Martin Grimmer

Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit

Evaluation of a soft exosuit designed to reduce metabolic requirements during walking. When defining requirements for any wearable robot for walking assistance, it is important to maximize the user’s metabolic benefit resulting from the exosuit assistance while limiting the metabolic penalty of carrying the system’s mass. Thus, the aim of this study was to isolate and characterize the relationship between assistance magnitude and the metabolic cost of walking while also examining changes to the wearer’s underlying gait mechanics. The study was performed with a tethered multiarticular soft exosuit during normal walking, where assistance was directly applied at the ankle joint and indirectly at the hip due to a textile architecture. The exosuit controller was designed such that the delivered torque profile at the ankle joint approximated that of the biological torque during normal walking. Seven participants walked on a treadmill at 1.5 meters per second under one unpowered and four powered conditions, where the peak moment applied at the ankle joint was varied from about 10 to 38% of biological ankle moment (equivalent to an applied force of 18.7 to 75.0% of body weight). Results showed that, with increasing exosuit assistance, net metabolic rate continually decreased within the tested range. When maximum assistance was applied, the metabolic rate of walking was reduced by 22.83 ± 3.17% relative to the powered-off condition (mean ± SEM).

Effects of unidirectional parallel springs on required peak power and energy in powered prosthetic ankles: Comparison between different active actuation concepts

Peak power, peak torque and energy requirements are amongst the main issues for powered prosthetic ankles. Including series elastic actuators (SEA) can reduce peak power and energy requirements by reducing motor speed in comparison to a Direct Drive (DD). Parallel elastic actuation concept (PEA, the parallel spring can be compressed and elongated) can reduce the peak power even more by reducing required motor torque. However, the energy consumption increases in comparison to SEA. In this paper, we investigated if unidirectional parallel springs (UPS) that act only at special ankle angles could combine the positive features of both SEA and PEA concepts in terms of motor peak power and/or energy reductions. Therefore, the motor peak power and energy requirements are compared between active actuation concepts SEA, SEA+PS, SEA+UPS, DD+PS and DD+UPS. For the minimum motor peak power requirements, we found that the reduction was similar between PS and UPS. However, the corresponding energy requirements were less or similar in UPS for corresponding speeds. For the minimum energy requirements, it was found that the UPS reduced the energy requirements but increased the corresponding required peak power for the majority of speeds in comparison to PS. The ultimate goal of combining positive characteristics of SEA and PEA was identified mainly for lower walking and running speeds.

Active lower limb prosthetics: a systematic review of design issues and solutions

This paper presents a review on design issues and solutions found in active lower limb prostheses. This review is based on a systematic literature search with a methodical search strategy. The search was carried out across four major technical databases and the retrieved records were screened for their relevance. A total of 21 different active prostheses, including 8 above-knee, 9 below-knee and 4 combined knee-ankle prostheses were identified. While an active prosthesis may help to restore the functional performance of an amputee, the requirements regarding the actuation unit as well as for the control system are high and the development becomes a challenging task. Regarding mechanical design and the actuation unit high force/torque delivery, high efficiency, low size and low weight are conflicting goals. The actuation principle and variable impedance actuators are discussed. The control system is paramount for a “natural functioning” of the prosthesis. The control system has to enable locomotion and should react to the amputee’s intent. For this, multi-level control approaches are reviewed.

Mimicking Human-Like Leg Function in Prosthetic Limbs

Human upright locomotion is a complex behavior depending on manifold requirements. Bones, muscles, cartilage and tendons provide mechanical infrastructure. Central nervous commands, reflex mechanisms from the spinal cord level or also preflexes defined by actuator properties provide input to create motion patterns like walking or running. Due to dysvascularity, infections or traumatic events parts of the biological framework can get lost. Until the end of the twentieth century mostly passive structures were used to replace amputees lower limbs. Full functionality like in the biological system can not be provided because of missing sensory information and power source. Innovations in actuator, battery and micro electronics technology make it possible to improve prosthetic design. A first innovation was introduced with semi-active devices using microprocessor controlled dampers to modulate prosthetic joint behavior similar to isometric or eccentric muscle function. A further step is to power the joints to emulate concentric muscle function. Combined with ingenious control mechanisms this could potentially provide every possible movement task. Twenty-six powered prosthetic systems and further passive prototypes are presented in this work. Mechanical and control solutions are introduced. Amputee gait in various daily life situations using passive, semi-active and powered prostheses is compared. Areas for improvements are discussed.

The myosuit: Bi-articular anti-gravity exosuit that reduces hip extensor activity in sitting transfers

Muscle weakness—which can result from neurological injuries, genetic disorders, or typical aging—can affect a persons mobility and quality of life. For many people with muscle weakness, assistive devices provide the means to regain mobility and independence. These devices range from well-established technology, such as wheelchairs, to newer technologies, such as exoskeletons and exosuits. For assistive devices to be used in everyday life, they must provide assistance across activities of daily living (ADLs) in an unobtrusive manner. This article introduces the Myosuit, a soft, wearable device designed to provide continuous assistance at the hip and knee joint when working with and against gravity in ADLs. This robotic device combines active and passive elements with a closed-loop force controller designed to behave like an external muscle (exomuscle) and deliver gravity compensation to the user. At 4.1 kg (4.6 kg with batteries), the Myosuit is one of the lightest untethered devices capable of delivering gravity support to the users knee and hip joints. This article presents the design and control principles of the Myosuit. It describes the textile interface, tendon actuators, and a bi-articular, synergy-based approach for continuous assistance. The assistive controller, based on bi-articular force assistance, was tested with a single subject who performed sitting transfers, one of the most gravity-intensive ADLs. The results show that the control concept can successfully identify changes in the posture and assist hip and knee extension with up to 26% of the natural knee moment and up to 35% of the knee power. We conclude that the Myosuits novel approach to assistance using a bi-articular architecture, in combination with the posture-based force controller, can effectively assist its users in gravity-intensive ADLs, such as sitting transfers.

