KOREA UNIVERSITY
Intelligent Robotics Lab
Variable Stiffness Actuation
based on Dual Actuators
Connected in Series and Parallel
Prof. JaeJae-Bok Song ([email protected] )
Intelligent Robotics Lab.
(htt // b ti k
(http://robotics.korea.ac.kr)
k)
Depart. of Mechanical Engineering,
Korea University, Seoul
Seoul,, Korea
Various Variable Stiffness Devices at Korea Univ.
Serial-type Dual Actuator Unit
Safety Joint Mechanism
• Serial connection
• Passive compliance
• Position control
• 1 rotational DOF
• Stiffness control
• Joint type
• Force estimation
• pHRI
• Collision safety
• Environment estimation
Safety Link Mechanism
Parallel-type Dual Actuator Unit
• Parallel connection
• Passive compliance
• Antagonistic actuation
• 3 rotational
i
l DOFs
DOF
• Variable stiffness
• Link type
• Parallel actuation
• pHRI
KOREA UNIVERSITY
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Intelligent Robotics Lab
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Dual Actuator Unit (DAU)
ƒ Redundant Actuation
Actuator 1
• Simultaneous control of position and stiffness
Power
Transmission
for one DOF
• Improved safety
Actuator 2
ƒ Two types of DAUs
Proxima
al link
Distal link
D
Dual Actuator Unit
Serial connection
Parallel connection
Serial-type Dual Actuator Unit
(S-DAU)
Parallel-type Dual Actuator Unit
(P-DAU)
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Intelligent Robotics Lab
Variable Stiffness Actuators
ƒ VSA
VSA--II (variable stiffness actuation)
ƒ ANLES
• Univ. of Pisa (Bicchi
(Bicchi,, 2008)
• Torsion spring + 4-bar linkage
KOREA UNIVERSITY
(actuator with nonlinear elastic
system)
• Tokai Univ. (Koganezawa, 2006)
• Torsion spring + nonlinear guide
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Research Trends: Compliant Actuators
ƒ Antagonistically actuated joint with
quadratic series-elastic actuation
ƒ MACCEPA (mechanically adjustable
compliance and controllable equilibrium
position actuator)
• Georgia Tech. (DeWeerth, 2005)
• Tension spring and curved surface
• Vrije Univ. Brussel (Ham, 2008)
Spring
Roller
Curved surface
KOREA UNIVERSITY
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Intelligent Robotics Lab
Serial-type Dual Actuator Unit
(S-DAU)
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S-DAU : Introduction
ƒ S-DAU
• Connected in series
• Based on planetary gear train
ƒ Features
• Positioning actuator (PA) with high gear ratio
• Stiffness modulator (SM) with low gear ratio
• Force estimation
• Collision safety
• Stiffness estimation
• Environment estimation
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Max torque (Nm)
Max velocity (ra
ad/s)
• Indep. control of position and stiffness
Intelligent Robotics Lab
S-DAU : Principle of Operation
ƒ Planetary gear train
• Two inputs & One output
Æ Useful for actuator unit
with dual inputs
ƒ S-DAU based on planetary gear train
KOREA UNIVERSITY
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S-DAU : Principle of Operation
ƒ No contact with environment
θ DAU = θ PA + θ SM
ƒ Contact with environment
⎧ τ SM = k SM ⋅ θ SM
⎨
⎩τ SM = K T , SM ⋅ i SM
k
⋅θ
⇒ i SM = SM SM
K T , SM
KOREA UNIVERSITY
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Intelligent Robotics Lab
S-DAU : Construction
• Planetary gear train
PA
• Gear ratio
Version 2
29mm
- 690:1 for PA, 56:1 for SM
• Version 1 : 48x61x110 mm, 500g
(including clutch mechanism)
SM
• Version 2 : 26x61x110 mm, 450g
61mm
Version 1
Planetary
gear train
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Intelligent Robotics Lab
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Joint stiffness (N
Nm/deg)
Response to
stiffness change
Joint stiffness (N
Nm/deg)
S-DAU : Position Control / Stiffness Control
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KOREA UNIVERSITY
Intelligent Robotics Lab
S-DAU : Force Estimation
ƒ Force estimation
• No need for an expensive F/T sensor for force control
τ SM = k SM ⋅ θ SM
⇒ τ SM = J T ⋅ F
• kSM : user specified
• θSM : measured by the SM encoder
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Measured force
Estimated force
10
0
-10
-20
0
3
6
9
12
15
18
Time (sec)
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S-DAU : Collision Safety
o
ƒ Joint Stiffness : k SM = k SM
− β vel ⋅ Δω
o
• k SM
: initial stiffness,
Δω = ω SM − ω o
• Example
ωSM= 270 deg/s, ωo = 170 deg/s,
o
= 1.5 Nm/deg, βvel = 0.01,
k SM
Æ kSM = 0.5 Nm/deg just after collision
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KOREA UNIVERSITY
Intelligent Robotics Lab
S-DAU : Parallel Manipulator with Two SS-DAUs
ƒ Experimental Setup
• 5-linkage parallel manipulator with two S-DAUs.
