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For over three decades, the Brubotics team at Vrije
Universiteit Brussel has been at the forefront of human-centric
robotics research. Our extensive portfolio encompasses a wide
range of innovative approaches designed to enhance the
performance, safety, and efficiency of robots across various
applications. Through our expertise in developing cutting-edge
robotic devices such as exoskeletons, prostheses, and
collaborative robots, we have identified a critical issue:
conventional engineering design methods often produce
systems that are excessively heavy and bulky, resulting in
limited productivity, high costs, and significant energy
demands. These issues are predominantly linked to the
actuation systems. Consequently, our team has concentrated on
pioneering advancements in actuation technology to reduce
mass and size, lower energy consumption, and improve safety,
productivity, and cost-effectiveness. In this extended abstract,
we provide a concise overview of our lab's key achievements
in this area, both past and present.
Historically, compliant actuation has been an important
research avenue for Brubotics. A first step consisted in the
development of the Pleated Pneumatic Artificial Muscle
(PPAM), a membrane that expands radially and contracts
axially when inflated. This type of actuator achieves high force
density, controllable stiffness, and was most notably used for
the bipedal robot Lucy [1]. Since the 2010s, Brubotics has
directed its efforts towards electrical elastic actuators that have
high power density, are easy to control, and are tailored
towards human-centered robotics. We developed the patented
Mechanically Adjustable Compliant and Controllable
Equilibrium Position Actuation (MACCEPA) [2], an actuator
that has a simple construction but allows to control the
equilibrium position of the actuator and its stiffness separately.
Over the years, Brubotics has finetuned this concept for high
torque density and energy efficiency. The MACCEPA has
been implemented in active lower-limb prostheses [3],
exoskeletons [4] and bipedal robots [2]. The integration of
locking mechanisms to these systems can result in a
substantial upgrade of their performance [5].
Another relevant research avenue for the Brubotics team
has been the development of redundant actuators. Unlike
traditional servo-drive actuators, redundant actuators utilize
two or more motors to drive the output. This additional design
degree of freedom can be exploited to increase the energy
efficiency and safety of the actuator. Redundancy can be
introduced in two ways. Kinematic redundancy implies that the
output speed is a weighted sum of the speeds of the input
*This research was partially supported by Research Foundation – Flanders
(grant no. S001821N, 1505820N).
All Authors are with the Robotics and Multibody Mechanics team of the
Vrije Universiteit Brussel, Ixelles, 1050, Belgium, and members of IEEE.
V.B. is with IMEC, Leuven, 3001, Belgium
motors. Static redundancy implies that the torque of the input
motors is added up to generate the output torque.
A specific type of kinematically redundant actuator studied
in our lab, which we dubbed the “dual-motor actuator”
(DMA), consists of two DC motors connected via a differential
mechanism. This setup results in a kinematically redundant
actuator [6]. By integrating brakes after each motor, DMAs as
the one shown in Fig.2 can operate in distinct regions: high
torque at low speeds and high speeds at moderate torque. This
adaptability is particularly beneficial for applications such as
collaborative robots (cobots) and exoskeletons. By distributing
power between motors optimized for specific speed-torque
regions, DMAs achieve high energy efficiency. The brake
system allows one motor to be idle without power
consumption, and disengaging the brakes activates full dual-
motor operation, enhancing maneuverability albeit at the cost
of increased energy use. Selecting lightweight yet powerful
motors and gearboxes is essential for balancing mass reduction
and energy efficiency [7]. This is crucial for wearable robots
where weight impacts usability. Optimizing gear ratios and
employing high-efficiency motors minimizes energy
consumption, making the system more sustainable. DMAs can
optimize speed distribution, thus reducing reflected inertia
compared to single-drive actuators. Lower reflected inertia
allows the actuator to respond swiftly to control inputs and
external forces, reducing impact forces during collisions and
enhancing human-robot interaction safety [8].
The Series-Parallel Elastic Actuator (SPEA) is another
successful actuator concept from our lab. Its statically
redundant variant, the +SPEA, places multiple series elastic
actuator units with locking mechanisms in parallel [9]. Parallel
springs provide part of the output force without electrical
energy. Each locked unit acts as a parallel spring with a
All other Authors are with Flanders Make, Heverlee, 3001, Belgium
Corresponding author:
P.L.G., email: pablo.lopez.garcia@vub.be, phone: +32 499 741816.
Advancements in Actuation Solutions for Human-Centric Robotics*
Pablo Lopez Garcia, Amin Khorasani, Stein Crispel, Raphaël Furnémont, Muhammad Usman, Anand
Varadharajan, Dirk Lefeber, Bram Vanderborght, and Tom Verstraten
Figure 1: Remote Actuation at Brubotics
public
configurable resting length, allowing the average force to be
actively varied. This reduces total energy consumption for
tasks with high forces and low accelerations but adds
complexity due to the large number of motors. The iSPEA
variant uses an intermittent mechanism to reduce the number
of motors to one [10], whereas the SPECTA variant explores
the additional potential of using a constant torque spring [11].
Although this decreases versatility, it maintains high energy
efficiency for tasks like pick and place.
In recent years, Brubotics has also explored various
hardware solutions based on the combined use of elastic
elements and remote actuation to reduce moving mass and
energy consumption. Typical solutions combine spring
elements with cable/belt pulley transmissions, resulting in a
relatively high mechanical complexity [12]. Flexible shafts are
slender and torsionally compliant, offering a simpler, single-
element solution. Our team has developed several devices that
incorporate these flexible shafts as transmission components
(Fig. 1). In the context of the ELYSA project, a robot arm with
a 1:1 payload-to-weight ratio was developed, capable of lifting
18 kg using a torque-dense joint remotely actuated with a
flexible shaft [13, 14]. We also designed an occupational upper
body exoskeleton and a lower limb rehabilitation exoskeleton
incorporating flexible shaft-based remote actuation [15]. These
solutions relocate the mass of the motors, drive electronics and
batteries to the waist to decrease metabolic energy
consumption. Furthermore, the flexible shaft introduces
intrinsic joint compliance, a desirable property in wearable
robots.
