Active Disturbance Rejection Based Robust Trajectory Tracking Controller Design in State Space

This paper proposes a new Active Disturbance Rejection based robust trajectory tracking controller design method in state space. It can compensate not only matched but also mismatched disturbances. Robust state and control input references are generated in terms of a fictitious design variable, namely differentially flat output, and the estimations of disturbances by using Differential Flatness and Disturbance Observer. Two different robust controller design techniques are proposed by using Brunovsky canonical form and polynomial matrix form approaches. The robust position control problem of a two mass-spring-damper system is studied to verify the proposed robust controllers.Comment: Accepted by ASME Journal of Journal of Dynamic Systems, Measurement, and Control in 201

A 3D printed monolithic soft gripper with adjustable stiffness

Soft robotics has recently gained a significant momentum as a newly emerging field in robotics that focuses on biomimicry, compliancy and conformability with safety in near-human environments. Beside conventional fabrication methods, additive manufacturing is a primary technique to employ to fabricate soft robotic devices. We developed a monolithic soft gripper, with variable stiffness fingers, that was fabricated as a one-piece device. Negative pressure was used for the actuation of the gripper while positive pressure was used to vary the stiffness of the fingers of the gripper. Finger bending and gripping capabilities of the monolithic soft gripper were experimentally tested. Finite element simulation and experimental results demonstrate that the proposed monolithic soft gripper is fully compliant, low cost and requires an actuation pressure below -100 kPa

A Sliding Mode Force and Position Controller Synthesis for Series Elastic Actuators

This paper deals with the robust force and position control problems of Series Elastic Actuators. It is shown that a Series Elastic Actuator's force control problem can be described by a second-order dynamic model which suffers from only matched disturbances. However, the position control dynamics of a Series Elastic Actuator is of fourth-order and includes matched and mismatched disturbances. In other words, a Series Elastic Actuator's position control is more complicated than its force control, particularly when disturbances are considered. A novel robust motion controller is proposed for Series Elastic Actuators by using Disturbance Observer and Sliding Mode Control. When the proposed robust motion controller is implemented, a Series Elastic Actuator can precisely track desired trajectories and safely contact with an unknown and dynamic environment. The proposed motion controller does not require precise dynamic models of the actuator and environment. Therefore, it can be applied to many different advanced robotic systems such as compliant humanoids and exoskeletons. The validity of the motion controller is experimentally verified.Comment: Accepted by Robotica in 201

Ultra-stretchable MWCNT-Ecoflex piezoresistive sensors for human motion detection applications

Ultra-stretchable sensors are highly desirable for wearable electronic applications. In this paper, we manufactured self-standing piezoresistive sensors using a simple method based on blending multiwall carbon nanotubes (MWCNTs) with a stretchable elastomeric matrix (Ecoflex). The sensors showed a low electrical percolation threshold of 0.3 wt%, with an elastic modulus as soft as the human skin in the forearm and palm dermis. An increase in the cross-linking degree of the matrix from 17.43 ± 0.20 mol/m 3 up to 28.55 ± 2.07 mol/m 3 was observed with the incorporation of MWCNTs, revealing that the conductive filler is covalently bonded to the elastomeric matrix. The piezoresistive sensors showed high stretchability with an outstanding linearity between the resistance change with the applied strain, up to 200%, with significant sensitivity which is essential to use these sensors in human motion applications, e.g. finger bending, walking or speaking, and even detecting a hot liquid poured into a cup. Finally, MWCNT-Ecoflex sensors showed remarkable mechanical and electromechanical response features which are essential for wearable applications to monitor human motion with minimal discomfort

3D printed flexure hinges for soft monolithic prosthetic fingers

Mechanical compliance is one of the primary properties of structures in nature playing a key role in their efficiency. This study investigates a number of commonly used flexure hinges to determine a flexure hinge morphology, which generates large displacements under a lowest possible force input. The aim of this is to design a soft and monolithic robotic finger. Fused deposition modeling, a low-cost 3D printing technique, was used to fabricate the flexure hinges and the soft monolithic robotic fingers. Experimental and finite element analyses suggest that a nonsymmetric elliptical flexure hinge is the most suitable type for use in the soft monolithic robotic finger. Having estimated the effective elastic modulus, flexion of the soft monolithic robotic fingers was simulated and this showed a good correlation with the actual experimental results. The soft monolithic robotic fingers can be employed to handle objects with unknown shapes and are also potential low-cost candidates for establishing soft and one-piece prosthetic hands with light weight. A three-finger gripper has been constructed using the identified flexure hinge to handle objects with irregular shapes such as agricultural products

