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In a bilateral control system, reducing the information-transmission channel between the master and slave systems has advantages, such as suppressing data traffic, reducing stored data, and simplifying the control-system design. The position/force hybrid-control-type bilateral control system can realize stable and highly transparent bilateral control; however, it requires four transmission channels between the master and slave systems to transmit the position and force information. Several types of conventional two-channel controllers are difficult to operate and contact with hard environments. A two-channel controller with a performance equivalent to that of a four-channel controller exists, but it still requires local position control loops. Therefore, this study proposes a force-symmetric type two-channel bilateral control system based on force control systems with robust acceleration controllers using second-order disturbance observers (DOBs) among higher-order DOBs. Bilateral control systems have two control goals: synchronization of the position of the master-slave system, which is a position-related control goal, and artificial realization of the law of action-reaction between the master-slave system, which is a force-related control goal. Methods using low-order DOBs, such as zero- and first-order DOBs, cannot achieve the position-related control goal. In contrast, the proposed method can achieve both the position and force control goals simultaneously, even though it only has force transmission channels and control loops. This study also showed that the performance of the proposed two-channel bilateral control system is equivalent to that of a four-channel bilateral control system that transmits both position and force information. The effectiveness of the proposed method was confirmed through experiments.

This paper presents a methodology and generalized motion framework for quantifying passive biomechanical stability and Range of Motion of human cadaveric specimens, using a position-controlled industrial robot and a wrist-mounted force/torque sensor. Many biomechanical studies on diarthrodial joints using human cadaveric specimens are published in the literature, using various test protocols and machines to apply the loading conditions. In these studies, laxity or mobility of the joints are quantified by measuring the magnitude of translations and rotations with respect to force and torque. The protocols and anatomical motions of the specimens are usually described high-level, textually, and from a medical perspective to a broad audience. The present paper aims to describe, from a technical perspective to a robotics audience, our method to perform biomechanical studies and how existing protocols can be replicated through parameterization using the existing textual descriptions. To accomplish this, we propose a generalized task space motion framework for performing biomechanical studies on diarthrodial joints. The generalization is made by defining the robot Tool Center Point at the cadaveric joint rotation center and aligning the specimen so the anatomical motions can be modeled in world frame or tool frame. The framework was successfully evaluated in a technical pilot study on the shoulder, using one cadaveric shoulder specimen and an established protocol from the literature. The specimen was tested in the intact state and in an injury state, with increased passive instability observed for the injury state compared to intact state.

Offshore wind farms are a crucial source of renewable energy, but maintenance and repair can be challenging due to their remote locations and harsh environmental conditions. Professional divers or Remotely Operated Vehicles (ROVs) are commonly used to conduct maintenance operations, but they come with high daily operational costs. Autonomous Underwater Vehicles (AUVs) have the potential to improve the efficiency, safety, and costs of maintenance operations. This project evaluates the feasibility of using an AUV to conduct a cathodic protection (CP) survey, which involves measuring the corrosion potential of underwater structures to prevent deterioration. The AUV is equipped with a manipulator that has a CP probe with a sharp tip to puncture through the structure’s coating and make contact with the steel underneath. To ensure high accuracy and reduce environmental perturbances, the AUV attaches to the structure while conducting the survey. The technology and methods used in this project are demonstrated in a water tank using a Girona1000 AUV. Task Priority kinematic control is combined with a custom force control strategy based on admittance control to enable tracking of the end-effector configuration and contact force during the probing operation. The mission flow control is implemented using behavior trees. The results show that the use of AUVs for CP surveys is feasible and has the potential to significantly improve the efficiency, safety, and costs of maintenance operations in offshore wind farms.

The traditional method of milling which uses constant spindle speed and feedrate to prevent machine, workpiece, and machining tool failure. However, this method prevents engineers from further improving machining efficiency. It is possible to increase machining efficiency by appropriately adjusting feedrate during the machining process. Although there are plenty of researches regarding adaptive machining force control, those researches focus on the design of controller, and few of them discuss the relation between control signals and tool wear for parameters setting. This study presents a method for setting process parameters in constant load machining, including feedrate upper and lower limits, as well as reference spindle current. The reference spindle current and feedrate upper limit were selected to sustain cutting force up to tool strength. The feedrate lower limit was established to maintain tool wear conditions within a given range, preventing severe and unstable tool wear. The experimental results showed that when the feedrate was reduced to the lower limit value, the machining parameters setting method presented in this study could keep the tool wear within the set value, avoiding the risk of tool failure. The strategy for setting the machining parameters can be used to end milling, slot milling, or a combination of both milling processes. The constant load machining parameters presented in this study outperformed constant feedrate machining of the identical milling conditions in terms of tool life and material removal rate. The traditional method estimated tool life by cutting lengths, but we used the feedrate change to estimate the tool wear for change. The result showed that this method had better tool life than traditional way.

