Force-based Impedance Control Research Articles

Adaptive End-Effector Buffeting Sliding Mode Control for Heavy-Duty Robots with Long Arms

This study aims to resolve the problems of low precision, poor flexibility and unstable operation in the control performance of loading robots with long telescopic booms and heavy loads. Firstly, the kinematics and dynamics of long-arm heavy-duty robots are analyzed, and the dynamics model of a long-arm heavy-duty robot is established using the Lagrange method. A new power-hybrid sliding-mode approach law is proposed, and a hybrid force/position control strategy is used to control long-arm heavy-duty robots. The position control of long-arm heavy-duty robots uses a new sliding-mode adaptive control to improve the position accuracy of important joints, and PD control is used to force control the other joints. The two-stage telescopic arm is flexible and the long-arm heavy-load robot is simulated. The simulation results show that the long-arm heavy-load robot obtained using the improved sliding-mode adaptive control algorithm has good track-tracking and jitter-suppression effects. The new power-hybrid sliding-mode controller designed in this paper reduces the jitter amplitude of the end-effector of long-arm heavy-duty robots by 28.75%, 10.92% and 16.22%, respectively, compared with the existing new approach law sliding-mode controller. The simulation results show that the proposed power-hybrid reaching law sliding-mode controller can effectively reduce the amplitude difference of the end-effector. Finally, the force/position control strategy is combined with force-based impedance control, and the design process of impedance controller parameters is introduced, which provides a reference for the trajectory-tracking and vibration-suppression of end-effectors of long-arm heavy-duty robots.

Research on Feedforward Disturbance Rejection Control for Force-based Impedance Control System of Hydraulic Drive Unit

Research on Feedforward Disturbance Rejection Control for Force-based Impedance Control System of Hydraulic Drive Unit

Dynamics compensation of impedance-based motion control for LHDS of legged robot

Aimed at the negative effect of dynamics characteristics of leg hydraulic drive system (LHDS) on the accuracy of motion control of the hydraulic drive legged robot, a dynamics compensation control method is proposed. First, according to the mechanical structure of LHDS, the kinematics and statics models of LHDS are analyzed and obtained respectively. Based on the principle of force-based impedance control of LHDS, an impedance based motion control simulation model of LHDS is built and analyzed. The simulation results show that the dynamics characteristics have a great influence on the accuracy of impedance based motion control. Then, a dynamics compensation method considering gravity and inertia force is proposed to solve this problem. Finally, the effect of the dynamics compensation method is verified on the robot single leg test platform. The experimental results show that the compensation method reduces the negative effect of the dynamics characteristics of LHDS on impedance based motion control accuracy, and the position tracking accuracy of the robot’s foot end can be improved by more than 65%. The theory proposed in this paper provides a theoretical basis for the motion control of the whole robot prototype.

Adaptive Variable Parameter Impedance Control for Apple Harvesting Robot Compliant Picking

In order to reduce the damage of apple harvesting robot to fruits and achieve compliant picking, an adaptive variable parameter impedance control method for apple harvesting robot compliant picking is proposed in this paper. Firstly, the Burgers viscoelastic model is used to characterize the rheological properties of apples and study the variation of mechanical properties of apple grasping at different speeds. Then, a force-based impedance control system is designed. On this basis, aiming at the influence of impedance controller parameters on contact force, three impedance parameters self-tuning functions are constructed to complete the design of an improved force-based impedance control system based on the hyperbolic secant function. The simulation and experimental results show that the proposed control makes the desired force smoother, and its overshoot is about 2.3%. The response speed is faster, and the adjustment time of contact force is shorter of about 0.48 s. The contact force overshoot is about 2%, which is 37.5% less than that of the traditional force-based impedance control. This research improves the control performance for apple harvesting robot compliant picking.

Second order matrix sensitivity analysis of force-based impedance control for leg hydraulic drive system

In this paper, the mathematical expressions of a second-order matrix sensitivity analysis (SOMSA) is derived. This method has higher accuracy and requires less calculation works than previous analysis methods. Based on the SOMSA, when the traditional force-based impedance control is applied in leg hydraulic drive system (LHDS) of legged robots, the effects of parameter variations on control performance are studied by sensitivity dynamic analysis under nine working conditions. Then, combined with two measurable sensitivity indexes, the results of the sensitivity dynamic analysis are studied quantitatively. Finally, the above results are verified experimentally by using LHDS test platform. The conclusions contribute to the optimization of the LHDS structure and provide theoretical references for compensation control strategies of the traditional force-based impedance control.

Design and Application of MVIC for Hydraulic Drive Unit of Legged Robot

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.

