Design and Assist-as-Needed Control of Flexible Elbow Exoskeleton Actuated by Nonlinear Series Elastic Cable Driven Mechanism

Exoskeletons can assist the daily life activities of the elderly with weakened muscle strength, but traditional rigid exoskeletons bring parasitic torque to the human joints and easily disturbs the natural movement of the wearer’s upper limbs. Flexible exoskeletons have more natural human-machine interaction, lower weight and cost, and have great application potential. Applying assist force according to the patient’s needs can give full play to the wearer’s remaining muscle strength, which is more conducive to muscle strength training and motor function recovery. In this paper, a design scheme of an elbow exoskeleton driven by flexible antagonistic cable actuators is proposed. The cable actuator is driven by a nonlinear series elastic mechanism, in which the elastic elements simulate the passive elastic properties of human skeletal muscle. Based on an improved elbow musculoskeletal model, the assist torque of exoskeleton is predicted. An assist-as-needed (AAN) control algorithm is proposed for the exoskeleton and experiments are carried out. The experimental results on the experimental platform show that the root mean square error between the predicted assist torque and the actual assist torque is 0.00226 Nm. The wearing experimental results also show that the AAN control method designed in this paper can reduce the activation of biceps brachii effectively when the exoskeleton assist level increases.

Design of a Quadratic, Antagonistic, Cable-Driven, Variable Stiffness Actuator

Antagonistically actuated Variable Stiffness Actuators (VSAs) take inspiration from biological muscle structures to control both the stiffness and positioning of a joint. This paper presents the design of an elastic mechanism that utilizes a cable running through a set of three pulleys to displace a linear spring, yielding quadratic spring behavior in each actuator. A joint antagonistically actuated by two such mechanisms yields a linear relationship between force and deflection from a selectable equilibrium position. A quasi-static model is used to optimize the mechanism. Testing of the fabricated prototype yielded a good match to the desired elastic behavior.

Vibration analysis of nonlinear and linear elastic systems

Objective. In this study, the task is to establish the theoretical prerequisites for the operability of a regressive-progressive elastic mechanism by comparing the amplitude-frequency characteristics and phase trajectories with a linear elastic system of comparable stiffness in a static equilibrium position.Methods. The article presents a comparative dynamic analysis of vibrations of elastic systems with linear rigidity and regressive-progressive characteristics obtained as a result of the use of elastic elements in the form of high flexibility rods with longitudinal eccentric compression. Such elastic elements in various design variants have been tested and patented as damping elements for use in the construction of vibration dampers for construction structures and vehicle suspensions, and have experimentally shown their effectiveness in damping vibrations.Results. The regressiveprogressive elastic characteristic obtained by the elliptic parameters method and using the ANSIS calculation complex is used in the dynamics equations in an approximated form, which expands the capabilities of the method. It is shown that increasing the energy intensity of a curvilinear system reduces the vibration amplitude.Conclusion. The regressive-progressive change of the stiffness of curvilinear elastic systems can be achieved using an elastic element with eccentric longitudinal compression; the regression plot of elastic properties is achieved due to eccentric compression; the progressive plot – through the use of a guide or other design solutions. The implementation of this characteristic allows using such elastic mechanisms in systems where the accumulation of potential energy occurs with a smaller compression stroke for the same perturbation than for linear systems.

An Optimized 3D Probe Using Sensitivity and Compliance Analysis

High-precision micro/nano probe plays an increasingly important role in manufacturing and measurement. The multi-arm elastic mechanism that can produce deformation under the contact force is widely used in the design of probe. The striking feature of this mechanism is that multiple variables are coupled to each other and are not easily separated. However, the transfer matrix of probe, rather than a multivariable decoupling model, is widely used as a measurement model in traditional research. Transfer matrix appears as a “black box” and does not reveal working principles of probe. Our previous research proposed a 3D decoupling model. The 3D model presents the coupling relationship between input and output variables, and also finishes a theoretical explanation of complex features of 3D probe. Recent studies have found that this decoupling model has practical value in parameter and shape optimization of probe. As the optimized purpose, two indicators — sensitivity and compliance (reciprocal of stiffness) are proposed from the model. The increased sensitivity means the probe has a lower resolution requirement for the capacitive sensor used. High compliance of probe means small contact force between the stylus ball and workpiece. Excessive stiffness can cause excessive contact forces that damages surface of workpiece. Combined with theoretical model and finite element analysis (FEA), the key parameters affecting sensitivity and compliance of probe are extracted, and a new optimized elastic mechanism based on an original Hexflex mechanism. The new optimized probe has better performance with sensitivity, input compliance, output compliance increased by 78.6%, 48.4%, 157.7%, respectively.