Actuator with Angle-Dependent Elasticity for Biomimetic Transfemoral Prostheses (original) (raw)
Related papers
Journal of Computational and Nonlinear Dynamics, 2023
Robotic leg prostheses and exoskeletons have traditionally been designed using highly-geared motor-transmission systems that minimally exploit the passive dynamics of human locomotion, resulting in inefficient actuators that require significant energy consumption and thus provide limited battery-powered operation or require large onboard batteries. Here we review two of the leading energy-efficient actuator design principles for legged and wearable robotic systems: series elasticity and backdrivability. As shown by inverse dynamic simulations of walking, there are periods of negative joint mechanical work that can be used to increase efficiency by recycling some of the otherwise dissipated energy using series elastic actuators and/or backdriveable actuators with energy regeneration. Series elastic actuators can improve shock tolerance during foot-ground impacts and reduce the peak power and energy consumption of the electric motor via mechanical energy storage and return. However, actuators with series elasticity tend to have lower output torque, increased mass and architecture complexity due to the added physical spring, and limited force and torque control bandwidth. High torque density motors with low-ratio transmissions, known as quasi-direct drives, can likewise achieve low output impedance and high backdrivability, allowing for safe and compliant human-robot physical interactions, in addition to energy regeneration. However, torque-dense motors tend to have higher Joule heating losses, greater motor mass and inertia, and require specialized motor drivers for real-time control. While each actuator design has advantages and drawbacks, designers should consider the energy-efficiency of robotic leg prostheses and exoskeletons beyond steady-state level-ground walking.
Concept of a Series-Parallel Elastic Actuator for a Powered Transtibial Prosthesis
Actuators, 2013
The majority of the commercial transtibial prostheses are purely passive devices. They store energy in an elastic element during the beginning of a step and release it at the end. A 75 kg human, however, produces on average 26 J of energy during one stride at the ankle joint when walking at normal cadence and stores/releases 9 J of energy, contributing to energy efficient locomotion. According to Winter, a subject produces on average of 250 W peak power at a maximum joint torque of 125 Nm. As a result, powering a prosthesis with traditional servomotors leads to excessive motors and gearboxes at the outer extremities of the legs. Therefore, research prototypes use series elastic actuation (SEA) concepts to reduce the power requirements of the motor. In the paper, it will be shown that SEAs are able to reduce the power of the electric motor, but not the torque. To further decrease the motor size, a novel human-centered actuator concept is developed, which is inspired by the variable recruitment of muscle fibers of a human muscle. We call this concept series-parallel elastic actuation (SPEA), and the actuator consists of multiple parallel springs, each connected to an intermittent mechanism with internal locking and a single motor. As a result, the motor torque requirements can be lowered and the efficiency drastically increased. In the paper, the novel actuation concept is explained, and a comparative study between a stiff motor, an SEA and an SPEA, which all aim at mimicking human ankle behavior, is performed.
Actuators, 2014
A monoarticular series elastic actuator (SEA) reduces energetic and peak power requirements compared to a direct drive (DD) in active prosthetic ankle-foot design. Simulation studies have shown that similar advantages are possible for the knee joint. The aims of this paper were to investigate the advantages of a monoarticular SEA driven hip joint and to quantify energetic benefit of an SEA driven leg (with monoarticular hip, knee and ankle SEAs) assuming that damping (negative power) is passively achieved. The hip SEA provided minor energetic advantages in walking (up to 29%) compared to the knee and the ankle SEA. Reductions in required peak power were observed only for speeds close to preferred walking speed (18 to 27%). No energetic advantages were found in running where a DD achieved the best performance when optimizing for energy. Using an SEA at each leg joint in the sagittal plane reduced positive work by 14 to 39% for walking and by 37 to 75% for running. When using an SEA instead of a DD, the contribution of the three leg joints to doing positive work changed: the knee contributed less, the hip contributed more positive work. For monoarticular SEAs the ankle joint motor did most of the positive work.
Stiffness Modulation in a Humanoid Robotic Leg and Knee
IEEE Robotics and Automation Letters
Stiffness modulation in walking is critical to maintain static/dynamic stability as well as to minimize energy consumption and impact damage. However, optimal, or even functional, stiffness parameterization remains unresolved in legged robotics. We introduce an architecture for stiffness control utilising a bioinspired robotic limb consisting of a condylar knee joint and leg with antagonistic actuation. The joint replicates elastic ligaments of the human knee providing tuneable compliance for walking. Further, it locks out at maximum extension, providing stability when standing. Compliance and friction losses between joint surfaces are derived as a function of ligament stiffness and length. Experimental studies validate utility through quantification of: 1) hip perturbation response; 2) payload capacity; and 3) static stiffness of the leg mechanism. Results prove initiation and compliance at lock out can be modulated independently of friction loss by changing ligament elasticity. Furthermore, increasing co-contraction or decreasing joint angle enables increased leg stiffness, which establishes cocontraction is counterbalanced by decreased payload. Findings have direct application in legged robots and transfemoral prosthetic knees, where biorobotic design could reduce energy expense while improving efficiency and stability. Future targeted impact involves increasing power/weight ratios in walking robots and artificial limbs for increased efficiency and precision in walking control.
