IEEE Robotics & Automation Magazine - September 2016 - 99

Shake Table Motion

Micropatterned Ribbons
Load Cell
Soft Polyurethane

Shake Table

(i)
250 µm

500 µm

(b)
+Q

A

Φ

∆θ
-Q
(ii)

(i)

S N

S N
S N

S

Electric
Displacement

S N

10-35 mT

18.00
15.00
12.00
9.00
6.00
3.00
0.00

f (θ )

S N

F

θ

L mL
H θ ε
nε

10 mm

500 µm
Ribbon Surface
(ii)

Zero Magnetic Field
(iii)

Power
Supply

MRE Base Isolator

ε nε

(iii)

Magnetic Microdomains
(iv)

50
40
50
30
20
10
10
0
0.0 0.5

m=1
n = 100

θ

1.0

Simulation

1.5
Theory

(iv)
(a)

(c)

Figure 4. (a) In (i), an ultrasoft polyurethane elastomer is embedded with rigid micropatterned ribbons that slide past each other.
The tabs are enclosed in a chamber filled with MR fluid. In (ii), the surface of each ribbon is patterned with an array of aligned
microchannels. In (iii), in the absence of a magnetic field, the MR microparticles are randomly dispersed. In (iv), under an external
field of 10-35 mT, the microparticles form magnetic domains that are confined in the microchannels [45]. (b) The experimental setup
for evaluating and characterizing the performance of the MRE base isolator prototype proposed in [48] (courtesy of Y. Li). (c) In (i),
an ERE with layered mesostructures in an undeformed state and (ii) undergoing simple share deformation by an applied force and
voltage. In (iii), the distribution of electric displacement in the layered mesostructures calculated by a finite-element model. In (iv), the
normalized effective permittivity f as a function of the layer orientation i calculated by the numerical model and theory [53].

settling, sealing issues, and environmental contamination,
which makes another member of the MR material family more
preferable to enable tunable stiffness: MR elastomers (MREs)
and foams. While MRFs can be coupled with elastomers
through fluidic channels, a sensibility to external magnetic
fields can be directly embedded in the elastomer material by
including the MR particles during the fabrication/curing process. This is the case with MREs, which consist of a polymeric
matrix with embedded micro- or nanosized ferromagnetic particles, such as carbonyl iron [46]. Such materials have been
already investigated and applied in various engineering fields,
especially as vibration reducers or isolation systems. A recent
and comprehensive survey of current applications, manufacturing processes, and modeling approaches can be found in [47],
but so far studies have focused on MREs' static properties of
stiffness change, aiming at developing adaptive layered structures. In [48], an adaptive MRE base isolator shaped as a laminated structure demonstrated the capability of remarkably
changing the lateral stiffness of the isolator by up to 1,630%
under a medium level of magnetic field [Figure 4(b)]. MREbased sandwich beams with adjustable rigidity have been theoretically evaluated in [49] and [50], while experimental
verifications are reported in [51].

As with MRFs, ER fluids have their elastomeric versions, but the use of ER elastomers (EREs) is much less
widespread. This is probably due to the shortcomings of
the use of ferroelectric particles, as the maximum-yield
stress they generate is, on average, two orders of magnitude lower. A few studies have focused on the experimental evaluation of the stiffness change, and an interesting
area is represented by the theoretical study of the distribution of the ferroelectric particles in the elastomers. The
role of anisotropy has been deepened from a theoretical
point of view by Liu et al. [52] in the microscopic structure
and by Cao and Zhao [53] in the mesoscopic structure
[Figure 4(c)]. Even if not validated, the theoretical values
suggest stiffness variations of 300 kPa if the layered mesostructures are arranged in parallel configuration.
Low Melting Point Materials and Glass
Transition-Based Softening
A very recent and impressive example of the exploitation of
low melting point materials (LMPMs) as variable-stiffness
systems has been provided by Cheng et al. [54]. They found
that an extraordinary functionality is enabled by an
extremely low-cost and commercially available material:
SEpTEMBER 2016

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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99
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016
