Hybrid Impedance Control Research Articles

DESIGN, DEVELOPMENT AND HYBRID IMPEDANCE CONTROL OF AN ANKLE REHABILITATION ROBOT

DESIGN, DEVELOPMENT AND HYBRID IMPEDANCE CONTROL OF AN ANKLE REHABILITATION ROBOT

Hybrid impedance and admittance control for optimal robot–environment interaction

AbstractCompliant interaction between robots and the environment is crucial for completing contact-rich tasks. However, obtaining and implementing optimal interaction behavior in complex unknown environments remains a challenge. This article develops a hybrid impedance and admittance control (HIAC) scheme for robots subjected to the second-order unknown environment. To obtain the second-order target impedance model that represents the optimal interaction behavior without the accurate environment dynamics and acceleration feedback, an impedance adaptation method with virtual inertia is proposed. Since impedance control and admittance control have complementary structures and result in unsatisfactory performance in a wide range of environmental stiffness due to their fixed causality, a hybrid system framework suitable for the second-order environment is proposed to generate a series of intermediate controllers which interpolate between the responses of impedance and admittance controls by using a switching controller and adjusting its switching duty cycle. In addition, the optimal intermediate controller is selected using a mapping of the optimal duty cycle to provide the optimal implementation performance for the target impedance model. The proposed HIAC scheme can achieve the desired interaction and impedance implementation performance while ensuring system stability. Simulation and experimental studies are performed to verify the effectiveness of our scheme with a 2-DOF manipulator and a 7-DOF Franka EMIKA panda robot, respectively.

Robotic abrasive belt grinding of complex curved blades based on a novel force control architecture integrating smooth trajectories

Constant force control and smooth trajectory planning are regarded as key strategies to improve the surface quality of blades in robotic abrasive belt grinding. However, some technical challenges still remain: (1) external normal grinding force with time-consuming sensing, large fluctuations, and force overshoot, and (2) lack of global C2 continuous grinding trajectories. In this paper, a unified architecture is proposed to concurrently address the above issues. The main contribution of this paper is to provide a complete solution for robots to automatically accomplish the grinding of complex curved blades. The grinding process is arranged as follows: Firstly, cubic B-spline and C2 continuous quaternion spline curves are used to generate the global C2 continuous trajectory, thereby avoiding vibrations and unexpected slowdowns during grinding. Secondly, the actual grinding force is sensed based on a low-speed Fourier series trajectory and the least square algorithm. Finally, a novel force control scheme consisting of an adaptive hybrid impedance (AHI) control scheme and two nonlinear tracking differentiator (NTD) modules is proposed to overcome the problems of large force fluctuations and force overshoot. Experimental results demonstrate that the proposed solution delivers better performance. Furthermore, the surface roughness Ra of the machined blade is controlled within 0.8 μm, which meets the requirements of real-world industrial applications.

Neural network-based variable stiffness impedance control for internal/external forces tracking of dual-arm manipulators under uncertainties

The desired interaction between manipulators, objects, and environments has resulted in the internal/external force control for dual-arm manipulators being in increasing demand. Consequently, this study focused on the internal/external force tracking for dual-arm manipulator systems under external disturbances, geometries, and stiffness uncertainties which continuously lead to unsatisfactory internal force tracking. The proposed scheme is based on a two-level adaptive impedance control scheme, where the stiffness coefficient is adjusted to adapt to uncalibrated objects. An object-level hybrid impedance controller was used to regulate the external disturbance to produce a compliant response. A manipulator-level neural network-based variable stiffness impedance controller (NNVSIC) was proposed to regulate the internal force under various uncertainties. Additionally, an adaptive wavelet neural network was designed to compensate for the geometric estimation errors of the object. The variable stiffness coefficient could automatically adapt to an unknown object during the cooperation process. One advantage of the proposed method is that no prior knowledge was required. The same controller parameters could be adapted to various objects. The asymptotic stability of the proposed NNVSIC was proven via Lyapunov stability analysis. A series of experiments were conducted using two self-developed nine-degrees-of-freedom redundant manipulators. Furthermore, hard and soft objects of various geometries and stiffnesses were used to verify the effectiveness of the algorithm. The experimental results demonstrated the efficiency and superiority of the proposed controller through performance comparison with various algorithms.

