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This article presents a framework for systematic, stable and passive, dynamical balancing and locomotion control of flexible-joint bipedal robots. In order to achieve stability/passivity of the full flexible-joint, floating-base model with contacts, several novel control designs are proposed, whose ability to guarantee regulation and tracking stability is mathematically and practically demonstrated. The proposed designs enable usage of full-state feedback terms, thereby increasing both link tracking and oscillation suppression performance. These constitute the only control schemes reported in the literature, which are capable of asymptotically stabilizing flexible-joint, floating-base systems with contacts, during trajectory-tracking tasks. Moreover, a novel linear quadratic regulator (LQR) tuning approach is proposed, which permits the creation of models characterized by distinct kinetic chain and impedance combinations. Stable switching between these gain sets is guaranteed, as it is demonstrated that the proposed controllers enable unconstrained and stable, variable impedance control (VIC). The proposed control methods are corroborated through practical, balancing and locomotion experiments on the COmpliant huMANoid, as well as via dynamical simulations; these results demonstrate stability maintenance during tracking and VIC tasks. The ability to stably modulate a legged robot's active impedances could enable closer replication of biologically inspired behaviors. [ABSTRACT FROM AUTHOR]

The grid-connected inverter may get unstable due to variation of grid impedance. To improve the system stability, an adaptive active damper, which emulates an adaptive tuning virtual resistor, can be introduced at the point of common coupling. However, subject to the control bandwidth, the virtual resistor is actually a frequency- dependent impedance, and its damping performance is impaired. In this article, we propose an impedance shaping method to make the virtual impedance be resistive in a wider frequency range, and it is implemented under hybrid frame for directly controlling the active power of the active damper and eliminating the influence of the cross-coupling terms on the emulation of a virtual resistor. Finally, the prototypes of a 3-kVA three-phase active damper and a 6-kVA three-phase LCL-type grid-connected inverter are fabricated and tested in the lab. The experimental results are provided to verify the effectiveness of the proposed control methods for the three-phase adaptive active damper. [ABSTRACT FROM AUTHOR]

Steer-by-wire (SBW) system has been introduced to reduce the number of parts in a vehicle by eliminating the mechanical link between the steering wheel and the road wheel. Even though intensive research has been conducted on the SBW system, the performance of the actuator system that provides the desired steering feel still needs improvement. Therefore, this article presents a compact size torque controllable steering wheel module for the SBW system, where the actuator that provides the steering feel to the driver is modularized with the gear mechanism and the spring. The steering torque sensing mechanism is compactly integrated by placing the spring inside the gear mechanism. Thanks to the advantage in the torque measurement and control ability by the spring mechanism, high gear-ratio transmission can be employed in the proposed module, which can also address the low inherent mechanical impedance problem that may cause fail-safety issues in the SBW system. To enhance the impedance rendering performance of the proposed module, an internal model compensator which is composed of inertia and friction compensation is applied. The kinematics, statics, and dynamics of the proposed module are theoretically analyzed, and a model-based control algorithm is proposed based on this analysis result. Finally, the increase in the inherent impedance and the torque control performance of the proposed module are verified through experiments. [ABSTRACT FROM AUTHOR]

This article proposes a method for single-leader–dual-follower teleoperation, where one robot (direct-follower robot, DFR) is directly teleoperated and the other robot (assisting-follower robot, AFR) can autonomously cooperate with the DFR to hold and move a deformable object, with the contact force regulated to a desired value. Since the AFR does not know its partner’s movement, first, it achieves position alignment with the DFR by using the contact force. Second, we develop an adaptive movement estimation algorithm according to the Lyapunov theory, such that DFR’s position is estimated in the presence of dynamic uncertainties. Finally, we adopt the impedance control to generate a reference trajectory for the AFR to track, in order to maintain the desired contact force. Several simulations are conducted on a platform with two three-degrees-of-freedom robots. Experiments are performed with a dual-arm robot (Baxter), results of which demonstrate the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In complex robot applications, such as human−robot interaction and robot machining, robots should interact with an unknown environment. To learn the interactive skill, a model-based actor−critic learning algorithm and a safety-learning strategy are proposed in this article to find the optimal impedance control, in which the learning process is safe and fully automatic and does not know the system parameter. In the learning algorithm, a critic is defined as a quadratic form of the system states and the external force. A modified deterministic policy gradient algorithm is presented to improve the learning efficiency. The proposed approach utilizes a model-based constraint and a highly efficient learning algorithm. In the safety-learning strategy, the robot is trained under a constant force, and the learned impedance control can transfer to different interaction situations by choosing the suitable impedance index. The effectiveness of the learning algorithm and the performance of the learned impedance control are validated in a UR5 robot. The robot can perform human−robot interaction and robot machining tasks after the training process with 100 s training time. [ABSTRACT FROM AUTHOR]

