This paper presents a robust nonlinear control scheme with time-varying output constraint for the electro-hydraulic force control system (EHFCS). Two typical double-rod symmetrical hydraulic cylinders are employed to simulate force environments in the EHFCS. Therefore, in order to improve the performance of the EHFCS, firstly, the model of the EHFCS is established with taking external disturbances, parameter uncertainties as well as structural vibrations into consideration. Secondly, in order to estimate external disturbances, parameter uncertainties and structural vibrations in the EHFCS and compensate them in the following robust controller design, two disturbance observers (DOs) are designed according to the nonlinear system model. Thirdly, with two estimation values from two DOs, a time-varying constraint-based robust controller (TVCRC) is presented in detail. Moreover, the stability of the proposed controller is analyzed by defining a proper Lyapunov functions. Finally, in order to validate the performance of the proposed controller, a series of simulation studies are conducted using the MATLAB/Simulink software. These simulation results give a fine proof of the efficiency of the proposed controller. What’s more, an experimental setup of the EHFCS is established to further validate the performance. Comparative experimental results show that the proposed controller exhibits better performance than the TVCRC without two DOs and a conventional proportional integral (PI) controller.

As a parallel mechanism with six-degree-of-freedom, an electro-hydraulic shaking table is endowed with strong coupling characteristics among different degree-of-freedoms. When the electrohydraulic shaking table moves to one direction, there will be some unnecessary related motions in other directions, which will affect the tracking accuracy. The kinematics model, the dynamic model, and the electro-hydraulic model are all established based on analyzing the components and working principles of the electro-hydraulic shaking table. After that, a modal space controller is built on the basis of a threevariable feedback controller by relying on the designed modal space matrix on the premise of modal control theory. In this regard, a multiple-input multiple-output system with strong coupling within the physical space is transformed into a set of single-input single-output systems independent from each other within the modal space. Moreover, the accuracy of vibration control can be improved by introducing a discrete feedforward controller. It is evident from the experimental results that the proposed control strategy can effectively improve the accuracy of vibration control.INDEX TERMS Electro-hydraulics, parallel mechanism, vibration control, coupling, modal space control.

This paper firstly presents a novel 4-SPS/S (active chain/passive chain) parallel manipulator (PPM) driven by four pneumatic artificial muscles (PAMs). SPS denotes a spherical pair-prismatic pair-spherical pair chain and S denotes a spherical pair chain. The PPM proposed can be utilized as shoulder, wrist, waist, hip and ankle simulators or ankle rehabilitation robot. And we present a comprehensive analysis on the PPM proposed, including degrees of freedom (DOF), position, kinematic, Jacobian, singularity, decoupling and workspace analysis. In order to decrease the influence of the maximum angle of spherical pairs in four active chains on the workspace, a novel connector for PAM is proposed. The structure of the PPM is explicitly described, and DOF of the PPM are analyzed based on constraint screw theory. The posture of the moving platform (MP) of the PPM is obtained though Z-Y-X (α-β-γ) type Euler angles and the inverse position solution is obtained. When the MP moves toward any a single Euler angle direction, the closed-form direct position solution is presented by geometric analysis. A back propagation (BP) neural network model for the whole direct position solution is established. Two methods for the Jacobian matrix, one velocity composition, one differentiation, are addressed, and the acceleration inversion is obtained. Based on the Jacobian matrix, the singularity and kinematic decoupling of the PPM are both analyzed. The geometric model of expanded PAM is introduced, and the change of diameters of four PAMs in active chains is taken into consideration when chains of the PPM interfere. Based on length ranges of four active chains, the maximum angle of spherical pairs and possible interferences between chains of the PPM, the configuration workspace is presented. Finally, the characteristic of the workspace and the influence of the maximum angle of spherical pairs in active chains on the configuration workspace are both analyzed.

This article presents a novel control strategy on an electro-hydraulic shaking table under the acceleration control combining an amplitude phase controller and a zero phase error tracking controller with a discrete feed-forward compensator. Because of the electro-hydraulic system’s nonlinearity, phase delay and amplitude attenuation exist in the acceleration response signal inevitably when the electro-hydraulic shaking table system is excited by a sine vibration signal. Moreover, the phase delay of the electro-hydraulic shaking table is composed of phase deviation and actuator delay. For improving the acceleration tracking accuracy, an amplitude phase controller is employed to compensate the phase deviation and amplitude attenuation by introducing weights to adjust the reference signal. Meanwhile, the discrete feed-forward compensator is applied to compensate the actuator delay. As an offline compensator, the zero phase error tracking controller is employed to compensate the phase delay of the response signal and improve the convergence speed of the proposed controller. Overall, the proposed control strategy combines the merits of these three controllers with better tracking performance demonstrated by simulation and experimental results.

The objective of this paper is to pursue a wire rope control methodology for reducing the tension difference between two wire ropes of a hoisting system. As we know, complicated disturbances exist in the complex electro-hydraulic hoisting system, notably, some of these disturbances are coupled, such as high-speed airflow disturbances, structure vibrations and vibrations in flexible wire ropes. Furthermore, there are model errors in force modeling due to the Coulomb friction between two wire ropes and two moveable head sheaves in the real physical hoisting systems. To eliminate disturbances, two types of disturbance observers (DOs) are employed: a traditional disturbance observer (TDO) and a coupled disturbance observer (CDO), both of which are utilized to estimate and compensate for the Coulomb friction and coupled disturbances online. As a result, a nonlinear backstepping control scheme is presented with estimation values from the TDO and the CDO. The experiment’s results demonstrate the effectiveness of the proposed control methodology.