Adaptive Impedance Control for Force Tracking in Manipulators Based on Fractional-Order PID

Force tracking control in robot arms has been widely used in many industrial applications, particularly in tasks involving end effectors and environmental contact, such as grinding, polishing, and other similar operations. However, these environments are not always precisely known. In order to address the force tracking control problem in unknown environments, this paper proposes a fractional-order PID adaptive impedance control strategy based on traditional impedance control. The unknown environmental information is estimated online using the adaptive impedance control algorithm, and the estimated parameters are used to generate reference trajectories to reduce force tracking errors. Fractional-order PID control is then introduced into the system to improve the control performance of the system model, and the theoretical proof of strategy stability is conducted. Finally, a comparison of four strategies was conducted through simulations: traditional impedance control, adaptive hybrid impedance control, adaptive variable impedance control, and the fractional-order PID impedance control proposed in this paper. The simulation results demonstrate that the strategy proposed in this paper exhibits robustness, virtually eliminates overshoot, and enhances response speed. In contrast, both adaptive hybrid impedance control and adaptive variable impedance control exhibit approximately 30% to 45% overshoot during interactions with the environment. Furthermore, in terms of force tracking error, the proposed strategy in this paper outperforms the above two strategies by approximately 29% to 60%, achieving excellent force tracking control performance.

Hybrid impedance and admittance control of robot manipulator with unknown environment

Hybrid impedance and admittance control of robot manipulator with unknown environment

Design and hybrid impedance control of a new compliant wrist rehabilitation device

Over a decade, wrist rehabilitation robots have been proved to be efficient in the rehabilitation of patients with stroke or wrist injuries. Wrist rehabilitation devices compensate for deficiencies in manual rehabilitation training and reduce the workload of physiotherapists with their repetitive and gradual training necessary to rehabilitate patients with wrist impairments properly. Two design approaches have been reported in literature for wrist rehabilitation robotic devices. In one design approach, the strategy of placing the actuators, either linear or rotary, on the forearm to minimise the weight and inertia has been proposed. This necessitates a use of sophisticated transmission systems. In the other design approach, rotary motors have been proposed to be placed directly on the wrist joints, which results in a high weight and inertia. In this paper, a new 2-DOF compliant wrist rehabilitation device that can achieve the flexion-extension and radial-ulnar rehabilitation movements compliantly is proposed. In the proposed design, two series elastic actuators (SEAs), each consists of a servomotor and a custom-designed series elastic unit, are proposed to be placed directly on the wrist joints. In order to minimise the inertia and the required torques, the device is counterweight balanced by optimising the centre of mass of the device in the SolidWorks model. Based on the device dynamics modelled by ADAMS software, hybrid position and force tracking impedance control with the incorporation of a disturbance observer controller for the rotary SEA is developed for position and force tracking through the different therapeutic exercises. The disturbance observer controller is incorporated for high precision force control and robustness against exogenous disturbances. Simulation results demonstrate high performance of the proposed wrist rehabilitation device in terms of precise position and force trajectories tracking and robustness against exogenous disturbance as well as patient safety during all therapeutic exercises.

Adaptive hybrid impedance control for dual-arm cooperative manipulation with object uncertainties

In this paper, a novel adaptive hybrid impedance control for a dual-arm robot is proposed to cooperatively manipulate unknown objects. The proposed adaptive hybrid control scheme could be employed to improve the control performance of motion tracking and internal force regulation when dual-arm cooperative robot grasp an unknown object. Along with the analysis of human motion in the process of transferring, the controller has a master–slave structure in three levels: the first level is an absolute-relative motion controller satisfying the closed-chain constraints; the second level introduces an adaptive impedance control with variable stiffness to regulate the internal force; the motion and force control signals are fused in the third level with a select matrix. The controller uses the forces sensed at the end-effectors to compute the internal force with only kinematics, and the dynamics of the object is not required. To verify the effectiveness of the algorithm, a dual-arm robotic system is set up with two UR5 arms and the experimental results illustrate the performance and feasibility of the proposed control strategy.

