18-06-2013, 12:15 PM
Flexonics
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Abstract
We introduce a new approach to design and fabrication of mechatronic
systems called flexonics. Flexonics integrates structural, mechanical and electronic
elements based on 3D printing techniques, in particular inkjet printing. This
paper outlines some principles for flexonic design, which is based on flexion
rather than sliding or rolling motion as in traditional mechanics. It describes our
preliminary explorations of materials and processes. And it describes a kinematic
approach to joint design that has so far led to one promising new design. Finally,
we give a brief prospectus of future applications of flexonics.
Introduction
In this paper, we describe a new class of mechatronic devices, and propose methods
for their fabrication and design. Our overall goal is to build fully functional
mechatronic devices without assembly. These devices will integrate structural,
mechanical, and electronic components during fabrication using an all inkjet printing
process. In order to achieve the above goal, we intend to develop a complete
3D printer, and create a vocabulary of flexonic components (both mechanical and
electronic).
Flexonics are radically different from traditional electromechanical systems in
both form and manner of construction. The term “flexonics” derives from the fact
that the proposed mechatronic systems achieve motion through flexion (bending).
There will be no traditional sliding or rolling movements in actuators, sensors, or
joints. Thus, flexonic devices will not utilize or need gears, bearings, or any sliding
surface. This limits traditional design drawbacks such as friction, backlash,
and mechanical wear. However, new issues arise when designing with flexures:
material fatigue, inherent elastic energy storage, limited range of motion, and undesirable
degrees-of-freedom require a different design strategy.
Related Work
Various methods of 3D printing have emerged in recent years [8,9]. They are
used for rapid prototyping during design and for building molds for manufacturing.
Various systems use materials and layering processes ranging from fragile
photo-cured resins (Stereolithography or SLA), to powders melted by a powerful
laser (Selective Laser Sintering or SLS), to extruded thin thermoplastic filaments
(Fused Deposition Modeling or FDM – Stratasys, Inc.). With FDM, fully functional
devices can be made from either rigid ABS plastic or flexible elastomer.
Complex parts with regions of overhang and irregular surfaces can be easily built
using a system that integrates build material and sacrificial support material. We
have built several flexure-based designs using FDM. These devices are shown in
subsequent figures as a demonstration of flexonic components (Sect. 4). However,
FDM has disadvantages when applied to the problem of fabricating a broad
range of flexonic devices, including dielectric elastomer actuators. Few materials
are available, resolution is relatively poor, part strength and properties are influenced
by build orientation, and electronic components are not realizable.
Shape Deposition Manufacturing (SDM) [1] has similar goals to flexonics
manufacturing: integrating functional components with a 3D manufacturing process
so that complex devices can be built without assembly. The difference is that
SDM employs several traditional technologies – electronics or mechanical components
are built separately and then “dropped in” during the build. SDM does
not employ one, but a large family of manufacturing processes. By contrast, our
goal is to build complete, fully functional electromechanical systems using a 3D
inkjet printing process.
Fabrication of flexonic devices
Process
We are developing a novel manufacturing process requiring two steps. The first
step is software based and follows a procedure similar to traditional rapid prototyping
methods. Initially, an appropriate 3D CAD model of the device must be
created. Because flexonics integrates all components without assembly, this
model can be viewed as a single part. Depending on the orientation and design of
a device, there may be regions of unsupported overhang. Therefore, within this
step, locations for support material should be determined and added to the model.
This support material will be sacrificial; following fabrication, it will be removed
either by dissolving or breaking. Continuing within the software stage, the model
would be divided into horizontal layers.
Materials
The 3D printing process requires materials meeting two criteria: First, the material
viscosity must be low enough to allow controlled ejection from the inkjet during
fabrication. Second, the material must remain a liquid for sufficient time before
jetting so as not to clog the print orifice, yet solidify within a reasonable time
after jetting. These requirements constrain the usable material set to those that can
be dissolved into solution, or those that will flow when heated. Polymer and oligomer
solutions, polymer resins, molten solders, and nanoparticle suspensions are
all suitable for printing.
In Sect. 4, our current work using organic materials for electronics is discussed
in greater detail. These materials include solution-based oligomers and nanoparticles.
Materials for mechanical components that are being explored include thermoplastics
like ABS and silicone thermoplastic and solvent-based elastomers.
Flexonics integrates joints, actuators, structural volumes, and electronics. Therefore,
it is necessary to match material properties to component function. Joints require
both rigid and flexible regions; two polymers with high and low elastic
moduli are necessary. Actuators require the properties of joints, but also need
conductive and non-conductive polymers as well. Structural volumes tend to be
rigid; however, the overall stiffness and anisotropy might be tuned using a selection
of polymers. Flexonic manufacturing also requires a suitable sacrificial material
for supporting overhanging geometry and complex forms during fabrication.
Water-soluble thermoplastics are commonly used in FDM machines for sacrificial
volumes and give a good starting point for our work as well.
Electronics
Active electronic components based on organic semiconductors are the natural
choice for use as control elements in flexonics. Organic electronic devices may be
fabricated using solution-based processing technologies such as inkjet deposition
to produce complete electronic circuits without the use of lithography and/or vacuum
processing. Such properties make organic electronic circuits ideally suited
for use as integrated control elements in flexonics applications. Circuits may be
fabricated on the polymer surfaces of the flexonics using inkjet-based processing
facilities integrated into the flexonics manufacturing flow.
Mechanical components
Traditional mechatronic devices require several components: structural support,
mechanical joints and linkages, actuators, and control circuitry. These requirements
do not change within the flexonic environment. Rather, the components
take on different forms. For some, this is simply a matter of designing a suitable
flexonic replacement component. For others, the discrete parts of the traditional
design are replaced with one integrated, highly complex mechanism.
Simple joints
One of the requirements for most mechanical designs is some form of rotary joint.
A traditional joint might employ some rolling or sliding interaction between the
two connected links in the form of a hinge or bearing assembly. The flexonic
counterpart moves by deflection, where elastic energy storage provides an increasing
resistance to rotation. Other issues when designing flexonic joints include
range of motion, axis-drift, off-axis stiffness, and stress concentrations [11]. We
have identified several joint designs for possible inclusion within flexonic mechanisms;
these are shown in Figs. 3, 6, and 7.
Complex mechanisms
While flexonics can provide a base set of simple mechanical joints, it also encourages
highly complex specialized devices. Traditional motion generation mechanisms
can be redesigned within the flexonics design space. As an example, consider
the Peaucellier linkage illustrated in Fig. 8. The function of the linkage is to
convert rotational motion into pure translation. Under traditional design methods,
the links are separate parts, connected to each other via pivots at their ends. The
flexonic counterpart, however, can be one continuous piece. A partial realization
of this device is shown in Fig. 8 with traditional pivots being replaced by smalllength
flexures (also called notch flexures or “living hinges”).
Mechanisms for specific functions like gripping are also relatively easy to design.
One such possibility is shown in Fig. 9. Instead of discrete regions of flexion
as used in the Peaucellier linkage, we implement distributed flexion along the
gripper “fingers”. A simple translation of the center rigid beam brings both fingers
together.