International Journal of Advanced Robotic Systems, 2013
A safe interaction is crucial in wearable robotics in general, while in assistive and rehabilitation applications, robots may also be required to minimally perturb physiological movements, ideally acting as perfectly transparent machines. The actuation system plays a central role because the expected performance, in terms of torque, speed and control bandwidth, must not be achieved at the expense of lightness and compactness. Actuators embedding compliant elements, such as series elastic actuators, can be designed to meet the above-mentioned requirements in terms of high energy storing capacity and stability of torque control. A number of series elastic actuators have been proposed over the past 20 years in order to accommodate the needs arising from specific applications. This paper presents a novel series elastic actuator intended for the actuation system of a lower limb wearable robot, recently developed in our lab. The actuator is able to deliver 300 W and has a novel architecture making its centre of mass not co-located with its axis of rotation, for an easier integration into the robotic structure. A custom-made torsion spring with a stiffness of 272.25 N·m·rad −1 is directly connected to the load. The delivered torque is calculated from the measurement of the spring deflection, through two absolute encoders. Testing on torque measurement accuracy and torque/stiffness control are reported.
A biomimicking design for mechanical knee joints
Bioinspiration & Biomimetics
In this paper we present a new bioinspired bicondylar knee joint that requires a smaller actuator size when compared to a constant moment arm joint. Unlike existing prosthetic joints, the proposed mechanism replicates the elastic, rolling and sliding elements of the human knee. As a result, the moment arm that the actuators can impart on the joint changes as function of the angle, producing the equivalent of a variable transmission. By employing a similar moment arm-angle profile as the human knee the peak actuator force for stair ascent can be reduced by 12% compared to a constant moment arm joint addressing critical impediments in weight and power for robotics limbs. Additionally, the knee employs mechanical 'ligaments' containing stretch sensors to replicate the neurosensory and compliant elements of the joint. We demonstrate experimentally how the ligament stretch can be used to estimate joint angle, therefore overcoming the difficulty of sensing position in a bicondylar joint. PAPER Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.
Frontiers in Neurorobotics, 2018
The CYBERLEGs Beta-Prosthesis is an active transfemoral prosthesis that can provide the full torque required for reproducing average level ground walking at both the knee and ankle in the sagittal plane. The prosthesis attempts to produce a natural level ground walking gait that approximates the joint torques and kinematics of a non-amputee while maintaining passively compliant joints, the stiffnesses of which were derived from biological quasi-stiffness measurements. The ankle of the prosthesis consists of a series elastic actuator with a parallel spring and the knee is composed of three different systems that must compliment each other to generate the correct joint behavior: a series elastic actuator, a lockable parallel spring and an energy transfer mechanism. Bench testing of this new prosthesis was completed and demonstrated that the device was able to create the expected torque-angle characteristics for a normal walker under ideal conditions. The experimental trials with four amputees walking on a treadmill to validate the behavior of the prosthesis proved that although the prosthesis could be controlled in a way that allowed all subjects to walk, the accurate timing and kinematic requirements of the output of the device limited the efficacy of using springs with quasi-static stiffnesses. Modification of the control and stiffness of the series springs could provide better performance in future work.
A novel biomimetic actuator system
Robotics and Autonomous Systems, 1998
The design of a biomimetic actuation system which independently modulates position and net stiffness is presented. The system is obtained by arttagonisticaUy pairing contractile devices capable of modulating their rate of geometric deformation relative to the rate of deformation of a passive elastic storage element in series with the device's input source. A mechanical model is developed and the properties of the device are investigated. The theoretical results developed are then compared with experimental evidence obtained from a simple prototype model of the system. upon similar experimental results, Alexander [2] asserts that humans store energy in their Achilles tendons and the ligaments that support the arch of the foot, and that compliant legs and feet reduce the peak forces that occur when the foot strikes the ground. In a similar vein, Cavagna et al. suggest that running is essentially bouncing.
Conceptual design of a novel variable stiffness actuator for use in lower limb exoskeletons
2015 IEEE International Conference on Rehabilitation Robotics (ICORR), 2015
A novel modular variable stiffness actuator (VSA), for use in the knee joint of lower limb exoskeletons, is presented. The actuator consists of a combination of a spindledriven MACCEPA (Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator) and a spring acting in parallel, (dis)engaged by means of a simple on/off mechanism depending on the phase of the gait cycle. Such design approach is inspired by two clearly distinctive gait phases of a knee joint, one with a high velocity and low torque, and another one with low velocity and high torque profiles. By tackling each of these two phases separately, energy consumption and torque requirements of an active part of the actuator have been decreased, while keeping the size and the weight of the actuator at a reasonable size for use in wearable robots (WR).
Mechanism and Machine Theory, 2018
Compliant actuators are increasingly being designed for wearable robots (WRs) to more adequately address their issues with safety, wearability, and overall system efficiency. The advantages of mechanical compliance are utilized in a new actuator designed to exploit inherent gait dynamics. Unlike any other orthotic power unit, it combines Variable Stiffness Actuator (VSA) and Parallel Elasticity Actuation (PEA) unit into a single modular system. This way, the actuator has the potential to provide the benefits of VSA when net-positive work is necessary and efficiently store energy during energetically conservative tasks. A novel real-time torque controller allows the two units to work together throughout the gait cycle. The design aspects and experimental evaluation of the actuator and its lowlevel torque controller are presented in this paper. The actuator characterization, carried out in two benchmarking environments, highlights the actuator's high torque density and favorable energetic performance, providing evidence for its applicability in a standalone or multiple-joint lower limb orthoses.