To fully leverage lightweight electric motors, Brubotics
also explored high-ratio transmissions to convert the high
speeds of small and light motors into high torques. In most
high-torque, moderate-speed applications, current high gear
ratio transmissions suffer from low efficiency, non-
backdrivability, and excessive weight [16]. To address these
issues, we have developed the patented R2poweR Compound
Planetary Gear Train (C-PGT) gearbox technology [17] that is
currently involved in its valorization process through our spin-
off AILOS and can be seen in Fig. 2. This innovative gearbox
enhances actuator energy efficiency by reducing internal
power flows and optimizing gear meshing stages [18]. The
R2poweR technology achieves a gear ratio exceeding 200:1
without compromising efficiency (>80%) or weight (<700g).
Additionally, the usability of this gearbox as a power-split
device for redundant actuation principles has also been
explored to further reduce energy consumption. The
integration of a redundant actuation mechanism through a
high-ratio gearbox introduces extra design flexibility to
optimize performance criteria. Redundant kinematic DOF can
be introduced to reduce the reflected inertia of the drivetrain,
in addition to static DOFs, which can be exploited to is used to
minimize actuator weight. As such, the operating range of the
motor can be reshaped to match the task requirements [5].
In conclusion, throughout the past 30 years, Brubotics has
emerged as one of the leading research teams in the domain of
actuation for human-centered robotics. Our research group will
continue these endeavors, with a particular focus on addressing
the challenges of wearable robotics and collaborative robots.
Building on our extensive expertise in robotic needs and
actuator components and concepts, we aim to surpass the
performance of current devices by implementing more holistic
design processes. Additionally, we will persist in seeking
breakthroughs in component designs to push the boundaries of
what is achievable with today’s technology.
R
EFERENCES
[1] Vanderborght, Bram, et al. "Overview of the Lucy project: Dynamic
stabilization of a biped powered by pneumatic artificial muscles."
Advanced Robotics 22.10 (2008): 1027-1051.
[2] Van Ham, Ronald, et al. "MACCEPA, the mechanically adjustable
compliance and controllable equilibrium position actuator." Robotics
and Autonomous Systems 55.10 (2007): 761-768.
[3] Flynn, Louis, et al. “Ankle–knee prosthesis with active ankle and
energy transfer: Development of the CYBERLEGs Alpha-Prosthesis.”
Robotics and Autonomous Systems, 2015, vol. 73, p. 4-15.
[4] Moltedo, Marta, et al. “Mechanical design of a lightweight compliant
and adaptable active ankle foot orthosis.” 2016 6th IEEE Conference
on Biomedical Robotics & Biomechatronics (BioRob) p. 1224-1229.
[5] Plooij, Michiel, et al. "Lock your robot: A review of locking devices in
robotics." 2015, IEEE Robotics & Automation Magazine, 106-117.
[6] Verstraten, Tom, et al. "Kinematically redundant actuators, a solution
for conflicting torque–speed requirements." The International Journal
of Robotics Research 38.5 (2019): 612-629
[7] Khorasani, Amin, et al. "A methodology for designing a lightweight
and energy-efficient kinematically redundant actuator." IEEE Robotics
and Automation Letters 7.4 (2022): 10786-10793.
[8] Khorasani, Amin, et al. "Mitigating collision forces and improving
response performance in human-robot interaction by using dual-motor
actuators." IEEE Robotics and Automation Letters (2024).
[9] Furnémont, Raphaël, et al. "Novel control strategy for the+ SPEA: A
redundant actuator with reconfigurable parallel elements."
Mechatronics 53 (2018): 28-38.
[10] Mathijssen, Glenn, et al. "Study on electric energy consumed in
intermittent series–parallel elastic actuators (iSPEA)." Bioinspiration &
biomimetics 12.3 (2017): 036008.
[11] Saerens, Elias, et al. "Novel SPECTA actuator to improve energy
recuperation and efficiency." Actuators. Vol. 11. No. 3. MDPI, 2022.
[12] Verstraten, Tom, et al. "Redundancy in Biology and Robotics: Potential
of kinematic redundancy and its interplay with elasticity." Journal of
Bionic Engineering 17 (2020): 695-707.
[13] Usman, Muhammad, et al. "Flexible Shaft as Remote and Elastic
Transmission for Robot Arms." 2024, IEEE Robotics & Autom. Letters
[14] Usman, Muhammad et al. “Design and Development of Highly Torque
Dense Robot Joint using Flexible Shaft based Remote Actuation.”,
2024, IEEE/ASME (AIM) International Conference on Advanced
Intelligent Mechatronics.
[15] Rodriguez-Cianca, D., et al. "A Flexible shaft-driven Remote and
Torsionally Compliant Actuator (RTCA) for wearable robots."
Mechatronics 59 (2019): 178-188.
[16] García, Pablo López, et al. "Compact gearboxes for modern robotics: A
review." Frontiers in Robotics and AI 7 (2020): 103.
[17] Crispel, Stein, et al. "A novel Wolfrom-based gearbox for robotic
actuators." 2021, IEEE Transactions on Mechatronics, 1980-1988.
[18] Varadharajan, Anand, et al. "Unconventional Gear Profiles in Planetary
Gearboxes." (2022).
Figure 2: Brubotics’ dual motor actuation (left) and high-ratio,
backdrivable
R2poweR
gearbox (right).