With the increasing use of robots in industry, the use of multi-robot systems, i.e., systems in which multiple robots work together to perform a single task, has become common. In an environment where multiple robots cooperate, the relative position and orientation error between robots must be minimized to improve the accuracy of tasks. In multi-robot systems, accurate kinematic parameters (KP) and base frame transformation (BFT) are required to minimize the position and orientation error between the robots. In this study, a method for KP and BFT identification method that can minimize the position and orientation error of the multi-robot system without using a position measurement device is proposed. A cost function representing the position and orientation error of the multi-robot system was constructed, and the differential evolution algorithm was used to determine the parameters that minimize the cost function. Furthermore, a system consisting of two industrial robots was constructed to verify the effectiveness of the proposed identification method. Experimental results show that the proposed identification method can reduce the relative position error of the dual-robot system by 79% compared to before calibration.

In grinding tasks, the contact force has a significant impact on product surface quality. Therefore, force-sensing technology to detect contact force is important. Although force sensors are widely used for contact force detection, the response of the force sensor includes sensor-specific errors such as offset. In this paper, we propose a contact force detection method based on the combination of frequency information and the differential feature ( $\Delta F$ ) of the force signal. The use of high-frequency information reduces the influence of force sensor-specific errors. However, contact force detection using only high-frequency information causes a time delay in the detected value relative to the measured value depending to the frame size of time window used for frequency analysis. To reduce the time delay, high-frequency information and $\Delta F$ are integrated by inputting them into an long short-term memory (LSTM)-based force detection model. To verify the effectiveness of the proposed method, we compared it with a force detection model based on an FNN and CNN on a dataset of plane grinding tasks. Consequently, the detection accuracy of the LSTM-based model was superior to that of the FNN and CNN models. Compared to the LSTM model using only high-frequency information as input, the detection accuracy was 26% higher when the error was small and 57% higher when the error was large. In addition, the time delay was reduced from 166 ms to 30 ms using $\Delta F$ as the input. The frequency information and $\Delta F$ are features calculated from the same force information dataset; therefore, no additional dataset is required.

This paper proposes a robust hybrid position/force control (HPFC) system for multi-degree of freedom manipulators (MDoFMs) with torque constraints on each joint. General HPFC systems can control the interactive contact forces and positions of manipulators in various environments. In HPFC systems, to improve the control performance for MDoFMs, motor torque saturation should be considered. Thus, a robust HPFC system that considers torque constraints by predictive functional control (PFC) is proposed in this paper. The proposed method simultaneously handles the response characteristics and torque constraints of actuators by using PFC as joint space position controllers. Additionally, the robustness of actuators against external forces is enhanced, and the model parameter errors are compensated by the disturbance observer technique. Moreover, implicit force control law and inverse kinematics are introduced for the workspace position/force control to implement the joint space position controllers. Consequently, the stability and robustness of the actuators are considered, and suitable position/force control can be performed even if the torque is saturated. The validity of the proposed method is tested with three planar manipulator joints with a uniaxial force sensor.

The slippage occurs between the fingers and object will result in grasp failure that badly affects the prosthetic hand's operation. In this paper, a new slippage prevention strategy is proposed to improve the prosthetic hand's grasp stability. The proposed method, which comprises a slipping process model, a multi-threshold detection scheme, and a force regulation policy, can have a minimal loading force intervention after stabilization. First, the slipping process is modeled with special considerations on the nonlinear and noise characteristics of the system. Then, the multiple thresholds are selected for slippage detection to reduce the influence of loading force variation and system nonlinearity. Lastly, a loading force regulation policy consisting of a serials of constant, slight force increments is applied to suppress the detected slippage and minimize the force growth. One merit of our method is that the parameters selected in the algorithm largely depend on the system characteristics of the hand system, but not the grasped object. Experimental results show that the proposed method has better detection accuracy on slippage detection and minimal loading force intervention after slippage suppression.

