IEEE Robotics & Automation Magazine - September 2022 - 48

in the various actuators. Then we study the fields of application
of these devices and analyze how different design aspects
correlate with one another and applications. This analysis
yields insight into how design choices can influence the characteristics
of actuators and the robots using them, from the
viewpoint of robot resilience, human safety, power consumption,
and energy storage. Finally, we provide an annotated
database of the papers considered to conduct this review.
Overview
The field of compliant robotic actuation has been growing [1],
[2], motivated by the intention of building robots able to cope
with unknown environments [3], behave safely in human-
robot interaction [4], tolerate shocks [5], store energy [6], [7],
and move agilely on legs [8]-[10]. Nevertheless, when compared
to rigid equivalent robots, compliant actuation underperforms
in some domains. Among other aspects, reduced
control authority bandwidth can lead to undesired oscillations
and vibrations [11], which can, in turn, reduce precision,
compromise stability, and waste energy. Through the
years, it became clear that to mitigate such limitations [12],
which can jeopardize the very advantage of using compliant
actuation in the first place, the inclusion of some form of
damping action [13] is mandatory.
A possible method to suppress vibrations is through active
control [11], [14]. However, such an approach requires the
actuator to move very fast and consume a lot of input power
to stably dissipate the mechanical energy of the system [15],
[16]. To overcome this, engineers began to include physical
damping elements to recover system performance and facilitate
stabilization [17]. They drew inspiration from the
dynamic behavior of human muscles [18]-[20] and natural
systems [21], [22] as well as technologies such as magnetorheological
(MR) [23] and fluid dynamic [24] car suspensions,
which are extensively studied in the automotive world [25].
This led to the proposition of actuators that combine soft
behavior with engineered damping effects.
As shown by Figure 1(a), the interest in this topic arose
almost 30 years ago, almost simultaneously with the birth of
the first generations of compliant actuators. Nevertheless,
studies of damped robotic actuators progressed slowly until
the past 15 years, during which researchers began investigating
the technology more thoroughly. This article acknowledges
the maturity of the field by presenting a survey of damping
technologies and their applications in robotic actuation. In
particular, to narrow the breadth of our analysis and walk in
the shoes of an actuator designer, we focus on solutions that
include physical damping elements, referring the interested
reader to [28] and [29] as access points to explore the vast literature
on active control approaches to dampen compliant robots.
The first product of our analysis is a carefully crafted database
of 50 research papers published during the past 30 years.
These papers concern the design, use, and application of passive
and semiactive damping in robotic actuation, describing 42
devices. The database categorizes the entries based on the
design of the damping systems and their intended application.
48 * IEEE ROBOTICS & AUTOMATION MAGAZINE * SEPTEMBER 2022
In particular, concerning damper design, the literature screening
led to subdividing actuators according to the technology
(i.e., the physical principle) they use to implement the damping
action, their ability to change the damping ratio, and the connection
topology of their architecture. While the first two classifications
are commonplace in the robotic and automotive
literature, the third, which is based on the possible ways to connect
the main motor and its gearbox, the spring, and the damper
(see the " Topology " section), is the second contribution of
this article, which we hope will help guide the design and dissemination
of future damped compliant actuators. The categorization
based on intended applications is organized into two
layers. The finer subdivision identifies 12 categories based on
paper keywords. These are then grouped into three families
based on a correlation analysis: medical haptic and wearable
(MHW) robots, industrial and collaborative robots (ICRs), and
humanoid assistive and legged (HAL) systems (see the " Applications "
section). This grouping is also a result of our work.
The chronological study of the number of publications in
each of these categories yields an important perspective in
terms of research trends for design aspects and applications.
Nevertheless, we derive the main results of our analysis by
arranging the publications based on category pairs (see the
" Discussion " section) to highlight relevant combinations
among architectural aspects and application domains. We hope
this review will provide a perspective on the past 30 years of
research into using physical damping elements in the design of
compliant robots and their actuators. We believe that our
results could help future users of damped compliant robots
understand the architectures of their systems and their motivations.
Moreover, our approach aspires to guide future designers
toward harnessing past insights and creating novel solutions.
Motivations
The optimal damping design for robots with soft actuators is
not trivial to find since its inclusion is usually motivated by
various, application-specific requirements. Often, one can
narrow a task to yield the definition of a precise damping
value. This is true for systems that exploit resonance and antiresonance
to optimize cyclic tasks (e.g., hammering [30]) and
cutting off band-specific vibrations (e.g., screwdriving [31]).
But in general, most applications require the simultaneous
optimization of several parameters of the task in question. As
expected, not all these parameters behave in the same way as
a function of the amount of damping.
Table 1 reports some notable examples of cost functions
that are typically used to measure these parameters, showing
how different their trends can be. We distinguish the effects of
damping based on the architecture of the actuator (see the
" Topology " section). The relative position of the main motor
and its gearbox, the spring, and the damper affects the way
damping interacts with the output and therefore the trend of
the cost functions. Some cost functions, such as the overshoot
percentage in row 1, which can be related to the accuracy of
rapid motions of arms [32] and legs [33], tend to decrease
with larger damping values until they reach a minimum and
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