Compensation Platform Research Articles

A novel inertial parameter identification method for the ship-mounted Stewart platform based on a wave compensation platform

To meet the requirement of time-varying inertial parameter identification of control algorithms for shipborne Stewart wave compensation platforms, this study proposes an innovative inertial parameter identification method based on a non-inertial system. Unlike traditional Stewart identification methods with a fixed frame, the proposed method does not require designing excitation trajectories to consider the condition number of the observation matrix. Namely, the proposed method requires only performing two-dimensional dynamic parameter collection for parameter identification in a six-dimensional non-inertial motion. In addition, a coupling error dimension reduction method is developed to improve the identification accuracy under the condition of coupling motion. Finally, experiments are conducted to verify the effectiveness of the proposed identification method. The experimental results show that under the coupling motion conditions, the coupling errors of the corrected singularity points are reduced by 7.9–28.1% compared to before correction, and the axial force average error correction is below 5.32%.

FPID-RCP: A Control Method for a Swing-Type Wave Compensation Platform System

With the rapid development of marine engineering in recent years, offshore operations have become increasingly common, making wave compensation platforms crucial for safe operations at sea. This paper presents a pendulum-type wave compensation platform specifically designed for wave compensation applications. The main components of this wave compensation platform include a chassis, support base, hydraulic cylinders, telescopic rods, upper platform, three sets of balancing mechanisms, three sets of tilt angle sensors, and a control system. Firstly, to thoroughly understand the compensatory motion of the pendulum-type three-degree-of-freedom wave compensation platform, kinematic analysis of the entire system was conducted, and the motion inverse solution curves of the mechanism were obtained through simulation using motion simulation software. Secondly, to enhance the compensatory response performance of the platform, a fuzzy PID control algorithm was employed to control the system and achieve attitude control of the platform. Finally, through control system simulation, compared to PID control, fuzzy PID reduces system response delay and successfully meets the expected technical requirements and application needs.

Anti-Swaying Control Strategy of Ship-Mounted 3-RCU Parallel Platform Based on Dynamic Gravity Compensation

It is essential to ensure stability during marine transportation or the installation of high center of gravity loads. The heavy loads increase gravity disturbance, affecting the steady-state-error control of the multiple degrees of freedom (DOFs) motion compensation platform. In this paper, we propose a proportional derivative (PD) controller with dynamic gravity compensation (PDGC) for a 3-RCU (revolute–cylindrical–universal) parallel platform to improve the control effect of marine motion compensation for high center of gravity loads. We introduce an evaluation parameter of load stability and a weighting coefficient of anti-swaying control to tune the controller performance. The controller can set its control target between the two, keeping the load contact surface level and allowing the load center of gravity with the least movement. By deriving the Jacobian matrix, the gravity disturbance in the joint space is calculated and is compensated in the controller. First, we verify the control superiority of this controller over the PD controller under sinusoidal excitation in simulation and validate the effectiveness of the proposed anti-swing strategy. Then, the experiments are conducted with random excitation. The root mean square (RMS) value of the load’s residual angle with the proposed controller is reduced to 32.2% and 17.6% in two directions, respectively, compared with the PD controller under class 4 sea state excitation. The proposed method is effective for the anti-swaying control of ship-mounted 3-RCU parallel platforms.

Design and analysis of 3-UPU wave compensation platform based on improved sliding mode control

ABSTRACT In this paper, the 3-UPU parallel mechanism platform is designed through SolidWorks software, and the assembly model of the 3-UPU parallel mechanism is imported into the Simulink environment through Simscape/Multibody. Aiming at the heave motion of the ship at sea, an improved sliding mode controller is designed in this research, named as global fast terminal sliding mode controller, to track and compensate the ship heave motion data. In order to verify the effect of the designed controller, the traditional PID controller is selected for comparison. The results show that: when the traditional PID controller is used to track and compensate it, if the appropriate parameters are selected, the tracking effect is better, the tracking error is small but fluctuates up and down, and the tracking error accuracy is poor. After the introduction of the improved sliding mode controller proposed in this paper, when entering the stable stage, the tracking compensation effect of the improved controller is better than that of the traditional PID control, and the tracking error is smaller than that of the traditional PID control. The global fast terminal sliding mode control has good robustness and can be applied to the ship heave compensation system.

