High Backdrivability Research Articles

Mechatronic Design and Implementation of a Low-Noise Powered Knee Prosthesis With High Backdrivability

Working noise is an important consideration that hinders the acceptance of powered prostheses by amputees. In this article, we present a powered knee prosthesis with low noise and high backdrivability. The powertrain of the proposed prosthesis consists of a motor and a four-stage synchronous belt to decrease working noise. At the same time, benefiting from the low impedance of the motor and low impedance design, the proposed prosthesis is endowed with the characteristics of low impedance, and thus, high backdrivability. A three-layer control method based on a finite state machine is implemented, and walking experiments have been carried out on a transfemoral amputee. The results show the characteristics of the proposed prosthesis in low impedance, low noise, and the ability to reproduce a natural gait. In particular, the working noise of the proposed prosthesis is less than 46 dBA most time, which is lower than common household appliances, and may have positive effects on the acceptance of daily uses.

A Large Force Haptic Interface with Modular Linear Actuators

This paper presents a haptic interface with modular linear actuators that addresses the limitations of conventional devices based on rotary joints. The proposed haptic interface is composed of parallel linear actuators that provide high backdrivability and small inertia. The performance of the haptic interface is compared to those of conventional mechanisms in terms of force capability, reflected inertia, and structural stiffness. High stiffness, large range of motion, and high force capability, which are in trade-off relationships in traditional haptic interfaces, are achieved. The device can apply up to 83 N continuously, i.e., three-fold more than most haptic devices. The theoretical minimum haptic force density and stiffness of the proposed mechanism are 1.3 to 1.9 and 37 times those of the conventional mechanisms under similar conditions, respectively. The system is scalable because the structural stiffness depends on only the timing belt stiffness, whereas that of conventional haptic interfaces is inversely proportional to the cube of the structural length. The modular actuator enables changes in the degrees of freedom (DOFs) for different applications. The proposed haptic interface was tested through an interaction experiment in a virtual environment with virtual walls.

Design and Development of a Spherical 5-Bar Thumb Exoskeleton Mechanism for Poststroke Rehabilitation.

This paper presents the kinematic design and development of a two degree-of-freedom (2DOF) spherical 5-bar thumb exoskeleton to augment the finger individuating grasp exercise robot (FINGER) rehabilitation robot, which assists the index and middle fingers individually in naturalistic grasping. The thumb module expands the capabilities of FINGER, allowing for broader proprioceptive training and assessment of hand function. The design process started by digitizing thumb-grasping motions to the index and the middle fingers separately, recorded from multiple healthy subjects utilizing a motion capture system. Fitting spheres to trajectory data of each subject allowed normalization of all subjects' data to a common center and radius. A two-revolute joint serial-chain mechanism was synthesized (intermediate optimization step) to reach the normalized trajectories. Next, the two resulting grasping trajectories were spatially sampled as targets for the 2DOF spherical 5-bar synthesis. Optimization of the spherical 5-bar included symmetry constraints and cost-function penalties for poor manipulability. The resulting exoskeleton assists both flexion/extension and abduction/adduction of the thumb enabling a wide range of motions. Consistent with FINGER, the parallel structure of the spherical 5-bar places the actuators at the base of the module, allowing for desirable characteristics, including high backdrivability, high controllable bandwidth, and low mechanical impedance. The mechanical design was developed from the kinematic solution, including an adjustable thumb cuff to accommodate different hand sizes. Fit and function of the device were tested on multiple subjects, including survivors of stroke. A proportional-derivative force controller with gravity and friction compensation was implemented to reduce resistance to motion during subject testing.

Energy-Efficient Actuator Design Principles for Robotic Leg Prostheses and Exoskeletons: A Review of Series Elasticity and Backdrivability

AbstractRobotic 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 during daily locomotor activities besides continuous level-ground walking.

