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The electro-hydraulic servo system (EHSS) demonstrates numerous advantages in size and performance compared to other actuation methods. Oftentimes, its utilization in industrial and machinery settings is limited by its inferior efficiency. In this paper, a nonlinear backstepping control algorithm with an energy-saving approach is proposed for position control in the EHSS. To achieve improved efficiency, two control valves including a proportional directional valve (PDV) and a proportional relief valve (PRV) are used to achieve the control objectives. To design the control algorithm, the state space model equations of the system are transformed to their normal form and the control law through the PDV is designed using a backstepping approach for position tracking. Then, another nonlinear set of laws is derived to achieve energy-saving through the PRV input. This control design method, based on the normal form representation, imposes internal dynamics on the closed-loop system. The stability of the internal dynamics is analyzed in special cases of operation. Experimental results verify that both tracking and energy-saving objectives are satisfied for the closed-loop system.

Electro-hydraulic servo systems (EHSS) are used in many industrial applications for position and force control. Force control with a hydraulic actuator is challenging and requires complicated control algorithms used along with high crossover frequency electro-hydraulic valves, even for simple force control tasks. In this paper, a different hydraulic structure is proposed to improve the force tracking quality and increase efficiency in the EHSS. This comes at the cost of a new model with linearization and uncertainty challenges. To address these challenges, a robust H∞ control design approach is followed to control the proposed EHSS. Model linearization uncertainties are approximated by a polytope and a robust controller is designed to keep the system stable and satisfy the H∞ performance conditions within this polytope. Experimental results verify that the objectives of the paper are satisfied after using the proposed system.

Among the prevalent methods in linear bilateral teleoperation systems with communication channel time delays is to employ position and velocity signals in the control scheme. Utilizing force signals in such controllers significantly improves performance and reduces tracking error. However, measuring force signals in such cases, is one of the major difficulties. In this paper, a control scheme with human and environment force signals for linear bilateral teleoperation is proposed. In order to eliminate the measurement of forces in the control scheme, a force estimation approach based on disturbance observers is applied. The proposed approach guarantees asymptotic estimation of constant forces, and estimation error would only be bounded for time-varying external forces. To cope with the variations in human and environment force, sliding mode control is implemented. The stability and transparency condition in the teleoperation system with the designed control scheme is derived from the absolute stability concept. The intended control scheme guarantees the stability of the teleoperation system in the presence of time-varying human and environment forces. Experimental results indicate that the proposed control scheme improves position tracking in free motion and in contact with the environment. The force estimation approach also appropriately estimates human and environment forces.

In micromanipulation applications, controlling the force exerted on the object is of great importance. In such cases, any uncontrolled forces may damage the object or cause system failure. However, the presence of disturbances such as impedance uncertainties and hysteresis can strongly degrade force control performance and even lead to instability. Therefore, accurate force control when internal and external disturbances occur is a significant challenge. Conventional control methods usually have a number of restrictive conditions especially on the disturbance bounds. To rectify those issues, a modified robust disturbance rejection-based force control approach is proposed in this paper. For this purpose, an appropriate disturbance observer is utilized to estimate the disturbance effect regardless of amplitude. Then a robust control method is employed to achieve the disturbance-free desired dynamic. A modification is also performed to rectify the need for acceleration measurement in the control design. Finally, the force control for an unknown environment in the presence of disturbances is accomplished. The efficiency of the proposed approach is evaluated through simulation studies and compared with the well-known PI method. The experimental results validate the force control performance for the micropositioning piezoelectric actuator.

Precise control of piezoelectric actuators used in micropositioning applications is strongly under the effect of internal and external disturbances. Undesired external forces, unmodelled dynamics, parameter uncertainties, time variation of parameters and hysteresis are some sources of disturbances. These effects not only degrade the performance efficiency, but also may lead to closed-loop instability. Several works have investigated the positioning accuracy for constant and slow time-varying disturbances. The main concern is controlling performance and also the presence of time-varying perturbations. Considering unknown source and magnitude of disturbances, the estimation of the existing disturbances would be inevitable. In this paper, a compound disturbance observer-based robust control is developed to achieve precise positioning in the presence of time-varying disturbances. In addition, a modified disturbance observer is proposed to remedy the effect of switching behaviour in the case of slow time variations. A modified Prandtl–Ishlinskii (PI) operator and its inverse are utilized for both identification and real-time compensation of the hysteresis effect. Experimental results depict that the proposed approach achieves precise micropositioning in the presence of estimated disturbances.

