Smooth transition adaptive hybrid impedance control for connector assembly

Physiotherapy is the treatment to recover a patient’s mobility and limb function after an injury, illness, or disability. Rehabilitation robots can be used to replace human physiotherapists. To ensure safety during robot physical therapy, the patient’s limb needs to be controlled to track a desired joint trajectory, and the torque due to interaction force/torque needs to be measured and regulated. Therefore, hybrid impedance and admittance with position control (HIPC) is required to track the trajectory and simultaneously regulate the contact torque. The literature describes two structures of HIPC: (1) a switched framework between admittance and impedance control operating in parallel (HIPCSW); and (2) a series connection between admittance and impedance control without switching. In this study, a hybrid adaptive impedance and position-based admittance control (HAIPC) in series is developed, which consists of a proportional derivative-based admittance position controller with gravitational torque compensation and an adaptive impedance controller. An extended state observer is used to estimate the interaction joint torque due to human stiff contact with the exoskeleton without the use of force/torque sensor, which is then used in the adaptive algorithm to update the stiffness and damping gains of the adaptive impedance controller. Simulation results obtained using MATLAB show that the proposed HAIPC significantly reduces the mean absolute values of the actuation torques (control inputs) required for the shoulder and elbow joints in comparison with HIPC and HIPCSW.

Trajectory tracking of tail-sitter aircraft by ℓ₁ adaptive fault tolerant control

The interaction force control problem for the object manipulated by the dual-arm manipulator with the unknown environment is addressed in this paper. An adaptive dual-loop impedance control scheme is designed to meet the three control targets, including the trajectory tracking control of the object, the internal force control of the object with the two arms, and the interaction force control of the object with the unknown environment. Firstly, an adaptive variable impedance control method is proposed in the outer loop to copy with the unknown stiffness and position of the environment. Secondly, an impedance control scheme based on the internal force is designed in the inner loop to plan the internal force and the modified position supplied with the outer-loop control. Thirdly, to realize the reference trajectory supplied with the inner-loop control, the virtual decomposition control (VDC) method is incorporated, which is a recursive adaptive control progress, so that the unknown dynamics of the manipulators is acceptable and the computation complexity is also very low to satisfy the real-time control requirement. The stability verifications of the inner-loop impedance control and the VDC method are given based on the Lyapunov method, and the stability analysis of the outer-loop impedance control is obtained using the Routh criterion. Finally, a plane dual-arm robotic manipulator with three degrees of freedom is used to verify the effectiveness of the proposed control scheme.

In this paper, adaptive impedance control is developed for an ${n}$ -link robotic manipulator with input saturation by employing neural networks. Both uncertainties and input saturation are considered in the tracking control design. In order to approximate the system uncertainties, we introduce a radial basis function neural network controller, and the input saturation is handled by designing an auxiliary system. By using Lyapunov’s method, we design adaptive neural impedance controllers. Both state and output feedbacks are constructed. To verify the proposed control, extensive simulations are conducted.

This paper presents the feasibility study of an adaptive impedance force control application to deburring/milling process. The proposed control regulates desired force on nonhomogeneous workpiece in regardless of unknown location and stiffness of the workpiece. The adaptive technique of minimizing the force error between the desired and the actual force is used to compensate for uncertainties in unknown workpiece. Effect of selecting the impedance damping gain with different workpiece stiffness on force tracking is analytically determined. Analyzing the stability of the proposed adaptive force control yields the bound of the adaptive gain for stability. Simulation studies of deburring/milling process show the excellent force tracking and confirm stability bound for stable force tracking.

Reference trajectory learning based adaptive iterative impedance control for a lower limb rehabilitation exoskeleton with actuator saturation

Dead-zone is one of the most common hard nonlinearities ubiquitous in master–slave teleoperation systems, particularly in the slave robot joints. However, adaptive control techniques applied in teleoperation systems usually deal with dynamic uncertainty but ignore the presence of dead-zone. Dead-zone has the potential to remarkably deteriorate the transparency of a teleoperation system in the sense of position and force tracking performance or even destabilizing the system if not compensated for in the control scheme. In this paper, an adaptive bilateral control scheme is proposed for nonlinear teleoperation systems in the presence of both uncertain dynamics and dead-zone. An adaptive controller is designed for the master robot with dynamic uncertainties and the other is developed for the slave robot with both dynamic uncertainties and unknown dead-zone. The two controllers are incorporated into the four-channel bilateral teleoperation control framework to achieve transparency. The transparency and stability of the closed-loop teleoperation system is studied via a Lyapunov function analysis. Comparisons with the conventional adaptive control which merely deal with dynamic uncertainties in the simulations demonstrate the validity of the proposed approach.

