND INTRODUCTION
The global movement to reduce energy consumption and
increase efficiency is also a large topic in the industrial
environment. Parallel to this, with the development of industry
4.0, more digitalization is entering all large companies. That
includes the need for smarter machines with more sensor
integration and intelligent condition monitoring.
Besides solenoids and electric engines, pneumatics is still
the state-of-the-art actuation technology in most companies.
Although it has many benefits, pneumatics is known to be
inefficient mainly due to losses in the piping, valves and
connectors. Especially in clean productions the pressured air
brings additional particles and oil to the process. Monitoring and
control of pneumatic systems is based on external sensors that
only supply basic data. Especially in assembly lines a large
number of these pneumatic grippers is used to position, handle
and transport workpieces. The goal of the presented work is to
develop a functional prototype that displays the potential to
replace these grippers in the future. The developed gripper must
be an energy-efficient, smart and lightweight alternative that
completely eliminates pneumatic systems and components but
comes with similar functionality like todays standard grippers.
On the way to new solutions that address both challenges,
efficiency as well as digitalization, smart materials can play a
large role. Besides their actuator properties they can
simultaneously be used as a sensor. This leads to sensor-less
actuation systems, which are lightweight, compact and energy-
efficient. While Dielectric Elastomers (DEs) are known for their
high energy efficiency and fast actuation frequencies [1], Shape
Memory Alloys (SMAs) feature the highest known energy
density, which again leads to high force outputs in small
installation spaces [2]. As the activation is based on a thermal
effect, the frequency and efficiency of an SMA itself is restricted.
Proceedings of the ASME 2020 Conference on Smart Materials,
Adaptive Structures and Intelligent Systems
SMASIS2020
September 15, 2020, Virtual, Online
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Therefore, smart control and design strategies have been
developed to overcome those drawbacks. Recent research shows
various approaches for energy-efficient SMA based actuator
systems [3], [4].
Furthermore, the design of traditional industrial grippers
is based on an assembly of moving parts with linear guides,
bearings and hinges [5]. The need for precision parts combined
with the high level of assembly effort, makes traditional grippers
expensive and complex. All guides and bearings are prone to
wear, especially in process environments with abrasive media. In
compliant mechanisms on the other hand, which are already
known from micro grippers, all joints and guides are replaced by
flexible parts. The whole gripper kinematic can be fabricated as
a single part, which can nowadays be realized with a variety of
3D printing processes. Because of the absence of tolerances that
are inherent to mechanical guides and bearings, the precision and
repeatability is increased by the use of solid state joints [6].
2. FUNDAMENTALS
SMA actuators in are in most applications used in the form
of thin wires. They are easily deformed at low temperatures and
return to their original shape when heated above their transition
temperature. This behavior is called the two-way shape memory
effect (SME) and is based on a phase transformation from
martensite (low temperature) to austenite (high temperature) [7].
Todays most investigated and commonly used alloy is Nickel-
Titanium (NiTi) with a transition temperature of approx. 90°C
and a maximum stroke of approx. 5 %. There are already a
couple of applications for SMA based actuators in automotive
industry and customer products. For example valves and latching
mechanisms [8], [9]. In recent research concepts, SMA driven
pick-and-place systems, robotic applications and medical
devices have been developed [10]–[13]. In these concepts, the
advantages of SMA actuators like high energy density, low
weight, silent actuation, high force outputs and short activation
times have been shown. 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. This can for example be achieved in two different
ways: with high speed activation and energy-free position
holding [3], [4], [11]. For a fast activation, the current for heating
the wire is a short pulse with large amplitude, which leads to an
adiabatic heating behavior of the wire. Various control strategies
can then be applied to keep the SMA from overheating. Energy-
free position holding can easily be achieved with the help of
friction and, as presented in this work, with a bistable
mechanism. For both methods antagonistic SMA wires are
needed. The bistable mechanism brings the advantage of a
transmission ratio of the SMA stroke and high retention forces at
the actuator output. Combining a bistable SMA actuator with
high speed activation reduces the energy consumption of the
system to a minimum.
In the industrial production, grippers are the active
interface between handling system and workpiece. Their
functions can be to temporarily maintain, as well as change a
definite position and orientation of the workpiece, retain process
forces and moments and fulfill specific technical operations [16].
The most relevant types of grippers for this work are so called
jaw grippers. The prehension of an object or workpiece with
these grippers can be organized in three groups: pure enclosing
without clamping, partial form fit combined with clamping force
and pure force closure [16]. The jaw movement itself can mainly
be classified in two forms that are depicted in FIGURE 1:
angular jaw movement a) and parallel jaw movement b). Both
can be inside and outside gripping.
FIGURE 1: SCHEMATICS OF THE JAW MOVEMENT OF
COMMON INDUSTRIAL GRIPPERS [16]
The drive system that moves the gripper kinematics and thus the
gripping jaws is in most cases pneumatic driven. Also, electric
drives and solenoids are used as actuators. Up to now, smart
materials could not yet be established as actuators for macroscale
grippers, although research on the field has been done. In the area
of micro grippers, there are several examples that show the
benefits of piezoelectric elements and shape memory alloys [17],
[18].