Robotic manipulators have been used inindustry for decades. However, their heavy mass and rigidity make them unsuitable for environments where they may coexist with humans. This has spurred interest in robotic arms with compliance in their joints using methods, such as impedance control. However, inherent compliance is safer, less computationally expensive, and does not require complex sensing solutions. In this article, the design of a sensorized hybrid hard-soft articulated joint using inflatable soft rubber bellows for actuation and simple angular potentiometers for angular position sensing is presented. The joint is hybrid in the sense that it combines rigid motion with soft actuation. This design allows for a simple yet effective solution that combines the precise sensing of rigid manipulators with the compliance of soft ones. Closed-loop control of this joint was implemented and it was capable of following a given trajectory while rejecting disturbances. This design was then extended to a two degrees of freedom (DOFs) joint with the path following capability, and then, integrated with a rigid base joint to form a 6-DOFs hybrid robotic manipulator for use in small retail businesses. This manipulator can handle payloads up to 1 kg over a large work area and was tested for pick-and-place operations. [ABSTRACT FROM AUTHOR]

Rendering a high impedance on a series elastic actuator (SEA) can improve motion control performance while retaining collision safety by a lower physical stiffness. The safe impedance range can be increased with high-gain velocity feedback control, but, in practice, this is limited by noise. This article applies the noise reduction disturbance observer to inner-loop velocity control of an SEA, attenuating noise and allowing higher safe rendered stiffness compared with standard torque and torque/velocity hierarchical control. Closed-form expressions for maximum passive stiffness and $Z$ -region are found, shown to depend strongly on the high-frequency noise gain, and used to optimize the control gains. Performance is experimentally verified on a reaction-force SEA, validating the passivity of the high-stiffness control in impact (free space and stiff environment) while rendering a safe stiffness 3.0 times the intrinsic stiffness. [ABSTRACT FROM AUTHOR]

This article proposes a novel interaction force estimation scheme capable of estimating the interaction forces for industrial robots with high precision in the absence of force sensors. Specifically, we first establish the dynamic model of a class of industrial robots via model identification methods. Furthermore, a high-order finite-time observer (HOFTO) is established to estimate the interaction forces, where HOFTO serves as an interaction force observer and is designed to accurately estimate external time-varying interaction forces. In addition, we strictly analyze the stability of our approach in two different cases, endowing HOFTO with reliable estimation abilities in broader applications. Subsequently, we integrate the estimated force information into the applications of force control frameworks (e.g., collision detection and impedance control). Several evaluations on a real 6-axes robot demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Depth, proximity, and tactile sensors are often used in robotic object manipulation. The three types of sensors are combined to address each sensor limitation: Depth use is limited by blind spots and measurable range, proximity sensor is limited to millimetric distances, and the tactile sensor requires direct contact with the object. Here we present a novel sensor that can simultaneously measure millimetric distance and surface tilt, as well as contact force. We demonstrated the advantage of this novel sensor during a contact task. There, we implemented an admittance control and succeeded in achieving smooth transition from zero- to infinite-impedance contact condition and vice-versa. This novel sensor affords the realization of advanced interactions between robot and objects and contrary to previous approaches—limited by distance, blind spots, and impact force during initial contact—demonstrated good performance at all times with smooth and continuous measurements during contact. [ABSTRACT FROM AUTHOR]

Most dc power distribution architectures are organized as a typical cascaded converter system that is composed of source and load converters, and may experience a system oscillation due to the negative impedance effect of constant power loads. The load-side virtual impedance method is a possible solution to such stability problem, which however makes the dynamic response of the load converter slow, and thus increases the requirement of the filter capacitance. In this article, an auxiliary circuit that is connected to the output port of the load converter is proposed to enhance the transient performance of the power converter system, while the system can be stabilized by the virtual impedance method. The control and parameter design of the proposed auxiliary circuit is introduced. The capacitance requirement of the system for an expected transient deviation of the output voltage is significantly reduced by using the proposed auxiliary circuit. An example design is given to verify the feasibility of the proposed method with simulation analysis and experimental results. [ABSTRACT FROM AUTHOR]

This article presents a computed-torque impedance control architecture for robotic-assisted mold polishing based on human skills.The mold polishing process for plastic injection is critical since tiny surface imperfections are easily visible in the plastic product, having strong implications in the overall quality. A Cartesian impedance control referred to the end-effector frame with posture optimization is proposed to tackle this problem. A collaborative robot manipulator from Franka Emika and a steel mold are used in the experiments to validate our approach. Human skills are captured both in comanipulation and free-hand modes, where position and force patterns are analyzed. Those patterns are then autonomously replicated by the robotic system, validating the proposed methodology. [ABSTRACT FROM AUTHOR]

