Design and Development of a Lightweight Ankle Exoskeleton for Human Walking Augmentation

RESUMÉ La plupart des exosquelettes motorisés de la cheville ont une masse distale considérable, ce qui limite leur capacité à réduire l’énergie dépensée par l’utilisateur durant la marche. L’objectif de notre travail est de développer un exosquelette de chevilles avec le minimum de masse distale ajoutée comparé aux exosquelettes motorisés de chevilles existants. Aussi, l’exosquelette doit fournir au moins 50 Nm de support au couple de flexion plantaire. L’exosquelette développé dans le cadre de ce mémoire utilise deux câbles Bowden pour transmettre la force mécanique de l’unité d’actionnement attachée à la taille aux deux tiges en fibre de Carbonne attachées à la botte de l’utilisateur. Quand les deux tiges sont tirées, ils génèrent un couple qui supporte le mouvement de flexion plantaire à la fin de la phase d’appui du cycle de marche. Une pièce conçue sur mesure et imprimé en plastique par prototypage rapide a été attachée au tibia pour ajuster la direction des câbles. Une étude d’optimisation a été effectuée pour minimiser la masse des tiges limitant ainsi la masse distale de l’exosquelette (attaché au tibia et pied) à seulement 348 g. Le résultat principal obtenu à partir des tests de marche est la réduction de l’activité des muscles soléaire et gastrocnémien du sujet par une moyenne de 37% et 44% respectivement lors de la marche avec l’exosquelette comparée à la marche normale. Cette réduction s’est produite quand l’exosquelette a fourni une puissance mécanique de 19 ± 2 W avec un actionnement qui a commencé à 38% du cycle de marche. Ce résultat démontre le potentiel de notre exosquelette à réduire le cout métabolique de marche et souligne l’importance de réduire la masse distale d’un exosquelette de marche.----------ABSTRACT Most of powered ankle exoskeletons add considerable distal mass to the user which limits their capacity to reduce the metabolic energy of walking. The objective of the work presented in this master thesis is to develop an ankle exoskeleton with a minimum added distal mass compared to existing autonomous powered ankle exoskeletons, while providing at least 50 Nm of assistive plantar flexion torque. The exoskeleton developed in this master thesis uses Bowden cables to transmit the mechanical force from the actuation unit attached to the waist to the carbon fiber struts fixed on the boot. As the struts are pulled, they create an assistive ankle plantar flexion torque. A 3D-printed brace was attached to the shin to adjust the direction of the cables. A design optimization study was performed to minimize the mass of the struts, thereby limiting the total added distal mass, attached to the shin and foot, to only 348 g. The main result obtained from walking tests was the reduction of the soleus and gastrocnemius muscles activity by an average of 37% and 44% respectively when walking with the exoskeleton compared to normal walking. This reduction occurred when the exoskeleton delivered a mechanical power of 19 ± 2 W with an actuation onset fixed at 38% of the gait cycle. This result shows the potential of the proposed exoskeleton to reduce the metabolic cost of walking and emphasizes the importance of minimizing the distal mass of ankle exoskeletons

