Impedance-based Control Research Articles

Impedance Adaptive Controller for a Prototype of a Whiplash Syndrome Rehabilitation Device

The objective of this study was to design of an output based impedance adaptive controller for a special class of cervical orthoses, a class of biomedical devices for the rehabilitation of neck illnesses. The controller used the adaptive sliding mode theory to enforce the tracking of the reference trajectory if the patient was not resistant to the therapy. If the patient rejects the orthosis activity, a second impedance-based controller governs the orthosis movement allowing the patient to take the leading role in the orthosis sequence of movements. The proposed controller considers a weighted controller combining the tracking and the impedance controls in a single structure. The monitoring of the external force was evaluated on a novel weighting function defining on-line the role of each controller. The proposed orthosis was motivated by the prevalence of whiplash, which is a syndrome that is produced by forced hyperextension and hyperflexion of the neck. This study included the development of a technological prototype of the orthotic type to support the recovery of patients diagnosed with whiplash. The sections that make up the orthotic device are two independent systems that move the patient’s head in the sagittal and frontal planes. For this purpose, the mechanical structure of the cervical orthosis was made up of 7 pieces printed in 3D with polylactic acid (PLA). The operation of the cervical orthosis was evaluated in two sections: (a) using a simulation system, which consists of a spring with an artificial head and the development of a graphic interface in Matlab, and (b) evaluating the controller on the proposed orthosis. With these elements, the follow-up of the trajectory proposed by the actuators was evaluated, as well as its performance in the face of the opposition that a patient generates. The superiority of the proposed controller was confirmed by comparing the tracking efficiency with proportional-integral-derivative and first-order sliding variants.

Automatic virtual impedance adaptation of a knee exoskeleton for personalized walking assistance

This paper attempts to address the problem of online modulation of virtual impedance for an assistive robot based on real-time gait and activity measurements to personalize the assistance for different users at different states. In this work, smart shoes and inertial sensors are introduced to measure ground contact forces and knee joint kinematics, respectively. An automatic impedance tuning (AIT) approach is presented for a knee assistive device (KAD) based on real-time activity recognition and gait phase detection. The activities considered in this paper are level, uphill, and downhill walking. A Gaussian mixture model (GMM) is employed to map the fuzzy likelihood of various activities and gait phases to the desired virtual impedance of the KAD. The prior estimate of virtual impedance is defined using human knee impedance identified with the walking data collected on different users. The AIT approach is integrated into the high-level impedance-based controller of the KAD for assistance during the stance phase. Finally, to evaluate the benefit of the proposed algorithm in stance phase, an EMG sensor is placed on the vastus medialis muscle group of three participants. The proposed approach is compared with two baseline approaches: constant impedance and finite state machine, and the results demonstrate that the profiles of impedance parameters and robot assistive torque are smoother and the muscle activity of vastus medialis is reduced. It is also noticed that the participants reduce their step lengths and increase walking cadence with assistance from the KAD.

An Integrated Framework for Cooperative Aerial Manipulators in Unknown Environments

This letter presents a viable framework that integrates control, estimation of unknown payload, safety management, and obstacle avoidance for cooperative transportation in unknown environments using multiple aerial manipulators. Unlike existing approaches for cooperative manipulation based on force decomposition or impedance-based control that often require heavy or expensive force/torque sensors, this letter suggests a method without such sensors, by exploiting the decentralized dynamics. First, an online estimator is designed to estimate the mass and inertial properties of an unknown payload, and an adaptive controller based on the estimated parameter is developed. For planning, trajectory in joint space is generated to satisfy unilateral constraints for safety, and dynamic movement primitives (DMPs) modify the trajectory to avoid obstacles on the fly. By integration of these components, the proposed framework can be executed in real time. The proposed framework is validated with experiments using camera-equipped aerial manipulators combined with a two-degree of freedom robotic arm in environments containing obstacles. Experimental results show that the proposed approach can be utilized for safe cooperative aerial transportation in realistic environments.