Modular Magnetorheological Actuator With High Torque Density and Transparency for the Collaborative Robot Industry

In the last decade, the development of simple and easy-to-use robotics has provided a significant step towards the democratization of collaborative robots, which can now work closely alongside humans. However, state-of-the-art cobots still provide safety at the cost of work-speed, which is purposely limited to avoid high-energy collisions. In order to increase safety and work speed simultaneously, force/impedance-controlled actuators with high dynamic performance, backdrivability and low-reflected inertia are of interest. In this letter, a novel robotic actuator architecture with two independent magnetorheological (MR) power chains is proposed, built and experimentally characterized. The modular 30 Nm robotic joint is shown to have similar size (85 X 85 X 124 mm), weight (1.1 kg) and torque density (27 Nm/kg) as the Universal Robot (UR) size 1 robotic joint, which relies on a Harmonic Drive (HD) gearbox, but with 150 times less inertia, high backdrivability (0.7% of max torque), high torque bandwidth (>70 Hz) and inherent torque-limiting that makes it immune to impacts. The dual MR power chain allows ultra-fast transparent interactions with bluezero backlash over half its torque range and peak torque when motor reversal is allowed. To illustrate the force/impedance control performance, a 3-DOF robotic arm is built and used to simulate virtual objects in both open and closed loop.

Toward Improving Actuation Transparency and Safety of a Hip Exoskeleton With a Novel Nonlinear Series Elastic Actuator

Actuation transparency and safety are important requirements in the design and control of assistive exoskeletons for individuals who suffer lower limb deficits but still maintain a certain level of voluntary motor control. In recent years, series elastic actuator (SEA) has been regarded as a promising solution for transparent actuation and safe human–robot interaction, thus SEAs are widely developed and applied in assistive exoskeletons. However, existing SEAs designed for assistive exoskeletons still lack both actuation transparency and safety because of high stiffness of the elastic element and the high mechanical impedance of the actuators. To address this problem, a novel nonlinear SEA (nSEA) is presented in this article. The optimized nonlinear series elastic element coupled with a quasi-direct drive motor creates the nSEA with low mechanical impedance, high backdrivability, and less acoustic noise. Besides, a new torque control, based on cascade PI control, is proposed for the nSEA to control the interaction torque with high accuracy and robustness. Finally, an experimental evaluation with human subjects is performed to validate the advantages of the nSEA-driven hip exoskeleton in the realization of actuation transparency and safety. The root-mean-square interaction torque in zero-impedance mode is as low as 0.051 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m during walking conditions, leading to a negligible negative influence on the hip joint's range of motion, walking speed, and energy expenditure when wearing the hip exoskeleton.

A Powered Hip Exoskeleton With High Torque Density for Walking, Running, and Stair Ascent

Powered exoskeletons need actuators that can generate substantial assistive torque and orthoses that can efficiently transfer the assistive torque to the user. Powered exoskeletons also need to be lightweight and ergonomic to minimize the negative effects wearing the exoskeleton has on the user's effort and comfort. Here we present the design, development, and validation of an autonomous powered hip exoskeleton with high torque density. The exoskeleton actuator is based on a four-bar mechanism with integrated composite springs. A compact carbon fiber frame encloses the custom actuator, doubling as the exoskeleton thigh linkage. A self-aligning mechanism is used to avoid uncomfortable spurious forces and torques on the user's limb. Custom pelvis and thigh braces are developed using composite materials to reduce weight. A custom embedded electronic system is integrated into the pelvis brace to minimize the device weight and electrical consumption. Experiments show that the proposed powered hip exoskeleton can produce high nominal torque (41.9 Nm repetitive peak torque), high backdrivability (0.16 Nm back-driving torque), high bandwidth (23.8 Hz), and high control accuracy (2.1% steady-state error). Human tests show that the proposed exoskeleton can assist in walking, running, and stairs climbing.

Compact, backdrivable, and efficient design of linear electro-hydrostatic actuator module

An electro-hydrostatic actuator (EHA) has high backdrivability and is suitable for robots that interact with the environment, including human. However, the challenging problem in its mechanical design is that it is difficult to achieve compact size and high power transmission efficiency, compared with standard gear reducers. To tackle this problem, this study presents the design method by macro- and micro-scale designs. A linear-type EHA consists of a cylinder and a hydraulic pump. The former includes cylinder parameters that are dominant in the total size, and the latter includes gap width in the gear pump that is important for reducing power loss. We propose the hierarchical design to determine these parameters. In the macro-scale design, we determine the cylinder radius and differential pressure to maximize the efficiency-to-volume ratio. Based on the result of the macro-scale design, the micro-scale design determines the width of internal gaps to minimize the power loss in the gear pump. We derive mathematical formulations for the designs and develop the EHA by utilizing modular design and 3D printing. Moreover, we evaluate the fundamental properties of the developed EHA module, focusing on its power transmission efficiency and backdrivability.