In this paper, we consider the problem of accurate force control using actuated winches, intended for use in real-time hybrid hydrodynamic model testing. The paper is also relevant to other cable-driven parallel robot applications that use force control in an inner control loop. For this problem, conventional strategies typically use actuated winches with torque-controlled servomotors directly connected to the cabled drum. In contrast, we propose using actuated winches with position-controlled servomotors that connect to the cabled drum via a clockspring. The servomotors are position controlled at drive-level and are rapid, accurate, robust, and simple to install. We show how this, combined with an accurate estimate of the clockspring deflection and stiffness, can yield fast and precise force tracking on moving objects. This includes proposing associated feedforward force-controllers that compensates for damping, angle-dependent force variations, delays, and non-constant clockspring characteristics. Extensive experimental testing on a 1 degree of freedom actuated mass-spring system supports the work.

Most industrial robots are provided by the manufacturer with a controller that cannot be modified by the user (e.g. a standard PID). This arrangement is commonly referred to as closed control architecture, since it is not possible to program arbitrary control laws. For the implementation of novel algorithms, it is on the contrary necessary to employ an open control architecture, which allows programming any control scheme. For that reason, it is customary to have testbeds that are made up of robot manipulators specially designed for this goal. Another disadvantage of the closed control architecture is that the controller provided by the manufacturer usually does not include a force control term since it allows only to program desired motion trajectories. To overcome these drawbacks without physically modifying the closed control architecture, this contribution presents a novel approach to simultaneously follow position and force trajectories by employing only motion planning, i.e. only by choosing the desired position trajectory. The approach is especially well suited for DC motor actuators with large gear reduction ratios as those of many industrial robots. The convergence of the manipulator position and applied force depends exclusively on the performance of the controller provided by the manufacturer. The approach is tested on a dual-arm cooperative manipulation system made up of two ABB IRB-2400 industrial robots with closed control architecture.

This paper presents a fault-tolerant control (FTC) based on impedance control and full state feedback backstepping sliding mode control (FBSMC) algorithm for an n degree of freedoms (n-DOF) serial hydraulic manipulator under the presence of matched and mismatched uncertainties and sensor faults in the constrained framework. These faulty signals, generated from unknown constant or time-variant offset values, happen on both manipulator joint angles and force sensors; thereby degrading the system performance. Therefore, to address both matched and mismatched uncertainties and signal faults, the system dynamics subjects to the sensor faults is mathematically modeled. Then, the robust fault estimation algorithm based on extended state observer (ESO) is proposed to estimate the system state and faulty signals for the FTC design to achieve the force and position tracking performance. System stability of the proposed control scheme is theoretically proven by performing Lyapunov theorems. Finally, comparative simulation results are given on a 3-DOF serial hydraulic manipulator to evaluate the effectiveness of the proposed fault estimation and FTC methodology.

This paper analyses Disturbance Observer- (DOb-) based robust force control systems in the discrete-time domain. The robust force controller is implemented using velocity and acceleration measurements. A DOb is employed in an inner-loop to achieve robustness, and another DOb, viz. Reaction Force Observer (RFOb), is employed in an outer-loop to estimate interaction forces and improve the performance of force control. First, the inner-loop is analysed. It is shown that the DOb works as a phase-lead/lag compensator tuned by the nominal design parameters in the inner-loop. The phase margin of the inner-loop controller and the bandwidth of the velocity-based (i.e., conventional) DOb are constrained not only by noise-sensitivity but also by the waterbed effect. This explains why we observe unstable responses as the bandwidth of the conventional DOb increases in practice. To eliminate the design constraint due to the waterbed effect, this paper proposes an acceleration-based DOb. Then, the robust force controller is analysed. It is shown that the design parameters of the RFOb have a notable effect on the stability of the robust force control system. For example, the robust force controller has a non-minimum phase zero (zeros) when the RFOb is not properly tuned. This may cause severe stability and performance problems when conducting force control applications. By using the stability and robustness analyses, this paper proposes new design tools which enable one to synthesize a high-performance robust force control system. Simulations and experiments are presented to validate the proposed analysis and synthesis methods.

