Exoskeleton helps arm-based physical therapy
Inside machines and robotics: Looking inside an exoskeleton shows advanced robotics technologies used to help physical therapy related to loss of arm movement. It’s a high-technology motion control application.
A robotic exoskeleton is helping those with paralysis in arms to move, helping with physical therapy. It has 23 degrees of freedom (DoF) enabled by more than 50 sensors, sensing actuators, multiple processors and high-speed networking.
Please describe a recent robotics implementation.
Harmonic Bionics of Austin, Texas, developed the Harmony exoskeleton for the upper limbs that helps people who have suffered a stroke or neuromuscular damage to improve their functionality and healing. Physical therapists use the device to assist patients in their rehabilitation.
What was the scope of the project and goals?
The goal was to maximize the range of motion of the human shoulder and the shoulder girdle on the hemiparetic arm (the arm impacted by the stroke). The exoskeleton had to fit with people of all shapes and sizes (Harmony can be adjusted to fit more than 95% of the U.S. population.) It also includes advanced actuator and controller technology that allows Harmony to customize gravity compensation, assistance and resistance, as needed, for each patient.
What types of robotics and other automation, controls, or instrumentation were involved?
The device is a 23 DoF robotic exoskeleton with 14 degrees of closed-loop torque sensing actuators capable of impedance control. The remaining 9 actuators have position feedback for active size adjustment. The robot control system was designed from the ground up by Harmonic Bionics using the EtherCAT protocol to enable cycle times across distributed processors. For instrumentation, the device has more than 50 sensors. Each joint has two encoders, active load sensing, and motor feedback, in addition to other required miscellaneous instrumentation.
What were particular challenges outlined in the project?
To be effective in a clinical setting, it was important for the exoskeleton to quickly and smoothly change its physical dimensions to align with different body sizes. In addition, due to the robot’s complexity, the resizing mechanisms need to be highly compact to allow for room for the electronics and actuators. All linear bearing mechanisms need to be able to lock once in position to prevent unwanted changes in size. Also, development of high-power, robust, and compact actuators capable of closed-loop impedance control at high refresh rates was a challenge.
How were those issues resolved?
To assist with sizing differences and the exoskeleton’s weight, Harmonic Bionics developers chose maintenance-free plastic bearings. The bearings allow for smooth and bind-free size adjustment without lubrication, as well as size and weight reduction. Oil-based lubrication for the linear bearings cause many issues, including the accumulation of dirt, which allows for bacteria to accumulate. Harmonic also used liners and rail guide systems. The guide systems are manufactured to ensure high rigidity in the stand for the exoskeleton and are extremely resistant to dirt and offer a low coefficient of friction and wear.
The design team built out all of the motor control and communication electronics in-house with a closed-loop cycle time of over 2,000 Hz across distributed processors, in addition to custom designing the actuator module to enable closed-loop torque feedback for accurate and reliable torque and impedance control. This has enabled the level of performance therapists require.
Can you share some positive metrics associated with the project?
Pilot research findings based on healthy and stroke subjects showing positive results were recently published at the International Stroke Conference. There is growing traction in the medical community on the need to bring this robotic technology into the clinical market as rapidly as possible.
What were the resulting lessons learned you’d like to share?
Harmony will be a powerful tool to use as a modality to aide in progressing the stroke patient towards normalizing movement. Clinics are limited and it sometimes takes years of therapy to become functional. The misconception is normal use and movement will be fully restored. The hope is to provide an adjunct to existing strategies in clinics and at home with the stroke population. Harmony helps make a therapist’s job easier to handle the patient’s upper limb paralysis and gives immediate feedback.
KEYWORDS: Robotic exoskeleton, motion control
A robotic exoskeleton design helps with arm-movement therapy.
Closed-loop torque-sensing actuators help with control.
Industrial Ethernet provided high-speed communications.
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