IEEE Robotics & Automation Magazine - March 2015 - 113

-
++
Nonlinear transfer ratio

Singularity

Passive

2

++

++

++

-

-

-

-

+

-

-

++
Active
Singularity
Four-bar linkages

2

++

++

+

-

0

0

-

+

-

++
+
Passive
Nonbackdrivable gearing Friction

2

0

++

++

++

++

++

+

++

+
0

+

0

+

+
+

+
++

++
++

-
-

++
++

+
2

Passive
Friction
Overrunning

Active
Friction
Thermic

1

-

0

++

-

0
Active
Friction
Statically balanced

2

++

++

++

++

0

0

0

-

++

0
-
Active
Friction
Bistable

2

++

++

-

++

0

0

0

0

-
++

-
++
0

0
0

0
++

++
+

+
+

++
+
1

++
Active
Friction
Piezoelectric

Active
Friction
Capstan

2

+

0

-

-

++
Active
Friction
Self-amplifying

1

+

++

+

++

0

0

+

0

++

++
0
Active
Friction
Overrunning

1

0

++

+

++

+

+

+

0

-
++

+

0

0

-
0

0
-

++
+

++
++

-
++

++

Friction
EM

1
Mechanical Passive
Cam based

Active

Mechanical Passive
Ratchets

2

-

+

-

-

++

Mechanical Passive
Latches

1

-

++

++

+

+

+

++

++

-

++
++

Mechanical Active
Hydraulic locks

1

-

++

++

-

++

++

++

++

-
++

0

+

+

+
+

+
-

++
+

+
++

++
-

-
Mechanical Active
Dog clutches

2

Mechanical Active
Ratchets

2

+

+

-
++
+

-
++
+

++

++
+

+
+

+
-

+
+

+
++

++
-

-
Mechanical Active
Latches

1

Weight
Type

1

Locking
Locking Torque
Torque Adjustable
Switching
Time
Price
Switching
Number
Power
of Locking
Consumption Positions Size
Number
(Un)locking
of
While Under Cont. Power
Activation Directions Load
Consumption
Locking
Principle

Table 1. The comparison between the different devices mentioned in this article. In all cases, ++ means that the property of the ideal locking mechanism is satisfied. For instance, with respect to the energy consumption, ++ means that the locking device (almost) does not consume energy.

mechanism with a variable pivot
axis. When the pivot moves, the
nonlinear amplification ratio of the
lever changes from zero to infinity.
Third, the knee of the humanoid
Poppy by Lapeyre et al. [92] uses a
spring in parallel to the knee, which
locks the knee during the stance
phase in a certain singular position.
Comparison
In this section, the different types of
locking mechanisms are compared
on the criteria given in the "What Is
an Ideal Locking Device?" section.
Table 1 lists all types of locking
devices and shows how well they
score on the criteria, with a score of
++, +, 0, -, or --. A + always
indicates that the device scores well.
For instance, if the energy consumption scores ++, this means that the
device uses (almost) no energy.
Mechanical locking devices typically have low energy consumption.
Even when they are actuated, the
only thing the actuator has to do is
to position the blocking part, for
instance, the pawl. Furthermore,
mechanical locking devices typically
have a low weight, are small, have a
low price, and their locking torque is
only limited by the strength of the
parts. However, such locking devices
also have disadvantages. First, they
are hard to unlock while being
under load because of the friction
between the two interfering parts.
Second, their number of locking
positions is limited (except for the
hydraulic lock and the ratchet).
Finally, the impacts that occur when
a joint is blocked will lead to shocks
in the system.
Friction-based locking devices
have fewer problems unlocking
under load than mechanical locking
devices. This is due to the fact that
the two friction surfaces can often
be disengaged, releasing the lock.
Another advantage of friction-based
locking devices is that two friction
surfaces can be engaged at any position, and therefore, the number of
locking positions is infinite. Finally,
since the locking torque depends on

march 2015

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IEEE ROBOTICS & AUTOMATION MAGAZINE

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113
Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2015
