Real-time Hybrid Testing Research Articles

Real-time hybrid testing using iterative control for periodic oscillations

Real-time hybrid testing is a method in which a substructure of the system is realized experimentally and the rest numerically. The two parts interact in real time to emulate the dynamics of the full system. Such experiments, however, are often difficult to realize as the actuators and sensors, needed to ensure compatibility and force–equilibrium conditions at the interface, can seriously affect the predicted dynamics of the system and result in stability and fidelity issues. The traditional approach of using feedback control to overcome the additional unwanted dynamics is challenging due to the presence of an outer feedback loop, passing interface displacements or forces to the numerical substructure. We, therefore, advocate for an alternative approach, removing the problematic interface dynamics with an iterative scheme to minimize interface errors, thus, capturing the response of the true assembly. The technique is examined by hybrid testing of a bench-top four-storey building with different interface configurations, where using conventional hybrid measurement techniques is very challenging. A case where the physical part exhibits nonlinear restoring force characteristics is also considered. These tests show that the iterative approach is capable of handling even scenarios which are theoretically infeasible with feedback control.

A novel real-time hybrid testing method for Twin-Rotor Floating Wind Turbines with Single-Point Mooring systems

The Floating Wind Turbines (FWT) are at the forefront of innovation, aiming to enhance power capacity and reduce costs. Among these innovations, the Twin-Rotor Single-Point Mooring Floating Wind Turbines (TR-SPM FWT) stands out. However, the unique structural design and dynamic characteristics of TR-SPM FWT pose challenges for traditional FWT model testing methods. To address this, a real-time hybrid testing (RTHT) method is proposed, employing the Multi-drive Aerodynamic Loading Simulator (MALS), specifically tailored for TR-SPM FWT model tests. MALS incorporates load distribution rules to accurately simulate aerodynamic loads, with its precision and response speed comprehensively evaluated. Furthermore, to enable multiple MALS units to replicate complicated aerodynamic behaviors concurrently, interference between channels and synchronization of units are examined. A load correction model is introduced and a RTHT system is devised, considering the floater yaw feedback of TR-SPM FWT. Ultimately, the proposed method is successfully applied to investigate the dynamic response of a TR-SPM FWT under diverse wind and wave conditions, demonstrating the reliability of the proposed test method and system.

Novel multi-degree-of-freedom force–displacement mixed control framework for low loading rate substructure hybrid testing

In substructure hybrid tests, imposing realistic multi-degree-of-freedom (MDOF) displacement and force boundary conditions on a specimen involves controlling several actuators to emulate the MDOF boundary conditions of the experimental substructure. This setup presents significant challenges for decoupling control in MDOF systems. Due to the typically high stiffness of the specimen in the axial direction, force control is preferred over displacement control, which has limited accuracy in this direction. Typically, force control in the axial direction is combined with simultaneous displacement control in the softer direction, forming an MDOF force-displacement mixed control scheme. This scheme transforms force control into displacement control in the inner loop in the axial direction to ensure safe loading. The scheme involves addressing two major challenges: the nonlinear force–displacement transformation and the nonlinear coordinate transformation from the local actuator space (LAS) to the global Cartesian coordinate (GCC) system. To address these challenges, the Modified Newton–Raphson iteration integrated with proportional integral plus feed-forward (MNR-PIFF) and incremental kinematic transformation integrated with proportional integral plus feed-forward (IKT-PIFF) methods are proposed. Based on these methods, a novel MDOF force–displacement mixed control (NMFDC) framework is developed for substructure hybrid testing. A 6-DOF cyclic test simulation using the NMFDC framework was conducted to demonstrate its superiority. Furthermore, a 6-DOF cyclic test and substructure hybrid test at low loading rates were performed to validate the NMFDC framework because of the loading equipment limit. Experimental results and numerical simulations indicate that the NMFDC framework achieves swifter response with lower error levels. For applications in fast and real-time hybrid testing, dynamic loading equipment need to be used, the PIFF controller in the NMFDC framework should be replaced by adaptive time delay compensation in future developments.

Evaluation of control accuracy for a boundary-coordinating device in a real-time hybrid test

Multi-axial real-time hybrid simulation (ma-RTHS) utilizes multiple loading devices to realize boundary control with multiple degrees of freedom (MDOF), thus being capable of handling complex dynamic scenarios and multi-dimensional problems. In this paper, a new control technique was developed by using a parallel configuration of double shaking tables to implement shear force and bending moment at the boundary between substructures. The dynamic forces are combined by inertia forces of controlled mass driven by electromagnetic shaking tables. The two shaking tables are packaged as a boundary-coordinating device (BCD). An enhanced three-variable control (ETVC) was proposed to consider the coupling effect between two shaking tables and incorporated with the adaptive time series (ATS) compensator to improve the synchronization of the two shaking tables. The proposed control method was verified by three rounds of hybrid tests on a four-story steel shear frame using different ground motions. Nine criteria were utilized to evaluate the performance of RTHS including both tracking performance and global performance indexes. It was proved that RTHS was successfully implemented, and the boundary forces were well-tracked by the proposed control strategy. Good tracking performance was achieved to prove the effectiveness of the strategy.