Piezoelectric actuators are widely used for precise micro-positioning. In most applications, there is contact with the environment. Sensing external forces is a challenging problem in controlling mechanical systems, especially in micro scales. The external force could cause dual hysteresis non-linearity effects, which will disrupt the actuator positioning accuracy. Therefore, an effective controller should be designed to guarantee the trajectory tracking in the presence of external forces and perturbations. In this paper, a modified Prandtl–Ishlinskii operator and its inverse is utilized for both identification and real time compensation of the hysteresis effect. A variable structure controller is proposed for trajectory tracking of the actuator position. A sliding-based observer would estimate full states from the only measurable position trajectory. An adaptive perturbation estimation approach is proposed so that external forces can be separated from disturbances. The stability of the controller in the presence of estimated states and external forces is demonstrated analytically. The experimental results depict that the proposed approach achieves precise trajectory tracking and good external force estimation.

Piezoelectric actuators (PEAs) are frequently used in a wide variety of micromanipulation systems. However, the hysteresis non-linearity and the creep reduce their fidelity and cause difficulties in the micromanipulation control procedure. Besides, variation of temperature and external loads could change the model parameters identified for the piezo actuator. In this paper, a robust feedforward–feedback controller is proposed for precise tracking control of PEAs, under external disturbances. A Bouc–Wen hysteresis model is integrated with a second-order linear dynamic, to represent the behaviour of a PEA. Based on the Bouc–Wen model, a hysteresis observer is developed to estimate the hysteresis non-linear effect. Then, for real-time compensation of the observer error and the thermal and external load disturbances, a sliding mode control (SMC) strategy with a perturbation estimation function is utilized. To verify the efficiency of the proposed method, a comparison between the proposed approach and a classical SMC is presented.

Piezoelectric actuators are widely used in micro manipulation applications. However, hysteresis nonlinearity limits the accuracy of these actuators. This paper presents a novel approach for utilizing a piezoelectric nano-stage as the slave manipulator of a teleoperation system based on a sliding mode controller. The Prandtl–Ishlinskii (PI) model is used to model actuator hysteresis in feedforward scheme to cancel out this nonlinearity. The presented approach requires full state and force measurements at both the master and slave sides. Such a system is costly and also difficult to implement. Therefore, sliding mode unknown input observer (UIO) is proposed for full state and force estimations. Furthermore, the effects of uncertainties in the constant parameters on the estimated external forces should be eliminated. So, a robust adaptive controller is proposed and its stability is guaranteed through the Lyapunov criterion. Performance of the proposed control architecture is verified through experiments.

Piezoelectric actuators are widely used in applications of micropositioning. In the most situations, there is a contact with an environment. Existence of external forces is a challenging problem in controlling mechanical system, especially in micro scales such as piezoelectric actuators. In such a situation, dual hysteresis nonlinearity effects could disrupt the actuator micropositioning accuracy. In this paper a modified Prandtl-Ishlinskii (PI) operator and its inverse is utilized for both identification and real time compensation of the hysteresis effect. A variable structure controller is proposed for trajectory tracking of the actuator position. A sliding-based observer would estimate full states from the only measurable position trajectory. Proposing an adaptive perturbation estimation approach, the pure external forces would be observed. The stability of the controller in the presence of the estimated states and external forces is demonstrated analytically. The experimental results depict that the proposed approach is achieved in precise trajectory tracking and external force estimation.

In a master-slave telerobotic system controllers are inevitably implemented digitally. In addition to discretization as an outcome of digital implementation, the presence of delays in communication channels compromises stability as well as transparency. Therefore the designer has to take into account both of these factors in designing a stable teleoperator with a great degree of transparency. In this paper two impedance controllers are designed for master and slave with the objectives of delayed force and position tracking respectively. Linear stability analysis in z-domain has been applied in order to examine the effect of discretization. This also allows inspecting the influence of delay in a simple framework. Finally a plain set of rules are produced to tune the parameters so as to warrant stability with the best achievable transparency.

In this paper, a robust control approach using a novel hysteresis observer is developed for precise tracking control of piezoelectric actuators (PEAs). This control methodology considers the problems of unknown or uncertain system parameters, hysteresis non-linearity, and external load disturbances. For representing the behaviour of a PEA, a Bouc–Wen hysteresis model is integrated with a second-order linear dynamic. It is shown that the non-linear response of the model owing to the hysteresis effect acts as a bounded disturbance. Based on this fact a hysteresis observer is proposed to estimate the hysteresis effect. Then for real-time compensation of the observer error, parametric uncertainties, and external disturbances, a sliding mode control strategy with a perturbation estimation function is utilized. By using the proposed control approach the asymptotical stability in displacement tracking and robustness to the dynamic load disturbance can be provided. Finally, the proposed approach and conventional controllers are compared by experiments.