Purpose Interaction force control is crucial for robotic manipulators in contact tasks. For different tasks, robotic manipulators are expected to possess adaptive capability to modulate interaction force performance. To this end, this paper aims to propose an interaction force control scheme for robotic contact tasks via prescribed performance-enhanced impedance regulation. Design/methodology/approach The proposed scheme integrates Dual Prescribed Performance boundary in impedance outer loop and position inner loop in parallel, named DPP-IC. Based on the performance boundary, the interaction force deviation is transformed into matching position adjustment compensation for enhancing impedance regulation in contact tasks. The compensation together with the output of the standard impedance model is then used as desired input in our designed prescribed performance PID controller (PPC-PID) in position loop. This can confine interaction force within prescribed performance boundary, thereby simultaneously improving transient and steady-state control performance. Findings Comparison experiments with three mainstream impedance control methods of standard impedance control, adaptive law-based impedance control and learning-based variable impedance control are conducted on our self-designed robotic manipulators. The results of this study prove that the advantages of the proposed scheme in improving interaction force control. Originality/value This paper first introduces prescribed performance boundary into standard impedance model in parallel, improving inter-action force control performance for robotic contact tasks on a wide range of environmental stiffness.

In this paper, an image-based impedance control strategy for force tracking of an unmanned aerial manipulator (UAM) is presented. Firstly, image features with nice decoupling characteristics are designed and the relationship between the camera motion and the image features is derived. Then, a two-stage strategy is proposed to achieve force tracking of the UAM on a planar object in an arbitrary pose. The first stage drives the end-effector perpendicular to the object’s planer surface by pure visual servoing. To achieve force tracking under the visual guidance, an adaptive visual impedance control method which adjusts the target stiffness according to the force tracking error and the visual feature error is proposed in the second stage. The closed-loop system is proved asymptotically stable by means of Lyapunov analysis. Further, the stability in free flight phase of the stage two is also analyzed and ensured. Finally, experiments were carried out including a whiteboard cleaning task in different poses. The experimental results illustrate the validity and effectiveness of the proposed approach. Note to Practitioners—This work is motivated by the contact force tracking problem of an unmanned aerial manipulator (UAM) without the position measurement. In recent years, impedance control is widely used in force tracking problems. However, position measurement is needed both for the robot and the object in most studies. To generate desired force under visual guidance, image-based visual servoing is combined with a novel impedance controller with variable stiffness. The idea is intuitive. For a human in contact with a wall, the contact force is controlled by adjusting his arm stiffness, which means the stiffness is adapted to the difference between the desired and actual contact force. Furthermore, the stiffness is also adapted to the visual tracking error, removing the assumption that no tracking error is in the inner loop of the impedance controller in the previous works. The proposed strategy is a promising solution for real applications and is validated by a board cleaning experiment in this paper.

To solve the problem of insufficient adjustment ability of traditional impedance control due to uncertainty of environmental parameters when the robot grinds castings, an adaptive variable impedance constant force control method for robot grinding with unknown environmental parameters is proposed. In the proposed method, first, self-impedance impedance control is used as inner loop control to realize fast tracking response of actual trajectory to reference trajectory and reduce overshoot of grinding force; secondly, fuzzy adaptive variable impedance control is used as outer loop control to realize real-time prediction of unknown environmental parameters and real-time adjustment of impedance parameters; finally, the constant force control of robot grinding in unknown environment is realized by combining inner and outer loop control. In order to verify the effectiveness of the proposed method, the simulation model of the algorithm is constructed based on the MATLAB-SIMULINK simulation platform for simulation analysis, and experimental verification is carried out on the casting surface defects robot grinding platform. The simulation and experimental results show that the algorithm can control the grinding force well when grinding unknown environmental parameters, the force control accuracy is within ±1N, the average grinding force error of casting fin grinding is about 7.59%, the average grinding force error of casting riser grinding is about 5.31%, and the contour accuracy of casting surface defects is good after grinding by force control, and the surface quality after grinding is improved.