Optimal detumbling strategy for a non-cooperative target with unknown inertial parameters using a space manipulator

Capturing tumbling target is considered as one of the greatest challenges in on-orbit service missions due to tumbling motion and parameter uncertainty of the space non-cooperative target. The tumbling motion of the target should be attenuated as quickly as possible before grasping to avoid the unexpected collisions and possible damages to the space manipulator or the target, which means the shortest-time detumbling trajectory is necessary. In many studies about the optimal detumbling trajectory planning the inertial parameters of the target have been assumed to be accurately known, which is usually difficult for a non-cooperative target so that parameter identification is needed before planning a optimal detumbling trajectory. In this paper, a novel strategy for parameter identification and detumbling of a tumbling target by a space manipulator is proposed. The strategy includes two phases. First, unknown inertial parameters of the target are identified in finite-time by a new estimation method with the assumption of interval excitation (IE), which relaxes the strict persistent excitation (PE) condition required in most prior studies. Second, the shortest-time detumbling trajectory of the tumbling target is generated by formulating a time-optimal control problem (OCP) in which the target attitude bounds and the limitations of the contact force as well as the relative velocity between the space manipulator and the target are represented as extended dynamical sub-systems and saturation functions. Then, the Calculus of Variations method is used to solve the OCP to obtain a high accuracy solution. During two phases, the contact force between the space manipulator and the target is utilized to generate external moment for identifying the inertia parameters of the target and detumbling the target. Meanwhile, a unified hybrid impedance control framework is applied to the space robot to track the desired contact force and motion trajectory. The whole strategy is verified by a space manipulator capturing a tumbling target, and numerical simulations demonstrate the effectiveness of the proposed methods.

Neural network-based adaptive hybrid impedance control for electrically driven flexible-joint robotic manipulators with input saturation

In this paper, a neural network (NN)-based adaptive hybrid impedance control (AHIC) scheme is proposed for the electrically driven flexible-joint robotic manipulators (EDFJRM) with input saturation, where the hybrid impedance is designed to obtain the reference trajectory by combining the position/force control with the impedance control. The proposed scheme integrates hybrid impedance control into backstepping technique, where a smooth saturation function is employed to facilitate input saturation. Moreover, the reference input trajectory of the robot is obtained by different target dynamics in the free space and the constrained space, and NNs are employed to approximate the saturation errors term, uncertain parts and external disturbances. In this way, the control objective of the position and force tracking can be realized in both free space and constrained space. Lyapunov stability analysis shows that all the signals in closed-loop system are guaranteed to be uniformly ultimately bounded, and the steady-state tracking errors of the position in the free space and the force in the constrained space are converged to zero. Finally, simulation results of two-rigid-link EDFJRM demonstrate the effectiveness of the proposed NN-based AHIC scheme.

Novel fractional hybrid impedance control of series elastic muscle-tendon actuator

Purpose This paper aims to develope a novel fractional hybrid impedance control (FHIC) approach for high-sensitive contact stress force tracking control of the series elastic muscle-tendon actuator (SEM-TA) in uncertain environments. Design/methodology/approach In three different cases, the fractional parameters of the FHIC were optimized with the particle swarm optimization algorithm. Its adaptability to the pressure of the sole of the foot on real environments such as grass (soft), carpet (medium) and solid floors (hard) is far superior to traditional impedance control. The main aim of this paper is to derive the dynamic simulation models of the SEM-TA, to develop a control architecture allowing for high-sensitive contact stress force control in three cases and to verify the simulation models and the proposed controller with experimental results. The performance of the optimized controllers was evaluated according to these parameters, namely, maximum overshoot, steady-state error, settling time and root mean squared errors of the positions. Moreover, the frequency robustness analysis of the controllers was made in three cases. Findings Different simulations and experimental results were conducted to verify the control performance of the controllers. According to the comparative results of the performance, the responses of the proposed controller in simulation and experimental works are very similar. Originality/value Origin approach and origin experiment.

A Hybrid Impedance Controller for Series Elastic Actuators to Render a Wide Range of Stable Stiffness in Uncertain Environments

Accurate and wide-range stiffness control is important for safe human-robot interaction. Accurate stiffness control can be better achieved using series elastic actuators (SEAs) than conventional rigid actuators. However, the stable range of virtual stiffness rendered by SEAs is limited by the stiffness of the actual spring, which cannot be too high in order to ensure good force control accuracy. Adding a virtual damper or derivative gain can increase the stable range of virtual stiffness, but the stable range would highly depend on the environmental stiffness. To relax the stiffness limitation in uncertain environments and explore more merits of SEAs, this letter proposes a hybrid impedance controller. This new controller linearly combines the spring force feedback and inertia force feedback. The stable range of virtual stiffness can be easily increased to ten times the actual spring stiffness with minimum effect on the force control accuracy. Unlike typical impedance controllers, the environmental stiffness can be used to raise the stable range of stiffness and hence the robustness of the controller can be ensured. Experiments will be provided to verify the hybrid impedance controller. We expect that the hybrid impedance controller can be used for SEAs in unstructured environments to provide a wide range of virtual stiffness.