Impedance control in multiple cooperative space robots pulling a flexible wire

With the interaction between the space robot tip and the environment, the base’s position and orientation are disturbed leading to force and trajectory control complexity. Impedance control is a technique for force and trajectory control in a robotic system. This paper presents a strategy of impedance control in a multiple cooperative space robots pulling a flexible wire. First, the control strategy was developed for one degree of freedom multiple space robots. The developed control strategy was extended to two degree of freedom cooperative space robots. A flexible wire is pulled by a group of space robots with one degree of freedom and two degree of freedoms, respectively. In the impedance-based control strategy design, the process of modulating the robot tip impedance is a very significant feature. In this work, impedance control strategy between tip and environment is applied for pulling a wire by one degree of freedom and two degree of freedoms. Impedance depends on the gain compensation for passive degree of freedom dynamics. Simulation and the animation studies are carried out to validate the proposed control scheme. The results achieved are quite satisfactory and reveal that impedance controllers in a multiple cooperative space robots with one and two degree of freedoms can limit the interaction forces to a predefined value of 8 N. The bond graph modeling methodology is used in the dynamic system model for the generation of system equations.

Planning and Control for Collision-Free Cooperative Aerial Transportation

This paper presents planning and control synthesis for multiple aerial manipulators to transport a common object. Each aerial manipulator that consists of a hexacopter and a two-degree-of-freedom robotic arm is controlled by an augmented adaptive sliding mode controller based on a closed-chain robot dynamics. We propose a motion planning algorithm by exploiting rapidly exploring random tree star (RRT*) and dynamic movement primitives (DMPs). The desired path for each aerial manipulator is obtained by using RRT* with Bezier curve, which is designed to handle environmental obstacles, such as buildings or equipments. During aerial transportation, to avoid unknown obstacle, DMPs modify the trajectory based on the virtual leader–follower structure. By the combination of RRT* and DMPs, the cooperative aerial manipulators can carry a common object to keep reducing the interaction force between multiple robots while avoiding an obstacle in the unstructured environment. To validate the proposed planning and control synthesis, two experiments with multiple custom-made aerial manipulators are presented, which involve user-guided trajectory and RRT*-planned trajectory tracking in unstructured environments. Note to Practitioners —This paper presents a viable approach to autonomous aerial transportation using multiple aerial manipulators equipped with a multidegree-of-freedom robotic arm. Existing approaches for cooperative manipulation based on force decomposition or impedance-based control often require a heavy or expensive force/torque sensor. However, this paper suggests a method without using a heavy or expensive force/torque sensor based on closed-chain dynamics in joint space and rapidly exploring random tree star (RRT*) that generates the desired trajectory of aerial manipulators. Unlike conventional RRT*, in this paper, our method can also avoid an unknown moving obstacle during aerial transportation by exploiting RRT* and dynamic movement primitives. The proposed planning and control synthesis is tested to demonstrate performance in a lab environment with two custom-made aerial manipulators and a common object.

Intuitive Clinician Control Interface for a Powered Knee-Ankle Prosthesis: A Case Study.

This paper presents a potential solution to the challenge of configuring powered knee-ankle prostheses in a clinical setting. Typically, powered prostheses use impedance-based control schemes that contain several independent controllers which correspond to consecutive periods along the gait cycle. This control strategy has numerous control parameters and switching rules that are generally tuned by researchers or technicians and not by a certified prosthetist. We propose an intuitive clinician control interface (CCI) in which clinicians tune a powered knee-ankle prosthesis based on a virtual constraint control scheme, which tracks desired periodic joint trajectories based on a continuous measurement of the phase (or progression) of gait. The interface derives virtual constraints from clinician-designed joint kinematic trajectories. An experiment was conducted in which a certified prosthetist used the control interface to configure a powered knee-ankle prosthesis for a transfemoral amputee subject during level-ground walking trials. While it usually takes engineers hours of tuning individual parameters by trial and error, the CCI allowed the clinician to tune the powered prosthesis controller in under 10 min. This allowed the clinician to improve several amputee gait outcome metrics, such as gait symmetry. These results suggest that the CCI can improve the clinical viability of emerging powered knee-ankle prostheses.

