Impedance Control Method Research Articles

Programming by Demonstration Using the Teleimpedance Control Scheme: Verification by an sEMG-Controlled Ball-Trapping Robot

Impedance control allows robots to manipulate physical interactions delicately. However, issues associated with path planning and impedance remain unresolved. Herein we propose a path and impedance planning method for impedance control in a robot based on programming by demonstration through telemanipulation using a surface electromyogram. We considered a task that requires quick and precise adjustment of path and impedance, that is, ball trapping. We implemented a teleoperated robot that can deliver an operator's impedance as well as position during the ball trapping task to the slave side. The operators were asked to perform demonstrations of ball-trapping tasks using the implemented teleoperated robot, where the slave side is a vertical robot with one degree of freedom. The path and impedance were recorded and programmed as control input profiles from a set of successful demonstrations using Gaussian mixture regression. The result showed that using the human demonstration, the robot could learn how to catch a dropped ball without rebounding.

Impedance Control Method with Capability of Singular Configuration Avoidance

Impedance Control Method with Capability of Singular Configuration Avoidance

Development and Hybrid Control of an Electrically Actuated Lower Limb Exoskeleton for Motion Assistance

This paper describes a system design and hybrid control algorithm of an electrically actuated lower limb exoskeleton (LLE). The system design mainly includes three parts: mechanical structure design, actuation system design and sensor system design. According to the initial state of the joint angle, LLE can be divided into Non-anthropomorphic state (NAS) and anthropomorphic state (AS). The human motion intention (HMI) estimation can be divided into gait phase classification and reference trajectory estimation. The fuzzy logic is used to detect different phases in the gait phase classification. In the reference trajectory estimation, the kinematic model of the LLE is utilized to obtain a continuous joint trajectory, which is used as input of the control law. To make the LLE accurately follow the movement of people and remain stable, a hybrid dual-mode control strategy is proposed in this paper, i.e., the adaptive impedance control (AIC) method is used to improve the stability and resistance to shock in stance phase, and the active disturbance rejection control with the fast terminal sliding mode control (ADRC-FTSMC) method is employed to improve the response speed and the tracking precision in swing phase. Furthermore, in order to solve the torque discontinuity in the switching process, a smoothing method is proposed during the transition. Finally, the prototype experiments were set up to verify the tracking performance and power-assisted effect of the proposed exoskeleton. The experiments results show the LLE can achieve excellent tracking performance and power-assisted effect based on the proposed HMI methodology and hybrid dual-mode control strategy.

Impedance Control Method for Avoiding Singularities and Preserving Manipulation Performance

Impedance Control Method for Avoiding Singularities and Preserving Manipulation Performance

High-Accuracy Force Control With Nonlinear Feedforward Compensation for a Hydraulic Drive Unit

The hydraulic drive unit (HDU) applied in a hydraulic drive legged robot joints adopts an outer loop impedance control method based on the hydraulic control's inner loop during motion so that the entire system obtains certain compliance characteristics. There are two types of inner loops, namely, the position closed loop control and the force closed loop control; this paper studies a system in which the inner loop is a force closed loop control system and improves its accuracy in order to provide references for the outer loop impedance control. Therefore, designing a compensation method that improves the inner loop force control's accuracy has important research significance. In view of the above research significance, this paper first deduces a mathematical model for the force closed loop control and simplifies the sixth-order mathematical model to find the transfer function of each part. Second, combined with influence factors, such as pressure-flow nonlinearity, friction nonlinearity, and complex and variable loads on the system, the feedforward compensation controller of the force control input is derived. Considering the practical application in engineering, the controller order is reduced, and a partial compensation is achieved. Finally, on the HDU performance test platform, the force control performance is quantitatively analyzed by inputting typical signals and random signals. The experimental results show that the feedforward compensation controller can greatly improve the system force control performance with different input signals. The above research results can be combined with a corresponding disturbance rejection strategy for the force control system to provide an important reference and experimental basis for the hydraulic inner loop control strategy of force-based impedance.

Disturbance Observer‐Based Robot End Constant Contact Force‐Tracking Control

A disturbance observer‐based hybrid sliding mode impedance control method is proposed in this paper, which is able to achieve robot end constant contact force‐tracking control without force/torque sensors. The method requires only the values of joint torque, joint angle, and joint angular velocity, which are converted by robot servo motor signals, to implement the control. The control scheme consists of two parts: one is a disturbance observer and the other is a hybrid sliding mode impedance controller. The disturbance observer, which takes robot internal signals mentioned above as the inputs to estimate the robot end contact force, is designed based on generalized momentum, thus improving the estimation accuracy. The hybrid sliding mode impedance controller, which uses the values estimated by the disturbance observer and the robot internal signals as the inputs to calculate the corresponding position adjustment, integrates both the impedance control and sliding mode control, thus improving the force‐tracking performance and robustness. Experimental results show that the proposed disturbance observer‐based hybrid sliding mode impedance control method possesses high control precision.