A Prediction Method of Ship Motion Based on LSTM Neural Network with Variable Step-Variable Sampling Frequency Characteristics

In active heave compensation, in order to realize the smooth control of the heave compensation platform, it is necessary to use the ship motion measurement system to accurately obtain the ship displacement signal, invert the ship displacement signal, and then control the expansion and contraction of the electric cylinder so that the compensation platform remains horizontal. The ship displacement measurement system generally adopts the second integral of the acceleration sensor to obtain the ship displacement signal. During the acquisition process of the ship displacement signal, the quadratic integration process of the acceleration, and the communication process of the output control command, there is a processing lag which makes the error accumulate, resulting in a delay in the measurement of the ship motion. In order to collect the ship motion more accurately and control the heave compensation platform more precisely, this paper proposes a ship motion prediction method based on a variable step-variable sampling frequency characteristic LSTM (Long Short-Term Memory) neural network. First, we use the autocorrelation function algorithm to calculate the inherent delay of the lag in the process of signal acquisition by the measurement system. Secondly, the LSTM neural network is used to predict the inherent delay step of the lagging ship displacement signal. During the prediction process, it is found that the difference in the sampling frequency of the displacement signal will lead to a change in the step of the inherent delay—experiment in the laboratory to verify. By controlling the motion platform to simulate the motion of the ship and using the ship motion measurement system and the laser sensor system to measure the displacement signal of the motion platform synchronously, it is verified that the ship motion measurement system does have an inherent delay. Thirdly, on a sailing ship, ship displacement signals are collected by setting multiple sets of ship motion measurement systems. Finally, multiple sets of sampling frequency and multiple steps are set, and the ship motion is predicted based on the variable step-variable sampling frequency LSTM neural network. It is verified that the prediction accuracy is related to the sampling frequency of the signal collector and the prediction step of the LSTM neural network, which improves the prediction accuracy of the model and the timeliness of ship motion acquisition.

A novel motion planning algorithm for a three DoF foldable parallel compensation platform based on prediction and B-spline

Foldable parallel mechanisms are suitable to compensate for the motion of vessels due to their compact structure, multidimensional motion, and large carrying capacity. However, the vessel sways violently under harsh sea conditions, challenging the rational utilization of the mechanism workspace. This work proposes a novel motion planning algorithm based on motion prediction and the B-spline for a three-degrees-of-freedom foldable parallel compensation platform. First, for harsh sea conditions, the motion law of a guardian vessel is studied, and the encoder-decoder model is used to predict the roll, pitch, and heave motions of the vessel. Second, we propose a multidimensional motion compensation strategy under the limits of the mechanism dimensions. Then, a backward kinematics model is developed. Its workspace under the structural and actuator limitations is proposed. Finally, based on the theory of the nonuniform rational B-spline curve and target motion prediction, a dynamic motion planning algorithm is proposed, and experiments are conducted. The motion planning algorithm can provide smooth motion commands under the mechanism workspace, velocity, acceleration constraints. It can be realized using asynchronous calculation, which meets real-time motion control requirements.

Control system research in wave compensation based on particle swarm optimization

For the offshore wave compensation control system, its controller setting will directly affect the platform's compensation effect. In order to study the wave compensation control system and optimization strategy, we build and simulate the wave compensation control model by using particle swarm optimization (PSO) to optimize the controller's control parameters and compare the results with other intelligent algorithms. Then we compare the response errors of the wave compensation platform under different PID controllers; and compare the particle swarm algorithm's response results and the genetic algorithm to the system controller optimization. The results show that the particle swarm algorithm is 63.94% lower than the genetic algorithm overshoot, and the peak time is 0.26 s lower, the adjustment time is 1.4 s lower than the genetic algorithm. It shows that the control effect of the wave compensation control system has a great relationship with the controller's parameter selection. Meanwhile, the particle swarm optimization algorithm's optimization can set the wave compensation PID control system, and it has the optimization effect of small overshoot and fast response time. This paper proposes the application of the particle swarm algorithm to the wave compensation system. It verifies the superiority of the method after application, and provides a new research reference for the subsequent research on the wave compensation control systems.