This study presents a new linear motor with an E-core stator and a homopolar permanent magnet (PM) mover. The velocity and force ripples of the motor are effectively reduced by an optimized structure and effective force compensation. Firstly, the mechanical structure, magnetic paths and operation principles are introduced. Force analysis is carried out via magnetic circuit method and finite element method (FEM). The magnetic structure of the motor is optimized to reduce the cogging force. Secondly, a force compensation approach is developed to control the velocity and reduce the force ripples of the motor. Parameter identification is employed for the force control to observe the force of the motor so as to compensate the force ripples. Finally, the velocity ripples and force ripples are calculated and measured by simulation and experimentation. Both the simulation and experimental results show high feasibility of the force compensation method. By optimizing the magnetic structure of the motor and developing the force compensation block of the control part, the estimated force output suggests that the force ripples of the motor can be limited within 5%.

Intelligent interaction control is required in many fields of application, in which different operative situations have to be faced with different controllers. Being able to switch between optimized controllers is, indeed, of extreme importance to maximize the task performance in the different operative conditions (i.e., free-space motion and contact), especially when considering sensorless robots. To deal with the proposed context, a sensorless optimal switching impact/force (OSIF) controller is proposed. The low-level robot control is composed of an inner joint position controller, fed by an outer Cartesian impedance controller with a reference position. The estimation of the external wrench is implemented by means of an Extended Kalman Filter (EKF). The high-level controller (feeding the Cartesian impedance controller with the setpoint) is composed of an optimized impact controller (LQR-based controller), an optimized force controller (SDRE-based controller), and a continuous switching mechanism (Fuzzy Logic-based). In addition, the output of the switching mechanism is used to adapt the Cartesian impedance control parameters (i.e., stiffness and damping parameters). Experimental tests have been performed on a Franka EMIKA panda robot to validate the proposed controller. Obtained results show the capabilities of the OSIF controller, being able to detect task phase transitions while satisfying the target performance.

A series elastic actuator (SEA) includes an elastic spring in series with an actuator. SEAs provide more accurate force and impedance control than conventional rigid actuators. They are ideal for robots and machines that need to interact safely with the environment. The majority of existing SEAs uses brushless or brushed DC motors as the actuators. The advantages of using step motors as the actuators of SEAs have not received enough attention. Step motors have much higher torque-to-weight ratio and torque-to-inertia ratio than other DC motors. Hence they can provide better stability and high-speed accuracy of force control while maintaining lightweight. When the rotor position feedback is used, step motors can achieve accurate dynamic position response smoothly. This paper develops the dynamic model of a linear series elastic step motor and presents its prototype. Force and impedance control responses will be provided to show the advantages due to the high torque-to-inertia ratio of step motors. It is expected that the results presented here can offer a better actuator selection of SEAs when high-performance dynamic force control is required.

A technique named Learning from Demonstration allows robots to learn actions in a human living environment from the demonstrations directly. In a learning method from demonstrations directly, however, teaching actions cannot be reused between situations with different restrictions. In this study, we propose a method for training a success judgment model based on Learning from Demonstration and use this as a differentiable loss function of tasks. By formulating the constraints of the action in a manner in mathematical optimization and combining these constraints with the learned success judgment model into a loss function, an action generation model can be trained by the gradient method. This system was verified with the action of scooping up a pancake.

The ability of aerial manipulation robots to reach and operate in high altitude workspaces may result of interest in a wide variety of applications and scenarios that nowadays cannot be accessed easily by human operators. Consider for example the installation of sensors in polluted areas, the insulation of leaks in pipe structures, or the corrosion repair in power lines and wind turbines. This paper describes the application of a human-like dual arm aerial manipulator for the inspection of pipe structures, typical of chemical plants, involving the installation and retrieval of sensor devices. The goal is to reduce the time, cost, and risk with respect to conventional solutions conducted by human workers. Two configurations of the aerial robot are considered and compared: the standard, in which the arms are installed at the base of the multirotor, and the long reach configuration in passive pendulum, which extends the effective workspace of the manipulator and increases safety during the operation on flight. The kinematic and dynamic models of both configurations are derived, proposing a unified notation for the equations of motion, and a force/position control scheme that relies on the servo controller and the mechanical joint compliance. The paper also describes a simulation framework used for validating the execution of the aerial manipulation task before the realization of the real experiments, which contributes to reducing the probability of failure. The potential application of the standard and long reach configurations is evaluated in two sensor installation tasks carried out in an indoor testbed.