Kinematic analysis of electroactive polymer actuators as soft and smart structures with more DoF than inputs

Electroactive polymer (EAP) actuators have been attracting the attention of researchers due to their muscle-like behaviour and unusual properties. Several modelling methods have been proposed to understand their mechanical, chemical, electrical behaviours or ‘electro-chemo-mechanical’ behaviour. However, estimating the whole shape or configuration of the EAP actuators has always been challenging due to their highly non-linear bending behaviour. This paper reports on an effective method to estimate the whole shape deflection of the EAP actuators by employing a so-called backbone approach. Tri-layer configured polypyrrole (PPy) based EAP actuators were used as a soft and smart structure with more degrees of freedom than its input. After deriving the inverse kinematic model of the actuator, its complete shape is estimated by solving the inverse kinematic model with an angle optimization (AngleOPT) method. The experimental results and numerical results have demonstrated the effectiveness of the method in estimating the highly non-linear bending behaviour of the PPy actuators and applicability of this modelling approach to other EAP actuators

Electroactive polymers as soft robotic actuators: Electromechanical modeling and identification

Biologically inspired robotic applications have recently received significant attention due to developments in novel materials and actuators with an operation principle similar to the natural muscles’. Electroactive polymer (EAP) actuators, also known as artificial muscles, possess extraordinary properties such as low efficiency consumption, compliance, bio-compatibility and ability to be miniaturized. Although several methodologies have been proposed for modeling and identification of their quasi-static bending behavior, a negligibly small attention has been given to their dynamic behavior. In this paper, we, therefore, report on their electromechanical modeling and parameter identification. We model the tri-layer EAP actuators as a soft robotic actuator consisting of a significant number of rigid links connected with compliant revolute joints. The experimental and numerical results presented suggest that the soft robotics approach is an effective way to model the EAP actuator and subsequently identify its dynamic parameters accurately. We have previously employed the same soft robotic approach to estimate the whole shape of the EAP actuator as a function of time

Reusable Flexible Concentric Electrodes Coated With a Conductive Graphene Ink for Electrotactile Stimulation

Electrotactile stimulation is a highly promising technique for providing sensory feedback information for prosthetics. To this aim, disposable electrodes which are predominantly used result in a high environmental and financial cost when used over a long period of time. In addition, disposable electrodes are limited in their size and configurations. This paper presents an alternative approach based on a 3D printed reusable flexible concentric electrode coated with a conductive graphene ink. Here, we have characterized the electrode and demonstrated its effective performance in electrotactile stimulation and sensory feedback for robotic prosthetic hands

A Disturbance Observer-Based Robust Controller Design for Systems with Right Half Plane Zeros and Poles

This paper analytically derives the bandwidth limitations of Disturbance Observer (DOB) when plants have Right Half Plane (RHP) zero(s) and pole(s). If the plant is non-minimum phase, then the bandwidth of DOB should be set at a lower value than its upper bound to improve the robust stability and performance. If the plant is unstable, then the bandwidth of DOB should be set at a higher value than its lower bound to achieve the robust stability. The upper and lower bounds are analytically derived by using Poisson integral formula. It is shown that the bandwidth limitation of DOB is directly related to the locations of the RHP zero(s) and pole(s) and becomes more severe as they get close each other. A minimum phase approximation of the non-minimum phase nominal plant model is proposed by using Genetic Algorithm (GA) to tackle the internal stability problem of the DOB-based robust control systems. Simulation results are given to verify the proposed robust controllers

A stiffness enhancement methodology for artificial muscles

Electroactive conducting polymer actuators have been proposed as alternative to conventional actuators due to their extraordinary properties. This paper reports on a stiffness enhancement methodology for cantilever type conducting polymer actuators based on a suitably designed contact surface with which the actuators are in contact during operation. Finite element analysis and modeling are used to quantify the effect of the contact surface on the effective stiffness of a tri-layer cantilevered beam, which represents one-end free, the other end fixed polypyrrole (PPy) conducting polymer actuator under a uniformly distributed load. After demonstrating the feasibility of the stiffness enhancement concept, experiments were conducted to determine the stiffness of bending-type conducting polymer actuators in contact with a range (20-40 mm in radius) of circular contact surfaces. The simulation and experimental results demonstrate that the stiffness of the actuators can be varied in a nonlinear fashion using a suitably profiled contact surface. The larger is the radius of the contact surface, the higher is the stiffness of the polymer actuators