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Febin Sathar

on 15 July 2014

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The applications include artificial skins for humanoid robots, robotic prosthetics, soft wearable robots human-friendly robots for human-robot interactions and human-computer interface.
Although the smallest channel size is 200 μm, limited by the resolution of the 3-D printer used for making molds, this could be further reduced by implementing different manufacturing methods such as micromachined molds and soft lithography.Compact and sensitive skins could be achieved.
The nonlinearity in pressure sensing is due to the nonlinear areal reduction rate of the rectangular microchannels when contact pressure is applied. Improvement on the linearity in pressure sensing is currently being investigated by changing the channel geometry.
A new manufacturing method that does not require the EGaIn injection step is another ongoing effort to enable faster and higher volume production.
The prototype is able not only to measure the magnitude of the stimulus but also to identify the type of stimulus. One strategy is comparing the signs of the three sensor signals such as
1) If V1 > 0 and V2 < 0, the stimulus is x-axis strain,
2) If V1 < 0 and V2 > 0, the stimulus is y-axis strain,
3) If all three sensor signals are positive, the stimulus is z-axis strain.
Once the type of the stimulus is identified, we can estimate the magnitude of the stimulus from the calibration results.
The limitation of this strategy is that it is not capable of decoupling more than one type of stimuli at the same time.
Stimulus differentiation
Euteric Gallium-Indium
An alloy of Gallium and Indium, Liquid state at room temperature.
High surface tension
High electrical conductance
Therefore an ideal conductor for a soft sensor
Sensors functional at strains over 100%t
Why EGaIn?
A highly elastic artificial skin was developed using an embedded liquid conductor. Three hyper-elastic silicone rubber layers with embedded microchannels were stacked and bonded.
Channel dimensions : 200 μm (width) × 300 μm (height)
The overall size : 25 mm × 25 mm
Thickness : 3.5 mm.
The characteristic modulus : 63 kPa(approx)
The sensor is functional up to the strain of approximately 250%.
For strain sensing, the calibration results showed linear and repeatable sensor signal. The gauge factors of the skin prototype are 3.93 and 3.81 in x and y axes, respectively
the minimum detectable displacements are 1.5 mm in x-axis and 1.6 mm in y-axis.
For pressure sensing, the prototype showed repeatable but not linear sensor signals.
BONDING : The cured layers are bonded to make a single sensor structure by spincoating the same liquid silicone between the layers
EGaIn INJECTION: Using 2 syringes EGaIn is injected into the microchannels, where one syringe injects EGaIn and the other extracts air captured in the microchannels

OVERALL SIZE of the artificial skin :
25mm x 25mm
Using the combination of the signals from the three sensors, the device is able to detect and distinguish three different stimuli: x-axis strain, y-axis strain, and z-axis pressure
All three sensor layers are connected through interconnects [p2 & p3] between layers, making one circuit that is electrically equivalent to three variable resistors connected in series.
Pressing the surface of the elastomer skin decreases the cross-sectional area of the microchannels and increases their electrical resistance.
Pressure Sensing
Sensing principle : When microchannels filled with EGaIn are deformed by either pressing or stretching, the electrical resistance of the microchannels increases due to their reduced cross sectional areas, increased channel lengths, or both.
Highly deformable
Multi modal sensing capable of detecting strain and contact pressure simultaneously
Fabricated using the combined concept of hyperelastic strain and pressure sensors with embedded microchannels filled with liquid conductor.
Liquid conductor : Euctectic Indium Gallium (EGaIn)
Artificial Skin : Characteristics
Plastic molds are prepared using a 3-D printer & liquid silicone is poured into the molds
The liquid silicone is cured at room temperature for >3 hrs
Separate sensor layers are cast
The development of highly
deformable artificial skin with
contact force (or pressure)
and strain sensing capabilities is
a critical technology to the areas
of wearable computing, haptic interfaces, and tactile sensing in robotics.
The microcontroller transfers the resistance changes to the computer which after being read by a MATLAB program, a virtual model of the skin was generated
System integration
While the prototype displays negligible hysteresis in strain sensing, it shows noticeable hysteresis in pressure sensing especially in a high pressure range over 40 kPa.
As the compression rate increases from 1 mm/sec to 2 mm/sec, the hysteresis level decreases
Compression was applied multiple times at the center of the top surface of the skin with a flat circular surface (diameter: 10 mm) up to 50 kPa, with various compression rates (1 mm/sec, 1.5 mm/sec, and 2 mm/sec).
When the material experiences strain in the axial direction of the channels:
The overall channel length increases
The cross-sectional areas of the channels decrease
Causes an increase in the overall resistance of the channel
The theoretical relationship between resistance change and strain
Strain sensing
3 soft sensor layers made of silicone rubber
Layers 1 & 2 :
have straight-line microchannels with a strain gauge pattern that results in directional sensitivity in axial directions as well as surface pressure sensitivity.
Layer 2 is placed on top of Layer 1 with a 90° rotation
Layer 3: has circular patterned microchannels that are sensitive to surface pressure but are not directionally sensitive to strains along any axis
Skin Design
Since the sensor signals for strain sensing are approximately linear, simultaneous strain measurement of both axes can be achieved by determining a calibration matrix C, when y and s are measured reference and sensor signals,respectively

[1]. Design and Fabrication of Soft Artificial Skin Using Embedded Microchannels and Liquid Conductors, IEEE Sensor Journal, August 2012

[2]. Soft artificial skin with multimodal sensing capability using embedded liquid conductors,” in Proc.IEEE Sensors Conf., Limerick, Ireland, Oct. 2011, .

[3].Feature detection for haptic exploration with robotic fingers, Int. J. Robot. Res
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