FLEXIBLE ELECTRONICS
Soft magnetic “skin” deforms,
provides localized force sensing
IBy Bill Schweber n a proof-of-concept project based on measuring magneticfield
changes, a research team developed a way to sense
location and depth of deformation of an elastomeric “skin”
embedded with millions
of magnetic microparticles.
The quest to replicate
some of the many
functions of human
skin involves multiple
unique approaches and
forms. One of the many
challenges is that skin
is soft and deformable,
and senses at thousands
of points, so using
discrete sensors and
interconnecting wiring is
impractical.
However, researchers
at Carnegie Mellon
University (CMU) are
pursuing a very different
approach to meet one of
the needs of an electronic
Fig. 1: An overview of the sensor showing the elastomer composite loaded
with magnetic microparticles cured under a field (a); the composite retains
the stretchability and flexibility of the host substrate and is compatible with
stretchable circuitry (b); and the magnetic field measured at the magnetometer
changes with the deformation of the elastomer (c). (Source: Carnegie Mellon
University).
skin (e-skin). They’re using a deformable, silicone-based
covering that’s embedded with micro-magnetic particles. They
then sense changes in
the orientation of these
particles via changes in
the overall magneticfield
intensity and orientation,
and process the
data using complex, sophisticated
classification
algorithms to determine
the nature and location
of the deformation.
This soft skin is composed
of silicone rubber
elastomer embedded
with millions of microparticles
that move as
Fig. 2: In the experimental setup, a 4-DoF desktop robotic arm is used to apply
pressure with a plastic indenter (r = 1.5 mm) while the load-cell output, location,
and magnetic field are sampled and stored. (Source: Carnegie Mellon University).
the skin-like material
makes contact with an object and deform (as shown in figure
1). The magnetic Ne-Fe-B microparticles were on the order
of 200μm in diameter. They used these microscale magnetic
particles to reduce the intensity of internal stress concentrations
when a mechanical load is applied and to retain the flexible or
stretchable properties of the “skin.”
An arrangement based on a Melexis MLX90393 micropower
Triaxis magnetometer with electronics on a standard SparkFun
board was attached to the elastomer to sense changes in its
magnetic field, which are the result of localized pressure and
deformation. The magnetic skin required no electrical connection
to the underlying magnetometer board; instead, it required
proximity for the magnetic flux changes to be detected.
Although millions of individual magnetic particles are distributed
throughout the skin contributing to the magnetic field,
they measured it using
a simple three-axis
output that preserves
information about
the deformation. By
implementing advanced
pattern-recognition and
classification algorithms
on the changes in magnetometer
output, the
arrangement can infer
the location and direction
of the deformation,
somewhat analogous to
what real skin (and the
brain) can do.
To quantitatively validate
this proof-of-concept
sensing-skin project,
they programmed
a 4-degree-of-freedom
(DOF) robotic arm to
apply pressure of an indenter (pressure measured on a load cell)
and collect data on magnetic field change (see figure 2).
They collected this
data over a 15mm2 and
5mm diameter circle.
Using a model-based
analysis, they developed
classification algorithms
and were able to localize
pressure with an accuracy
greater than 98% to
within a 3mm2 area (on
average) for both grid
and circle patterns.
The CMU team is also
focused on applicationspecific
scenarios, such
as “soft robotics” in
medicine for gastrointestinal endoscopy. Their next objective
is to enhance the ability of this “skin” to sense and localize the
force and deformation when handling cylindrical objects such
as catheter endoscopes. Full details, including modeling of
circular and square indents, numerical analysis, and results, are
in their paper “Soft Magnetic Skin for Continuous Deformation
Sensing” published in Advanced Intelligent Systems. Further
details of the modelling and arrangement are in their posted
Supplemental Material.
This article first appeared on Electronic Design –
www.electronicdesign.com
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