Speed control of 3-phase induction motor using rotor impedance

This paper introduces a method of rotor impedance control by using controlled capacitor/inductor connected in series in the rotor circuit. The proposed system can be operate as a speed control system or a soft starting system to limit the starting current of the motor. An approach to a three MOSFETs-based a rotor circuit of induction motor is presented. The experimental realization of the drive posed some serious problems, notably with regard to the successful driving pulses of MOSFETs over a wide speed range. Analysis of the secondary MOSFETs-controlled motor raised many interesting challenges. It was decided to use an equivalent circuit approach to the analysis because the primary voltage and current were both largely sinusoidal. The equivalent circuit used is based on the single-phase equivalent of a balanced sinusoidal three-phase system. The effective value of the external rotor inductance or capacitance is calculated. Moreover, the motor performance specially speed, motor starting current, and torque during the control of duty cycle of MOSFETs are studied. Also, MOSFETs performance has been studied. Closed loop speed control of the motor speed have been investigated using hysteresis control based on controlling MOSFETs duty cycle. Experimental results have been carried out in the Laboratory to verifying the analysis. The simulation results are given and proved to yield good agreement when compared with the experimental results.

Time-Varying Impedance Control of Port Hamiltonian System with a New Energy-Storing Tank

In order to guarantee the passivity of a kind of conservative system, the port Hamiltonian framework combined with a new energy tank is proposed in this paper. A time-varying impedance controller is designed based on this new framework. The time-varying impedance control method is an extension of conventional impedance control and overcomes the singularity problem that existed in the traditional form of energy tank. The validity of the controller designed in this paper is shown by numerical examples. The simulation results show that the proposed controller can not only eliminate the singularity problem but can also improve the control performance.

An improved force-based impedance control method for the HDU of legged robots

Hydraulic drive mode enables legged robots to have excellent characteristics, such as greater power-to-weight ratios, higher load capacities, and faster response speeds than other robots. Nowadays, highly integrated valve-controlled cylinder, called hydraulic drive unit (HDU), is employed to drive the joints of these robots. However, various robot control issues exist. For example, during the walking process of legged robots, different obstacles are encountered, making it difficult to control such robots because the load characteristics of the ends of their feet change with the environment. Furthermore, although the adoption of HDU has resulted in high-performance robot control, the hydraulic systems of these robots still have problems, such as strong nonlinearity, and time-varying parameters. Consequently, robot control is very difficult and complex. This paper proposes an improved second-order dynamic compliance control system, impedance control, for HDU. The control system is designed to rectify the issues affecting the impedance control accuracy of the dynamic compliance serial–parallel composition between the HDU force control inner loop and the impedance control outer loop. Specifically, it consists of a compliance-enhanced controller and a feedforward compensation controller for the force control inner loop. Furthermore, the dynamic compliance composition of the inner and outer HDU control loops is rearranged. The results of experiments conducted indicate that the proposed method significantly improves the control accuracy compared to that of traditional force-based impedance control.

Stabilization of a Cascaded AC–DC–DC System With Small Bus Capacitance Based on Small Signal Analysis

In this paper, the source converter (voltage source rectifier) of a cascaded system is modeled based on small signal analysis, and the closed-loop transfer functions of the converter are obtained. Then, the output impedance of the source converter is calculated. The analysis of the output impedance indicates that the smaller the bus capacitance, the worse the stability of the cascaded system, and the smaller the power converted by the cascaded system. Three kinds of virtual impedance control methods are proposed to reduce the output impedance of the source converter; all of them improve the stability and increase the maximum power of the cascaded system. The parameters of the three kinds of methods are designed and their effects are compared. Simulation and experimental results show a good agreement with the theoretical analysis, which proves the validity of the virtual impedance control methods.

