Upper-limb active exoskeleton BITERS

Abstract

Stroke can result in injury and death, with impairments such as loss of somatosensation, loss of fractionated movement, impaired motor speed, and paresis being common post-stroke. The upper-limb is important to many activities of daily living (ADLs), with these impairments often limiting the execution of basic tasks. Rehabilitation works to develop motor movements and develop neuroplasticity, which can be approached in many different ways and forms. Stroke rehabilitation robotics and exoskeletons have many benefits over traditional methods and other technology. This field has a range of approaches in terms of actuation, sensing, and control systems. Exoskeletons are wearable devices that directly interact with the human body, exerting force onto the wearer to excite movement.

The BITERS exoskeleton is a 5-degree of freedom (DOF) active upper-limb exoskeleton, which features two active joints and three passive joints. The kinematic model of the exoskeleton has been modeled as an open-loop chain encompassing from the centre of the torso out to the upper arm (humerus). Two passive DOFs accommodate the translation of the glenohumeral (GH) joint in the frontal plane. The rotational joint at the centre of the torso allows for 40◦ of shoulder elevation from active human movement. The prismatic joint is a linear slider bearing in series with a 40mm compression spring. The inclusion of this prismatic joint is a novel feature of the exoskeleton, allowing for the displacement of the GH joint along the direction of scapula sliding, and accommodating for a large range of discrete shoulder movements. This DOF can translate the device’s work-space anteriorly (against a restorative force) or posterally while encouraging correct shoulder posture. The GH joint allows the rotation of the humeral head in the shoulder girdle and is approximated by three rotational DOFs which form a spherical assembly whose axis intercept with the centre of the humerus head. The first and third of these DOFs (parallel to the coronal and sagittal planes respectively) are actuated by a DC motor driven bidirectional Bowden cable system, allowing for actuated abduction/adduction and flexion/extension. The remaining DOF in the spherical assembly allows the passive free movement of the other DOFs, internal/external rotation is theorised to occur autonomously given the freedom to passively move at this joint.

Portability was a focus area during development. The exoskeleton is lightweight with its arm unit weighing 500g and back unit weighing 3500g. The 3D polycarbonate (PC) build of the exoskeleton arm provides high tensile strength while reducing weight, a 50% infill is used in these links to improve flexibility. All larger loads are positioned distally on the backplate, whose force is distributed across the user’s shoulders via padded yoke straps. Compliance is desired in systems for safety and comfort. Soft actuation, the shift of weight away from the actuation points, compliant exoskeleton-body attachment (cuffs), and flexible filament material, are all used to increase the compliance of the system.

A Teensy 3.5 microcontroller is used to read sensor feedback, implement control, and provide a user interface. This system is powered by a 14.8V 4000mAh battery and has an operating life of 25 to 45 minutes. The exoskeleton’s proportional control system uses joint angle feedback and EMG to provide real-time adjusted assistance based on the effort outputted by the user. The posterior deltoid was determined to be a suitable EMG control input, due to its significant response to shoulder flexion, extension, and abduction, and its low response to passive movements such as internal/external rotation.

BITERS successfully modelled the human shoulder motion and allowed for active actuation of the shoulder with DC motor actuation and Bowden cable power transmission. Both manual proportional control and EMG assist-as-need based control were implemented by software. Validation of the exoskeletons range of motion (ROM) was successfully performed through the use of CAD simulation, modelled work-space analysis, and 3D motion tracking technology. The developed exoskeleton allows for 157◦ flexion, 30◦ extension and 177◦ abduction, with a work-space volume of 0.00446m3. The peak torque achieved was 7.71Nm at the actuated joints, enabling assisted shoulder motion in flexion/extension and adduction/abduction.