Hybrid Impedance Control of Robot Manipulators based on Generalized Momentum

In the hybrid impedance control strategy, the task space is divided into position control subspace and force control subspace, and the constraint direction in the manipulator task space is controlled by torque-based impedance control, while the free movement direction is controlled by position-based impedance control. The hybrid impedance control has strong force tracking characteristics in the direction of force control and strong flexibility in the direction of position control. In this paper, we use the generalized momentum observer instead of the torque sensor to estimate the torque exerted by the external environment on the robot. Simulation results are shown for a three-link SCARA robot.

Smooth adaptive hybrid impedance control for robotic contact force tracking in dynamic environments

PurposeThe purpose of this paper is to take transient contact force response, overshoots and steady-state force tracking error problems into account to form an excellent force controller.Design/methodology/approachThe basic impedance function with a pre-PID tuner is designed to improve the force response. A dynamic adaptive adjustment function that combines the advantages of hybrid impedance and adaptive hybrid impedance control is presented to achieve both force overshoots suppressing and tracking ability.FindingsThe introduced pre-PID tuner impedance function can achieve more than the pure impedance function in aspects of converging to the desired value and reducing the force overshoots. The performance of force overshoots suppression and force tracking error are maintained by introducing the dynamic adaptive sigma adjustment function. The simulation and experimental results both show the achieved control performance by comparing with the previous control methods.Practical implicationsThe implementation of the controller is easy and convenient in practical manufacture scenes that require force control using industrial robots.Originality/valueA superior robot controller adapting to a variety of complex tasks owing to the following characteristics: maintenance of high-accuracy position tracking capability in free-space (basic capabilities of modern industrial robots); maintenance of high speed, stability and smooth contact performance in collision stage; and presentation of high-precision force tracking capability in steady contact.

Nonlinear Hybrid Impedance Control for Steering Control of Rack-Mounted Electric Power Steering in Autonomous Vehicles

In this paper, we present an innovative approach to steering control, based on torque overlay, for the smooth and efficient transfer of the steering control from an autonomous driving system to the driver, and to control the pinion angle of the rack-mounted electric power steering for lateral control of an autonomous vehicle. The novelty of our approach lies in the formulation of hybrid impedance control, which employs steering control for both lateral and impedance controls. The proposed method, nonlinear hybrid impedance controller, consists of desired impedance pinion angle generator and a super-twisting sliding mode controller for pinion angle tracking. The desired impedance transfer function of the driver’s torque to the steering wheel angle reflects the driver’s torque to the steering control. The desired impedance pinion angle is generated using the desired impedance transfer function and the desired pinion angle derived by the lateral controller. The sliding mode control method is designed, based on a super-twisting algorithm for the pinion angle, to track the desired impedance pinion angle. Consequently, without the driver’s torque, the proposed method is activated as a pinion angle tracking controller for lateral control of an autonomous vehicle. Moreover, with the driver’s torque, the proposed method is activated as an impedance controller.

Detumbling strategy based on friction control of dual-arm space robot for capturing tumbling target

The rotational motion of a tumbling target brings great challenges to space robot on successfully capturing the tumbling target. Therefore, it is necessary to reduce the target’s rotation to a rate at which capture can be accomplished by the space robot. In this paper, a detumbling strategy based on friction control of dual-arm space robot for capturing tumbling target is proposed. This strategy can reduce the target’s rotational velocity while maintaining base attitude stability through the establishment of the rotation attenuation controller and base attitude adjustment controller. The rotation attenuation controller adopts the multi-space hybrid impedance control method to control the friction precisely. The base attitude adjustment controller applies the dual-arm extended Jacobian matrix to stabilize the base attitude. The main contributions of this paper are as follows: (1) The compliant control method is adopted to achieve a precise friction control, which can reduce the target angular velocity steadily; (2) The dual-arm extended Jacobian matrix is applied to stabilize the base attitude without affecting the target capture task; (3) The detumbling strategy of dual-arm space robot is designed considering base attitude stabilization, realizing coordinated planning of the base attitude and the arms. The strategy is verified by a dual-arm space robot with two 7-DOF (degrees of freedom) arms. Simulation results show that, target with a rotation velocity of 20 (°)/s can be effectively controlled to stop within 30 s, and the final deflection of the base attitude is less than 0.15° without affecting the target capture task, verifying the correctness and effectiveness of the strategy. Except to the tumbling target capture task, the control strategy can also be applied to other typical on-orbit operation tasks such as space debris removal and spacecraft maintenance.

