Design principles of the assistive soft exosuit

Harvard published details of their soft exosuit that they are making with DARPA funding.

A soft lower-extremity robotic exosuit is intended to augment normal muscle function in healthy individuals. Compared to previous exoskeletons, the device is ultra-lightweight, resulting in low mechanical impedance and inertia. The exosuit has custom McKibben style pneumatic actuators that can assist the hip, knee and ankle. The actuators attach to the exosuit through a network of soft, inextensible webbing triangulated to attachment points utilizing a novel approach we call the virtual anchor technique. This approach is designed to transfer forces to locations on the body that can best accept load. Pneumatic actuation was chosen for this initial prototype because the McKibben actuators are soft and can be easily driven by an off-board compressor. The exosuit itself (human interface and actuators) had a mass of 3500 g and with peripherals (excluding air supply) is 7144 g. In order to examine the exosuit’s performance, a pilot study with one subject was performed which investigated the effect of the ankle plantar-flexion timing on the wearer’s hip, knee and ankle joint kinematics and metabolic power when walking. Wearing the suit in a passive unpowered mode had little effect on hip, knee and ankle joint kinematics as compared to baseline walking when not wearing the suit. Engaging the actuators at the ankles at 30% of the gait cycle for 250 ms altered joint kinematics the least and also minimized metabolic power. The subject’s average metabolic power was 386.7 W, almost identical to the average power when wearing no suit (381.8 W), and substantially less than walking with the unpowered suit (430.6 W). This preliminary work demonstrates that the exosuit can comfortably transmit joint torques to the user while not restricting mobility and that with further optimization, has the potential to reduce the wearer’s metabolic cost during walking.

Work published in 2014 [Multi-joint Actuation Platform for Lower Extremity Soft Exosuits], the system delivered 4.01W on average over a gait cycle of 1.08 seconds with the ankle suit, and 3.27 W on average over a gait cycle of 1.06 seconds for the hip suit. The total energy delivered by the system over a single gait cycle was 4.33J and 3.47J respectively. During each gait cycle, the ankle joint received 3.02 J for an efficiency of 70%. The hip joint received 1.67 J for the flexion with an efficiency of 48%. Compared to biological joint power, the system provided 14.3% of energy needed by the ankle power and 9.6% of energy needed by the hip power on flexion.

In general, an exosuit should be able to create paths to transfer load between the assisted joint and other parts of the body where those forces can be handled without impeding natural human walking dynamics and comfort.

The first engineered soft exosuit [with a paper published in 2013], greatly reduced mechanical impedance and inertia compared to previous exoskeletons and wearable assistive devices.

The lightweight Soft Exosuit is designed to overcome the challenges of traditional heavier exoskeleton systems, such as power-hungry battery packs and rigid components that can interfere with natural joint movement. It is made of soft, functional textiles woven together into a piece of smart clothing that is pulled on like a pair of pants and intended to be worn under a soldier’s regular gear. Through a biologically inspired design, the suit mimics the action of the leg muscles and tendons when a person walks, and provides small but carefully timed assistance at the joints of the leg without restricting the wearer’s movement.

In a current prototype, a series of webbing straps positioned around the lower half of the body contain a low-power microprocessor and network of supple strain sensors that act as the “brain” and “nervous system” of the Soft Exosuit, respectively — continuously monitoring various data signals, including the suit tension, the position of the wearer (e.g., walking, running, crouched), and more.

Boosting the strength of those who are weaker or do not have walking endurance

Besides military applications, other applications for this design methodology include assisting the elderly, rehabilitation for children and adults with disorders such as Cerebral Palsy. In these applications, rather than augment healthy performance, the system has the potential to provide assistance for limited function, where smaller forces have the potential to achieve greater changes in performance.

Soft versus rigid exoskeletons

Previous exoskeletons all relied on rigid frameworks of linkages, coupled to the body at select locations via pads, straps, or other interface techniques. As the wearer flexes or extends their biological joints, these rigid links add considerable inertia to movement which must be overcome by motors or by the user. Though great effort has been made to minimize these effects, they still add considerable impedance to the natural gait dynamics and kinematics. Also, static misalignment of the biological and exoskeleton joints can result in dynamic misalignments of up to 10 cm duringnormal movement, causing pain and even injury to users. One solution has been to include redundant, passive degrees of freedom to accommodate these misalignments; however, this adds further weight to the system. It is partly for these reasons that these systems do not typically reduce the metabolic power required for locomotion. In order to address these issues there has recently been work on developing active soft orthotics that show great promise in reducing the impedance experienced by the wearer and allowing more natural movement.

