Series Elastic Actuators

This thesis presents the design, construction, control and evaluation of a novel force controlled actuator. Traditional force controlled actuators are designed from the premise that "Stiffer is better''. This approach gives a high bandwidth system, prone to problems of contact instability, noise, and low power density. The actuator presented in this thesis is designed from the premise that "Stiffness isn't everything". The actuator, which incorporates a series elastic element, trades off achievable bandwidth for gains in stable, low noise force control, and protection against shock loads. This thesis reviews related work in robot force control, presents theoretical descriptions of the control and expected performance from a series elastic actuator, and describes the design of a test actuator constructed to gather performance data. Finally the performance of the system is evaluated by comparing the performance data to theoretical predictions

Horses Damp the Spring in Their Step

The muscular work of galloping in horses is halved by storing and returning elastic strain energy in spring-like muscle–tendon units1, 2.These make the legs act like a child\u27s pogo stick that is tuned to stretch and recoil at 2.5 strides per second. This mechanism is optimized by unique musculoskeletal adaptations: the digital flexor muscles have extremely short fibres and significant passive properties, whereas the tendons are very long and span several joints3, 4. Length change occurs by a stretching of the spring-like digital flexor tendons rather than through energetically expensive length changes in the muscle5. Despite being apparently redundant for such a mechanism5, the muscle fibres in the digital flexors are well developed. Here we show that the mechanical arrangement of the elastic leg permits it to vibrate at a higher frequency of 30–40 Hz that could cause fatigue damage to tendon and bone. Furthermore, we show that the digital flexor muscles have minimal ability to contribute to or regulate significantly the 2.5-Hz cycle of movement, but are ideally arranged to damp these high-frequency oscillations in the limb

Exotendons for Assistance of Human Locomotion

Background: Powered robotic exoskeletons for assistance of human locomotion are currentlyunder development for military and medical applications. The energy requirements for such devices are excessive, and this has become a major obstacle for practical applications. Legged locomotion in many animals, however, is very energy efficient. We propose that poly-articular elastic mechanisms are a major contributor to the economy of locomotion in such specialized animals. Consequently, it should be possible to design unpowered assistive devices that make effective use of similar mechanisms. Methods: A passive assistive technology is presented, based on long elastic cords attached to anexoskeleton and guided by pulleys placed at the joints. A general optimization procedure is described for finding the best geometrical arrangement of such exotendons for assisting a specific movement. Optimality is defined either as minimal residual joint moment or as minimal residual joint power. Four specific exotendon systems with increasing complexity are considered. Representative human gait data were used to optimize each of these four systems to achieve maximal assistance for normal walking. Results: The most complex exotendon system, with twelve pulleys per limb, was able to reduce the joint moments required for normal walking by 71% and joint power by 74%. A simpler system, with only three pulleys per limb, could reduce joint moments by 46% and joint power by 47%. Conclusion: It is concluded that unpowered passive elastic devices can substantially reduce the muscle forces and the metabolic energy needed for walking, without requiring a change in movement. When optimally designed, such devices may allow independent locomotion in patients with large deficits in muscle function

Exotendons for assistance of human locomotion

BACKGROUND: Powered robotic exoskeletons for assistance of human locomotion are currently under development for military and medical applications. The energy requirements for such devices are excessive, and this has become a major obstacle for practical applications. Legged locomotion in many animals, however, is very energy efficient. We propose that poly-articular elastic mechanisms are a major contributor to the economy of locomotion in such specialized animals. Consequently, it should be possible to design unpowered assistive devices that make effective use of similar mechanisms. METHODS: A passive assistive technology is presented, based on long elastic cords attached to an exoskeleton and guided by pulleys placed at the joints. A general optimization procedure is described for finding the best geometrical arrangement of such "exotendons" for assisting a specific movement. Optimality is defined either as minimal residual joint moment or as minimal residual joint power. Four specific exotendon systems with increasing complexity are considered. Representative human gait data were used to optimize each of these four systems to achieve maximal assistance for normal walking. RESULTS: The most complex exotendon system, with twelve pulleys per limb, was able to reduce the joint moments required for normal walking by 71% and joint power by 74%. A simpler system, with only three pulleys per limb, could reduce joint moments by 46% and joint power by 47%. CONCLUSION: It is concluded that unpowered passive elastic devices can substantially reduce the muscle forces and the metabolic energy needed for walking, without requiring a change in movement. When optimally designed, such devices may allow independent locomotion in patients with large deficits in muscle function

The design and construction of a modular force control actuator

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1996.Includes bibliographical references (leaf 37).by Michael B. Wittig.B.S

High Compliant Series Elastic Actuation for the Robotic Leg ScarlETH

This paper presents the actuation system of the robotic leg ScarlETH. It was developed specifically for a quadrupedal robot and is designed to achieve fast position control as well as accurate joint torque control. It introduces strong passive dynamics to create an efficient running behavior. High spring compliance with low damping in combination with a cascaded, motor velocity based, control structure was successfully tested in simulation and experiments. Final tests with the entire leg demonstrate that the system can perform a hopping motion providing only positive actuator power

The design and application of a nonlinear series compliance actuator for use in robotic arms

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999.Includes bibliographical references (p. 55-56).by Arrin Katz.S.M

تصميم ركبة صناعية فعالة متكيفة مع الأراضي غير المستوية

تحاكي الركب الصناعية التجارية المتوافرة حالياً عمل الركبة البشرية من خلال أداء حركة دورانية واحدة (امتداد/انثناء)، كما وتركز الدراسات البحثية في هذا المجال على نفس الأمر، يهدف هذا البحث إلى الوصول لتصميم مفصل ركبة صناعي يقوم بحركتي دوران، ذلك بالإضافة لاقترابه قدر الإمكان من المواصفات الحركية والتحريكية للركبة الطبيعية السليمة. المفصل في هذا البحث هو من نوع U-Joint يؤمن حركتي دوران، يُتحكم بالحركة الدورانية الأولى (الامتداد/انثناء) عن طريق مشغل متسلسل مرنSeries Elastic Actuator SEA، ويُتحكم بالحركة الدورانية الثانية (الإبعاد/ اقتراب) عن طريق الميكانزم المسمى بنابض الغاز. تم التصميم على أساس المواصفات لشخص متوسط الطول والوزن، وتمت دراسة التصميم ستاتيكياً وديناميكياً باستخدام طريقة العناصر المنتهية للتأكد من قدرته على القيام بالوظيفة المطلوبة عند تعرضه للأحمال الممكنة

DESIGN AND CHARACTERISATION OF A CONTINUOUS ROTARY DAMPER WITH IDEAL VISCOUS DAMPING PROPERTIES

Master'sMASTER OF ENGINEERIN

Series elastic actuators

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 77-80).by Matthew M. Williamson.M.S