Design and testing a highly backdrivable and kinematic compatible magneto-rheological knee exoskeleton

Lower limb exoskeletons (LLE) have been successfully used in robotic-assisted rehabilitation to reduce the burden of locomotor impairment of disabled people. However, the design limitations of LLE mechanisms, such as the lack of kinematic compatibility relative to the user’s joints and the use of high stiff or heavy actuators, limit the outcomes of treatment and increase the risk of injury. To address these shortcomings, in this work we present the design of the MRKneeExo, a highly backdrivable and kinematically compatible active knee exoskeleton. The powertrain of the system is composed of a BLDC 70 W motor associated with a harmonic drive gearbox and a customized magneto-rheological (MR) clutch. To improve kinematic compatibility with the user’s knee, a crossed four-bar linkage mechanism (FBLM) was optimally designed to follow the trajectory of the instant center of rotation (ICR) of the knee projected in the parasagittal plane where the joint is placed. The MR clutch is used to decouple the motor-reducer from the FBLM, thus enabling high backdrivability. The results showed a small error (&lt;3 mm) between the FBLM and the knee ICR. Furthermore, the MR clutch allowed for low back-drive torque (1.28 N m) compared to the torque to back-drive the motor-reducer (18.51 N m). This paper is presented in a framework that can be generalized to support the design of other knee exoskeletons.

Hybrid 3K Compound Planetary Reduction Gearbox With a Roller Transmission Mechanism

Reduction gears are used in the joints of the robots. While precision and efficiency are required for industrial robot gearboxes, humanoid robots are expected to become more popular in the future. Flexibility must be considered in humanoid robots, and high back drivability shall be required. In this study, we aimed at realizing high efficiency, and high-precision reduction gears with high reduction ratios by maximizing the efficiency equation, hybridizing a 3K compound planetary reduction gearbox and traction drive, experimentally determining the efficiency, backlash, and angular velocity variation, and demonstrating the effectiveness of the proposed mechanism from a control perspective. In the experiments, we achieved a maximum forward driving efficiency of 90.90%, backward driving efficiency of 90.58%, and a minimum backlash of 1.18 arcmin.

Development of Bipedal Robot for Immediate Adaption Mechanism by Reflex

Reflexes contribute to many motor functions of humans. In order to understand the reflex mechanisms in posture control of humans, we develop a bipedal robot platform that can reproduce muscle properties and reflexes. This robot with BLDC motors has high back-drivability and torque output ability to reproduce these two characteristics. In this study, to quantitatively investigate the contribution of various types of posture reflexes with different sensory information (muscle length, force, and no input), we compared the settling time and steady-state deviations of the posture response under disturbances.

Design and Validation of a Powered Knee-Ankle Prosthesis with High-Torque, Low-Impedance Actuators.

We present the design of a powered knee-ankle prosthetic leg, which implements high-torque actuators with low-reduction transmissions. The transmission coupled with a high-torque and low-speed motor creates an actuator with low mechanical impedance and high backdrivability. This style of actuation presents several possible benefits over modern actuation styles in emerging robotic prosthetic legs, which include free-swinging knee motion, compliance with the ground, negligible unmodeled actuator dynamics, less acoustic noise, and power regeneration. Benchtop tests establish that both joints can be backdriven by small torques (~1-3 Nm) and confirm the small reflected inertia. Impedance control tests prove that the intrinsic impedance and unmodeled dynamics of the actuator are sufficiently small to control joint impedance without torque feedback or lengthy tuning trials. Walking experiments validate performance under the designed loading conditions with minimal tuning. Lastly, the regenerative abilities, low friction, and small reflected inertia of the presented actuators reduced power consumption and acoustic noise compared to state-of-art powered legs.

Functional Evaluation of a Force Sensor-Controlled Upper-Limb Power-Assisted Exoskeleton with High Backdrivability.