Three-DoF Vibration Compensation Platform for Robot-Based Precision Inline Measurements on Free-Form Surfaces

This article presents a three-degree-of-freedom (DoF) magnetically levitated vibration compensating measurement platform (MP) with arbitrary operation orientation for robot-based surface inspection of free-forms. The MP design comprises an internal position sensor system, which is used in feedback control to maintain a free-floating position in six DoFs with respect to its supporting frame when approaching a sample (stabilization mode). In addition, the integrated design includes three tracking sensors measuring the out-of-plane position of the MP to a sample surface. The entire system can be mounted as an end-effector of an industrial robot with the MP carrying a desired measurement tool. Disturbing environmental vibrations are actively compensated by maintaining a constant position of the MP with respect to a sample surface (tracking mode). The control design includes single-input single-output position controllers for all DoFs, achieving a bandwidth of about 130 Hz. By means of a cross-fading error gain, an efficient control transition between stabilization and tracking mode is enabled. A vibrating target is used to evaluate the vibration compensation performance of the tracking module for standardized floor vibration profiles. Attenuation between 35 and 20 dB for out-of-plane vibrations below 40 Hz is achieved. In the time domain, vibration profiles with 15.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m rms and 95 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> rad rms are reduced to 1.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m rms and 21 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> rad rms, respectively.

A Novel Three-SPR Parallel Platform for Vessel Wave Compensation

A wave compensation platform based on 3-SPR parallel platform is designed for marine ships with a dynamic positioning system. It can compensate for the heave, rolling, and pitching movement of a vessel under level 4 sea state. The forward kinematics of the mechanism is used to draw the central point position workspace and the attitude workspace of the moving deck of the compensation platform. The compensation effects of the 3-RPS parallel compensation platform and the 3-SPR parallel compensation platform are compared, and the feasibility and superiority of the compensation scheme using the 3-SPR parallel compensation platform are proved. To lower the working height of the upper deck of the compensation platform and reduce the extension range of the support legs, the structure of the compensation platform is optimized, and a novel 3-SPR parallel platform is designed. Finally, a simulation model was established. Using the inverse kinematic model as a compensation movement solver which can online calculate the length of branch legs based on the measured heaving, rolling, and pitching values of vessels, the compensation effect of the new structure under a certain sea state is simulated. The result demonstrated the efficiency of the ship motion decoupling movement of the newly designed compensation platform and proved the competence of it.

Optimization and High-performance Hardware Implementation of Multipath Interference Compensation Algorithm

Our previous sparse regularization method for multipath interference (MPI) compensation in pulsed Time-of-Flight (ToF) depth camera works well on desktop computer. However, pulsed ToF depth cameras are mainly applied in fields such as autopilot vehicles and robots. In these situations, exporting depth images to a computer and running MPI compensation in MATLAB every time are not realistic. Therefore, according to the hardware structure of the multipath interference real-time compensation platform, we use QR decomposition to optimize the sensing matrix in our algorithm for better hardware implementation, while maintaining the accuracy of our algorithm. For the optimized algorithm, we designed an efficient parallel data processing hardware structure that matches the features of our algorithm to optimize the calculation efficiency. Then, we implemented it on the realtime compensation platform. The experimental results show that this algorithm architecture runs stably under the clock frequency and it runs faster than desktop computer.