System Configuration and Control Using Hydraulic Transformer

University of Minnesota Ph.D. dissertation. May 2018. Major: Mechanical Engineering. Advisor: Perry Li. 1 computer file (PDF); xii, 294 pages.Hydraulic power transmission offers multiple benefits over competing technologies including an order of magnitude higher power density than electric systems, relatively low cost, fast response, and flexible packaging. Hydraulics are often used in high-performance mobile robots that demand power, precision, and compactness. However, typical hydraulic systems suffer from low system efficiency from the wide usage of throttle valves. The research described in this dissertation focuses on developing hydraulic transformers that transforms hydraulic power from one set of pressure and flow to the other set of pressure and flow to replace throttle valves such that a compact and efficient fluid power system can be realized. A dynamic model capable of capturing operating characteristics and losses is developed to establish a quantitative comparison between two major designs of the hydraulic transformer. A traditional design where a pump and motor are coupled together in a single package is chosen for the research. This design has three possible configurations with unique operating characteristics, and if these configuration modes can be switched, the resulting transformer is shown to be more compact and efficient. A trajectory tracking controller for a cylinder and force controller for a hydraulic human power amplifier is developed to demonstrate potential applications for the hydraulic transformer. The controller developed proves that utilizing hydraulic transformer need not sacrifice the control performance. Control methodologies ensuring efficiency of the transformer driven system are developed. Transformer operating speed is optimized to minimize the power loss through the transformer. Transformer configuration is switched actively to operate the transformer in its most optimal mode. These methods further improve the efficiency benefit of using the transformer. A hydraulic transformer system utilizing developed controllers compared against a throttle valve system tracking a trajectory with various loading conditions reveals that transformer system can achieve an efficiency of 81.2% which is more than threefold increase over the throttling system with an efficiency of 26.2%. This efficiency improvement is possible with the ability of a transformer to capture regenerative energy to reduce the net energy consumption. This dissertation successfully presents the controller development for a hydraulic transformer that captures both precision and efficiency

A novel hydraulic energy-storage-and-return prosthetic ankle : design, modelling and simulation

In an intact ankle, tendons crossing the joint store energy during the stance phase of walkingprior to push-off and release it during push-off, providing forward propulsion. Most prostheticfeet currently on the market – both conventional and energy storage and return (ESR) feet –fail to replicate this energy-recycling behaviour. Specifically, they cannot plantarflex beyondtheir neutral ankle angle (i.e. a 90° angle between the foot and shank) while generating theplantarflexion moment required for normal push-off. This results in a metabolic cost ofwalking for lower-limb amputees higher than for anatomically intact subjects, combined witha reduced walking speed.Various research prototypes have been developed that mimic the energy storage and returnseen in anatomically intact subjects. Many are unpowered clutch-and-spring devices thatcannot provide biomimetic control of prosthetic ankle torque. Adding a battery and electricmotor(s) may provide both the necessary push-off power and biomimetic ankle torque, butadd to the size, weight and cost of the prosthesis. Miniature hydraulics is commonly used incommercial prostheses, not for energy storage purposes, but rather for damping and terrainadaptation. There are a few examples of research prototypes that use a hydraulic accumulatorto store and return energy, but these turn out to be highly inefficient because they useproportional valves to control joint torque. Nevertheless, hydraulic actuation is ideally suitedfor miniaturisation and energy transfer between joints via pipes.Therefore, the primary aim of this PhD was to design a novel prosthetic ankle based on simpleminiature hydraulics, including an accumulator for energy storage and return, to imitate thebehaviour of an intact ankle. The design comprises a prosthetic ankle joint driving two cams,which in turn drive two miniature hydraulic rams. The “stance cam-ram system” captures theeccentric (negative) work done from foot flat until maximum dorsiflexion, by pumping oil intothe accumulator, while the “push-off system” does concentric (positive) work to power pushoff through fluid flowing from the accumulator to the ram. By using cams with specific profiles,the new hydraulic ankle mimics intact ankle torque. Energy transfer between the knee andthe ankle joints via pipes is also envisioned.A comprehensive mathematical model of the system was defined, including all significantsources of energy loss, and used to create a MATLAB simulation model to simulate theoperation of the new device over the whole gait cycle. A MATLAB design program was alsoimplemented, which uses the simulation model to specify key components of the new designto minimise energy losses while keeping the device size acceptably small.The model’s performance was assessed to provide justification for physical prototyping infuture work. Simulation results show that the new device almost perfectly replicates thetorque of an intact ankle during the working phases of the two cam-ram systems. Specifically,78% of the total eccentric work done by the prosthetic ankle over the gait cycle is returnedas concentric work, 14% is stored and carried forward for future gait cycles, and 8.21% is lost.A design sensitivity study revealed that it may be possible to reduce the energy lost to 5.83%of the total eccentric work. Finally, it has been shown that the main components of the system– cams, rams, and accumulator - could be physically realistic, matching the size and mass ofthe missing anatomy