The current paper presents a new approach for cooperative control of a robotic system for grasping and handling an object with unknown geometry. The robotic system consists of three Cartesian robots with three degrees of freedom. The unknown object is approximately represented by three two-degrees-of-freedom virtual links of unknown length. Considering each of the virtual links as part of the corresponding robot, three five-degrees-of-freedom robots result that coordinate with each other as a closed kinematic chain. To cope with the kinematic uncertainty of the object, an adaptive algorithm consisting of a kinematic adaptation law is developed herein. For designing the controller, the dynamic equations of motion are transformed from joint space to the object space. In other words, the system dynamic is decoupled into a locked system and a shaped system. A proportional–derivative controller and a proportional–integral–derivative controller are developed for the shaped and locked systems, respectively. The object is modelled in the SimMechanics toolbox of MATLAB, and the controllers are simulated in the Simulink environment. Simulation results show that the proposed kinematic adaptation law together with the control system gives good performance for the cooperative system.

Most of the research conducted on snake robots has been on movement, control or dynamics. There is only some research dealing with the reduction of actuators' sizes. Actuator size usually depends on the force/torque it can provide. Small actuators imply a more efficient, long lasting, lighter and more flexible robot. The required force/torque and energy consumption consequently is directly affected by the mechanism design. Mother nature has always presented optimum systems and has inspired engineers. In this paper, we have adopted the snake anatomy to design a snake robot. The results show a reduction in torque demand. This robot is an extension of our previous research on building a snake without including the anatomy. The new robot weighs about only one-third of the previous version.

Piezoelectric actuators are widely used in micro manipulation applications. However hysteresis nonlinearity limits accuracy of these actuators. This paper presents a novel approach for utilizing a piezoelectric nano-stage as slave manipulator of a teleoperation system. The Prandtl-Ishlinskii (PI) model is used to model actuator hysteresis in feedforward scheme to cancel out this nonlinearity. To deal with the influence of parametric uncertainties, unmodeled dynamics, and PI identification error a perturbation term is added to the slave model and apply a sliding mode based impedance control with perturbation estimation. The stability of the entire system is guaranteed by Llewellyn’s absolute stability criterion. Performance of the proposed controllers is verified through experiments.

SUMMARYThis paper presents the research work on a 1 degree of freedom (DOF) force reflecting tele-micromanipulation system. This system enables a human operator to position remote objects very precisely having haptic feedback. The slave robot is a nano-positioning piezo-actuator with hysteretic dynamics. This intrinsic nonlinearity results in positioning inaccuracy and instability. Hence, a LuGre friction model is employed to model and compensate for this undesirable behavior. By means of a transformation, the 2-DOF master–slave system (1-DOF each) is decomposed into two 1-DOF new systems: the shape system, representing the master–slave position coordination, and the locked system, representing dynamics of the coordinated system. A key innovation of this paper is to generalize this approach to the hysteresis-type nonlinear teleoperated systems. For the shape system, a position tracking controller is designed in order to achieve position coordination. This position coordination is guaranteed not only in free space motion, but also during contact at the slave side. Furthermore, a force tracking controller is designed for the locked system in order to achieve tracking of the force exerted on the master and slave robots. Using this force controller, transparency is remarkably enhanced. Based on the virtual flywheels concept, passivity of the closed-loop teleoperator is guaranteed against dynamic parameter uncertainties and force measurement inaccuracies. The simulation and experimental results verify the capability of the proposed control architectures in achieving high-level tracking of the position and force signals while the system remains stable.

In this article, a novel control method is proposed for feedforward compensation of hysteresis non-linearity in various frequency ranges. By integrating a multirate hysteresis compensator controller with PID feedback control, a combined controller is developed and experimentally validated for a piezoelectric micro-positioning system. Piezoelectric materials show non-linear hysteresis behaviour when they experience an electrical field. A fundamental study of a piezoelectric actuator (PEA) shows that the hysteresis effect deteriorates the tracking performance of the PEA. This paper presents a non-linear model which quantifies the hysteresis non-linearity generated in PEAs in response to the applied driving voltages. The tracking control method is based on multirate feedforward control. The proposed multirate control method uses an inverse modified Prandtl-Ishlinskii operator to cancel out hysteresis non-linearity. The controller structure has a simple design and can be quickly identified. The control system is capable of achieving suitable tracking control and it is convenient to use and can be quickly applied to practical PEA applications. Experimental results are provided to verify the efficiency of the proposed method.

