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This research develops a multiple-surface sliding mode control (MSSC) approach for position control of a servo-pneumatic system in the presence of mismatched uncertainties due to the friction force of the cylinder sealing. Servo-pneumatic actuators have many applications such as industrial automation, haptic interfaces, rehabilitation robots and non-invasive surgeries. Non-linearities due to internal and external disturbances, such as the friction force between the piston seal and cylinder wall, make it difficult to achieve adequate performance from these actuators. Thus, modelling and identification of friction parameters is an essential part of the controller design procedure. A simple model for friction such as the Stribeck model may be used in order to reduce the complexity of the identification procedure. In addition, a bounded uncertainty owing to the unmodelled dynamics of friction is considered. The lack of direct measurement for systems velocity necessitates finding a solution to estimate this parameter. One answer to this problem is to employ a high-gain observer. Whereas mismatched uncertainties appear in the state space equation of the system before control input, control input cannot apply directly to them. In the proposed framework, an MSSC scheme has been used to cope with these types of uncertainties. Asymptotic stability of the closed-loop system is proven by using the Lyapunov method; experimental results show that the proposed controller can deliver a good tracking performance and is robust to uncertainties. Experimental results validate the controller performance.

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.

One of the prevalent methods in linear bilateral teleoperation systems with communication channel time delays is using position and velocity signals in the control scheme. Utilization of force signals in such controllers improves the performance significantly and reduces the tracking error. Measuring force signals in such cases is one of the major problems. In this paper, a control scheme in the presence of human and environment force signals for the linear bilateral teleoperation is proposed. Due to elimination of measuring forces in the control scheme, a force estimation approach based on disturbance observers has been utilized. The proposed approach guarantees asymptotic estimation of constant forces. The estimation error would only be bounded for time varying external forces. To cope with the variation of the human and environment force, a sliding-mode-controller is used. The stability and transparency condition in the teleoperation system with the designed control scheme is derived from absolute stability concept. The designed control scheme guarantees the stability of the teleoperation system in the presence of time varying human and environment forces. Experimental results show that the proposed control scheme improves position tracking in the free motion and in contact with the environment. In addition, the force estimation approach appropriately estimates human and environment forces.

Many researchers have focused on force tracking improvement in teleoperation systems. Various methods have been utilized to reach this aim. This paper focuses on force-reflecting passivity-based architecture. Adding a force error term to conventional passivity-based architecture, a controller has been introduced to improve transparency. A condition has been obtained, which guarantees the stability of the system, while it holds. Moreover, a sliding mode perturbation estimation algorithm has been proposed. The method proved to be asymptotically converging to perturbations. The transparency of system has been compared in simple and force-reflecting passivity-based architectures, and also with and without perturbation estimator. Experiments on a 2-DOF non-linear delayed teleoperation system were conducted to investigate the system’s performance. Experimental results have shown that the controller is stable and force tracking has been improved compared with previous research. Furthermore, the perturbation in the system has been successfully estimated and cancelled. Conclusively, the paper introduces a controller that is stable and compares its performance in force tracking with previous research.

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.