In the application of piezoelectric jet dispensing, piezoelectric ceramics and flexure hinges cooperate to form a piezoelectric actuator. The dispensing quality of the piezoelectric jet is closely associated with the output force of the piezoelectric actuator. In this study, a corresponding piezoelectric actuator output force experiment platform is built, and the voltage-force hysteresis curve of the piezoelectric actuator is analyzed. The voltage-force hysteresis curve exhibits memory characteristics and rate dependence. A modeling method for the dynamic hysteresis output force model is proposed, and the numerical method of the model is presented. The corresponding output force model of the piezoelectric actuator is established based on experimental data to predict the output force under different input voltages. According to the dynamic hysteresis model, the feedforward control methodology of the dynamic hysteresis inverse model is proposed, and experimental tests are performed. Experimental results show that the proposed model can express the complex nonlinear characteristics of piezoelectric actuators, and the output force feedforward control method based on the dynamic hysteresis inverse model effectively decreases the nonlinear characteristics of the piezoelectric actuator.

To promote operational intelligence, improve surface quality, and reduce manpower dependence, a novel high-bandwidth end-effector with active force control for robotic polishing was proposed. Using this end-effector as a mini robot, a macro-mini robot for polishing processing was constructed, in which the macro robot provides posture control during polishing operations, whereas the mini-robot provides constant force control. By minimizing the inertia along the spindle in this configuration, the end-effector obtains a force control bandwidth of 200 Hz. Through a series of comparative experiments with different contact forces and feed rates, the proposed design was proven to have a smaller overshoot, a faster response, and a shorter settling time than the conventional method based on macro robot (KUKA iiwa) controlled force. The roughness of the workpiece reached $0.4~\mu \text{m}$ after polishing with the macro-mini robot, indicating the efficiency of this end-effector in high-precision material removal and surface polishing operations.

Each joint of hydraulic drive legged robot adopts highly integrated electro-hydraulic servo system, and the system has two common forms, namely, valve-controlled cylinder system and pump-controlled cylinder system. The former has a large energy loss, which restricts the endurance of the legged robot in the field. Although the latter has the advantages of energy saving and high efficiency, the overall response ability of control system is unsatisfied. In this paper, a novel pump-valve compound drive system (PCDS) is designed. It combines the advantages of pump-controlled cylinder system and valve-controlled cylinder system, which can not only effectively reduce the energy loss of the system, but also ensure the electro-hydraulic servo system of the robot joint has the ideal response ability. This paper consists of three parts. In the first part, the mathematical model of nonlinear force control of PCDS is established, which takes into account the pressure-flow nonlinearity of servo valve, the asymmetry of servo cylinder and the complexity and variability of load. So this model has a high accuracy. In the second part, On the basis of the above model, a force control method combining tracking error compensation and load compensation control is designed. And this control method can improve the control precision of the robot. In the third part, the performance of the designed control method is verified by experiments. The experiments show that the designed force control method can effectively reduce the influence of external position disturbance on the tracking accuracy of the force control system and improve the control accuracy of the robot joint movement.

Demand for robot-assisted therapy has increased at every stage of the neurorehabilitation recovery. This paper presents a controller that is suitable for the assist-as-needed (AAN) training of the wrist when performing the spatial motion. A compact wrist exoskeleton robot is presented to realize the AAN controller. This wrist robot includes series elastic actuators with high torque-to-weight ratios to provide accurate force control required for the AAN controller. In addition to assist-as-needed rehabilitation, the parameters of the AAN controller can be adjusted to deliver passive, active, or resistive rehabilitation. Experimental results demonstrate that the proposed AAN controller can provide the total solution to cover each stage of wrist spatial-motion rehabilitation.

A hydraulic drive unit (HDU), applied in each joint of a hydraulic driven legged robot, usually adopts the impedance control outer loop based on the hydraulic position or the force control inner loop during the motion process. When the hydraulic control adopts closed-loop force control, its control performance can directly determine the control performance of the impedance control outer loop. Therefore, it is significant to design a high-accuracy force control method aimed at HDUs. In this paper, aiming at the above research concept, the force control system of the HDU is introduced first, and the lack of the force control performance is studied under different working conditions on the HDU performance test platform. Second, a model-based variable input controller (MVIC) is designed to improve the force control performance. Finally, the control performance of the MVIC is verified on the HDU performance test platform. The experimental results show that the MVIC can greatly improve the force control accuracy under different working conditions and that the controller has good robustness. The above research results can provide important reference and lay the experimental foundation for force-based impedance control.