Grid-forming (GFM) converters exhibit voltage-source characteristics. During grid faults, the converter may be forced to deliver excessive fault current, which can exceed the safe operating area of the semiconductor devices. The adaptive virtual impedance control strategy is a method to address the overcurrent issue of grid-forming converters under grid faults. This strategy is capable of dynamically adjusting the virtual impedance value in accordance with the magnitude of the fault current, so as to limit the fault current. Nevertheless, this approach fails to limit the fault current beneath the preset current threshold, causing inconvenience for the design of controller parameters and practical application. To enhance the current-limiting effect, this paper proposes an improved adaptive virtual impedance control strategy by introducing an integrator to eliminate the steady-state error of the fault current. In order to analyze the stability of the proposed control strategy, this paper establishes a dq-axis impedance model, and the impedance stability criterion and the generalized Nyquist criterion are used to analyze the influence of control parameters on the stability of grid-forming converter. The effectiveness of the proposed improved adaptive virtual impedance strategy on fault current suppression and the correctness of the stability analysis theory were verified through Matlab/Simulink simulation and OPAL-RT semi-physical simulation platform.

The force tracking error is dependent on both the position tracking error and the estimated parameter error so that, in constrained adaptive robot control, the convergence of the estimated parameter error becomes more important than in unconstrained adaptive robot control. However, in the direct adaptive controller, the parameter adaptation is driven only by the tracking error in the joint motion, while in the indirect adaptive controller, the parameter estimation is driven only by the prediction error in the joint torque. Based on this observation a new combined direct and indirect adaptive controller driven by both the tracking error and the prediction error is developed for constrained robots with inertia parameter uncertainty. Stability of the combined adaptive controllers is established in the Lyapunov sense. The combined control improves both position and force tracking performance compared with direct adaptive control. The computer simulations are presented to demonstrate the proposed schemes.

A torque controller based on adaptive robust control is proposed for trajectory tracking of nonholonomic wheeled mobile robot with kinematic and dynamic uncertainties. In this controller, the parametric uncertainties are estimated by the adaptive control, and the non-parametric uncertainties are compensated by the adaptive robust control. The operation efficiency of the system is improved by distinguishing the differences between parametric and non-parametric uncertainties. Furthermore, the transient performance of adaptive control is optimized by the adaptive robust control and the measurements of uncertainties bounds are avoided, this strengthened the applicability of the controller. The uniformly ultimately bounded of closed-loop control system is proved by the Lyapunov theory. Simulations show that the adversely impacts of complex system uncertainties can be eliminated effectively and the high-precision trajectory tracking is guaranteed.

This work introduces the design of an adaptive tracking control strategy for a 6-DOF Staubli TX2-60L industrial robot manipulator operating under dynamic model uncertainties caused by variable payloads. A complete kinematic and dynamic model of the robot is derived using Denavit–Hartenberg parameters and the Lagrangian formulation, respectively. To address the limitations of conventional control methods under payload variations, an adaptive controller is designed, which continuously updates its parameters based on real-time tracking errors. A Lyapunov stability analysis is conducted to rigorously establish that the closed-loop system guarantees convergent tracking errors and bounded internal states. Simulation studies with different payload scenarios, including no load, step load variation, and continuous load changes, show the performance of the proposed control strategy. The results confirm that the adaptive controller maintains precise trajectory tracking and robust stability despite significant model uncertainties, highlighting its suitability for industrial pick-and-place and high-precision automation tasks. Keywords: Robot manipulator, kinematic model, dynamic model, variable payloads, adaptive control, stability analysis.

This paper describes an adaptive scheme for controlling the end-effector impedance of robot manipulators. The proposed control system consists of three subsystems: a simple "filter" which characterizes the desired dynamic relationship between the end-effector position error and the end-effector/environment contact force, an adaptive controller which produces the Cartesian-space control input required to provide this desired dynamic relationship, and an algorithm for mapping the Cartesian-space control input to a physically realizable joint-space control torque. The controller does not require knowledge of either the structure or the parameter values of the robot dynamics, and is implemented without calculation of the robot inverse kinematic transformation. As a result, the scheme represents a very general and computationally efficient approach to controlling the impedance of both nonredundant and redundant manipulators. Computer simulation results are given for a planar four degree-of-freedom redundant robot under adaptive impedance control. These results demonstrate that accurate end-effector impedance control and effective redundancy utilization can be achieved simultaneously by using the proposed controller.