This paper presents a new robotic joint actuation concept called the Variable Mechanical Fuse (VMF) that can achieve both high position accuracy and safety. The VMF uses springs and a mechanism to vary the preload on the springs, allowing it to behave either rigidly for precision control or elastically for safety. A prototype was designed, modeled, and tested that demonstrated the VMF's ability to reduce impact forces by 30-60% compared to a conventional rigid joint, showing its intrinsic safety.

1. A Novel Robotic Joint Actuation Concept:
The Variable Mechanical Fuse, VMF
Yoichiro Dan1
and Oussama Khatib2
1
Artificial Intelligence Laboratory, Stanford University, Stanford, CA 94305, USA
yoichir2@stanford.edu
2
Artificial Intelligence Laboratory, Stanford University, Stanford, CA 94305, USA
ok@cs.stanford.edu
Abstract. This paper presents a novel robotic joint actuation concept, the Variable
Mechanical Fuse, VMF. This actuation system realizes a rigid behavior for high
control performance and an elastic behavior for safer human-robot interaction.
The VMF adapts its behavior throughout a motion to ensure that both endpoints
have high position accuracy and the intermediate trajectory maintains safety.
Since the VMF changes its behavior passively from rigid to elastic, this actuation
system is intrinsically safe even when unexpected collisions occur. A compact
prototype has been designed and fabricated to verify its performance and safety.
Keywords: Elastic joint, SEA, variable stiffness joint, compliant motion and
torque control
1 Introduction
Industrial robots have been used in unmanned environments for a long time.
Safety is enforced by not having humans within close proximity of robots in oper-
ation. However, applications that require robots to cooperate with humans are ex-
pected to increase in the near future. This has sparked a wide variety of research
on safer robots. The research on human safe robot design started with SEA [1] de-
veloped by Pratt et al. SEA has springs as elastic components that allow compliant
motions against external forces. Edsinger et al. developed a robot that is equipped
with SEAs for safe human-robot interactions [2]. Due to the elasticity in SEAs,
they usually have limited position accuracy. In order to overcome this problem,
elastic mechanisms with variable elasticity were developed. Wolf et al. developed
a variable stiffness joint [3, 4] and realized a mechanism that has variable elastic

2. 2 Yoichiro Dan and Oussama Khatib
constants by non-linear cams and rollers. Morita et al. developed a variable stiff-
ness joint that has a cantilever with a variable active length [5]. Tsagarakis et al.
developed a variable stiffness joint that has springs and a lever [6]. Their design
uses a separate mechanism to vary the pivot point of the lever that connects to the
springs. Using a multi-stage elastic actuator concept, Kuan et al. developed a SEA
with two springs of different constants [7]. However it is observed that these de-
signs still do not have position accuracy that comes close to that of rigid joints.
A safe mechanism that does not sacrifice position accuracy while maintaining
acceptable level of safety is required in industry. Zinn et al. developed a macro-
mini actuation system that implements safety while being able to produce highly
responsive position control [8]. However the performances such as position accu-
racy and frequency response are still not high enough compared to conventional
rigid joints. The VMF concept attempts to solve the safety vs. performance trade-
off by having two vastly different behaviors.
2 Principle
High position accuracy and safety of robot arms can be achieved in a single
mechanism. The VMF actuation concept introduced here achieves both a rigid and
elastic behavior by setting the threshold to external forces.
Fig.1 shows the basic elastic joint principle of the VMF. The VMF has a spe-
cific stiffness and a variable threshold to external forces. When the external force
is lower than the threshold, the VMF shows a rigid behavior because the force is
not large enough to produce a displacement. When the external force is higher
than the threshold, the VMF shows an elastic behavior similar to that of an SEA.
Since the threshold can be set to any value, the VMF can display a purely elastic
behavior when the threshold is set to zero.
Fig. 1 Principle of VMF

3. The New Concept of Elastic Joint Mechanism: VMF (Variable Mechanical Fuse) 3
3 Analysis
Fig. 2 shows a schematic diagram of a VMF model. The VMF has springs as
elastic components between a motor and a link. The threshold is produced by ap-
plying preloads on the springs. The dynamic equation of the VMF is expressed by
the following two equations.
   
    exll
lm
fDJ
fDJ






,
,
(1)
where Jm and Jl show inertia of the motor and the link respectively, Dl shows fric-
tion coefficient between the motor and the link.  and  show angle of the motor
and the link respectively.  and ex show torque of the motor and external torque
that is applied on the link respectively.
The torque applied by the springs is described by the function f that contains
the preload on the springs.
     