The same holds for kinematics that are based on flexure hinges.
Compliant mechanisms are well established in micro grippers,
due to the fact that existing bearings and guides are too large and
add too much tolerance to a high precision microgripper. But
there have already been approaches to establish compliant
kinematics in industrial macro-scale grippers [19]. Benefits like
smaller tolerances, better precision, less individual parts stand
against drawbacks like elastic counterforces and higher
development effort.
The combination of a drive unit based on SMA wires and
compliant gripper kinematics is a promising new approach for
smart and innovative gripping systems.
3. DESIGN OF THE COMPLIANT MECHANISM
As a first step in the development of the functional
prototype, a concept of the compliant kinematics is created. The
goal is to get a first working version, at his point without
requirements for construction space. The basic design of the
compliant mechanism is visualized in FIGURE 2. It features two
trapezoidal leaf hinges (3) and two leaf spring joints (2) that
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basically work as pushing rods and connect to the drive unit. The
whole prototype is made of one piece via FDM printing. The
chosen material is PETG, due to its high flexibility and
toughness combined with good printing results. The exemplary
workpiece (1) has a diameter of 15.9 mm, a thickness of 2.9 mm
and a mass of 0.005 kg.
FIGURE 2: FIRST 3D-PRINTED CONCEPT OF GRIPPER
KINEMATICS BASED ON FLEXURE HINGES
The concept shows angular jaw movement and has no large
transmission ratio from input to jaw stroke. After the prototype
is manually tested and shows the desired stroke behavior, several
iterations loops follow up. The design is modified to reduce the
spring forces of the flexure hinges, to accommodate a flat
actuation unit and interchangeable gripping jaws. The result is
described and displayed in section 5.
4. DESIGN OF THE BISTABLE SMA-ACTUATOR
The patent of Motzki and Seelecke, on which the developed
SMA actuator is based, includes a variety of basic designs for a
bistable SMA actuator [20]. To get relatively compact outer
dimensions for the gripper actuator, the SMA wires are set
parallel to the leaf spring as shown in FIGURE 3.
FIGURE 3: SKETCH OF AN SMA ACTUATOR WITH A
BISTABLE PIVOT MOUNTED LEAFSPRING AND IN-PLANE
ARRANGEMENT OF THE SMA WIRES [20]
Number 41 and 42 in the figure are the SMA wires, 2 the leaf
spring and 8 the lever arms the SMA wires are attached to in the
points 51 and 52. Key features of the actuator mechanism are a
transmission of the SMA stroke to the actuator stroke in 4 via the
length of the lever arms and most important, the two stable
energy-free positions of the system. The output force of the
actuator is determined by the leaf spring parameters. Here the
most relevant factor is its thickness. Although the exact force is
not important for this first functional prototype, it has to be
strong enough to overcome the counterforce of the compliant
mechanics and secure the workpiece in the gripper jaws. One
gripping cycle (closing jaws and opening them again) should last
about one second. As this frequency is only depending on the
wire diameter, if no additional cooling media is taken into
account, the maximum possible wire diameter is 100 µm [21].
To gain sufficient output force, multiple NiTi wires must be
bundled mechanically in parallel. The wire bundle made for this
actuator is displayed in FIGURE 4.
FIGURE 4: SMA BUNDLE OF 6 NITI WIRES THAT ARE
RESISTANCE WELDED TO A STAINLESS-STEEL SHEET AND
ADDITIONALLY SECURED WITH SUPERGLUE
Six 100 µm NiTi wires are joined to stainless steel sheets that are
5 mm wide. This is realized with a resistance welding process
and a controlled pretension procedure for each single wire. The
high joint strength of dissimilar resistance welding of thin NiTi
wires to stainless steel has been shown by Scholtes et al [22]. To
make the weld spots more durable against peel stress and more
robust during the assembly process of the actuator, a bead of
temperature resistant super glue is added. The length of these
bundles depends on the stroke needed to let the leaf spring snap
from one position to the opposite. With the desired stroke in the
middle of the bistable element and its length, the angle in the
pivot points can be modelled with the help of a spline function
in CAD. The presented actuator features 4 mm stroke of the
bistable spring. The distance between the pivot mounts is
39 mm. With a lever of 1.5 mm and an SMA stroke of 3.5%, the
resulting wire length is 35 mm. In FIGURE 5 the final CAD
assembly of the actuator is displayed. The lever for the NiTi
wires to rotate the spring clamp is a result of the clamp’s and the
steel sheet’s thickness.