A power-assisted exoskeleton should be capable of reducing the burden on the wearer’s body or rendering his or her work improved and efficient. More specifically, the exoskeleton should be easy to wear, be simple to use, and provide power assistance without hindering the wearer’s movement. Therefore, it is necessary to evaluate the backdrivability, range of motion, and power-assist capability of such an exoskeleton. This evaluation identifies the pros and cons of the exoskeleton, and it serves as the basis for its subsequent development. In this study, a lightweight upper-limb power-assisted exoskeleton with high backdrivability was developed. Moreover, a motion capture system was adopted to measure and analyze the workspace of the wearer’s upper limb after the exoskeleton was worn. The results were used to evaluate the exoskeleton’s ability to support the wearer’s movement. Furthermore, a small and compact three-axis force sensor was used for power assistance, and the effect of the power assistance was evaluated by means of measuring the wearer’s surface electromyography, force, and joint angle signals. Overall, the study showed that the exoskeleton could achieve power assistance and did not affect the wearer’s movements.

Comparative study of force control methods for bipedal walking using a force-sensitive hydraulic humanoid

ABSTRACT This paper describes an experimental comparison of force control methods, including resolved acceleration control (RAC), joint position control (JPC), and resolved viscoelasticity control (RVC), focusing on the biped walking performance. In previous studies, we reported a stable biped walking using the JPC and RVC methods on a hydrostatically driven humanoid, Hydra. All joints of Hydra are actuated by electro-hydrostatic actuators, showing the high backdrivability and force-controllability. However, we did not realize the RAC on Hydra, which is a standard method in state-of-the-art humanoid robotics. It is the first time that we have reported the RAC method realized a biped walking on Hydra. Moreover, the comparison between three methods is presented.

Proposal of Motion Judgment Algorithm Based on Joint Angle of Variable Elastic Assist Suit with High Back Drivability

In recent years, the burden per worker has increased due to a decrease in the working population. Wearable assist suits have been developed as one of the methods for solving the problem. To extend the assist suit to practical situations, it is necessary to provide a motion judgment interface for judging the motion of a wearer. Therefore, in our study, a motion judgment algorithm is proposed for assist suits, based on variable viscoelasticity. The proposed algorithm judges sitting, standing-up, stance, sitting-down, and gait using only the joint angle information of the suit and verification is performed using human joint angles obtained by motion capture. Thus, the motion judgment rate is 90% or more for sitting, standing-up, stance, and sitting-down, and 80% or more for gait, confirming the usefulness of motion judgment. Additionally, based on these results, further verification is performed on an actual machine. As a result, in a series of motions starting from the sitting to the standing-up, the stance, and the gait, the motion judgement is successful five times from the sitting to the standing-up, the stance, and once in gait. In a series of motions from sitting to standing-up, the stance, and sitting-down, the motion judgment is successful five times during sitting; five times during sitting, stance, and sitting-down; and three times during standing-up. In this way, it is confirmed that the proposed method can judge the motion only by angle information, although there is a problem in a success rate depending on the motion.

Experimental Study on Critical Design of Electro-Hydrostatic Actuators Small in Size and Light in Weight

Actuation systems for robots and other machines used in critical applications is an area that requires further research. In such applications, a machine works in a human environment and physically interacts with humans. Reliability and backdrivability are still insufficient in current systems. An electro-hydrostatic actuator has the potential advantage of high reliability by nature and high backdrivability in mechanical simplexity when it is designed to be small and light. This study provides a theoretical investigation of the methods for evaluating internal leaks and other mechanical losses, such as Coulomb and viscous friction, and experimentally evaluates two types (trochoid and involute gear) of prototyped hydraulic pumps.

Conceptual Design for Ankle Rehabilitation Robot by Using Morphological Chart and Pugh Method

Lateral ankle sprains are among the most common injuries in competitive sports and recreational activities according to studies. Ankle sprains accounted for 70% or more of all reported ankle injuries in high risks sports. However, ankle sprains often only partially treated and there are more than 40% of ankle sprains are recurrent. Even though the ankle rehabilitation treatment exists, but there are some limitations that relates to the current situations. Firstly, the cost for ankle rehabilitation treatments are high and may be a burden to the patients while the traditional rehabilitation treatment performance highly relies on a physiotherapist’s experience and the manual training method makes physiotherapist exhausted. Therefore, ankle rehabilitation robot can help to solve these problems. Morphological Chart are used to generate the concept design while Pugh Method are used to choose the best design. Concept design A (4 DOF Pulley Driven Cable Based Parallel Mechanism Ankle Rehabilitation Robot) are chosen as the best design among 5 concept design due to its advantages in aspect of low cost, easy to use, high portability, high back drivability and fulfilment of ankle rehabilitation exercises.