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Design principles of the assistive soft exosuit

Harvard published details of their soft exosuit that they are making with DARPA funding.

A soft lower-extremity robotic exosuit is intended to augment normal muscle function in healthy individuals. Compared to previous exoskeletons, the device is ultra-lightweight, resulting in low mechanical impedance and inertia. The exosuit has custom McKibben style pneumatic actuators that can assist the hip, knee and ankle. The actuators attach to the exosuit through a network of soft, inextensible webbing triangulated to attachment points utilizing a novel approach we call the virtual anchor technique. This approach is designed to transfer forces to locations on the body that can best accept load. Pneumatic actuation was chosen for this initial prototype because the McKibben actuators are soft and can be easily driven by an off-board compressor. The exosuit itself (human interface and actuators) had a mass of 3500 g and with peripherals (excluding air supply) is 7144 g. In order to examine the exosuit’s performance, a pilot study with one subject was performed which investigated the effect of the ankle plantar-flexion timing on the wearer’s hip, knee and ankle joint kinematics and metabolic power when walking. Wearing the suit in a passive unpowered mode had little effect on hip, knee and ankle joint kinematics as compared to baseline walking when not wearing the suit. Engaging the actuators at the ankles at 30% of the gait cycle for 250 ms altered joint kinematics the least and also minimized metabolic power. The subject’s average metabolic power was 386.7 W, almost identical to the average power when wearing no suit (381.8 W), and substantially less than walking with the unpowered suit (430.6 W). This preliminary work demonstrates that the exosuit can comfortably transmit joint torques to the user while not restricting mobility and that with further optimization, has the potential to reduce the wearer’s metabolic cost during walking.

Work published in 2014 [Multi-joint Actuation Platform for Lower Extremity Soft Exosuits], the system delivered 4.01W on average over a gait cycle of 1.08 seconds with the ankle suit, and 3.27 W on average over a gait cycle of 1.06 seconds for the hip suit. The total energy delivered by the system over a single gait cycle was 4.33J and 3.47J respectively. During each gait cycle, the ankle joint received 3.02 J for an efficiency of 70%. The hip joint received 1.67 J for the flexion with an efficiency of 48%. Compared to biological joint power, the system provided 14.3% of energy needed by the ankle power and 9.6% of energy needed by the hip power on flexion.

In general, an exosuit should be able to create paths to transfer load between the assisted joint and other parts of the body where those forces can be handled without impeding natural human walking dynamics and comfort.

The first engineered soft exosuit [with a paper published in 2013], greatly reduced mechanical impedance and inertia compared to previous exoskeletons and wearable assistive devices.

The lightweight Soft Exosuit is designed to overcome the challenges of traditional heavier exoskeleton systems, such as power-hungry battery packs and rigid components that can interfere with natural joint movement. It is made of soft, functional textiles woven together into a piece of smart clothing that is pulled on like a pair of pants and intended to be worn under a soldier’s regular gear. Through a biologically inspired design, the suit mimics the action of the leg muscles and tendons when a person walks, and provides small but carefully timed assistance at the joints of the leg without restricting the wearer’s movement.

In a current prototype, a series of webbing straps positioned around the lower half of the body contain a low-power microprocessor and network of supple strain sensors that act as the “brain” and “nervous system” of the Soft Exosuit, respectively — continuously monitoring various data signals, including the suit tension, the position of the wearer (e.g., walking, running, crouched), and more.

Boosting the strength of those who are weaker or do not have walking endurance

Besides military applications, other applications for this design methodology include assisting the elderly, rehabilitation for children and adults with disorders such as Cerebral Palsy. In these applications, rather than augment healthy performance, the system has the potential to provide assistance for limited function, where smaller forces have the potential to achieve greater changes in performance.

Soft versus rigid exoskeletons

Previous exoskeletons all relied on rigid frameworks of linkages, coupled to the body at select locations via pads, straps, or other interface techniques. As the wearer flexes or extends their biological joints, these rigid links add considerable inertia to movement which must be overcome by motors or by the user. Though great effort has been made to minimize these effects, they still add considerable impedance to the natural gait dynamics and kinematics. Also, static misalignment of the biological and exoskeleton joints can result in dynamic misalignments of up to 10 cm duringnormal movement, causing pain and even injury to users. One solution has been to include redundant, passive degrees of freedom to accommodate these misalignments; however, this adds further weight to the system. It is partly for these reasons that these systems do not typically reduce the metabolic power required for locomotion. In order to address these issues there has recently been work on developing active soft orthotics that show great promise in reducing the impedance experienced by the wearer and allowing more natural movement.

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