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.

For the nonlinear, time-variable and unpredictable environment (such as the manipulator workspace that has many random and moveable obstacles), a Hammerstein adaptive (HA) impedance controller based on a Hammerstein impedance model is proposed in this paper. The model consists of a static nonlinear element and a linear dynamic element, which can describe the nonlinearity of the environment. Then, an adaptive law is designed by a Lyapunov function, which can adjust the parameters of the static nonlinear element, so that the model has good adaptability and robustness for the environment. By introducing a parameters self-adjust model-free adaptive controller as the inner position controller, the HA impedance control system is built, which does not require the knowledge of the robot dynamics. In order to verify the force-control performance of the proposed control scheme, some simulations and experiments are done on a bionic wrist joint actuated by pneumatic muscles. Compared with the conventional impedance controller with constant parameters, experimental results demonstrate that HA impedance controller has better force tracking characteristic and can restrict the contact force effectively, no matter how the external environment changes, which increases the safety between robots and humans or operation objects.

A high-gain observer based sliding mode force control system for the single-rod servo actuator is presented in this paper. In order to track the desired force, full states for feedback are needed. Since only the output force is measured, a high-gain observer with easy implementation and calibration is designed for the estimation of the unavailable states. Then the sliding mode controller is proposed and a continuously varying function instead of the traditional sign function is used to constitute the switching term of the control input which takes the distance of the system states from the sliding surface into account. So the chattering phenomena are eliminated. The stability of the closed-loop force control system is analyzed by the singular perturbation method and simulations show the effectiveness of the proposed force control method for the single-rod electrohydraulic servo actuator.

When using the powered lower-limb prosthesis, it is important to obtain varying amputee's weight to prevent an excessive knee flexion in the early stance phase of finite state impedance control. This paper proposes a method for implementing an impedance controller that can operate independently of the varying load mass for reaction force sensing elastic actuator (RFSEA) with a linkage arm. The proposed controller estimates the load mass using recursive least square. We apply a Kalman filter and a disturbance observer to improve the accuracy of the measurement and the tracking performance of the force controller, respectively. The results of an experiment applying a lab-developed RFSEA system with a linkage arm validate the feasibility of this approach. It is also expected that the proposed system reduces the size and development costs of the prosthesis due to the advantages of using the RFSEA.

In this paper, we investigate the adaptive robust visual servoing/force control problem for the uncertain robot manipulator. The robot is considered to work with an unknown constraint surface and the unknown depth of feature point in image plane is assumed to be time-varying. The unknown constraint surface is linearly parameterized, and new adaptive laws are designed to estimate the unknown parameters online. In image based visual servoing control, the unknown time-varying depth plays a special role as it appears nonlinearly in the overall Jacobian matrix and cannot be adapted together with other uncertain kinematic parameters. To handle this problem, a compensatory depth-independent interaction matrix framework and the corresponding adaptive laws are proposed to compensate for the depth in the closed-loop dynamics. Moreover, the dead-zone input constraint is considered and handled by designing the robust compensatory terms and adaptive laws. With the proposed control scheme, it is proved that the image position and force tracking errors converge to zero asymptotically. Finally, numerical simulation and experimental results illustrate the effectiveness of the proposed approach.

Robotic grasping is a challenging area in the field of robotics. When interacting with an object, the dynamic properties of the object will play an important role where a gripper (as a system), which has been shown to be stable as per appropriate stability criteria, can become unstable when coupled to an object. However, including a sufficiently compliant element within the actuation system of the robotic hand can increase the stability of the grasp in the presence of uncertainties. This paper deals with an innovative robotic variable stiffness hand design, VSH1, for industrial applications. The main objective of this paper is to realize an affordable, as well as durable, adaptable, and compliant gripper for industrial environments with a larger interval of stiffness variability than similar existing systems. The driving system for the proposed hand consists of two servo motors and one linear spring arranged in a relatively simple fashion. Having just a single spring in the actuation system helps us to achieve a very small hysteresis band and represents a means by which to rapidly control the stiffness. We prove, both mathematically and experimentally, that the proposed model is characterized by a broad range of stiffness. To control the grasp, a first-order sliding mode controller is designed and presented. The experimental results provided will show how, despite the relatively simple implementation of our first prototype, the hand performs extremely well in terms of both stiffness variability and force controllability.