Adaptive Hybrid System Framework for Unified Impedance and Admittance Control

Impedance and Admittance Control are two well-known controllers to accomplish the same goal: the regulation of the mechanical impedance of manipulators interacting dynamically with the environment. However, they both are affected by a strong limitation deriving from their fixed causality, which causes their inability to provide good performance over a large spectrum of environment stiffnesses. In this paper an adaptive hybrid system framework is proposed to unify Impedance and Admittance formulations and consequently overcome this limit. Indeed, the hybrid framework interpolates the opposite performance and stability characteristics of the above-mentioned impedance-based control strategies leading to a family of controllers with intermediate properties, and thus suitable for several conditions. Moreover, the adaptivity allows the hybrid system to operate properly in an environment characterized by unknown and even time-varying stiffness. Especially, the work focuses on the development of this latter aspect and an adaptive solution based on a feedforward Neural Network is presented. The effectiveness of the novel control strategy is demonstrated by means of numerical simulations.

Stable Nonlinear Trilateral Impedance Control for Dual-User Haptic Teleoperation Systems With Communication Delays

A new nonlinear adaptive impedance-based trilateral controller is proposed to ensure the absolute stability of multi-degrees-of-freedom (DOFs) dual-user haptic teleoperation systems subjected to communication delays. Using this strategy, reference impedance models are realized for the trilateral teleoperation system represented by a three-port network to facilitate cooperation of two human operators in order to perform a remote physical task. For this purpose, an impedance model defines the desired haptic interaction between the two human operators, while another impedance model specifies the desired behavior of the slave robot in terms of tracking the mater robots' trajectories during interaction with the remote environment. It is shown that different performance goals such as position synchronization and force reflection can be achieved via different adjustments to the impedance parameters. The sufficient conditions for the trilateral haptic system's absolute stability are investigated in terms of the impedance models' parameters. Accordingly, guidelines for modification of the impedance parameters are obtained to guarantee the absolute stability of the trilateral haptic system in the presence of communication time delays. A trilateral nonlinear version of the model reference adaptive impedance control (MRAIC) scheme is developed for implementing the proposed reference impedance models on the masters and the slave. The convergence of robots' trajectories to desired responses and the robustness against modeling uncertainties are ensured using the proposed controller as proven by the Lyapunov stability theorem. The proposed impedance-based control strategy is evaluated experimentally by employing a nonlinear multi-DOFs teleoperated trilateral haptic system with and without communication delays.

Control of Fully Actuated Unmanned Aerial Vehicles with Actuator Saturation

A natural way to control fully actuated UAVs is the use of feedback linearization. Indeed, given a sufficiently good knowledge of UAV mechanical properties, the resulting closed-loop system is linear and well-suited for further developments (e.g. impedance-based control for physical interactions). In the presence of actuator saturation, however, feedback linearization can lead to undesired and sometimes even unstable behaviors. To overcome this problem, this paper introduces a novel Explicit Reference Governor scheme which is particularly well suited for closed-loop systems with linear dynamics and nonlinear state and input constraints. The advantages of the proposed scheme are illustrated using numerical simulations.