Impedance Control Approach on Leg Motion Speed Variation on Soft Surface Interaction

This article presents the leg speed variation control using impedance control approach on soft surface displacement motion. One of the challenging fields of designing a legged robot that can be equipped with adaptation ability is it dynamic control which majorly involved in interaction with the environment. Numerous researchers have been widely implemented impedance control as dynamic interaction but less emphasized in adapting soft terrain. Most of the impedance control implementation on the legged robot on rough terrain emphasized on position changes, and it may not practical for legged robot navigate on the soft terrain. Soft terrain contains different ground stiffness and medium viscosities. Thus, this study has taken the initiative to propose a speed variation control on a robot’s leg by using a force-based impedance control approach to increase the leg energy exchanges specifically on foot placement. The proposed control was validated in actual robot’s leg, and performances show that the energy in the leg increases as the velocity of leg motion increase due to increase in force feedback while maintaining the shape of the leg motion.

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.

Dynamic Compliance Analysis for LHDS of Legged Robot, Part B: Force-Based Impedance Control

Dynamic Compliance Analysis for LHDS of Legged Robot, Part B: Force-Based Impedance Control

Impedance Control for Legged Robots: An Insight Into the Concepts Involved

The application of impedance control strategies to modern legged locomotion is analyzed, paying special attention to the concepts behind its implementation which is not straightforward. In order to implement a functional impedance controller for a walking mechanism, the concepts of contact, impact, friction, and impedance have to be merged together. A literature review and a comprehensive analysis are presented compiling all these concepts along with a discussion on position-based versus force-based impedance control approaches, and a theoretical model of a robotic leg in contact with its environment is introduced. A theoretical control scheme for the legs of a general legged robot is also introduced, and some simulations results are presented.

A Bio-Mechanical Designed Under-Actuated Hand and Force Control Schemes

In order to mimic the natural appearance, motion and perception of the human hand, a biomechatronic approach to design an anthropomorphic Under-actuated hand-HIT/DLR Under-actuated Hand has been presented. It reproduces human hand in its fundamental structure such as appearance, weight, dimensions and inertia. Its thumb can move along a cone surface in 3-D space. Similar with humans’, it combines with abduction and adduction from palmar position to lateral position. Actuated by only one motor, the mid finger, ring finger and little finger can envelop complex objects. Furthermore by applying dynamics model, the force-based impedance control can realize more accurate and stable force control during grasp. Experiments displays dynamic control can make the finger grasp with more smooth and precise trajectory and stable grasp force, these would be particularly helpful for the widely application of underactuated hand.

UNDERACTUATED HAND DYNAMIC MODELING, ITS REAL-TIME SIMULATION, AND CONTROL

The forward and inverse dynamic models of the underactuated 2-DOF finger have been established in this article based on virtual spring approach. This approach not only avoids the solution of differential-algebraic equations but also leads to a completely decoupled dynamic model that is ideal for directly inverse dynamic analysis, real-time dynamic simulation and control. To verify this approach, an underactuated 3-joint finger has been brought forward. Simulation results from Matlab/Simulink are consistent with those obtained from ADAMS grasp simulations. For the hand real-time dynamic control, the velocity observer has been established based on the dynamic model, the adaptive curve fitting with the observer has obtained precise velocity signals, made up the uncertain parameters such as torsion spring, inertial, damps, etc. and achieved ideal results. By applying dynamics model and observer, the force-based impedance control can realize more accurate and stable force control during grasp.

Observer-Based Dynamic Control of an Underactuated Hand

The purpose of this paper was to construct a velocity observer based on the dynamic model and realize accurate dynamic curve and force control. Curve fitting with the observer obtained precise velocity signals. Compared with PID and factored moment methods, it decreased the fitting errors a lot and achieved ideal results. Compensated with the inverse dynamic equation, the force-based impedance control with the observer could not only realize accurate force tracking, but achieve finger dynamic control by the combination of curve fitting and force tracking. Furthermore, a static grasp model was established for appropriate force distribution. The finger could grasp slippery, fragile, comparatively heavy and large objects like an egg with only base joint torque and position sensors, which illustrated that the hand could accomplish difficult tasks by using the static grasp model and dynamic control.

Hybrid Impedance Control of Massage Considering Dynamic Interaction of Human and Robot Collaboration Systems

This paper proposes an intelligent massage control system that uses a multi-fingered robot hand with hybrid impedance control, which is able to recreate the movement and force of a human massage therapist. Therefore, various massage points, such as changes in the stiffness of human skin muscle, can be controlled by using an impedance control method. A hybrid impedance control, comprised of position-based and force-based control methods, was developed. The position-based impedance control is used to control the lateral position of massage on the human skin muscle. On the other hand, the force-based impedance control is used to control the force of the vertical direction on human skin muscle. This paper also identifies human skin muscle through robot perception of impedance to decide on the parameters of the impedance controller. A strategy using impedance control to implement an adaptive control system is presented, under the conditions of both soft and hard skin and muscle. The effectiveness of this massage control system using a multi-fingered robot hand with hybrid impedance control is demonstrated through realistic massage experiments involving pushing and rubbing motions.