Hybrid impedance control of a knee joint orthosis

Purpose This paper aims to deal with the design of new hybrid approach for the assistance of the flexion extension movement of the knee joint. Design/methodology/approach The control approach combines the use of a knee joint orthosis along with functional electrical stimulation (FES) within an assist-as-needed paradigm. An active impedance controller is used to assist the generation of muscular stimulation patterns during the extension sub-phase of the knee joint movement. The generated FES patterns are appropriately tailored to achieve flexion/extension movement of the knee joint, which allows providing the required assistance by the subject through muscular stimulation. The generated torque through stimulation is tracked by a non-linear disturbance observer and fed to the impedance controller to generate the desired trajectory that will be tracked using a standard proportional derivative controller. Findings The approach was tested in experiments with two healthy subjects. Results show satisfactory performances in terms of estimating the knee joint torque, as well as in terms of cooperation between the FES and the orthosis actuator during the execution of the knee joint flexion/extension movements. Originality/value The authors designed a new hybrid approach for the assistance of the flexion extension movement of the knee joint, which has not been studied yet. The control approach combines the use of a knee joint orthosis along with FES within an assist-as-needed paradigm.

Dynamic Adaptive Hybrid Impedance Control for Dynamic Contact Force Tracking in Uncertain Environments

The importance of robot contact operation control has been increasing recently due to a need for robots to interact more with the outside world. However, traditional robot compliance control cannot take both transient contact force overshoots and steady-state force tracking error problems into account. To address this problem, this paper aims to design a dynamic adaptive hybrid impedance (DAHI) controller to deal with dynamic contact force tracking in uncertain environments (e.g., polishing scenarios). Under the premise of analyzing the transient response and steady-state error in the hybrid impedance control (HI) and adaptive hybrid impedance (AHI) control, the DAHI control, which combines the advantages from HI and AHI control, is applied to improve the performance of AHI controller. The main goal of such a controller is to avoid force overshoots in the contact stage while maintaining force tracking error in the dynamic tracking stage. The proposed controller is capable of adapting its update rate parameter online in order to track a reference force in uncertain environments. Besides, it does not require any modeling or estimation of an environment's dynamics or the robot's dynamics. The simulation and experimental results both show the achieved control performance. The results have also been compared with the previous control methods.

The Applications of Compliant Motion Control Technique on 6-DOF Hydraulic Parallel Mechanism

Most interaction control formulations focused on electrically driven parallel mechanism. In this paper, the combining practice applications of compliant motion control technique on six degrees of freedom (6-DOF) hydraulic parallel mechanism (HPM) are investigated. According to the manipulation task specification and hydraulic control system characteristics, three different compliant motion control schemes are presented in three typical application scenarios, respectively: a parallel position/force control scheme in the joint space coordinate for train curve negotiation performance test, a hybrid position/force control scheme with the internal force suppression for shaking table with actuation redundancy, and a hybrid impedance control scheme for auto-docking mechanism based on force and vision. The feasibility and effectiveness of the proposed schemes were verified through a series of experiments. It showed that the 6-DOF HPM achieves good compliant behavior by applying the proposed compliant motion control techniques. These schemes could also be extended to other applications where the compliant motion of the 6-DOF HPM is required.

User-Adaptive Assistance of Assistive Knee Braces for Gait Rehabilitation

Patients suffering from neurological and orthopedic diseases or injuries usually have mobility impairment problems, and they require customized rehabilitation training to recover. In recent years, robotic assistive devices have been widely studied for gait rehabilitation. In this paper, methods to determine user-adaptive assistance of assistive knee braces (AKBs) in gait rehabilitation are investigated. A fuzzy expert system, which takes a patient's physical condition and gait analysis results as inputs, is proposed to configure suitable levels of different assistive functions of the AKB. During gait rehabilitation, the AKB generates a reference knee trajectory according to the patient's individual gait pattern, and the interaction force is controlled through a hybrid impedance controller considering the individual assistive function configuration. The proposed methods are verified through clinical pilot studies of a patient with lower limb weakness. Experimental results show that AKB with the proposed control strategies can provide effective assistance to improve the patient's gait performance during gait rehabilitation.