• Independent position and stiffness controllers based on DSP 2812.
• Verifies S-DAU’s
S DAU s force estimation ability using a F/T sensor
sensor.
F/T sensor
Link 5
DAU 1
Joint 1 &
Joint 2
y
DAU 2
Link 4
x
Link 2
Link 3
Position controller Stiffness controller
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S-DAU : Stiffness Estimation
ƒ Stiffness estimation for hard material
• Applied force : 3N Æ 10N
• Stiffness of environment Ke :
- 3.5kN/m (estimated), 3.75kN/m(measured)
•Stiffness of manipulator KSM: about 100N/m
Estimated stiffness
Position information
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KOREA UNIVERSITY
Intelligent Robotics Lab
S-DAU : Stiffness Adaptation
ƒ Stiffness
• Stiffness matrix Æ stiffness ellipse in Cartesian space
• Low stiffness in normal direction
Æ Good control of contact force
• High
Hi h stiffness
tiff
i tangential
in
t
ti l direction
di ti
Æ Good
G d performance
f
on trajectory
t j t
tracking
t ki
• Stiffness ellipse adaptable to surface normal using the estimated force
y
[u
,1, u
T
,2 ]
2
[u
1,1,
u
1,2
]T
Ellipse
x
: eigenvalue
[u1, u2 ]T: eigenvector
KOREA UNIVERSITY
Principal axis
Fixed
stiffness ellipse
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Adaptable
stiffness ellipse
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S-DAU : Surface Estimation
ƒ Surface estimation
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29
28
27
26
25
24
23
22
21
20
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iii
Trajectory of PA
Environment
iv
v
Trajectory of
end-effector
ii
i
0
1
2
3 4 5 6
Position x (cm)
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Intelligent Robotics Lab
Parallel-type Dual Actuator Unit
(P-DAU)
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P-DAU : Introduction
ƒ P-DAU
• Connected in parallel
• Antagonistic actuation
ƒ Features
• Linear spring + Cam-follower
Æ Nonlinear stiffness characteristics
• Compact design
• Parallel actuation available
Æ Combined torques from dual actuators
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Intelligent Robotics Lab
P-DAU : Principle of Operation
Agonist
ƒ Antagonistic actuation
• Basic principle of human motion
• Two muscles for control of a single joint.
• Muscles modeled as nonlinear springs .
Antagonist
F
Low Stiffness
F
High Stiffness
Spring 1
Spring 1
x
x
Spring 2
Spring 2
M i plate
Moving
l t
-x2
x
x1
P2
Spring 2
KOREA UNIVERSITY
-x2'
P1
Output link
x
P2
Spring 2
Spring 1
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x1'
P1
Output link
Spring 1
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P-DAU : Principle of Operation
ƒ Cam-Follower Mechanism
• Compact design.
• Cam profile Æ Various nonlinear characteristics.
Output link
Cam
movement
Cam
profile
Face
cam
Translating cam with
translating roller follower
KOREA UNIVERSITY
Face cam with
oscillating follower
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Translating
follower
Cam-follower
in P-DAU
Intelligent Robotics Lab
P-DAU : Principle of Operation
ƒ Variable Stiffness mechanism of P-DAU
• Antagonistic actuation
• Cam-follower + Linear spring Æ Nonlinear spring
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P-DAU : Variable Stiffness Mechanism
ƒ Low Stiffness (Æ Small compression of spring)
ƒ High Stiffness (Æ Large compression of spring)
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KOREA UNIVERSITY
Intelligent Robotics Lab
P-DAU : Parallel Actuation
ƒ Parallel actuation
• Antagonistic actuation: Only a
single actuator can apply a force
to an object.
j
• Parallel actuation: Both actuators
can apply forces to an object.