Hybrid Impedance Control of Human Skin Muscle by Multi-fingered Robot Hand

This paper proposes an intelligent massage control system by using multi-fingered robot hand with hybrid impedance control, which is able to create the movement and the force of robot such as the human's massage. Therefore, the various massage points, such as the change of the stiffness of human skin muscle, can be controlled by using impedance control method. The hybrid impedance control, comprised of the two methods of the position-based and the force-based impedance control, were applied. The position-based impedance control is used to control the lateral position of massage on the human skin muscle. On the other hand, the force-based impedance control is used to control the force of the vertical direction on human skin muscle. This paper also gives the identification of human skin muscle through robot perception of impedance to decide the parameter of impedance controller. The control strategy using impedance control to implement an adaptive control system is presented, when human condition is changed with soft and hard skin muscle. Effectiveness of massage control system by using multi-fingered robot hand with hybrid impedance control is demonstrated through actual massage experiments of pushing and rubbing motion.

An Experimental Study on Cartesian Impedance Control for a Joint Torque-Based Manipulator

The purpose of this paper is to present our results on experimental investigations on Cartesian impedance control and nonlinearity compensation for our harmonic drive robot based on joint torque sensors. The imperfection of the cubic model for harmonic drive friction is detected according to friction identification experiments. In addition, five different Cartesian impedance control schemes are considered and four of them are tested by corresponding experiments with/without friction compensation. Experimental results tell us that the force-based impedance control strategy is more suitable than the position-based strategy for the harmonic drive robot based on joint torque feedback.

Impedance Control of a Single Shaft-Type Clutch Using Homogeneous Electrorheological Fluid

This study demonstrates the application of a precise and variable impedance control to the homogeneous electrorheological (ER) clutch. The viscous coefficient of a homogeneous ER fluid can be changed by the application of an electric field. However, the ER effect of the fluid is easily changed by shear rate variation or temperature variation. Therefore, some feedback compensations are necessary for these systems to maintain a more precise viscosity.In this paper, we apply a force-based impedance control to the homogeneous ER clutch in order to use ER fluid more effectively. First, we study the fundamental characteristics of the homogeneous ER clutch. Next, we apply the viscous control based on the force control in order to maintain a more precise viscosity. The results of our experiment clearly show that the viscosity of the clutch has been accurately and stably controlled by the proposed control method.

Variable Damping Control of a Semi-Active Device Using Homogeneous ER Fluid

The purpose of this study is to apply the variable damping impedance control to semi-active system using the homogeneous ER (Electro-rheorogical) fluid. A homogeneous ER fluid is constituted by liquid crystal or polymer liquid crystal, and it can change the apparent viscosity under a Newtonian fluid by applying the electric field. Then, when the fluid is applied for device of the machine system, the system can be used as a variable viscous damper. However, these systems have a lot of parameter variations or nonlinear elements. Therefore, some feedback compensations are necessary for these systems to keep up more precisely viscosity.In this paper, we apply the force-based damping control to the homogeneous ER clutch in order to use the characteristics of the fluid enough. We apply the damping control based on the force control to keep up more precisely viscosity. The result of our experiment clearly has shown that the viscosity of the clutch is more certainly controlled by the force-based impedance control.

Control of whole finger manipulation utilizing frictionless sliding contact—theory and experiment

We propose a whole finger manipulation (WFM) to improve the robustness of manipulation by a multifingered robot hand. The WFM is characterized by enveloping an object by finger links of the multifingered robot hand, and utilizing the frictionless sliding contacts between the surfaces of finger links and the grasped object. Therefore, the WFM can achieve a large manipulation without regrasping the object and can be highly robust in nature because of form-closure due to multiple contact points on the grasped object. This paper discusses the property and control of the WFM. The WFM with frictionless contact is shown to be a manipulable and force closure grasp using kinematics and statics with frictionless sliding contact. We discuss that the force closure for the WFM with frictionless contact can be viewed as form closure. An internal force-based impedance control is proposed for the stable manipulation in consideration of sliding contacts between the object and the finger links. A prototype hand and sensing systems are presented. The validity of the proposed control law is verified through the experiments.

Internal force-based impedance control for cooperating manipulators

An internal force-based impedance control scheme for cooperating manipulators is introduced which controls the motion of the objects being manipulated and the internal force on the objects. The controller enforces a relationship between the velocity of each manipulator and the internal force on the manipulated objects. Each manipulator is directly given the properties of an impedance by the controller; thus, eliminating the gain limitation inherent in the structure of previously proposed schemes. The controller uses the forces sensed at the robot end effectors to compensate for the effects of the objects' dynamics and to compute the internal force using only kinematic relationships. Thus, knowledge of the objects' dynamics is not required. Stability of the system is proven using Lyapunov theory and simulation results are presented validating the proposed concepts. The effect of computational delays in digital control implementations is analyzed vis-a-vis stability and a lower bound derived on the size of the desired manipulator inertia relative to the actual manipulator endpoint inertia. The bound is independent of the sample time.