Dance Teaching by a Robot: Combining Cognitive and Physical Human–Robot Interaction for Supporting the Skill Learning Process

This letter presents a physical human-robot interaction scenario in which a robot guides and performs the role of a teacher within a defined dance training framework. A combined cognitive and physical feedback of performance is proposed for assisting the skill learning process. Direct contact cooperation has been designed through an adaptive impedance-based controller that adjusts according to the partner's performance in the task. In measuring performance, a scoring system has been designed using the concept of progressive teaching (PT). The system adjusts the difficulty based on the user's number of practices and performance history. Using the proposed method and a baseline constant controller, comparative experiments have shown that the PT presents better performance in the initial stage of skill learning. An analysis of the subjects' perception of comfort, peace of mind, and robot performance have shown a significant difference at the p < .01 level, favoring the PT algorithm.

Novel Functional Task-Based Gait Assistance Control of Lower Extremity Assistive Device for Level Walking

A novel approach to functional task-based gait assistance control architecture and its preliminary evaluations are presented. This approach utilizes a finite state machine to implement a gait period detector to estimate the current gait period of the user among the six major periods. Furthermore, an impedance-based controller is used to produce the functional gait assistance at the hip and the knee joint in each detected gait period. This is the first experimental trial to offer gait assistance based on gait period functionality. The goal of this controller is to assist the gait of the impaired persons in order to reduce the walking efforts in their daily living. The performance of this approach is investigated on a single-leg lower extremity assistive device with a group of healthy subjects walking on a treadmill. Joint kinematics, assistive torques from assisted trial, and heart rates from all trials are recorded. Preliminary evaluation shows that this approach is capable of providing functional task gait assistance at hip and knee joint of the subject under varying walking speeds during level walking. In particular, at a constant speed of 1 km/h with five healthy subjects, when assisted with the device, it reduces their heart rate to statistically resemble the free-walking heart rates. Additionally, it decreases their physical efforts to substantially improve their performance, compared with their unassisted conditions.

Novel Cooperative Teleoperation Framework: Multi-Master/Single-Slave System

In this paper, a novel multi-master/single-slave (MM/SS) teleoperated framework is proposed. The desired objectives for the MM/SS system are presented in such a way that both cooperative and training applications, e.g., surgical teleoperation and surgical training, can benefit. Passivity of the system is investigated, and it is shown that an ideal MM/SS system, depending on its structure, may not always be passive unlike a conventional SM/SS system. An impedance-based control methodology is developed to satisfy the desired objectives of the MM/SS system in the presence of communication delays. The small-gain theorem is used to analyze the closed-loop stability, deriving a sufficient condition to guarantee system stability in the presence of time delays. Experimental results conducted on an MM/SS system are presented to evaluate the performance of the proposed methodology.

Optimized Assistive Human-Robot Interaction Using Reinforcement Learning.

An intelligent human-robot interaction (HRI) system with adjustable robot behavior is presented. The proposed HRI system assists the human operator to perform a given task with minimum workload demands and optimizes the overall human-robot system performance. Motivated by human factor studies, the presented control structure consists of two control loops. First, a robot-specific neuro-adaptive controller is designed in the inner loop to make the unknown nonlinear robot behave like a prescribed robot impedance model as perceived by a human operator. In contrast to existing neural network and adaptive impedance-based control methods, no information of the task performance or the prescribed robot impedance model parameters is required in the inner loop. Then, a task-specific outer-loop controller is designed to find the optimal parameters of the prescribed robot impedance model to adjust the robot's dynamics to the operator skills and minimize the tracking error. The outer loop includes the human operator, the robot, and the task performance details. The problem of finding the optimal parameters of the prescribed robot impedance model is transformed into a linear quadratic regulator (LQR) problem which minimizes the human effort and optimizes the closed-loop behavior of the HRI system for a given task. To obviate the requirement of the knowledge of the human model, integral reinforcement learning is used to solve the given LQR problem. Simulation results on an x - y table and a robot arm, and experimental implementation results on a PR2 robot confirm the suitability of the proposed method.

