Classification of Exoskeletons and Orthoses

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Journal of NeuroEngineering and Rehabilitation (JNER) – Exoskeletons and orthoses: classification, design challenges and future directions by Hugh Herr

Hugh Herr was involved in making a motorless exoskeleton that was quasi-passive yet able to bear 80 pounds of weight

For over a century, technologists and scientists have actively sought the development of exoskeletons and orthoses designed to augment human economy, strength, and endurance. While there are still many challenges associated with exoskeletal and orthotic design that have yet to be perfected, the advances in the field have been truly impressive. In this commentary, I first classify exoskeletons and orthoses into devices that act in series and in parallel to a human limb, providing a few examples within each category. This classification is then followed by a discussion of major design challenges and future research directions critical to the field of exoskeletons and orthoses.

I classify exoskeletons and orthoses into four categories and provide design examples within each of these. I discuss devices that act in series with a human limb to increase limb length and displacement, and devices that act in parallel with a human limb to increase human locomotory economy, augment joint strength, and increase endurance or strength.

Exoskeletons and orthoses are defined as mechanical devices that are essentially anthropomorphic in nature, are ‘worn’ by an operator and fit closely to the body, and work in concert with the operator’s movements. In general, the term ‘exoskeleton’ is used to describe a device that augments the performance of an able-bodied wearer, whereas the term ‘orthosis’ is typically used to describe a device that is used to assist a person with a limb pathology.

1. Series-limb exoskeletons – like Springwalker and Powerskip

with an in-series leg exoskeleton device, the ground reaction forces are still borne by the human leg. In contrast, with a parallel mechanism, body weight could be transferred through the exoskeleton directly to the ground, decreasing the loads borne by the biological limbs and lowering the metabolic demands to walk, run, and hop. Furthermore, such a parallel exoskeleton would not increase limb length, thereby not increasing the overall energetic demand to stabilize movement.

2. Parallel-limb exoskeletons for load transfer

Examples are Berkeley Lower Extremity Exoskeleton (BLEEX) and HULC.

BLEEX can reportedly support a load of up to 75 kg while walking at 0.9 m/s, and can walk at speeds of up to 1.3 m/s without the load. A second generation of the Berkeley exoskeleton is currently in testing. The new device is approximately half the weight of the original exoskeleton (~14 kg), in part due to the implementation of electric actuation with a hydraulic transmission system.

3. Parallel-limb exoskeletons for torque and work augmentation

Here we discuss exoskeletons that act in parallel with the human joint(s) for torque and work augmentation. Many parallel-limb exoskeletons have been developed to augment
joint torque and work. In distinction to the load-carrying exoskeletons mentioned in the last section, this type of exoskeletal and orthotic device does not transfer substantial load to the ground, but simply augments joint torque and work. This type of leg exoskeleton could improve walking and running metabolic economy, or might be used to reduce joint pain or increase joint strength in paralyzed or weak joints.

The Japanese HAL 5 suit is a prime example.

4. Parallel-limb exoskeletons that increase human endurance

A crutch was constructed with an orthotic elbow spring to maximize the endurance of physically challenged persons in climbing stairs and slopes. When the crutch user flexes both elbows to place the crutch tips onto the next stair tread, orthotic elbow springs compress and store energy. This stored energy then assists the crutch user during elbow extension, helping to lift the body up the next step, and delaying the onset of bicep and tricep muscle fatigue. In future developments, robotic exoskeletons and powered orthoses could be put forth that actively vary impedance to optimally redistribute the body’s work load over a greater muscle volume, maximizing the efficiency with which the body is able to perform mechanical work and significantly augmenting human endurance.

Future Directions and Challenges

There are many factors that continue to limit the performance of exoskeletons and orthoses. Today’s powered devices are often heavy with limited torque and power, making the wearer’s movements difficult to augment. Current devices are often both unnatural in shape and noisy, factors that negatively influence device cosmesis. Given current limitations in actuator technology, continued research and development in artificial muscle actuators is of critical importance to the field of wearable
devices. Electroactive polymers have shown considerable promise as artificial muscles, but technical challenges still remain for their implementation

Another factor limiting today’s exoskeletons and orthoses is the lack of direct information exchange between the human wearer’s nervous system and the wearable device. Continued advancements in neural technology will be of critical importance to the field of wearable robotics. Peripheral sensors placed inside muscle to measure the electromyographic signal, or centrally-placed sensors into the motor cortex, may be used to assess motor intent by future exoskeletal control systems. Neural implants may have the potential to be used for sensory feedback to the nerves or brain, thus allowing the exoskeletal wearer to have some form of kinetic and kinematic sensory information from the wearable device.

Today’s interface designs often cause discomfort to the wearer, limiting the length of time that a device can be worn. A proposed solution is 3D body scans and custom fitting. An exoskeleton, customized to fit the wearer’s outer anatomical features and physiological demands, would then be designed as a ‘second skin’. Such a skin would be made compliant in body regions having bony protuberances, and more rigid in areas of high tissue compliance. The exoskeletal skin would be so intimate with the human body that external shear forces applied to the exoskeleton would not produce relative movement between the exoskeletal inner surface and the wearer’s own skin, eliminating skin sores resulting from device rubbing. Compliant artificial muscles, sensors, electronics and power supply would be embedded within the three dimensional construct, offering full protection of these components from environmental disturbances such as dust and moisture. Once designed, device construction would unite additive and subtractive fabrication processes to deposit materials with varied properties (stiffness and density variations) across the entire exoskeletal volume using large scale 3-D printers and robotic arms.

Exoskeleton for Parapalegics that simulates natural gait

Assisted Steps: A patient with paralysis stands with the aid of the Berkeley exoskeleton. The exoskeleton moves the patient’s hips and knees to imitate a natural walk. Credit: University of California, Berkeley
A Berkeley device, which houses a computer and battery pack, straps onto a user’s back like a backpack and can run six to eight hours on one charge. Pumps drive hydraulic fluid to move the hip and knees at the same time, so that the hip swings through a step as one knee bends. The device plans walking trajectories based on data (about limb angles, knee flexing, and toe clearance) gathered from people’s natural gaits. Pressure sensors in each heel and foot make sure both feet aren’t leaving the ground at the same time.

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