Resonance Propagation and Elimination in Integrated and Islanded Microgrids

In this paper, a micro grid resonance propagation model is investigated. To actively mitigate the resonance using DG units, an enhanced DG unit control scheme that uses the concept of virtual impedance is proposed. It can be seen that a conventional voltage-controlled DG unit with an LC filter has a short-circuit feature at the chosen harmonic frequencies, whereas a current-controlled DG unit presents an open-circuit characteristic. The application of underground cables and shunt capacitor banks may introduce power distribution system resonances. This paper additionally focuses on developing a voltage-controlled DG unit-based active harmonic damping technique for grid-connected and islanding micro grid systems. An improved virtual impedance control method with a virtual damping resistor and a nonlinear virtual capacitor is proposed. The nonlinear virtual capacitor is used to compensate the harmonic dip on the grid-side inductor of a DG unit LCL filter. The virtual resistance is principally answerable for micro grid resonance damping. The effectiveness of the proposed damping method is examined using each a single DG unit and multiple parallel DG units<strong>.</strong>

Efficient Force Control Learning System for Industrial Robots Based on Variable Impedance Control

Learning variable impedance control is a powerful method to improve the performance of force control. However, current methods typically require too many interactions to achieve good performance. Data-inefficiency has limited these methods to learn force-sensitive tasks in real systems. In order to improve the sampling efficiency and decrease the required interactions during the learning process, this paper develops a data-efficient learning variable impedance control method that enables the industrial robots automatically learn to control the contact force in the unstructured environment. To this end, a Gaussian process model is learned as a faithful proxy of the system, which is then used to predict long-term state evolution for internal simulation, allowing for efficient strategy updates. The effects of model bias are reduced effectively by incorporating model uncertainty into long-term planning. Then the impedance profiles are regulated online according to the learned humanlike impedance strategy. In this way, the flexibility and adaptivity of the system could be enhanced. Both simulated and experimental tests have been performed on an industrial manipulator to verify the performance of the proposed method.

An innovative robotic training system imitating the cervical spine behaviors during rotation–traction manipulation

The present study demonstrated an innovative humanoid robot applied during Rotation–Traction (RT) manipulation practice and evaluation process. A mass–damper–spring mechanical system with an electromagnetic clutch was designed to emulate the cervical spine and a 3-DOF non-planar model was built to replace the neck part. With the help of an excellent electromechanical system and appropriate control strategy, the robot could imitate the entire dynamic responses of the human cervical spine during the RT manipulation process. Moreover, a novel adaptive force tracking impedance control was adopted to ensure a variable contact force in the unknown environment to imitate the real biomechanics of the human neck. In comparison to existing impedance control methods, the proposed control scheme is not only utility but also robust against external disturbances such as varying stiffness or uncertainties of the robot. The stability of the proposed impedance control was theoretically examined. Test results revealed that the cervical spine robot could faithfully replicate the biomechanical properties of the human cervical spine during RT manipulation and it is helpful in training and evaluating interns.

A Novel Position-Based Impedance Control Method for Bionic Legged Robots’ HDU

In this paper, a novel impedance control method is designed for the hydraulic drive unit (HDU) equipped on the hydraulic driven legged robot. First, the position control mathematical model of HDU is built. Next, the HDU performance test platform is introduced and the performance requirements are given. Second, the system load pressure observer is designed and an accurate stiffness control method is proposed by combining the external load force feed forward control with the load force feedback control. Third, the damping control method based on the velocity feedback is designed. Finally, a novel impedance control method is proposed through combining the stiffness control and the damping control. The effects of all techniques listed above have been verified experimentally. Especially, the impedance control effects are verified by comparing the impedance characteristics with the natural characteristics of the second-order mass-spring-damper system. This research indicates that the system characteristics of HDU with the novel impedance control method is well equivalent to that of the second-order mass-spring-damper system, which can solve the stall problem of the robot joint movement process caused by the load force vanish instantaneously and the switch between position control and force control. The above research can provide references for the active compliance control of bionic legged robots.

Design and control of a diagnosis and treatment aimed robotic platform for wrist and forearm rehabilitation: DIAGNOBOT

Therapeutic exercises play an important role in physical therapy and rehabilitation. The use of robots has been increasing day by day in the practice of therapeutic exercises. This study aims to design and control a novel robotic platform named DIAGNOBOT for diagnosis and treatment (therapeutic exercise). It has three 1-degree-of-freedom robotic manipulators and a single grasping force measurement unit. It is able to perform flexion–extension and ulnar–radial deviation movements for the wrist and pronation–supination movement for the forearm. The platform has a modular and compact structure and is capable of treating two patients concurrently. In order to control the DIAGNOBOT, an impedance control–based controller was developed for force control, which was required for the exercises, as well as a proportional–integral–derivative controller for position control. To model the resistive exercise, an angle-dependent impedance control method different from traditional methods has been proposed. Experiments were made on five healthy subjects and it has been demonstrated that the proposed robotic platform and its controller can perform therapeutic exercises.