Expanded use of power electronics opens the possibility that input characteristics presented to the conventional utility as well as ac and dc microgrids could be controlled in ways that help support utility operation. Power factor correcting interfaces are one wide-spread example, used around the world to meet regulatory requirements for both power factor and also harmonic content injected to the utility. More support for ensuring healthy utility operation might be demanded from these power electronic interfaces. For example, the potentially destabilizing effects of constant power loads (CPLs) could be remediated on time scales useful for supporting utility operation. This article presents an equivalent circuit model for limited-bandwidth CPLs that quantifies the intrinsic damping properties of regulated converters. A control architecture based on this equivalent circuit model is then analyzed. The control scheme limits the impact of CPL operation by emulating the internal damping of CPLs through programmable input behavior. An intermediate, internal energy buffer is used to support CPL operation while enabling the programmable or selectable input impedance on designed time scales. The effectiveness of this strategy on system stability is shown using a stability analysis and experimental results. [ABSTRACT FROM AUTHOR]

Global Positioning System (GPS) based control has recently been reported as a replacement of droop control to achieve fixed frequency operation in islanded microgrids. This article presents the perspective that the power sharing performance of a general microgrid synchronized by GPS is essentially determined by equivalent output impedance. A novel adaptive virtual impedance control approach, which implements different values of virtual resistance for d- and q-axis, is proposed accordingly. The virtual resistance concept is implemented comprising a basic local implementation for output impedance shaping, and a sparse resistance tuning network for the compensation of mismatched feeder impedance, which demands no knowledge of actual output impedance. The resistance tuning network utilizes the consensus protocol and only requires neighboring interactions among DG units. A complete tuning of output resistance for a given load condition results in accurate active and reactive power sharing even after communication is interrupted and will still outperform conventional droop control methods if load changes during the interruption. Small-signal analysis based on delay differential equations model of the overall microgrid is performed to investigate the adverse impact of communication delays on system stability. The efficacy of the proposed approach is validated by both simulation and experimentation. [ABSTRACT FROM AUTHOR]

The phase locked loop (PLL) will introduce the negative resistance near the fundamental frequency, which deteriorates the stability of doubly fed induction generators (DFIG) system under the inductive weak grid. This article develops the impedance model of the DFIG system based on direct power control without PLL, and analyzes the frequency coupling characteristic of established impedance model. The phase margin around the fundamental frequency is enhanced after cancelling the PLL. However, there will be a high-frequency resonance issue related to the frequency coupling characteristic. Based on the simplified high-frequency impedance model and the Bode diagram of equivalent single-input single-output impedance, it can be found that the DFIG system is more prone to have stability issues when the degree of frequency coupling is enhanced. An impedance reshaping control strategy is proposed to reduce the degree of frequency coupling at high frequency and suppress the high-frequency resonance. The experimental results validate the availability of the proposed reshaping control strategy. [ABSTRACT FROM AUTHOR]

In microgrids, intermittency of renewable energy sources (RES) and uncertain state-of-charge (SoC) of energy storage systems (ESS) can cause power deficiency to some distributed generation units (DGs). In this case, DGs with power deficiency may not meet the power demand, resulting in voltage collapse or frequency divergence. Unfortunately, this is seldom considered in inverter control design in existing literature. Thus, in-depth investigation into the microgrid performance under renewable energy resource fluctuations and appropriate control methods are urgently needed. In this article, an adaptive droop and adaptive virtual impedance control strategy is proposed. Unlike conventional droop control where the droop coefficients are fixed by assuming the DGs can always meet the load demand, the droop coefficients here are adjusted according to actual solar PV power output. In this way, proper power sharing among DGs can be achieved under renewable energy variation. Furthermore, the impact of varying DG capacities on system stability is mathematically investigated. An adaptive virtual impedance is then incorporated into the adaptive droop method to deal with the system instability caused by renewable energy variations. The proposed strategy is analyzed theoretically and validated in MATLAB/Simulink simulation and laboratory experiments. The results demonstrate the advantages of the proposed method over conventional approaches under various scenarios. [ABSTRACT FROM AUTHOR]

