This document proposes a new robotic joint actuation concept called the Variable Mechanical Fuse (VMF) which aims to realize intrinsic safety for human-robot interaction through an elastic behavior and high position control through a rigid behavior. Simulation and experimental results on a prototype VMF joint show it achieves lower impact forces during collisions and greater position stability under external forces compared to no preload or partial preload configurations. Future work will investigate the effect of gravity, design a 4 DOF robot arm using the VMF concept, and validate the intrinsic safety and control capabilities of the VMF for human-robot cooperation applications.

1. A Novel Robotic Joint Actuation Concept
The Variable Mechanical Fuse, VMF
Yoichiro Dan and Oussama Khatib
Stanford University

2. Background
- Cooperation of Human and Robot
- Intrinsic safety is essential for safer interaction

3. Background

4. Robotic Joint Actuation - SEA
(Gill A. Pratt and Matthew m.Williamson, MIT)

5. Robotic Joint Actuation - VSA
(Toshio Morita and Shigeki Sugano, Waseda)(Nikolaos G. Tsagarakis, IIT)
(Sebastian Wolf, DLR)

6. Macro Mini Actuation

7. New Concept of Robotic Joint Actuation
External Torque: t
Displacement:q
O
External Torque: t
Displacement:q
O
External Torque: t
Displacement:q
O
SEA VSA New Concept
- Actuation system with springs and a preload mechanism
- Realize elastic behavior for safer human-robot interactions
and rigid behavior for high control performance

8. Variable Mechanical Fuse, VMF
External Force: F
Displacement:qO
rigid elastic
threshold
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link
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tex

9. Simulation
- Impact forces
Collisions with several preload conditions
p/2
q
f
Kl Dl
0% Preload 50% Preload 100% Preload

10. - Position Stability
Simulation
p/2
q
f
Kl Dl
Position history for step input with several preload conditions

11. Mechanism

12. Design
joint motor
encoder
link
elastic unit
80mm
160mm
Parameters Unit Value
Max. payload kg 0.5
Length of link m 0.25
Joint stiffness Nm/rad 7.3
Max. angular displacement rad (deg) 0.17 (10)
Torque capacity Nm 1.3
Max. preload Nm 1.1
- Specifications

13. Design
preload motor
potentiometer
slide screw slide screw nut
preload generator
coil spring
pushing plate
pushing rotor
base
10mm
- Cross sectional image

14. Experiment
- Impact Forces
Weight: 0.5[kg]
Velocity: 180 [deg/s]
Acceleration time: 0.2 [s]

15. Experiment
- Position Stability
Weight: 0.5[kg]
Velocity: 180 [deg/s]
Acceleration Time: 0.2 [s]
Joint Stiffness: 7.3 [Nm/rad]
Preload [%] Max. Amplitude[deg]
0 2.5
50 0.4
100 0.2

16. Experiment
- Position Stability
Weight: 0.5[kg]
Velocity: 180 [deg/s]
Acceleration Time: 0.2 [s]
Joint Stiffness: 3.6 [Nm/rad]
Preload [%] Max. Amplitude[deg]
0 11.3
50 5.1
100 0.5

17. Experiment
- 0% Preload

18. Experiment
- 100% Preload

19. Summary
- Proposed new robotic joint concept
Variable Mechanical Fuse
- Verified the VMF realizes intrinsic
safety and high position stability
- Investigate gravity effect
- Design 4 DOF robot arm with VMF
Future Plans
Achievements