RLS magnetic encoders enable Marsi Bionics to build ‘life-changing’ exoskeletons

Child wearing ATLAS 2030 exoskeleton in seated position

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 custom-made 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."

Marsi Bionics' ATLAS 2030 exoskeleton for children

Marsi Bionics selected the RLS Orbis encoder for its ATLAS exoskeleton and the RLS RM08 encoder for its MAK knee-joint exoskeleton. The Orbis encoder is a component-level absolute rotary encoder designed for space constrained applications: it has a through-hole design, which enables direct mounting on the joint motor shaft. The RM08 encoder is a miniature high-speed rotary magnetic encoder: it has an 8 mm diameter and is IP68-rated for use in harsh environments.

Both encoders feature low weight and volume to minimise inertias, a non-contact and frictionless design to eliminate mechanical wear, and high angular resolution and accuracy to ensure excellent servo performance.

"We needed encoders that met our performance criteria without adding excess weight and volume, as any excess in the structure could be detrimental to the user's mobility and might hamper the walking movement," says Mr Plaza.

Marsi Bionics' MB-Active Knee (MAK) single-joint exoskeleton for adults