Piezoelectric materials show nonlinear hysteresis behaviour when they are under high electrical field and mechanical load. Fundamental study of PEA depicts that the Hysteresis effect deteriorate the tracking performance of The PEA. This paper proposes a nonlinear model which quantifies the Hysteresis nonlinearity generated in Piezoactuators in response to applied driving voltages. A novel perfect tracking control method based on multirate feedforward control is proposed which uses the nonlinear model to compensate mentioned limiting factors in PEA. In this study a multirate control method based on modified Prandtle-Ishlinskii operator as nonlinear model is implemented. It compensates rate dependant hysteresis nonlinearity in PEA. The controller structure has a simple design and can be quickly identified. The control system is capable to achieve suitable tracking control and it is convenient to use and can be quickly applied to the practical PEA applications. Experimental results are provided to verify the efficiency of the proposed method.

Teleoperation systems have been developed in order to manipulate objects in environments where the presence of humans is impossible, dangerous or less effective. One of the most attractive applications is micro telemanipulation with micropositioning actuators. Due to the sensitivity of this operation, task performance should be accurately considered. The presence of force signals in the control scheme could effectively improve transparency. However, the main restriction is force measurement in micromanipulation scales. A new modified strategy for estimating the external forces acting on the master and slave robots is the major contribution of this paper. The main advantage of this strategy is that the necessity for force sensors is eliminated, leading to lower cost and further applicability. A novel control algorithm with estimated force signals is proposed for a general nonlinear macro–micro bilateral teleoperation system with time delay. The stability condition in the macro–micro teleoperation system with the new control algorithm is verified by means of Lyapunov stability analysis. The designed control algorithm guarantees stability of the macro–micro teleoperation system in the presence of an estimated operator and environmental force. Experimental results confirm the efficiency of the novel control algorithm in position tracking and force reflection.

Cooperative robotic systems have been widely studied recently. However, bilateral teleoperation of these systems is rarely considered. Here, an adaptive control algorithm for the single-master multi-slave robotic systems is studied. The slave robots cooperatively handle an object of unknown kinematic and dynamic uncertainties. The master and the slave manipulators also contain dynamic uncertainties. The dynamics of the cooperative system is obtained based on a measurable point of the object. The object's grasping forces are concealed in the derived dynamics and can be controlled independent of the motion space. The controlled system is robust against constant but unknown time delays in the communication channel. Although the environment is passive, the human operator could become active in the designed control algorithm. Asymptotic stability of the closed-loop tele-operator in guaranteed in free motion strategy. In addition, the uniformly ultimately boundedness of synchronization signals is verified on contact scenario. Hardware in the loop experiments explain the performance of the proposed control framework.

In this paper, a new control scheme for bilateral teleoperation system with flexible link slave robot is addressed. In this regard, Modified collocated Proportional-Derivative (MPD) controller is suggested for flexible slave robot. The MPD control structure includes some vibration feedback terms to enhance the response of the flexible arm without necessity of additional sensors and knowledge of dynamic parameters. This approach utilizes capability of strain feedback in controller to reduce vibration at robot's end tip while PD part guarantee stable rigid body motion of the system. Stability of the bilateral teleoperation system under variable communication delays is proved based on Partial Differential Equation's (PDE) of system dynamics using Lyapunov method. Hence, the drawbacks of truncated-model-based methods are prevented. Experimental results for one DOF flexible teleoperation system validate improvement properties of presented framework in terms of trajectory tracking and force reflection.

In telerobotic systems the goal is to project human abilities from master side to the environment. One of the most important human skills is the capability of acting with different impedances at various situations proportional to the task. In this paper we focused on problems of variable environment or operator's hand impedances that persuaded us to utilize time domain stability analysis, and it could cause unbounded environment force and then, instability.

In this paper, an adaptive control scheme for a cooperative system consisting of two manipulators grasping and manipulating an unknown rigid object is proposed. It is assumed that the object will be in contact with a constrained environment through an assembly process. The contact force between each robot and the object is decomposed into two independent forces, one of which is for grasping the object, and the other is for motion of the object. The object position and its contact force with the environment of unknown stiffness is controlled using a robust adaptive control algorithm. The Lyapunov's direct method is used in design and stability analysis of the proposed controller. Simulation results, confirm the performance and effectiveness of the proposed controller.

Piezoelectric actuators are widely used in micro manipulation applications. However, hysteresis nonlinearity limits the accuracy of these actuators. This paper presents a novel approach for utilizing a piezoelectric nano-stage as the slave manipulator of a teleoperation system based on a sliding mode controller. The Prandtl-Ishlinskii (PI) model is used to model actuator hysteresis in feedforward scheme to cancel out this nonlinearity. The presented approach requires full state and force measurements at both the master and slave sides. Such a system is costly and also difficult to implement. Therefore, sliding mode unknown input observer (UIO) is proposed for full state and force estimations. Furthermore, the effects of uncertainties in the constant parameters on the estimated external forces should be eliminated. So, a robust adaptive controller is proposed and its stability is guaranteed through the Lyapunov criterion. Performance of the proposed control architecture is verified through experiments.