Investigation and Suppression of Harmonics Interaction in High-Power PWM Current-Source Motor Drives

For high-power PWM current-source drive systems, the distortion of dc-link current cannot be ignored due to the low converter switching frequency and the relative small dc choke for reduced cost/weight. The distorted dc-link current may introduce interharmonics in the grid side and the motor side through the rectifier and the inverter, and may give rise to the system resonance at certain motor speeds. When resonance occurs, significant interharmonics are introduced in the drive system, which will degrade the grid-side power quality and produce the excessive torque ripples on the motor side. In this paper, the harmonics interaction in a high-power PWM current-source drive system is investigated, and a method for the system resonance estimation is proposed. Based on the estimation of resonance conditions, a dc-link virtual impedance-based control method is developed to mitigate the resonance resulted from the harmonics interaction. The investigation of harmonics interaction, the proposed resonance estimation method, and the dc-link virtual impedance-based suppression method are verified through both simulations and experiments.

Contributions of knee swing initiation and ankle plantar flexion to the walking mechanics of amputees using a powered prosthesis.

Recently developed powered prostheses are capable of producing near-physiological joint torque at the knee and/or ankle joints. Based on previous studies of biological joint impedance and the mechanics of able-bodied gait, an impedance-based controller has been developed for a powered knee and ankle prosthesis that integrates knee swing initiation and powered plantar flexion in late stance with increasing ankle stiffness throughout stance. In this study, five prosthesis configuration conditions were tested to investigate the individual contributions of each sub-strategy to the overall walking mechanics of four unilateral transfemoral amputees as they completed a clinical 10-m walk test using a powered knee and ankle prosthesis. The baseline condition featured constant ankle stiffness and no swing initiation or powered plantar flexion. The four remaining conditions featured knee swing initiation alone (SI) or in combination with powered plantar flexion (SI+PF), increasing ankle stiffness (SI+IK), or both (SI+PF+IK). Self-selected walking speed did not significantly change between conditions, although subjects tended to walk the slowest in the baseline condition compared to conditions with swing initiation. The addition of powered plantar flexion resulted in significantly higher ankle power generation in late stance irrespective of ankle stiffness. The inclusion of swing initiation resulted in a significantly more flexed knee at toe off and a significantly higher average extensor knee torque following toe off. Identifying individual contributions of intrinsic control strategies to prosthesis biomechanics could help inform the refinement of impedance-based prosthesis controllers and simplify future designs of prostheses and lower-limb assistive devices alike.

Control of Adjustable Compliant Actuators

Adjustable compliance or variable stiffness actuators comprise an additional element to elastically decouple the actuator from the load and are increasingly applied to human-centered robotic systems. The advantages of such actuators are of paramount importance in rehabilitation robotics, where requirements demand safe interaction between the therapy system and the patient. Compliant actuator systems enable the minimization of large contact forces arising, for example, from muscular spasticity and have the ability to periodically store and release energy in cyclic movements. In order to overcome the loss of bandwidth introduced by the elastic element and to guarantee a higher range in force/torque generation, new actuator designs consider variable or nonlinear stiffness elements, respectively. These components cannot only be adapted to the walking speed or the patient condition, but also entail additional challenges for feedback control. This paper introduces a novel design method for an impedance-based controller that fulfills the control objectives and compares the performance and robustness to a classical cascaded control approach. The new procedure is developed using a non-standard positive-real Η2 controller design and is applied to a loop-shaping approach. Robust norm optimal controllers are designed with regard to the passivity of the actuator load-impedance transfer function and the servo control problem. Classical cascaded and positive-real Η2 controller designs are validated and compared in simulations and in a test bench using a passive elastic element of varying stiffness.

Controlling Knee Swing Initiation and Ankle Plantarflexion With an Active Prosthesis on Level and Inclined Surfaces at Variable Walking Speeds.