Æ Combined torque from dual
actuators
Æ No variable stiffness
Clutching point
Antagonistic
actuation
Parallel
actuation
Antagonistic
actuation
Actuation mode
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Parallel actuation with
clutch mechanism
Intelligent Robotics Lab
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P-DAU : Parallel Actuation
ƒ Clutch mechanism
• Based on cam-profile.
• Operated by the difference in position between upper and lower plates.
Clutch
cam
Spring
Clutch
pin
Middle plate
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KOREA UNIVERSITY
Intelligent Robotics Lab
P-DAU : Construction
ƒ Actuation Mechanism of PP-DAU
• Compact design Æ power transmission
by two Internal ring gears
Connected
Shaft to middle plate
Upper plate
Variable stiffness module
Spur gears to
internal ring gears
Middle plate
Lower plate
Internal ring gear 2
Internal ring gear 2
Internal ring gear 1
Internal ring gear 1
to upper plate
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P-DAU : Construction
• φ 70x62 mm, 470g (without motors)
• Maximum payload: 5Nm
• Variable stiffness range: 0.01 ~ 0.6 Nm/deg
• Response time : < 1sec
(from min. stiffness to max. stiffness)
Upper/Lower plate
Ver. 1
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Intelligent Robotics Lab
P-DAU : Performance
ƒ Antagonistic Actuation Mode
KOREA UNIVERSITY
ƒ Parallel Actuation Mode
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25 m
m
Safe Joint Mechanism
(SJM)
R 30
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Intelligent Robotics Lab
SJM : Introduction
ƒ Safe robot arm (Compliant robot arm)
• Active compliance
• Collision detection by sensors
Æ Control of actuators
• Slow response, noise, malfunction
• Passive Compliance
•Spring, flexible link/joint, soft covering
Æ Absorbing collision force
•Fast response, high reliability
but positioning inaccuracy.
Flexible joint (Quanser)
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Rovie, ATR
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SJM : Principle of Operation
Dangerous
60
ƒ Safety vs Performance
Unsafe region
50
Force (N)
• Tradeoff
• Low stiffness for safety
• High
Hi h stiffness
tiff
ffor performance
f
IDEAL
40
Low
stiffness
High
stiffness
30
Safe
Sa
e region
eg o
20
10
ƒ Our approach
Operating region
00
10
20
30
40
Inaccurate Displacement (mm)
positioning
• Nonlinear stiffness characteristics
Æ Only by passive mechanical elements
• Normal operation
Æ Stiff arm for accurate positioning
• Collision situation (Large impact)
Æ Soft arm for shock-absorbing
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Intelligent Robotics Lab
SJM : Principle of Operation
Sttiffness (N/mm)
ƒ Nonlinear spring system
• 4-bar linkage + Pre-compressed spring
• Transmission angle of 4-bar linkage
Æ Low
L
spring
i
force
f
for
f static
t ti equilibrium
ilib i
• Threshold force: Transmitted force ≥ Spring force
Spring
FS
Output slider
A
Input slider
y
O1 x
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FE
B
: transmission angle
FE : External force FS : Spring force
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SJM : Performance
Force (N))
Accelerationn (g)
200
150
100
HIC = 450
50
HIC = 15
0
-50
0
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KOREA UNIVERSITY
w/o SJM
with SJM
Start of collsion
10
Time (msec)
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Intelligent Robotics Lab
SJM : Current Status
ƒ Safe manipulator
• 6 DOF manipulator with SJM
• SJM installed at the elbow joint.
3rd version
2nd
M t
Motor
version
Harmonic Drive
SJM
• Size : Ø65*25mm
• Weight : 125g
• Torque : 8.5 Nm
• Range : ± 25°
• HIC : below 100
KOREA UNIVERSITY
• Size : Ø75*35mm
• Weight : 180g
• Torque : 10 Nm
• Range : ± 23°
• HIC : below 100
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Thank you !!
Contact: Prof. JaeJae-Bok SONG at [email protected]
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