This article presents a novel control scheme that primarily provides a solution to the problem of simultaneously performing stable trajectory tracking and variable impedance control (VIC) of flexible-joint robots. The existing asymptotically stabilizing tracking controllers for compliant robots rely upon several model-based terms and their derivatives. Contrarily, the proposed minimally model-based trajectory tracking control scheme requires knowledge of only the gravitational torque vector model. Although interaction and tracking performance enhancements are achievable through VIC, its operation introduces energy injections, thereby impacting closed-loop stability/passivity. Existing VIC passivity preservation techniques deal exclusively with rigid-joint robots. To this end, the proposed method introduces the minimally model-based trajectory tracking variable impedance control scheme, which ensures passivity/stability when applying VIC to flexible-joint robots. Numerical and experimental results corroborate the controllers’ efficacy in terms of realizing VIC interaction and tracking tasks, thus enabling flexible-joint robots to stably replicate biologically inspired behaviors. [ABSTRACT FROM AUTHOR]

In a practical islanded microgrid system, the mismatched line impedance causes serious problems, such as inaccurate power sharing and circulating current. In order to overcome these issues, this article proposes a coordinated virtual impedance control strategy for distributed generation (DGs) units. In the proposed control strategy, both virtual resistance and virtual inductance are simultaneously tuned to compensate the mismatched line impedance among DGs, and the microgrid system stability is enhanced by increasing damping for the whole system. Also, the virtual impedance is adaptively regulated in order to guarantee a good power-sharing performance even if the communication is interrupted or the load changes suddenly. The controller design process and theoretical analysis of the whole system are presented in detail based on small-signal analysis. The feasibility and effectiveness of the proposed control strategy were validated by simulations and experiments with a 1.3-kVA prototype microgrid composed of three DGs. [ABSTRACT FROM AUTHOR]

This article offers a new insight to the achievable stiffness limitation of the impedance control of series elastic actuators (SEAs) by suggesting a comparative analysis of energy ports and passivity control framework at the energy port to enhance the maximum achievable rendered stiffness of SEAs. To this end, it is explored that SEAs have two different port definitions to assess energy and passivity of the system—spring port and load port; and conservatisms for passivity evaluation of two ports are investigated and compared utilizing frequency characteristics. The results reveal that the load port passivity exhibits less conservatism, which allows to render larger achievable stiffness in impedance-controlled SEAs. Moreover, key parameters that determine the passivity characteristic of the SEA impedance control are discovered based on the load port passivity analysis. A novel passivity control framework that incorporates the time-domain passivity observer and the passivity controller is designed utilizing the load port energy monitoring, which offers a less conservative assessment of the systems passivity. Throughout this novel port passivity analysis and the passivity control, it can be achieved to render maximum rendered stiffness of the SEA impedance control much higher than the limitation that has been perceived as the maximum value. [ABSTRACT FROM AUTHOR]

The second harmonic current compensator (SHCC) can be added into the single-phase converters for compensating the second harmonic current (SHC) and thus removing the undesired electrolytic capacitor. In this paper, a virtual impedance, which is in the form of a capacitor and a resistor connected in parallel, is introduced to be in parallel at the bus-side port of the SHCC for effectively compensating the SHC while guaranteeing the system stability. This virtual parallel impedance is realized by feeding forward the bus-side port voltage of the SHCC, and thus the SHCC can be modular designed. The closed-loop parameter design of the SHCC is also presented. A 3.3-kW prototype of a two-stage single-phase power factor correction converter is built and tested in the laboratory, and the experimental results verify the effectiveness and feasibility of the proposed control method. [ABSTRACT FROM AUTHOR]

For a conventional islanded microgrid, each distributed generation (DG) unit usually equips with an LC filter and a closed-loop filter capacitor voltage control is applied to ensure an accurate tracking of DG voltage. In this article, a cost reduction microgrid and DG units configuration is developed via the following two steps. First, an L-type filter is adopted by each DG unit and a centralized shunt capacitor is placed at point of common coupling (PCC). Then, the inherent synchronization feature of droop control is further exploited as an inherent synchronizer to identify the carrier angle relationships between parallel inverters, but without any communications between them. With the carrier information obtained, a decentralized interleaving pulsewidth modulation (PWM) among parallel inverter can be easily achieved via a slow proportional integral (PI) regulation of each DG unit relative carrier. Accordingly, the high-frequency switching harmonics of PCC voltage are suppressed even with simplified and size reduced DG output chokes. Second, a virtual impedance control is applied to DG units to cancel the voltage drops on the DG feeder in a direct PWM manner. Accordingly, mitigating the low-order harmonic PCC voltages is obtained without using any closed-loop voltage tracking at DG unit local processor. Comparative experimental results have been provided to validate the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