FIGURE 5: CAD MODEL OF THE BISTABLE SMA ACTUATOR:
1: LEAFSPRING WITH BOLT AS STROKE OUTPUT; 2: SMA
WIRE BUNDLES; 3. PRECISIONS PINS AS PIVOT BEARINGS
4. FRAME WITH BUSHINGS; 5: SPRING CLAMP WITH WIRE
BUNDLE ATTACHEMENT POINTS
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All parts, except the leaf spring and the wire bundles are 3D
printed with an SLA printer, due to better resolution and
precision than FDM printers. The clamps (5) are mounted on
steel pins (3) that rotate in bushings in the frame (4). The
electrical connection of the SMA wires is realized with adhesive
copper foil, which is added to the clamps. The bundles to each
side of the spring are connected electrically in series, while
isolated from the opposing bundle pair.
A microcontroller-based electronic system is developed to
supply and control the actuator. It also processes the resistance
signal. A 24 V Volt signal, like for example produced by a PLC
control, serves as input signal. With a PWM, a high voltage pulse
supplies the SMA wires with a specific amount of energy. The
activation period lasts about 100 ms. Depending on the ambient
temperature, 1400 – 1800 mJ of energy is needed for the bistable
element to snap to the opposite position. The change in resistance
of the NiTi wires is measured and sent back to the PLC control
as a position feedback of the actuator.
5. ASSEMBLY OF GRIPPER AND FIRST
VALIDATION
To mount the actuator to the gripper kinematics, a
clamping system is designed. It allows to disassemble the gripper
easily and facilitates maintenance and fine tuning of the system.
In FIGURE 6 an exploded view of the gripper assembly in CAD
is displayed. The actuator mounting parts are screwed to the
gripper body with integrated kinematics and thus the actuator is
held in place. It is then connected to the base of the leaf spring
joints with a bolt and two nuts.
FIGURE 6: CAD MODEL OF THE GRIPPER ASSEMBLY IN
EXPLODED VIEW: GREEN IS THE COMPLIANT MECHANICS.
BLUE ARE THE CLAMPING PARTS FOR THE ACTUATOR:
These nuts allow to adjust the distance between gripper
mechanics and actuator and thus the distance between the
gripping jaws in open and closed state as well as the “neutral
position” of the compliant gripper. In FIGURE 7, the fully
assembled gripper with electrical connections is displayed. The
actual gripping jaws are exchangeable to fit different workpieces
and also adjust the distance in between. For the workpiece used
in this project, jaws with a partial form fit combined with
clamping force are applied. The whole gripper weighs about
150 g.
FIGURE 7: PHOTOGRAPH OF THE FULLY ASSEMBLED
COMPLIANT GRIPPER WITH A BISTABLE SMA ACTUATOR
HOLDING A SAMPLE WORKPIECE
For first validation tests of the gripper, an experimental setup is
designed. The goal is to measure the movement of one gripping
jaw over time in comparison to the input signal. The position
measurement is achieved by a Keyence LK-G37 laser
triangulation sensor, while the gripper is fixed to a breadboard.
The data acquisition is done by means of NI LabVIEW.
FIGURE 8 displays an exemplary result of the measurements.
The 24 V input signal from the PLC control, which corresponds
to a logical 1, is visible in red and.
FIGURE 8: MEASUREMENT GRAPH OF TWO ACTIVATION
CYCLES OF THE SMA DRIVEN GRIPPER
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The input signal switches to 1 for 1.2 s and then back to 0. This
cycle is then manually repeated after about 4 s. The movement
of the gripper jaw follows the input signal. The maximum stroke
is 3.4 mm, but stabilizes at 3.2 mm. As only one jaw is observed
and because they move in parallel, an overall stroke of the jaws
of 6.4 mm is achieved. The overshoot that can be observed at
opening and closing is due to an SMA stroke that is larger than
necessary and bends the bistable spring further than its own
stable position. With reduced energy input or an improved
control electronics, this behavior can be optimized. The
measurements show that switching between opened and closed
state takes less than 100 ms. A cooling time for the wire bundles
of 1.2 s is sufficient and can even be reduced with further
optimized control of the activation current.
6. CONCLUSION AND OUTLOOK
By means of relatively new developments and research
results in the field of SMA actuator technology, a sophisticated
new gripping concept for the industrial environment is presented.
The feasibility and functionality of an SMA driven bistable
gripper based on a compliant kinematics is illustrated in the
presented work. The gripper achieves a gripping frequency of
0.8 Hz at a jaw opening stroke of 6.4 mm. The switching time
itself is under 100 ms. It can grip, hold and transport a small
workpiece safely. The system is lightweight, features low energy
consumption, is noiseless and works without any additional
media except electricity. The self-sensing properties of NiTi
wires allow state detection of the gripper as well condition
monitoring without any additional external sensors. The
resistance signal of the actuator wires will in the future also be
used for a closed loop control of the activation pulse. Therefore
the snapping point of the spring is detected as a characteristic
behavior of the wire resistance. In the moment of detection the
current supply is turned off. This leads to even further reduced
energy consumption, faster actuation and cycle times, as well as
the ability to adapt to changing ambient conditions. A first
comparison to a standard pneumatic gripper promises savings in
energy of up to 80 %.
ACKNOWLEDGEMENTS
The authors would like to thank Robert Bosch GmbH for
funding the research project.
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