In this paper a new stable adaptive control for a Macro-Micro bilateral teleoperation system is proposed. Our platform to implement the controllers consists of a servo DC motor (macro) as the master robot and a piezo-actuator (micro) as the slave robot. Piezo-actuator has some characteristics which disturb the transparency and stability of the teleoperation system. We add a nonlinear disturbance observer to the slave robot controller in order to observe and compensate the disturbances. It is recognized that the presence of time delay is one of the largest barriers in teleoperation systems. This problem is mainly due to the distance separating the master from the slave and also is due to lag effect of filters and motor drivers. Because the time delay is unknown and variable, it can make the system unstable. In this paper, all of the above controllers are discussed using variable time delay. The stability of the system under variable time delay is guaranteed by Lyapunov stability criterion and passivity based methods. Tracking of force/position is achieved by selecting the best design parameters. Performance of the proposed control is validated by experimental results.Copyright © 2010 by ASME

Piezoelectric actuators (PEAs) are frequently used in a wide variety of micromanipulation systems. However their accuracy is limited due to hysteresis nonlinearity. Also investigation of the fundamental properties of the piezoceramics depicts that external mechanical loads cause inclination in hysteresis loop which can deteriorate tracking performance furthermore. A novel modeling and control approach is proposed in this paper, for precision trajectory tracking control of piezoelectric actuators under dynamic load condition. First the hysteresis nonlinear function based on Bouc-Wen hysteresis model is approximated by a Taylor series expansion. Then an adaptive trajectory tracking control is proposed based on the backstepping method using the developed mathematical model. The asymptotical stability in displacement tracking and robustness to the dynamic load disturbance can be provided using the proposed control approach. Experimental results are illustrated to verify the efficiency of the proposed method for practical applications.Copyright © 2009 by ASME

In this paper a nonlinear disturbance observer (NDO) based impedance control is proposed for a teleoperation system. The unknown friction and uncertainties will be estimated by the observer and added to the control input for compensation. Although friction will improve the stability, it worsens the transparency of the system which is another major point in teleoperation systems. The stability of the system is guaranteed by Lyapunov stability criterion and selecting the best design parameters. Tracking of force/position is achieved by these parameters. Also a fixed time delay is added to the system because of delays in the cables and other sources, it is then compensated with the designed controller. Performance of the proposed control is validated by experimental results.Copyright © 2009 by ASME

Friction forces in the robot joints insert nonlinearity in the dynamic and make errors in tracking. In this paper a new impedance control for the master is proposed to achieve force tracking. In addition, an impedance control for slave combined with sliding mode, which is based on perturbation estimation, is anticipated to reach position tracking. Friction compensators typically include a friction observer that provides a cancellation term in order to be added to the control input. Estimating and compensating the friction and other uncertainties will change the nonlinear equation to a linear one. The stability of entire system under time delay is guaranteed by Llewellyn’s absolute stability criterion. Performance of the proposed controllers is investigated through experiments.Copyright © 2009 by ASME

Piezoelectric actuators are widely used in micromanipulation tasks such as Atomic Force Microscopy and Cell Manipulation. However, the hysteresis nonlinearity and the creep reduce their fidelity and cause difficulties in the micromanipulation control procedure. Besides, variation of temperature and external loads could change the model parameters identified for the piezo actuator. In this paper, a novel feedforward-feedback controller is proposed. The Prandtl-Ishlinskii model is utilized to linearize the actuator hysteresis in feedforward scheme and a sliding mode based impedance control with perturbation estimation is used to cancel out the thermal and external load disturbances in feedback scheme. The efficiency of the proposed controller is verified by experiments.

In this paper we present a linear time invariant controller for bilateral teleoperation of a pair of N-DOF linear robotic systems under constant time delay. Our framework uses position and velocity signals to achieve position tracking in free motion. It also uses force feedback for compensating the human/environment disturbances to achieve position coordination in contact tasks. In passifying position and velocity feedback terms, for simplicity reasons there is no need to consider the dynamics of the robotic systems as long as they satisfy Euler-Lagrangian dynamic system equation. However when the force feedback is applied, we have to consider the dynamics of the robot in controller design. We do passivity analysis in the frequency domain by using Parseval's identity. Simulation has been performed to illustrate the proposed algorithm and to clarify some points in the design procedure.