 






,
,sgn
,
pp
lp K
f





0
0






(2)
where sgn is the sign function implemented to express the preload on springs. p
and Kl show the preload and the spring constant respectively.
Fig. 3 shows a simulation result of position step responses with various preload
values. The preload can be set from zero to the maximum value that is large
enough to cancel acceleration torque and holding torque of payloads. From the
graph, the oscillation on the trajectory of  is reduced with increased preloads.
Eventually the trajectory of  matches the trajectory of  at 100% preload which
corresponds to the behavior of a rigid joint.
Fig. 4 shows a comparison of impact forces between an ordinary rigid joint and
the VMF. Here is assumed that a weight of 250g is placed at the tip of the link of
250mm length, and the weight is collided with an obstacle at the angular velocity
rad/s. The impact force of the VMF is reduced by approximately 40% to 60%
compared to a rigid joint. It indicates that the VMF is safer than ordinary rigid
joints even when unexpected collisions are occurred while the VMF shows rigid
behavior with the maximum preload.

4. 4 Yoichiro Dan and Oussama Khatib
Fig. 2 Schematic diagram of VMF model
Fig. 3 Position step response of VMF with different preloads
Fig. 4 Comparison of impact forces between a rigid joint and VMF

5. The New Concept of Elastic Joint Mechanism: VMF (Variable Mechanical Fuse) 5
4 Mechanism
4.1 Design of VMF
A VMF prototype for a single joint has been designed and assembled. Fig. 5
shows a picture of the prototype. This joint has an elastic unit with springs and a
mechanism to produce the preload on the springs for setting the threshold to ex-
ternal forces. The mechanism supports an encoder for sensing the angular dis-
placement of the elastic unit. An important characteristic of a VMF is that this unit
also works as a torque sensor and allows compliant motions by torque control.
Fig. 6 shows the cross-sectional CAD image of the elastic unit. This unit is
equipped with a small geared electric motor coupled to a slide screw that is placed
in the center of the unit. When the slide screw is rotated by the motor, it drives a
preload generator downward in the picture to deform the springs and produce the
preload. When an external force is applied to the link shown in Fig. 2, a pushing
rotor that is placed in the center is rotated to the direction of the blue arrow since it
is connected directly to the link. Then the pushing rotor pushes the spring on ei-
ther side via a pushing plate to the direction of the red arrows. When the force ap-
plied to the spring by the pushing rotor is smaller than the preload, this unit shows
a rigid behavior. When the force exceeds the preload, this unit shows an elastic
behavior.
This mechanism is applicable to joints of a robot arm. When precise motion is
required such as for positioning, the arm can show high position accuracy by the
VMF’s rigid behavior. When safe motion is required during any interaction with
the robot, the arm can show compliant motion by the VMF’s elastic behavior.
Fig. 5 Prototype of VMF

6. 6 Yoichiro Dan and Oussama Khatib
Fig. 6 Cross-sectional image of the elastic unit
4.2 Specification
Table 1 shows the specifications of the prototype. This prototype is designed to
be incorporated into an elbow joint of a human sized robot arm. This design as-
sumes a 0.5kg payload at the tip of the link with a length of 0.25m. The torque ca-
pacity is set at 1.3Nm by the joint stiffness that is determined by the spring con-
stant and the maximum angular displacement. The maximum preload is set at
1.1Nm by the spring constant and the maximum stroke of preload generator. Since
this value is large enough to cancel the holding torque and the acceleration torque
of the payload, the unit can realize rigid behavior at any posture and velocity.
Table 1 Specifications of VMF
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

7. The New Concept of Elastic Joint Mechanism: VMF (Variable Mechanical Fuse) 7
5 Experiment
An experiment of collisions was performed to investigate the VMF’s safety.
Fig. 7 shows the experimental setup. A weight of 250g is placed at the tip of the
link of 250mm length. The weight is collided with the load cell at the angular ve-
locity rad/s and stopped safely within the maximum displacement of the elastic
unit. A rigid joint collision is also performed in the same condition. Fig. 8 shows
the results of the experiment. Depending on the preload value, the collision forces
are reduced by 30% to 60% compared to a conventional rigid joint. The collision
forces vary from around 90N to 200N linearly. These results correspond approxi-
mately to the simulation results described above. It was confirmed that the VMF
has intrinsic safety in any value of the preload.
Fig. 7 Experimental setup
Fig. 8 Results of collision experiment with different preloads

8. 8 Yoichiro Dan and Oussama Khatib
6 Conclusion
In this paper, we proposed the concept of VMF (Variable Mechanical Fuse) to
address the challenge of achieving both performance and safety in an actuation
system. Our study has shown this novel actuation system to realize high control
performance while providing intrinsic safety. A compact single DOF prototype
has designed and an experiment of collisions has been performed to confirm the
intrinsic safety. The experiment has demonstrated reductions of 30% to 60% of
the collision forces compared to conventional rigid joints. Further experiments
such as step response and frequency response are going to be performed soon.
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