Keeping astronauts physically healthy in the harsh space environment is a key to the successful execution of a space mission. Long-term space missions in the weightless environment, however, can result in space adaptation syndrome, which seriously affects astronauts' health. To alleviate the adverse effects, this paper proposes a wire driven astronaut rehabilitative training robot that simulates the characteristics of the gravity environment and load force on the astronauts. The robot can realize multiple physical exercises including running, bench press, and deep squat. A dynamic model of the wire driven unit (WDU) was provided. On this basis, a hybrid force controller was designed to improve the precision and real-time performance of WDU. Furthermore, a dual-closed-loop control strategy was proposed to improve the loading precision of the robot. Running experimental results demonstrate that the robot can load force safely and reliably during the physical training, and the control strategies are effective.

High power-to-weight ratio soft artificial muscles are of overarching importance to enable inherently safer solutions to human-robot interaction. Traditional air-driven soft McKibben artificial muscles are linear actuators, and it is impossible for them to realize bending motions through use of a single muscle. More than two McKibben muscles are normally used to achieve bending or rotational motions, leading to heavier and larger systems. In addition, air-driven McKibben muscles are highly nonlinear in nature, making them difficult to be controlled precisely. An shape memory alloy (SMA)-fishing-line-McKibben (SFLM) bending actuator has been developed. This novel artificial actuator, made of an SMA-fishing-line muscle and a McKibben muscle, was able to produce the maximum output force of 3.0 N and the maximum bending angle (the rotation of the end face) of 61°. This may promote the application of individual McKibben muscles or SMA-fishing-line muscles alone. An output force control method for the SFLM is proposed, and based on MATLAB/Simulink software, an experiment platform is set up and the effectiveness of control system is verified through output force experiments. A three-fingered SFLM gripper driven by three SFLMs has been designed for a case study and for which the maximum carrying capacity is 650.4 ± 0.2 g.

Electro-hydraulic load simulator is a typical test-equipment for hardware-in-the-loop simulation, and usually performs periodic tasks, in which the modeling uncertainties will also present some periodicity. With this notification, in this paper, the system model of electro-hydraulic load simulator is established, afterward, all periodic uncertainties are transformed into linear-in-parameters form by applying Fourier series approximation, then an adaptive repetitive scheme with a robust integral of the sign of the error (RISE) feedback is synthesized, in which adaptive repetitive law is designed to handle periodic uncertainties and RISE robust term to attenuate unmodeled disturbances. The developed controller features depending on the desired trajectory rather than the system states, therefore it requires little information of the dynamic system and uncertain nonlinearities, which can apparently restrain the problems from noise pollution. In addition, because the periodic uncertainties are approximated as Fourier series and then compensated, the system performance can be greatly improved when performing periodic tasks. The resulting final control input is continuous while asymptotic tracking performance can be achieved with various uncertainties and disturbances by the proposed controller via Lyapunov stability analysis. In comparison to the other three controllers, the effectiveness and high performance of the proposed control method are validated by the experimental results sufficiently.

This paper presents a cable driven astronaut rehabilitative training (ART) robot, which can provide astronauts with multiple physical exercises including bench press, running, and deep squat to alleviate and resist the adverse effects induced by space adaptation syndrome. First, the modular reconfiguration of the ART driven by a cable is proposed to simulate the load characteristics of the gravity environment. Second, in order to improve the accuracy of the ART, the force control strategy is presented. The controller consists of two parts: high-level controller, which to calculate the desired cable tension and low-level controller, which to make each cable achieve the desired tension. Finally, to validate the effect of the ART, three (adult male) weight lifters were recruited to perform several bench press exercises with the barbell and ART. The analysis of the surface electromyography (sEMG) of triceps brachii and pectoralis major during real bench press and bench press simulated by the ART was conducted. The performance of the ART and the data of sEMG demonstrate the controller is effective and the ART can effectively assist astronauts to carry out bench press exercises in space.