IEEE Robotics & Automation Magazine - September 2022 - 59

industrial robots with ER dampers and a robotic joint with a
mechanical impedance adjuster for grasping. In these, damping
components are inserted to enhance precision, while the
possibility of unknown objects in the working space of the
robots requires a certain degree of resilience.
Collaborative Robots
A category of robots derived from the previous one while
importing several advances from robotic research is that of
collaborative robots, i.e., those that work while interacting
with humans in a shared space. Although their first application
was in industry, they are moving to several other sectors
since they guarantee human safety [108]. This assurance is
often thanks to the inclusion of nontraditional actuation systems.
For instance, the authors of [26], [44], and [48] present
a damping system design for an actuator used to build a
4-DoF arm with bioinspired elastic and damped behavior
[Figure 1(b)]. In [90], the authors present a 4-DoF manipulator
that employs clutches to limit the maximum torque to a
safe level [Figure 3(a)]. In [99], an MR actuator is designed to
enable safe interaction between humans and robots.
HAL Robots
Humanoid Platforms
Humanoid platforms include robots that resemble people and
carry out tasks with a level of performance comparable with
that of humans. Recently, to increase the robots' " humanlikeness, "
research has focused on design aspects concerning
actuation and resilience. In [55], the authors present
a variable-damping system for modular robotic actuators
[Figure 3(e)]. These actuators can be connected to create different
structures. Viscoelastic actuators have also been used to
create a bipedal system, such as [5], that exploits a viscoelastic
liquid-cooled actuator. In [58], the authors propose a variable-damping
actuator for compliant joints [Figure 3(c)]. In
[85], the authors introduce an actuator with controllable friction
damping. This category blends naturally with the next
one since most humanoid robots rely on legged locomotion.
Legged Systems for Locomotion
Legged platforms draw inspiration from natural systems to be
able to walk, run, and hop. In their development, compliance
and damping are used to optimize energy efficiency and
smooth movements to replicate natural behavior. Damping
provides other advantages. Indeed, the authors of [109] show
how it is possible to reduce the required control energy and
peak power consumption. Moreover, in [6], the authors
describe Blue, a bipedal walking robot with variable stiffness
and damping to enhance robustness against disturbances and
impacts [Figure 1(d)]. In [98], the authors present an artificial
ankle system capable of providing biologically realistic,
dynamic behaviors; it exploits passive compliance and a variable-damping
element. Also, for hopping robots, in [7] the
authors present a compliant variable-stiffness leg with damping
control, while in [70] and [27] they introduce, respectively,
the Hybrid Actuator Development leg for agile locomotion
[Figure 3(b)], made by exploiting series elastic actuation with
an MR damping component, and a viscoelastic liquid-cooled
actuator [Figure 1(c)].
The work in [37] introduces a leg that exploits damping to
smooth the force reaction on the ground during a jump, comparing
the efficiency of a friction damper with a compliant system
and a hydraulic one. Finally, there are legged robotic
systems capable of walking, running, and hopping. In [95], the
authors present a viscoelastic bipedal robot and its trajectory
generation strategy, while the authors of [110] introduce two
examples in which the same task is it is performed using only
compliance. The authors of [79] describe a variable-damping
module for walking application, used for energy regeneration.
Assistive Robots
Assistive robots perform physical tasks for people with disabilities
and senior citizens. Although the name might suggest
similarity to the first macro category, our analysis describes a
different reality. Assistive robotics is a term used mostly by
researchers close to the field of locomotion systems. In [66],
the authors present an MR fluid clutch for human-friendly
actuators. The authors of [102] introduce another MR damper
for high-performance physical human-robot interaction.
These works present actuators for assistive robot but without
implementations. Nevertheless, the work to design actuation
units for this purpose is valuable, and we think it is worthwhile
to present it. The lack of implementation is, in our opinion,
related to the fact that, even if assistive robotics is a vast
field, it has a low technology readiness level, with applications
that are speculative and in development.
Discussion
Trend Analysis
While Figure 1(a) gives the speed with which the interest in
using dampers in robotic actuation arose in the early 1990s,
Figure 2 breaks down the development trends, from the
viewpoint of each categorization described in the previous
sections. Figure 4(a) illustrates how a large part of the
research effort has always been dedicated to semiactive
damping systems. Nevertheless, since the late 2000s, we
observe the birth of a small but consistent degree of interest
in completely passive damping systems. In our interpretation,
the main motivation of the early and dominant interest
in semiactive systems is that the possibility of modifying
damping action makes robots able to work in different conditions
(see the " Motivations " section). The recent interest
in passive damping systems is due to the pursuit of simplicity,
a topic that has become increasingly prominent in modern
robotics [111]. Indeed, when the boundaries of an
application are narrowly defined, simpler design and control
can be preferable over tunability to favor usability, robustness,
and economy.
Figure 4(b) describes the trends in the adoption of different
damping technologies. Note how a substantial amount of
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IEEE Robotics & Automation Magazine - September 2022

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