Motion control is a fundamental technique used in automated mechanical systems. Classically, velocity, force, and impedance are controlled in motion control systems, but simultaneous control is difficult. This article proposes periodic/aperiodic (P/A) motion control based on periodicity and aperiodicity of motion. The P/A motion control separately applies different control methods to P/A motions using P/A velocity and P/A force, which are extracted using a periodic/aperiodic separation filter (PASF) from velocity and force. Accordingly, six types of P/A motion controls are constructed in this article, which correspond to different combinations of the P/A velocity, P/A force, and P/A impedance controls. To construct the P/A motion control systems, acceleration control based on a disturbance observer is used. The ACS, which rejects disturbances, enables the P/A motion control design to ignore disturbances. The experiments were conducted to validate the six P/A motion controls, which simultaneously realized two P/A motion control objectives. [ABSTRACT FROM AUTHOR]

It is expected that bilateral control will find broad applications such as mobile robot exploration and operations in extreme environments. However, the transmission of vivid haptic sensation requires a rapid data-transfer rate of haptic data due to the short sampling period and high-frequency communication. The objective of this paper is aimed at the traffic reduction of haptic data by exploiting an impedance field equivalence of a standard bilateral control. The proposed approach theoretically clarifies the equivalence of the standard bilateral control and impedance control in terms of haptic transmission. Based on this equivalence, the same performance of a standard bilateral control can be achieved by only communicating the force information that determines the equilibrium position of the impedance control. The validity of the proposed approach is confirmed through both simulations and experiments. The obtained results show that the proposed approach successfully reduced the amount of communicated haptic data without any disadvantages compared with the standard acceleration-based bilateral system. This paper shows novel interpretation that haptic transmission is realized by controlling the equilibrium force, which contributes reducing data traffic in haptic transmission and understanding motion generation. [ABSTRACT FROM AUTHOR]

Recently, a series elastic actuator (SEA) has emerged as a potential actuator system for various robotic applications where safe and precise interactive force control is required. Even though lots of research has been conducted on the mechanical/controller design and the development of applications for SEAs, the accurate force observation issue has not been highlighted much. Only the simple law, that is, the spring in an SEA can measure interactive force has been repeatedly mentioned and utilized. However, this is not true when the load-side dynamics affects the spring deformation significantly. This paper tackles this problem by demonstrating the imprecise force observation of the spring deformation and proposing two types of external force observers to address the problem. A reaction force-sensing SEA (RFSEA) is adopted in this paper, and its dynamic characteristic is analyzed in detail using the Lagrangian mechanics. Based on the analyzed dynamics, force observers are designed and verified through simulations and experiments. An XY stage driven by RFSEAs is developed so that the stage can be force controlled, and the proposed force observers are applied to this. Human interactive forces on the developed XY stage, the impedance of which is controlled in several ways, are estimated and compared with a force plate. Various experimental results validate the performance and potential of the proposed force observer for SEA systems. [ABSTRACT FROM PUBLISHER]

Series elastic actuators (SEAs) have become prevalent in torque-controlled robots in recent years to achieve compliant interactions with environments and humans. However, designing optimal impedance controllers and characterizing impedance performance for SEAs with time delays and filtering are still underexplored problems. This article addresses the controller design problem by devising a critically damped gain design method for a class of SEA cascaded control architectures, which is composed of outer impedance and inner torque feedback loops. We indicate that the proposed gain design criterion solves optimal controller gains by maximizing phase-margin-based stability. Meanwhile, we observe a tradeoff between impedance and torque controller gains and analyze their interdependence in terms of closed-loop stability and overall impedance performance. Via the proposed controller design criterion, we adopt frequency-domain methods to thoroughly analyze the effects of time delays, filtering, and load inertia on SEA impedance performance. A novel impedance performance metric, defined as “Z-region,” is proposed to simultaneously quantify achievable impedance magnitude range (i.e., Z-width) and frequency range (i.e., Z-depth). Maximizing the Z-region enables SEA-equipped robots to achieve a wide variety of Cartesian impedance tasks without alternating the control structure. Simulations and experimental implementations are performed to validate the proposed method and performance metric. [ABSTRACT FROM PUBLISHER]

In this paper, a new type of fault current limiter (FCL) called impedance-based fault current limiter (IBFCL) is proposed. The structure has the capability to limit the fault current to an extent predefined value by means of the system impedance. The control circuit uses voltage and current data to calculate the impedance value. In order to limit the fault current, several eliminating resistors, which are in parallel with auxiliary switches, are bypassed by a main semiconductor switch. In addition, the proposed FCL has a fast response time with good current-limiting characteristic. The analysis of normal and fault conditions of the IBFCL circuit using power system computer-aided design/electromagnetic transients, including dc (PSCAD/EMTDC) software have been presented. Moreover, an experimental prototype is provided to validate simulation results and the feasibility of structure to be implemented in power systems. [ABSTRACT FROM PUBLISHER]