Improving lower-limb prostheses is important to enhance the mobility of amputees. The purpose of this paper is to introduce an impedance-based control strategy (consisting of four novel algorithms) for an active knee and ankle prosthesis and test its generalizability across multiple walking speeds, walking surfaces, and users. The four algorithms increased ankle stiffness throughout stance, decreased knee stiffness during terminal stance, as well as provided powered ankle plantarflexion and knee swing initiation through modifications of equilibrium positions of the ankle and knee, respectively. Seven amputees (knee disarticulation and transfemoral levels) walked at slow, comfortable, and hurried speeds on level and inclined (10°) surfaces. The prosthesis was tuned at their comfortable level ground walking speed. We further quantified trends in prosthetic knee and ankle kinematics, and kinetics across conditions. Subjects modulated their walking speed by ±25% (average) from their comfortable speeds. As speed increased, increasing ankle angles and velocities as well as stance phase ankle power and plantarflexion torque were observed. At slow and comfortable speeds, plantarflexion torque was increased on the incline. At slow and comfortable speeds, stance phase positive knee power was increased and knee torque more flexor on the incline. As speed increased, knee torque became less flexor on the incline. These algorithms were shown to generalize well across speed, produce gait mechanics that compare favorably with non-amputee data, and display evidence of scalable device function. They have the potential to reduce the challenge of clinically configuring such devices and increase their viability during daily use.

Design and Control of a Pneumatically Actuated Lower-Extremity Orthosis

Lower-extremity orthosis is a type of wearable mechanical device that serves a wide variety of important biomedical purposes, such as gait assistance and rehabilitative training. Due primarily to the constraints associated with actuation technology, the majority of current lower-extremity orthoses are either passive or tethered to external power sources, limiting the functionality of such devices. In this paper, the authors present the research results towards a fully mobile (i.e., untethered) powered lower-limb orthosis, leveraging the high power density of pneumatic actuators for the joint power generation. The design of the orthosis is presented, with the objectives of providing full locomotive assistance in multiple common locomotive modes and generating a minimum level of restriction to the wearer's daily activities. For the control of the orthosis, a finite-state impedance-based controller is developed, which simulates an artificial impedance in order to enable the natural interaction with the wearer. Preliminary testing on a healthy subject demonstrated that the orthosis was able to provide a natural gait and a comfortable user experience in the treadmill walking experiments.

A High-performance Redundantly Actuated Parallel Mechanism for Ankle Rehabilitation

In this paper we present a redundantly actuated parallel mechanism for ankle rehabilitation. The proposed device has the advantage of mechanical and kinematic simplicity when compared with the state-of-the-art multi-degree-of-freedom parallel mechanism prototypes while at the same time it is fully capable of carrying out the exercises required by the ankle rehabilitation protocols. Optimization of the device workspace, dexterity, torque output and size was carried out during the design phase of the device. The development of the system involved the realization of a new customized linear actuator able to meet the speed and force requirements of the device functionality. We also discuss the impedance-based control scheme used for the redundantly actuated device, which allows the execution of both assistive and resistive strengthening rehabilitation regimes. Results from the control of a single linear actuator and further experimental tests including the position tracking of the fully actuated platform are presented. It is believed that the performance and the simplicity of the proposed mechanism will allow the widespread use of the system as a new aid tool for ankle rehabilitation.

Transporting an Object by a Passive Mobile Robot with Servo Brakes in Cooperation with a Human

In this paper, we introduce a passive mobile robot called PRP (Passive Robot Porter) to realize transportation of an object in cooperation with human, which is developed based on a concept of passive robotics. PRP consists of three omni-directional wheels with servo brakes and a controller. It can manipulate an object by controlling an external force/moment applied by a human based on the control of the servo brakes. We consider the characteristics of the servo brakes and control the brake torque of each wheel based on the brake force/moment constraint so that several motion functions of PRP are realized based on the applied force. This allows PRP to track a path which includes motion perpendicular to the pushing direction of the human without using servo motors. The impedance-based motion control is also realized with respect to the perpendicular to the pushing direction. These functions are implemented on PRP experimentally, and the experimental results illustrate the validity of PRP and its control method.