A Nonlinear Model-Based Variable Input Control for the Position Control System of HDU

During the motion process of a hydraulic drive legged robot, the hydraulic drive unit (HDU) of each robotic joint generally adopts an outer loop impedance control method based on a hydraulic control inner loop. The hydraulic control inner loop can be generally divided into two parts: position closed-loop control and force closed-loop control. When the position closed-loop control is adopted, the performance of the position control directly determines the performance of the outer loop impedance control. Therefore, it is important to design a high-accuracy position control method aimed at HDU. Based on the ideas above, this paper introduces the position control system. Through an HDU performance test platform, the deficiency of the position control system is studied, using different typical input signals and gait trotting input signals. Then, a model-based variable input controller (MVIC) is proposed and designed. This controller contains the hydraulic system’s built-in nonlinear and load characteristics. Finally, the control performance of the MVIC is proved experimentally under different working conditions. The experiment’s results indicate that the MVIC can improve the HDU position control performance significantly. These results can provide important guidance and lay the experimental foundation for control strategy for hydraulic inner loops with position-based impedance control.

Dynamic Compliance Analysis for LHDS of Legged Robot, Part A: Position-Based Impedance Control

In this paper, the dynamic compliance analysis for position-based impedance control is studied. First, the leg hydraulic drive system (LHDS) of legged robots is studied and the mathematical model of hydraulic drive unit (HDU) position control system is built. Then, combined with kinematics, statics and dynamics of the leg mechanical structure, the position-based impedance control method is applied to LHDS. Moreover, the serial–parallel composition of the dynamic compliance of position-based impedance control is analyzed and the three key factors influencing the impedance control accuracy are found, which is the main contribution of this paper. Finally, the key factors proposed in this paper are verified experimentally by using both LHDS and HDU performance test platform. The theories and experiments proposed in this paper provide a theoretical foundation for the compensation of position-based impedance control.

Parameters Sensitivity Analysis of Position-Based Impedance Control for Bionic Legged Robots’ HDU

For the hydraulic drive unit (HDU) on the joints of bionic legged robots, this paper proposes the position-based impedance control method. Then, the impedance control performance is tested by a HDU performance test platform. Further, the method of first-order sensitivity matrix is proposed to analyze the dynamic sensitivity of four main control parameters under four working conditions. To research the parameter sensitivity quantificationally, two sensitivity indexes are defined, and the sensitivity analysis results are verified by experiments. The results of the experiments show that, when combined with corresponding optimization strategies, the dynamic compliance composition theory and the results from sensitivity analysis can compensate for the control parameters and optimize the control performance in different working conditions.

Impedance control of a cable-driven SEA with mixed H2/H∞ synthesis

PurposeThis paper aims to present an impedance control method with mixed H2/H∞ synthesis and relaxed passivity for a cable-driven series elastic actuator to be applied for physical human–robot interaction.Design/methodology/approachTo shape the system’s impedance to match a desired dynamic model, the impedance control problem was reformulated into an impedance matching structure. The desired competing performance requirements as well as constraints from the physical system can be characterized with weighting functions for respective signals. Considering the frequency properties of human movements, the passivity constraint for stable human–robot interaction, which is required on the entire frequency spectrum and may bring conservative solutions, has been relaxed in such a way that it only restrains the low frequency band. Thus, impedance control became a mixed H2/H∞ synthesis problem, and a dynamic output feedback controller can be obtained.FindingsThe proposed impedance control strategy has been tested for various desired impedance with both simulation and experiments on the cable-driven series elastic actuator platform. The actual interaction torque tracked well the desired torque within the desired norm bounds, and the control input was regulated below the motor velocity limit. The closed loop system can guarantee relaxed passivity at low frequency. Both simulation and experimental results have validated the feasibility and efficacy of the proposed method.Originality/valueThis impedance control strategy with mixed H2/H∞ synthesis and relaxed passivity provides a novel, effective and less conservative method for physical human–robot interaction control.

An EMG-based variable impedance control for elbow exercise: preliminary study

As the medical field continues to evolve, the average life expectancy of the people increases. However, due to natural deterioration, the average muscle force of the arm decreases in respect to age. So, there are some needs for the people to exercise his/her body as he/she is getting older. There are many researches about the robotic rehabilitation of the upper limb for patients. However, only a few focus on the system of the upper limb muscle exercise for normal people. So, in this study, an electromyography (EMG)-based variable impedance control method is developed for elbow exercise in normal people. Four EMG sensors were attached to the designated right arm and their average EMG level was controlled by the proposed algorithm during an elbow flexion-extension exercise. A total of twenty nine experiments with 15 subjects were performed to verify the proposed control algorithm with NCCEES, an elbow exercise system developed by the authors for the elbow exercise. The experimental results showed that the proposed control algorithm could control the EMG level in approximately 24.1% percent error during the elbow exercise. However, the proposed control algorithm could not control the EMG level fluctuation which depended on the muscular characteristics of the subject. The effect of training with the proposed algorithm will be future work.