Detection of Normal and Shear Stresses by
MEMS Tactile Sensors with
Piezoresistive Micro-Cantilever and Elastomer
Haruo Noma, Group Leader ATR IRC
ATROsaka University
OMRON Corp.
Nitta Corp.

BackgroundIndustrial robots in the factory

Next generation robots
near our life environment
ASIMO (Honda)
RI-MAN (Riken)

Requirement of the tactile sensor inrobots to assure safety for human body
Tactile sense of human skinlSensitivity: tens of Pa ~ hundreds of kPalSpacial resolution: ~ 1 mm
With shear senselSensitive to both normal and shear stresses

W/o shear sense

Target: 50 kPa = 500 g/cm2

Proposed Tactile Sensor
Elastomer(polydimethylsiloxane; PDMS)
Micro-cantileverwith piezoresistor (p+-Si)

Within 1mm

Within 1mm

Si Substratel

Detect stress by deformation of micro-cantilevers

High sensor densitylStress detection by opposite inclined cantilevers

Simultaneously sensing ofnormal and shearstressesl

Embedded in elastomer

Human-friendly soft surface

Mechanism of Tactile Sensingz

Shear stress

Normal stress
x+ΔRn+ΔRn+ΔRs-ΔRs

Piezoresistive layer

ΔR: piezoresistive outputOpposed cantilevers deform in same way, Opposed cantilevers deform in different way,-> Piezoresistive outputs: same signs-> piezoresistive outputs: opposite signs
Applying normal stress and shear stress at the same time
Summation of outputs: cancel out shear stress ⇒ Normal stress measurement
Subtraction of outputs: cancel out normal stress ⇒ Shear stress measurement

Normal and shear stresses can be selectively detected.

Mechanism of Tactile Sensing
2D Shear stress
Y axial shear force

daX axial shear force

dcab

Cantilever a & cdeform dominantly

X-Y axial shear force

dcbCantilever b & d
deform dominantlyacbAll cantilever
deform

2D shear stresses can be separately detected.

Fabrication Process of Tactile Sensor
SOI wafer

Cross-sectional view of fabricated cantileverp + -Si piezoresistive layer(B + ion implantation)
SiN layer deposition and patterning(by low-pressure chemical vapor deposition; LPCVD)

Au/CrSi

SiN
SiO2

p+-Si

Au/Cr layer deposition (EB evaporation)and patterning (lift-off)

Holes for sacrificial etching(by reactive ion etching; RIE)SiTop-view of cantilever pattern
p+-Si
Etching holes

Sacrificial etching(by bufferd hydrofluoric acid; BHF)

200 µmElastomer(PDMS) coating, curing
300 µm3x3 unit arrange sample
Dimension of each unit is 0.8mm x 0.8mm

9 set of tactile sensor
is installed within 3mm2.

Laser scanning image

Photographs of Fabricated Sensors
Sensor element

1 mmSensor chip

1.3 cmEmbedded in PDMS

1.8 cmThe size of one sensor element: ~ 1 mm2lCantilevers are embedded in PDMSlThree sensor elements in one chipMeasurement of Response to Normal StressMeasurement SystemZ-stage

Z stage

6-axes force sensor

Force sensor

Tactile sensorz

XY-stageFzx
XY stage

Tactile sensorlTactile sensor is pressed by moving Z-stage.

l

Reference applied force is measured by force sensor.

l

Piezoresitive change of tactile sensor is measured by digital multimeterl

Reference normal stress: sz = Fz/A (A: pressed area)

Measurement of Response to Shear
StressMeasurement System
Z-stage

Z stage
Force sensor

6-axes force sensor

Tactile sensor

Lz

XY-stage

Mx
XY stageSamplexl

Shear is applied to tactile sensor by moving XY-stage.

l

Reference applied moment is measured by force sensor.

l

Piezoresitive change of tactile sensor is measured by digital multimeterl

Reference shear stress: sx = Mx/(AL) (A: pressed area, L: distance from moment center)Sensitivity of Rectangular Type Sensor
Response for normal stressResponse for shear stressSensitive to both normal and shear stresses and good directional characteristic to shear stressl Resistance change has good linearity to the stress.
l Resistance change to shear stress is smaller than calculated value.
2l Detection of stress under 5 kPa (~ 50 g/cm ) is available.
lMeasurement of 2-axial Shear Stressx-directional shear stressy-directional shear stress
0.02

0.02

Left0.01

BottomΔR/R(%)

ΔR/R (%)

0.01
0- 0.01- 0.020RightStress1020
Shear Stress (kPa)

0- 0.01- 0.02

30

Stress- 0.03
0Top
1020
Shear Stress (kPa)Changes of piezoresistance show opposite signs for each cantilever pair.
Summation of outputs from cantilever pairs ~ 0lSubtraction of outputs from cantilever pairs ⇒ shear stress	l

Direction and magnitude of shear stress can be detected30

Measurement by Built-in Bridge Circuit
Shear stress

Normal stress

Strain (x10- 4)

Strain (x10- 4)

2Bridge sensor
linear fit

Bridge sensor
linear fit

1001020
Normal Stress (kPa)

30

2

100

1020
Shear Stress (kPa)30

Sensor output can be obtained as voltage
frombridge circuit fabricated in the chip (1V=1X10-4 strain of NiCr gauge).
lBoth normal and shear stresses can be detected with the same sensitivity.
lTarget of detection: 5 kPa can be detected by fabricated sensor.
lToughness

ΔR/R

0.001

0.000500100200
Normal Stress (kPa)

Targetrange

300(3kg/cm2)

Tactile sensor can be measured up to 300 kPa without cantilever damage.

Target applications
Advantages of this tactile sensor		Density control : Controllable by process & mask
Measuring range control: depend on design of elastomer	
High durability: Mechanically safe	
Cost: Same as semiconductor rule 	Widely used in many field
Industry :Dexterous human hand for industrial robot.
Special tools with traceability function.
Sensor embedded handler belt.

Home: Floor, stair, doorknob or handrail embedded sensor
for daily motion monitor
Automobile: True wheel slip sensor embedded in a tire.
Handle embedded sensor for UI.
Medical： Traceable globes with sensor.
Bed mat sensor for care of decubitus (bedsore)
Human Interface：Touchpad