Most survivors will require rehabilitation to regain as much independence and quality of life as possible. However, the rapidly increasing demand for post-stroke rehabilitation cannot be addressed through the means of traditional physical therapy alone.
Rehabilitation often begins soon after the stroke – perhaps within a day or two of the event – with physical therapy in the intensive care unit to enable the patient to relearn how to use limbs which are often paralysed by the stroke. Effective therapy must be based on the principles of neuromuscular physiology and can often take advantage of known neurological coupling between human joints, using coordinated movements and forces to boost functional recovery. The complexity of shoulder rehabilitation poses a significant obstacle to traditional forms of therapy.
The newly developed exoskeleton – called Harmony – is designed to address these challenges by creating a highly sensitive and repeatable platform which can greatly improve the effectiveness of shoulder therapy for stroke survivors. The Harmony exoskeleton wraps around patients’ shoulders, arms and wrists and can power their joints to assist them as they perform various dynamic movement tasks. This helps them relearn the specific rhythm that happens between the shoulder’s girdle and its ball and socket joint to maximise range of motion, thereby correcting the impaired neuromuscular function.
The rehabilitation robot uses advanced actuator technology and a suite of sensors that collect motion data at a high speed of 2kHz, which is fed into an algorithm to give users responsive control of varying levels of resistance during exercises. The designers needed a bearing that could carry large-moment loads so the exoskeleton can move coaxially with the patients’ skeletal structure as they move their arms. The bearing also needed to be small, lightweight and offer a low coefficient of friction.
“An exoskeleton like ours has very unique challenges,” said Rohit Varghese, head of product development at Harmonics Bionics, referring to the robot’s load requirements and limited space. “Very few bearings can satisfy them. IKO’s CRBH and CRBF crossed roller bearings fit very well.”
For such conditions, IKO’s crossed roller bearing (CRB) might be the best choice. With rollers alternately crossed at right angles between inner and outer rings, they can handle radial, axial and moment loads at the same time with high rotational accuracy. Opposing V-shaped upper and lower raceways hold the rolling elements. IKO crossed roller bearings are available with basic dynamic load ratings ranging from 2.91kN-1,140kN; basic static load ratings from 2.43kN-2,640kN and shaft diameters from 10mm-800mm.
Harmonic Bionics specified a model with basic dynamic load rating of 8.61kN and basic static load rating of 10.6kN – more than enough load-carrying ability to meet its moment load requirement of 100Nm. Delivering that performance is a compact and rigid bearing with 40mm shaft diameter, 65mm outer diameter, 10mm width and 150g weight.
In this instance, the company found that the CRBH crossed roller bearing was the most straightforward and compact way to carry the moment loads imposed on the exoskeleton, providing “just enough assistance” to the patient. This is in large part due to the CRBH’s low coefficient of friction. As the rehabilitation robot’s 50-plus sensors monitor patients’ positions and the forces their joints exert, the system runs algorithms to customise the levels of resistance and support that the patient receives relative to gravitational forces. High friction can otherwise compromise sensor measurements.
In addition, as the engineers built the prototype robot, they determined IKO’s CRBF bearing was a valuable tool in the process. Like the CRBH Series, the CRBF has an alternatively crossed-roller design. It features mounting holes on both the inner and outer rings, so the engineers were able to install the bearing without machining, special housings or fixing plates to micron-level bearing tolerances. The mounting holes saved space and simplified assembly before the company shifted to the CRBH series for the production units.
To provide such performance and weight benefits for a customer with such unusual needs takes an extraordinary supplier. IKO offers high-quality products and relevant applications engineering insight, for rotational engineering issues simple and complex.
Contact IKO today at enquiries@iko.co.uk to discuss your motion control needs.
Tel: 01908 566144
