A desired total orientation workspace for a parallel manipulator is usually an essential requirement in a practical application. At present, for the multiobjective optimization method of 6-DOF parallel manipulator for desired total orientation workspace, it is needed to predefine maximal and minimal lengths of the legs to serve as the constraint, and then the numerical method is used to solve the length of the legs and judge whether the solved maximal and minimal leg lengths meet the constraint. Predefining maximal and minimal length of the legs limits of the optimal range, the numerical method has heavy calculation burden and low calculating accuracy. In this paper, a hybrid method for solving the maximal and minimal lengths of the legs of 6-DOF parallel manipulator with desired total orientation workspace is proposed, and the actuator stroke length is calculated according to the maximal and minimal leg lengths. By judging whether the actuator stroke length can be solved to serve as the constraint, without the predefined maximal and minimal leg lengths to serve as the constraint, the optimal range is enlarged. Aiming at the physical size of the parallel manipulator and the proposed desired workspace condition index (DWCI), the optimization of the geometric parameters of the parallel manipulator is conducted based on the multiobjective optimization algorithm (NSGA-II), which is subject to the actuator stroke length. Stewart platform is set as the example; the geometric parameters of the platform whose workspace contains the desired total orientation workspace are optimized and the hybrid method is proved to be more accurate and efficient compared to the traditional numerical method. This method provides the optimization guidance for engineering designers to design the parallel manipulator for desired total orientation workspace.

Shipborne stabilized platform is an important equipment to ensure the stability of shipborne equipment relative to inertial coordinate system. This paper presents a model predictive control strategy based on ship motion prediction (MPMPC) for ship stabilization platform. Firstly, the ship motion is simulated, and the autoregressive prediction model (AR model) is used to predict the ship motion. Then the kinematics analysis of the Shipborne stabilized platform is carried out and the mathematical model of the hydraulic drive unit (HDU) of the stable platform is established. Then the predicted ship motion is combined with model predictive control (MPC). The predicted trajectory of HDU can be obtained by the kinematics calculation of predicted ship motion. One part of the predicted trajectory is used to compensate the time delay of HDU, and the other part is used as the reference trajectory of the rolling optimization of MPC, instead of the reference trajectory using the measured ship motion at the current moment in traditional model predictive control. Compared with the reference trajectory using the measured ship motion at the current moment, the predicted trajectory of AR model can reflect the future state of the system better, and a better control sequence will be obtained by minimizing the objective function. Finally, the simulation and experiment show that the MPMPC has higher tracking accuracy than traditional MPC.

This article studies the cavitation performance and preventing method of the hydromechanical pressure compensation independent metering system (HPCIMS). Compared with the conventional load sensing system (CLSS), the meter-in and meter-out orifices of HPCIMS can be regulated independently. A quasi-static behavior analysis of cavitation performance was applied to the HPCIMS and CLSS. The meter-in pressure equation of HPCIMS showed that keeping the ratio of the meter-in and meter-out orifices greater than the minimum value can avoid the cavitation phenomenon. Systems parameters were then kept as constant, and the key parameters related to cavitation performance of the two systems were compared by varying external force. Comparison results show that the cavitation phenomenon in the meter-in chamber of CLSS with the external active load is inevitable, but in HPCIMS, it can prevent the cavitation phenomenon by changing the ratio of the meter-in and meter-out orifices, so the HPCIMS has the cavitation prevention potential.

Hydraulic drive unit (HDU) is a typical actuator, but the characteristic of input delay hinders the application of many advanced control methods in HDU. First, in this paper, a mathematical model of HDU with input delay is established and the parameters are identified. Then, aiming at the input delay problem in HDU, a Smith estimated compensation model predictive control (SECMPC) strategy is proposed. On the one hand, the input delay state equation is employed to be a mathematical pattern for the state observation and predictive model. However, the combination between model predictive control (MPC) and Smith estimated compensation (SEC) is realized, the system state at k + d (d is the time delay coefficient) time is estimated in advance at k time to compensate the delay of the state. And then the prediction model based on input delay state equation is used for model prediction and rolling optimization. Thus, the delay system which is unstable is promoted to a stable system without delay. The effectiveness of SECMPC is proved with the HDU experiment and simulation; the maximum experimental displacement error of traditional MPC control is 15 mm, while that of SECMPC control is 8 mm. The SECMPC have some guiding significance for the control of systems with input delay.

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