NEWS & EVENTS
REPLACING THE WHEELCHAIR
RENISHAW
RLS magnetic encoders enable Marsi Bionics
to build ‘life-changing’ exoskeletons
BACKGROUND
Marsi Bionics S.L. is a leading technology
start-up based in Madrid, Spain. It designs
and builds custom exoskeletons for medical
applications with the aim of potentially
replacing the wheelchair in everyday life for
some patients.
Millions of people suffer from debilitating
neurophysical conditions such as
paraplegia, cerebral palsy and spinal
muscular atrophy (SMA). Neurological
rehabilitation with passive aids such as
canes, crutches and walkers is vital in
the treatment of mobility issues caused
by these conditions. Recent advances
in robotics have allowed treatment with
powered (active) robot exoskeletons that
support the patient’s body and enable
greatly improved outcomes.
The exoskeletons, created by Marsi
Bionics, give physically disabled people the
freedom to stand, move and interact with
their environment.
Data collected from the encoders is
fundamental for generating the position
references. RLS and Renishaw provided
Marsi Bionies with the best encoder
feedback solutions for its applications. RLS,
a Renishaw associate company, has been
chosen by Marsi Bionics to supply the latest
in magnetic encoder technology for the
creation of two new products: the ATLAS
2030 exoskeleton for children and the MB-
Active Knee (MAK) single-joint exoskeleton
for adults.
CHALLENGE
The ATLAS 2030 exoskeleton has up to six
degrees of freedom per limb. This device
enables the user to perform both unaided
and self-actuated actions such as walking
and sitting. Full exoskeletons consist of
motorised joints, limbs, electronic control
and power systems.
The designer must find a compromise
between a lightweight and compact
structure that facilitates easy handling
by the user, who might be physically
weakened, and a robotic system that
implements a physiologically complete
biomechanical model.
For stable walking, equilibrium control of
the exoskeleton-user assembly is achieved
by tracking its zero-moment point (ZMP)
references, which are based on the desired
Normalized Dynamic Stability Margin
(NDSM). The exoskeleton’s controller can
subsequently adapt reference walking gait
patterns, stored in memory, to maintain
stability.
Successful dynamic walking requires
precise control of the legs’ joint angles in
terms of position, velocity and acceleration
via rotary encoder feedback. This is difficult
to achieve as each mechanical joint is
compliant and includes elastic elements to
help mimic and support the real joints and
muscles of the human user.
Alberto Plaza, R&D engineer and
manager of the MAK project at Marsi
Bionics, describes the stringent encoder
requirements of human exoskeletons: “The
most difficult challenge when developing
exoskeletons is the reliability of obtaining
accurate angular position references, as
they change from one structure to another,
complicating standardisation and assembly
of the devices.
Previously, we had used our own custommade
PCB encoders that were fully linked
to the kinematic structures of the MAK and
ATLAS exoskeletons. But problems regularly
occurred because the joint motors generate
stray magnetic fields that can interfere
with magnetic encoders and cause faulty
readings.
“When designing the ATLAS and MAK
devices, we decided that the components
that make up the joints, such as the encoder,
should be as compact as possible without
compromising performance as there are
significant space constraints. Another aspect
to bear in mind is functionality: we need
absolute rotary encoders to ensure that
the angular position of each axis is always
reliably known, even after a power failure.”
78 PECM Issue 46