Determination of Reaction Forces of a Deck Crane in Wave Motion Using Screw Theory

In this paper, we present a method for calculating reaction forces for a crane mounted on a ship moving in waves. The method is used to calculate the reaction forces between the crane base and the vessel deck. This includes the case where the crane is mounted on the platform that keeps the base of the crane horizontal when the vessel is moving in roll and pitch. The wave motion of the ship is modeled with force response amplitude operators (RAOs) based on the JONSWAP wave spectrum. The combined equations of motion for a vessel and a crane are derived using Kane’s equations of motion, where velocities and angular velocities are formulated in terms of twists, and the associated partial velocities and partial angular velocities are given as lines in Plücker coordinates. The unknown reaction forces are represented as wrenches and are determined using screw transformations. The method is used to study the effect of the roll and pitch compensation platform in numerical simulations. The efficiency of the platform is evaluated in terms of the magnitude of reaction forces and crane payload sway angles.

Analysis of Kinematics and Dynamics of 3UPU/PU Parallel Platform Used in Automatic Wave Compensation

3UPU/PU parallel platform is able to achieve a linear and two rotating motion (3-DOF ) platform,it can used in automatic wave compensation of ship deck equipment. The compensation platform is composed of dynamic and static plate, hydraulic cylinder and driven chain parts; it has strong capacity , simple structure and suitable for most conditions. This paper analysis the motion of the platform using one order and two order influence coefficient, establishes the dynamic model of this platform using the Newton Euler method , obtained the velocity, acceleration and force curve by simulating calculation, lay the foundation for the stable platform’s further dynamic analysis and control.

A Modified Pall-Typed Friction Damper for Vibration Control in Series Compensation Platform Structure

Based on the characteristics of series compensation platform structure, a method of installing modified Pall-typed friction dampers for seismic passive control was proposed. Compared with the conventional , the modified Pall-typed friction damper has the same effect on energy dissipation, and it is superior in imposing pretension at both ends and saving friction materials. The equations of relation between brace displacement and internal force of the modified Pall-typed friction damper were derived. Besides, the time-history analysis method for seismic responses of series compensation platform structures with modified Pall-typed friction dampers was established, and a sine amplitude-modulation wave train was applied to analyzing the dynamic responses of these structures. The results show that, the displacement of the series compensation platform structures with modified Pall-typed friction dampers decrease to some extent. The seismic control is effective.

SU-E-J-157: Simulation and Design of a Real-Time 6D Head Motion Compensation Platform Based on a Stewart Platform Approach.

Real-time sub-millimeter head motion compensation during frameless SRS delivery has the potential to achieve the accuracy of frame-based SRS while being significantly less invasive. Previously, we demonstrated real-time 6D head motion monitoring using an optical camera, however, at the time we were limited to only 3D (x-y-z) of head motion correction due to mechanical restrictions of the head platform. In this work we investigate the feasibility of using a compact 6D robotic Stewart platform (hexapod) placed under the patient's head to perform both translational and rotational motion compensation in real-time. Benefits of a hexapod approach over a conventional serial kinematics stage include less flex, compactness, high force to weight ratio, and fast response times. A hexapod is a parallel robotics device consisting of two platforms connected by six linear actuators oriented at particular angles. To provide accurate motion in 6D, the desired position of the top platform (head) was ascertained using inverse kinematics. MATLAB was used to simulate the six actuator positions for performing motion along x-y-z-phi -theta-psi. Prior recorded 6D human volunteer head motion data was used as an input for simulation of motion compensation. Six Firgelli L12-P linearservo actuators, together with a PCI-7344 motion controller and Labview software, were used for initial construction of a hexapod prototype. The necessary actuator lengths over time were computed for this data, simulating the required 6D movement of the hexapod for motion correction. Simulations on previously collected volunteer data indicate a hexapod system is capable of responding to subject head motion with corrections of precise movements, and solutions to the linear system can be computed at near real-time speeds. Based on simulated results, it was successfully demonstrated that a hexapod device can compensate for small patient head motions along all six degrees of freedom.