Impedance networks have been already investigated in various literature with the main goals of increasing the attainable voltage gain and reducing the components number. Recently, coupled inductors found popularity because they let converters with lower weight and cost. It seems that coupled inductances are a proper answer to the increasing voltage gain while keeping down the components number. This paper proposes a novel impedance network circuit based on three coupled inductors with a Δ connection. The proposed Δ-source converter offers smaller magnetizing current and winding losses compared to the successful Y-source circuit. Moreover, with the Δ-connected three coupled inductors, the adverse effect of leakage inductance on the converter performance is significantly reduced. The effectiveness of the proposed structure is analytically proved. The theoretical achievements over the conventional Y-source structure are confirmed through extensive simulations and experiments. [ABSTRACT FROM PUBLISHER]

The potential of inkjet printing technology (IjP) for the fabrication of coils for biomedical applications in inductively coupled power transfer systems is studied in terms of needed compensations, bifurcation phenomena, and power transfer efficiency. The effect of using coils manufactured with IjP in the secondary side has been analyzed by studying the effect of the increase in internal resistance. The present study makes it possible to select the best topology depending on the load impedance, the coupling coefficient, and coil design. In terms of the compensations needed at the primary side, IjP does not significantly affect the behavior of the system; however, the series–series topology is preferable since the compensating capacitance is independent from the internal resistance. In terms of bifurcation, a more restricted condition is obtained for parallel compensated secondary circuits. There is a decrease on the power transfer efficiency due to the increase of the internal resistance introduced by IjP. However, it is important to select the best topology according to the application since the decrease could be from 63% to only 6%. It is concluded that IjP is a promising fabrication technique for coils for biomedical applications. [ABSTRACT FROM PUBLISHER]

As more widebandgap (WBG) devices are becoming commercially available, it is beneficial to use WBG device to increase the switching frequency in order to reduce the passive components. For quasi-Z-Source (qZS) grid-tied inverters, the reduction of passive components raises stability concerns as the coupling effect between the dc side and ac side of qZS inverter will increase. In this paper, the coupling effect between qZS impedance network and the output filter is analyzed by modeling both dc and ac sides. Analysis reveals the resonant characteristic of the qZS inverter. Controller parameter boundaries are derived, and a design method to improve stability is then proposed. Case studies for a 2.5-kW 10-kHz Si-based qZS inverter and a 1-kW 100-kHz GaN-based qZS inverter are presented. Circuit simulations and experimental verifications results are provided to assess analysis and the control design. [ABSTRACT FROM PUBLISHER]

This paper presents human–robot cooperation with adaptive behavior of the robot, which helps the human operator to perform the cooperative task and optimizes its performance. A novel adaptive impedance control is proposed for the robotic manipulator, whose end-effector's motions are constrained by human arm motion limits. In order to minimized motion tracking errors and acquire an optimal impedance mode of human arms, the linear quadratic regulation (LQR) is formulated; then, integral reinforcement learning (IRL) has been proposed to solve the given LQR with little information of the human arm model. Considering human–robot interaction force during the robot performing manipulation, a novel barrier-Lyapunov-function-based adaptive impedance control incorporating adaptive parameter learning is developed for physical limits, transient perturbations, and time-varying dynamics. Experimental results validate that the proposed controller is effective in assisting the operator to perform the human–robot cooperative task. [ABSTRACT FROM PUBLISHER]

This paper presents adaptive impedance control of an upper limb robotic exoskeleton using biological signals. First, we develop a reference musculoskeletal model of the human upper limb and experimentally calibrate the model to match the operator’s motion behavior. Then, the proposed novel impedance algorithm transfers stiffness from human operator through the surface electromyography (sEMG) signals, being utilized to design the optimal reference impedance model. Considering the unknown deadzone effects in the robot joints and the absence of the precise knowledge of the robot’s dynamics, an adaptive neural network control incorporating with a high-gain observer is developed to approximate the deadzone effect and robot’s dynamics and drive the robot tracking desired trajectories without velocity measurements. In order to verify the robustness of the proposed approach, the actual implementation has been performed using a real robotic exoskeleton and a human operator. [ABSTRACT FROM AUTHOR]

This paper proposes a new prognostic method for the health state of proton exchange membrane (PEM) fuel cells. The method is designed to predict the state-of-health (SOH) of PEMs and provide root cause analysis of the predicted health degradation. In this method, an equivalent circuit model (ECM) is built to emulate the impedance spectrum of PEM fuel cells. Because the key degradation parameters in the ECM cannot be measured in situ, this method instead estimates the parameters indirectly using the output voltage. The estimation is based on the linear relationship between the key ECM parameters and the output voltage. Using the constructed ECM and the estimated parameters, an impedance spectrum at the current moment is produced. The historical voltage evolution is then extrapolated using linear and exponential models that represent the irreversible and reversible phenomena, respectively. The models are used to predict future ECM parameters and, eventually, the impedance spectrum at any moment in the future. Through these steps, the proposed method provides an online estimation of the current SOH and predicts the level of future degradation. The primary novel feature of the proposed method is its ability to diagnose the root causes of potential degradation using data from nondisruptive online monitoring. [ABSTRACT FROM AUTHOR]