Pneumatic AFO Powered by a Miniature Custom Compressor for Drop Foot Correction

For active AFO applications, pneumatic remote transmission has advantages in minimizing the mass and complexity of the system due to the flexibility in placing pneumatic components and providing high back-drivability via simple valve control. However, pneumatic systems are generally tethered to large stationary air compressors, which greatly limit the practical daily usage. In this study, we implemented a wearable custom compressor that can be worn at the trunk of the body and can generate up to 1050 kPa of pressurized air to power an unilateral active AFO for dorsiflexion (DF) assistance of drop-foot patients. In order to minimize the size and weight of the custom compressor, the compression rate of the custom compressor was optimized to the rate of consumption required to power the active AFO. The finalized system can provide a maximum assistive torque of 9.8 Nm at a functional frequency of 1 Hz and the average resistive torque during free movement was 0.03 Nm. The system was tested for five hemiplegic drop-foot patients. The proposed system showed an average improvement of 12.3° of ankle peak dorsiflexion angle during the mid to late swing phase.

Motion Control and External Force Estimation of a Pneumatically Driven Multi-DOF Robotic Forceps

Robotic forceps with a rigid-link joint mechanism is orthodox for current robotic-assisted surgery systems. However, external force estimation without force sensors during operations is difficult for such electrically driven forceps. This work introduces a pneumatically driven multi-DOF (DOF: degree of freedom) forceps using a rigid-link mechanism with less interference of the wire drive between joints and realizes external force estimation by utilizing high back-drivability of pneumatic cylinders. We developed a position controller with dynamic compensation of the mechanical friction, in which the rotational angles of the three movable joints of the forceps are independently controlled. Moreover, we designed an external force observer in the position controller by applying the disturbance observer scheme. The results of the performance evaluation experiments are as follows. First, in the joint position control experiments, smooth and stable controllability is confirmed for sinusoidal reference inputs with the mean absolute errors of less than 2°. The resolution of the joint position control is approximately 1° for the response of step increasing reference inputs, which is acceptable for laparoscopic surgery. Second, the external force observer can correctly estimate the translational and the grasping forces with less than 20% errors of the maximum output forces. The practical sensitivities of the external force estimation are better than 0.5 N for translational forces and 0.2 N for grasping forces. The achieved performance of the developed forceps can be applicable for interactive force control in some particular surgical tasks such as suturing, ligation, organ traction and exclusion.

Quasi-Direct Drive Actuation for a Lightweight Hip Exoskeleton with High Backdrivability and High Bandwidth.

High-performance actuators are crucial to enable mechanical versatility of wearable robots, which are required to be lightweight, highly backdrivable, and with high bandwidth. State-of-the-art actuators, e.g., series elastic actuators (SEAs), have to compromise bandwidth to improve compliance (i.e., backdrivability). We describe the design and human-robot interaction modeling of a portable hip exoskeleton based on our custom quasi-direct drive (QDD) actuation (i.e., a high torque density motor with low ratio gear). We also present a model-based performance benchmark comparison of representative actuators in terms of torque capability, control bandwidth, backdrivability, and force tracking accuracy. This paper aims to corroborate the underlying philosophy of "design for control", namely meticulous robot design can simplify control algorithms while ensuring high performance. Following this idea, we create a lightweight bilateral hip exoskeleton to reduce joint loadings during normal activities, including walking and squatting. Experiments indicate that the exoskeleton is able to produce high nominal torque (17.5 Nm), high backdrivability (0.4 Nm backdrive torque), high bandwidth (62.4 Hz), and high control accuracy (1.09 Nm root mean square tracking error, 5.4% of the desired peak torque). Its controller is versatile to assist walking at different speeds and squatting. This work demonstrates performance improvement compared with state-of-the-art exoskeletons.