Exo Hand
Motorised Mechanical wrist and finger brace for individuals suffering from central lesions
Background
This custom-made brace is a prototype designed for the CAMIN research project directed by Christine Azevedo, research director at INRIA from the Laboratory of Informatic, Robotic and Microelectronic of Montpellier (LIRMM).
The concept was to design a light weight, adjustable and non-reversible motorized brace to assist functional electrostimulation (FES) induced motion for functional grasping using patch electrodes placed onto the forearm of individuals suffering from central lesions.
Working principle:
While FES triggers the motion of the fingers and wrist, the brace, driven by a dedicated software
under development at the LIRMM has the following functions:
– Follow and assist the motion of the individual’s wrist and fingers
– Allow to completely “lock in place” the position of both the fingers and the wrist in the desired grasping position in order to stop the FES and therefore prevent rapid muscular fatigue induced by continuous FES.
Note: For the purpose of the photos and videos taken on a healthy arm without FES, the fingers ‘cables were set in a loose position which explain the lack of tension that can be seen between the palm of the hand and the wrist’s adjoining panel.
Design’s requirements
Main technical requirements:
Be driven by
electrical motors
Be non-reversible
Be adjustable
to different individual
Allow the placement of multiple FES electrodes on the skin
Be comfortable
Be easy to place
onto the individual
On top of the technical requirements that had to be addressed, it was essential for me to design a brace that would respect the hand’s biomechanics. I also wanted the electrical motors and dedicated gearing systems to be imbedded into the brace design in a compact, lightweight package capable of transmitting a descent amount of power.
The system also had to exhibit a good stiffness (minimum elasticity) in order to ensure a proper grasping of the object to be held in place (e.g book or water bottle)
Technical solution description
The architecture presented here can be seen as a hybrid approach between soft robotics and rigid mechanical motorized hand exoskeleton design.
The transmission of power through the fingers is insured by 5 micro stainless-steel cables running in a closed loop system inside adjustable and movable phalanges’ rings and linked to the fingers’ motor unit. The wrist motion is insured by a closed loop cable actuating a single pivot joint and controlled by the wrist motor unit.
The motor units are attached to a wrist and forearm adjoining panels inside which orthopedic cushioning sheaths are sutured. Dedicated straps and buckles running inside the latter allow for closing around the arm.
The phalanges’ rings are composed of a top half ring produced in a rigid material for maximum power transmission inside which the cables are inserted. The bottom part of the phalanges ’rings is composed of an adjustable Velcro band allowing to fasten the rings in place on each phalange and accommodate different fingers’ anatomy (diameter and length)
The travel upon closing of each finger can be customized via a specific mechanism enclosed in the motorized unit in order to compensate for the length difference between fingers.
The tension of each cable can also be adjusted on the individual prior to actuation using the cable tensioning screws to compensate for different hands anatomy.
Upon opening, the fingers are brought back to a flat position using silicon elastics bands (in white color) located on top of each finger and inserted inside each phalanges’ top half rings. The tension of each elastic band is adjustable using a Velcro band attached to the top part of the hand and the glove for the thumb.
The palm of hand is equipped with a flexible armed silicon sheet doubled with orthopedics skin contact foam. Each finger’s cables run inside low friction braded tubing waved inside the palm armed silicon sheet.
The weight of the fully assembled brace was measure at 462g.
Manufacturing
Only a few mechanical parts were produced or modified using conventional CNC machining:
– Motors’ supports
– Motors’ unit custom gears allowing for the non-reversible characteristic of the brace
– Pivot axis supporting the wrist hinge area
– Axis supporting the fingers’ cables wrapping system
Beside threaded inserts, screws and pins, the rest of the parts, including the custom gearing stages integrated inside the motors’ wrist and fingers units, were produced using 3D printing technology.
3D printing material:
PA6 20% carbon charged
Continuous glass fiber reinforcement
For stiffness reasons, glass fiber reinforcements were used in the following parts:
– Both parts composing the hinge actuating the wrist motion
– The wrist adjoining panel
The illustration below shows the parts produced using 3D printing.
Videos of the operating brace associated with FES will be uploaded as soon as possible after the first trials.