In this paper, a walking robot that consists of its body and six legs is considered for trajectory control. The leg mechanism is a 1-UP&2-UPS (U-universal joint, P-prismatic joint, and S-spherical joint) parallel mechanism with the arm–leg conversion function. Due to the nonlinearity and uncertainties of the robot, it is difficult to obtain its exact dynamic model, therefore this paper utilizes the decentralized control strategy to design a dynamic hybrid controller, in which the outer loop is an impedance controller, the middle and inner loops are designed by using the sliding-mode control and the nonlinear proportional–integral–derivative (PID) control, respectively. The inverse kinematics of one leg is derived for mathematic model, based on which three control methods including the classical PID control, position–velocity loops cascade control, and dynamic hybrid control are examined. Contrast tracking experiments were performed with one robotic leg. The experimental results are shown to identify their differences and prove the effectiveness and applicability of the proposed dynamic hybrid control. [ABSTRACT FROM PUBLISHER]

Induction motors are the most common driving force for active mechanical components such as pumps and valves. When these motors are used in safety systems, which are normally on standby, monitoring the motors through nondestructive evaluation techniques is needed to guarantee their operability and system integrity. In this context, this study suggests a method for motor health monitoring at standstill through impedance analysis. Impedance change tendencies for various motor failure cases are investigated. To identify the tendencies, first an equivalent circuit (EC) of an induction motor is developed, and probable impedance changes for each failure case are estimated based on the developed EC. Then, the estimates are experimentally verified by reproducing the failures and measuring the impedance with an impedance analyzer. As the equivalence between the estimates and the experimental results is confirmed, unique tendencies in impedance changes for each failure case are characterized. The suggested method is expected to be useful in areas with access restriction, such as nuclear power plants (NPPs), as motor impedance can be measured from a distance without any disruption on-site. [ABSTRACT FROM PUBLISHER]

Robot-aided physical therapy should encourage subject’s voluntary participation to achieve rapid motor function recovery. In order to enhance subject’s cooperation during training sessions, the robot should allow deviation in the prescribed path depending on the subject’s modified limb motions subsequent to the disability. In the present work, an interactive training paradigm based on the impedance control was developed for a lightweight intrinsically compliant parallel ankle rehabilitation robot. The parallel ankle robot is powered by pneumatic muscle actuators (PMAs). The proposed training paradigm allows the patients to modify the robot imposed motions according to their own level of disability. The parallel robot was operated in four training modes namely position control, zero-impedance control, nonzero-impedance control with high compliance, and nonzero-impedance control with low compliance to evaluate the performance of proposed control scheme. The impedance control scheme was evaluated on 10 neurologically intact subjects. The experimental results show that an increase in robotic compliance encouraged subjects to participate more actively in the training process. This work advances the current state of the art in the compliant actuation of parallel ankle rehabilitation robots in the context of interactive training. [ABSTRACT FROM PUBLISHER]

High-speed simultaneous position and contact force control is vital for microgrippers dedicated to automated high-throughput microassembly tasks. This paper presents the design and validation of a new robust impedance control for high-speed position and force regulation of a compliant gripper which is driven by a piezoelectric stack actuator. Unlike traditional position/force switching control, the proposed scheme enables a smooth transition between the free and constrained motions of the gripper. A single controller is devised to regulate both the gripper tip position and the contact force, rendering a simplification in control design. The stability of the closed-loop system in the presence of piezoelectric nonlinearities is guaranteed by a digital sliding mode impedance control algorithm. Moreover, the proposed control scheme is applicable to high-order systems, and the control algorithm simplification enables a high-speed loop rate. The high-speed control is realized by deploying the algorithm to a field-programmable gate array controller. The effectiveness of the proposed scheme is verified through experimental studies on grasp–hold–release operation of a graphite microrod. The control scheme can be extended to other microassembly systems as well. [ABSTRACT FROM PUBLISHER]

In the paper by Jin et al. (IEEE Trans. Ind. Electron., vol. 62, no. 3, pp. 1363–1371, Mar. 2015), the authors present an expansion of Ohm's law in the nonsinusoidal ac circuit, based on which a distinct instruction for analyzing and designing nonsinusoidal circuit including ac/dc link circuit is provided. This paper is very interesting and a good work. However, some equations and expressions may be improved. In this paper, they are discussed and modified. [ABSTRACT FROM AUTHOR]

