IEEE Robotics & Automation Magazine - March 2015 - 107

What Is an Ideal Locking Device?
A locking device is a device that switches between allowing
and preventing relative motion between two parts. The
requirements of a locking device differ across applications.
This section lists all of the requirements one might have on a
locking device. In the rest of this article, locking devices are
evaluated based on how well they meet each of these requirements such that the reader will be able to select the most suitable locking device for his or her application. An ideal locking
device has the following properties (in random order):
● Adjustable locking directions: The device can switch
between locking in zero, one, or two directions.
● Unlocking while under load: While there is a load on the
locking device, it should still be able to unlock.
● Low energy consumption: While the device is unlocked or
while it is unlocking, it should not consume energy.
● Lockable in any position: The device has an infinite amount
of locking positions.
● Compact: The device should be small relative to its
application.
● Lightweight: The device should be lightweight relative to its
application.
● Short switching time: The device switches instantaneously.
● Inexpensive: The device should be inexpensive relative to its
application.
● High locking force: The device has unlimited locking torque.
Some locking devices can also be used as controllable
brakes, meaning that the locking torque can be controlled,
and when the external torque is higher than the locking
torque, the brake slips. Although this property is not necessary for a locking device, in some applications, it might be an
advantage, and, therefore, it will be considered as a side note
in this article.
Categorization
Numerous locking devices are presented in the literature. In
this article, the locking devices are categorized into three main
groups based on three locking principles (see Figure 1). The
three distinguishable categories are as follows:
● Mechanical locking: The position of a mechanical component determines the locking or unlocking. Examples of
such components are wedges and pawls. This position can

Active

Mechanical

Passive

Locking Devices

Locking Principle

Activation

are describing mechanisms that were implemented after 2010.
Therefore, this article discusses their potential in robotics
applications.
Each locking device principle has advantages and disadvantages, with no single design fulfilling all of the requirements of
the ideal locking device. On the other hand, not every application requires a locking device that fulfills all of these requirements. Therefore, this article provides an overview of the locking devices that are useful for robotic applications and
discusses their properties, advantages, and disadvantages,
starting with the description of an ideal locking device.

Friction

Singularity

Latches
Ratchets
Dog Clutches
Hydraulic Locks

Electromagnetic (EM) Four-Bar
Overrunning
Linkages
Self-Amplifying
Capstans
Piezoelectric
Bistable
Statically Balanced
Thermic

Latches
Ratchets
Cam Based

Overrunning
Nonbackdrivable

Nonlinear
Transfer Ratio

Figure 1. The three main categories of locking devices include
mechanical, friction-based, and singularity locking. All three can be
divided into actuated and passive devices.

be determined by an actuator or can depend on, e.g., the
position of a joint or the direction of the velocity. These
locking devices are discussed in the "Locking Devices
Based on Mechanical Locking" section.
● Friction-based locking: Engaging or disengaging two friction surfaces determines if the joint is locked or unlocked.
This engagement can be determined by an actuator or can
depend on, e.g., the position of a joint or the direction of
the velocity. These locking devices are discussed in the
"Friction-Based Locking Devices" section.
● Singularity locking: Singularities in mechanisms cause a
transfer ratio to go to infinity. In such a singular position, the
locking device features an infinitely high locking force and
an infinitely small unlocking force. These locking devices are
discussed in the "Singularity Locking Devices" section.
Each of these three groups can be subdivided into active
and passive lo cking
devices (see Figure 1).
Contrary to passive lockThe first and most often
ing devices, active locking
devices use an actuator to
cited reason for
change the timing of the
locking, the locking posiusing locking devices
tion, or the locking torque.
Therefore, while passive
is energy management
devices do not require any
electronics or control,
in robotic systems.
active devices often use
s om e k i n d of s t at e
machine controller. The "Comparison" section compares all
the different devices based on the properties of an ideal locking device described earlier, and the "Selection and Development" section provides a guide for the selection of a suitable
locking mechanism.
Locking Devices Based on Mechanical Locking
Mechanical locking devices all use some kind of obstruction of
a part by another part. For instance, in the latch in Figure 2(a),
march 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

107

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