Measurement of biomechanical interactions at the stump-socket interface in lower limb prostheses

This paper introduces a novel measuring approach for detecting relative movement between stump and socket in lower limb prostheses. The application of the motion capturing based measuring approach is shown at a single male trans-tibial amputee using a Patella Tendon Bearing (PTB) socket. It further investigates and assesses the feasibility of measuring the relative movement between stump and socket during level walking at different velocities and allocating it to the coinciding loads. Representative results for the two translational degrees of freedom in the sagittal plane are presented and discussed. For the proximodistal (pd) direction, a linear correlation between applied load and relative movement is found, while for the anteroposterior (ap) direction the stump movement is largely influenced by the motion sequence during the respective gait event. Additionally, the effect of walking speed is discussed.

Comparison of trajectory generation methods for a human-robot interface based on motion tracking in the Int 2 Bot

The acceptance of artificial devices like prostheses or other wearable robots requires their integration into the body schemas of the users. Different factors induce, influence and support the integration and acceptance of the device that substitutes or augments a part of the body. Previous studies have shown that the inducing and maintaining factors are visual, tactile and proprioceptive informations as well as their multi-sensory integration. This paper describes the vision-based part of the human-robot interface in the Int2Bot, which is a robot for the investigation of lower limb body schema integration during postural movements. The psychological approach and the technical setup of the robot, which is designed to imitate postural movements in the sagittal plane to imitate the human subject while performing squats, are outlined. To realize the imitation, an RGB-D sensor, in form of a Microsoft Kinect, is used to capture the subjects motions without contact and thereby avoid disturbances of body schema integration. For generation of the desired joint trajectories to be tracked by the control algorithm, different methods like an extended Kalman filter, inverse kinematics, an inverse kinematics algorithm using Jacobian transpose and approaches based on kinematic assumptions are presented, evaluated and compared based on human data. Benchmarking the results with data acquired using a professional motion capturing system shows that best overall joint angle estimations are achieved with the extended Kalman filter. Finally, the practical implementation within the robot is presented and the tracking behavior using the trajectories generated with the extended Kalman filter are analyzed.

Feasibility study of transtibial amputee walking using a powered prosthetic foot

Passive prosthetic feet are not able to provide non-amputee kinematics and kinetics for the ankle joint. Persons with amputations show reduced interlimb symmetry, slower walking speeds, and increased walking effort. To improve ankle range of motion and push off, various powered prosthetic feet were introduced. This feasibility study analyzed if predefined motor reference trajectories can be used to achieve non-amputee ankle biomechanics during walking with the powered prosthetic foot, Walk-Run Ankle. Trajectories were calculated using the desired ankle angle and ankle moment based spring deflection at a given spring stiffness. Model assumptions of the motor-spring interaction were well reflected in the experiment. The powered foot was able to improve range of motion, peak ankle power, average positive ankle power, peak ankle moment, and positive moment onset compared to a passive usage of the foot. Furthermore, symmetry improvements were identified for step length and duty factor. Further studies with an increased number of subjects are needed to show if the approach is also valid for other amputees. Using this method as a base, trajectories can be further individualized using human in the loop optimization targeting a reduction of user effort, improved stability, or gait symmetry.

Comparison of Ankle Moment Inspired and Ankle Positive Power Inspired Controllers for a Multi-Articular Soft Exosuit for Walking Assistance

Mobility can be limited due to age or impairments. Wearable robotics provide the chance to increase mobility and thus independence. A powered soft exosuit was designed that assist with both ankle plantarflexion and hip flexion through a multi-articular suit architecture. So far, the best method to reduce metabolic cost of human walking with external forces is unknown. Two basic control strategies are compared in this study: an ankle moment inspired controller (AMIC) and an ankle positive power inspired controller (APIC). Both controllers provided a similar amount of average positive exosuit power and reduced the net metabolic cost of walking by 15 %. These results suggest that average positive power could be more important than assistive moment during single stance for reducing metabolic cost. Further analysis must show if one of the approaches has advantages for wearers comfort, changes in walking kinetics and kinematics, balance related biomechanics, or electrical energy consumption.

Adding passive biarticular spring to active mono-articular foot prosthesis: Effects on power and energy requirement

Current active foot prostheses try to emulate the foot function by using a mono-articular series elastic actuator (SEA). In humans, a bi-articular actuation scheme is used to plantar-flex the foot through Soleus and Gastrocnemius muscles (in plantar-flexion the angle between foot and shank increases). In this study we investigated on using a bi-articular actuation scheme in active foot prostheses (where a passive bi-articular spring is added to an active mono-articular actuator). For design of these robotic systems, motor peak power (PP) and energy (E) requirement are two important factors. Lower PP or E requirement lead to a smaller motor or battery, hence decreasing the weight and increasing the performance capacity of the device. For this purpose, we compared the PP and E requirement of the mono- and the proposed bi-articular actuation models for active foot prostheses. The simulation results showed that for normal running (2.6 m/s), the PP requirement of the proposed bi-articular actuation can decrease about 54% in comparison to the mono-articular actuation. For normal walking (1.55 m/s), the required PP can reduce about 38%. On the other side, the corresponding E requirement can reduce about 7% for running, however, for walking, it can increase about 8%. In addition, it was found out that, if the spring is attached to the shank, the average results of the PP and E requirements are higher than the bi-articular scheme, however the differences were not very noticeable. Thus, our study shows that for designing a powered prosthesis both methods can be used. As the bi-articular spring can also influence the knee joint power of a trans-tibial amputee, the engineering advantages should be taken into account together with the users comfort.