This paper presents a method of implementing impedance control (with inertia, damping, and stiffness terms) on a dual-arm system by using the relative Jacobian technique. The proposed method significantly simplifies the control implementation because the dual arm is treated as a single manipulator, whose end-effector motion is defined by the relative motion between the two end effectors. As a result, task description becomes simpler and more intuitive when specifying the desired impedance and the desired trajectories. This is the basis for the relative impedance control. In addition, the use of time-delay estimation enhances ease of implementation of our proposed method to a physical system, which would have been otherwise a very tedious and complicated process. [ABSTRACT FROM PUBLISHER]

Due to the recent trend of the application of robots and other mechatronic devices to human life support, the force control draws more attention than ever. However, to use force sensors in all the required cases makes the system not only expensive but also bulky. Force-sensor-less power-assist control (FSPAC) which uses only encoders to obtain the external force information and provides force control performance can address this problem. Due to its simplicity and wide application, FSPAC is an essential technology to control a motor in a human-friendly way, but the design of FSPAC is different from the conventional feedback controllers so a new design methodology needs to be established. In order to attack this problem, this paper generalizes and analyzes the structure and characteristics of FSPAC. The generalized structure reveals how FSPAC can achieve the assistance, and the transfer function analysis based on the structure addresses the robustness and assistance performance evaluation problems. The robustness of FSPAC is analyzed in terms of the gain margin and robust stability, and the limitation of the power assistance to guarantee the robust stability is derived. Then, the evaluation way of feedback control design in FSPAC is provided. All the discussion in this paper provides the readers with understanding and appropriate design methodology of FSPAC. [ABSTRACT FROM PUBLISHER]

Battery emulation with a controllable high-power dc supply enables repeatable hardware-in-the-loop testing of powertrains for hybrid and electric vehicles. For this purpose, not only the power flow but also the nonlinear characteristic and dynamic impedance of batteries need to be emulated. In this paper, nonlinear local model networks are used to obtain dynamic battery models with high fidelity that can be computed in real time. This approach also allows the extraction of local linear impedance models for high-bandwidth impedance emulation, leading to a tighter coupling between the test bed and simulation model with predictable closed-loop dynamics. A model predictive controller that achieves optimal control with adherence to system constraints is extended to impedance control and robustness against constant power loads. This results not only in superior dynamic performance but also in stable dc-bus voltage control even for testing of tightly controlled electric motor inverters with negative differential input resistance. Since the controller design is based on a model of the test bed setup including the virtual battery model, emulator hardware, and input characteristics of the powertrain under test, it is possible to systematically analyze stability. [ABSTRACT FROM AUTHOR]

This paper aims at analyzing the effect of widely known impedance-shaping (IS) control method on the perception of soft tissues in telemicrosurgical applications. The generalized teleoperation control architecture has been modified to include the IS term. New performance index has been defined based on the two proposed indices for the detection and the discrimination of the soft environments to analyze the effect of this modified control on the kinesthetic perception of soft tissues. The effect is then theoretically analyzed on the conventional position–position, force–position, and four-channel control architectures based on the newly defined index. The effectiveness of this newly proposed kinesthetic perception index is also verified using psychophysics experiments. The theoretical analysis of the effects of the IS method on the perception of soft tissues is then validated using the proposed index by experiments with phantom soft tissues for conventional teleoperation architectures. [ABSTRACT FROM PUBLISHER]

The motion of a wheelchair is different from any other vehicle. It needs control in three dimensions: the longitudinal, the lateral, and the pitch direction. This paper takes this point into consideration and provides a 3-D control of a wheelchair. We focus on the control of a push-rim power-assist wheelchair. The main objective of this paper is to provide a wheelchair rider with 3-D assistance that guarantees power and safety. To this end, three kinds of assist controls are proposed: the disturbance attenuation controls are designed for the longitudinal and lateral directions while the tip-over prevention control is designed for the pitch direction. These controls for three directions can be integrated by appropriately taking advantage of system configuration. We demonstrate that all these controls can work independently for each purpose and we show experimental results. [ABSTRACT FROM AUTHOR]

A simple robust compliant-motion-control technique is presented for a robot manipulator with nonlinear friction. The control technique incorporates both time-delay-estimation technique and ideal velocity feedback; the former is used to cancel out soft nonlinearities, and the latter serves to reduce the effect of hard nonlinearities, including Coulomb friction and stiction. The proposed controller has a simple structure and yet provides good online friction compensation without modeling friction. The robustness of the proposed method has been confirmed through comparisons with other controllers in 2-DOF SCARA-type industrial robot experiments. [ABSTRACT FROM AUTHOR]