Real-time hybrid test method for floating wind turbines: Focusing on the aerodynamic load identification

Since the development of floating wind turbine (FWT) shows a rapid trend towards larger capacity and larger rotor size, the traditional full-model basin test method has encountered limits. Especially the balance between the wind generation system (WGS) size and scale ratio of the model FWT system. Under such circumstances, the hybrid model test method provides the possibility to improve the FWT model test. In the hybrid model test method, the original physical model system is divided into physical subsystem and numerical subsystem, and the data acquisition and process module between the 2 subsystems plays an important role. In this paper, a framework of real-time hybrid test (RTHT) system is setup firstly, which combines physical model wind turbine and motion platform. The damping modification to the corresponding numerical code is applied to improve the motion calculation accuracy. Delay implementation is employed to avoid motion divergence. Then a simulation loop is setup in numerical environment to study the identification of aerodynamic load. The influence of identification accuracy to the RTHT result is analyzed. Lastly, the dual-accelerometer method of aerodynamic load identification is employed in the proposed RTHT system. Decay tests and irregular wave only tests are carried out to validate the aerodynamic load identification method. The capability and potential of the RTHT method of floating wind turbine model test is preliminary proved in the work of this paper.

Bidirectional Real-Time Hybrid Test on a Steel Column Virtually Connected to a Reinforced Concrete Substructure

Bidirectional Real-Time Hybrid Test on a Steel Column Virtually Connected to a Reinforced Concrete Substructure

Shaking Tables Test on Seismic Responses of a Long-Span Rigid-Framed Bridge Considering Traveling Wave Effect and Soil–Structure Interaction

The traveling wave effect and soil–structure interaction have significant influence on the seismic response of large-span bridges with complex site conditions. In this paper, a 1/10 scaled-down large-span rigid-framed bridge model was designed and fabricated, and a shaking tables test considering the traveling wave effect and soil–structure interaction was carried out on a large-scale continuous rigid bridge model by a real-time substructure hybrid test technique. Influences of the traveling wave effect and soil–structure interaction on the seismic responses of the rigid-framed bridge specimen were systematically analyzed with experimental data. The test results showed that when the apparent wave speed was small, the traveling wave effect increased the seismic responses of the rigid-framed bridge. With the increase in apparent wave speed, the structural response under traveling wave excitation and uniform excitation was basically the same. The SSI effect lead to a great change in the seismic input peaks and spectral characters at the bottom of the pier, and increased the seismic responses of the rigid-framed bridge. When both traveling wave and the SSI effect were considered, there was a phase difference in the seismic excitation. The dynamic responses of a continuous rigid-framed bridge could not be simply obtained by superposition of the separate traveling wave effect or SSI effect. Meanwhile, the real-time substructure test method in this paper solved the problems that the traditional soil box experiment cannot be applied to the test of a large-scale model, the soil and bridge structure find it difficult to meet the unified similarity ratio, and the boundary conditions are difficult to simulate accurately.

Stability prediction method of time-varying real-time hybrid testing system on vehicle-bridge coupled system

In recent years, real-time hybrid testing (RTHT) has been applied for the dynamic testing of high-speed trains running on bridges. A guarantee of stability for the RTHT system is essential to achieve a safe and reliable result. However, the inherent time-varying characteristics of the vehicle-bridge coupled system pose challenges to RTHT stability prediction. This study aims to develop a stability prediction method specifically tailored for time-varying RTHT system. Firstly, the vehicle-bridge coupled RTHT was modelled using a discrete state–space representation with a comprehensive consideration of the time-varying vehicle-bridge interaction and the dynamics of the shaking table. Subsequently, a time-varying stability criterion was derived from the periodic time-varying state matrix, forming the basis for a relative stability prediction method. The validity of the proposed method was confirmed through simulations and experiments employing a single-axle interaction within the vehicle-bridge coupled RTHT system. The coupled system consisted of a quarter-car model and simply supported beams were used as an example to evaluate the stability and accuracy of the time-varying RTHT. The results showed that the stability increased with increasing vehicle speed. Reducing the pure time delay of the shaking table improved both stability and accuracy. Increasing the effective frequency of the shaking table improved the accuracy but may reduce the stability of the RTHT.

Stability prediction method for real-time hybrid test system based on the measured dynamics of physical test system

The stability analysis of real-time hybrid test (RTHT) is crucial for the division of numerical and physical substructures, performance evaluation of loading system controllers, and feasibility judgment of RTHT systems. Existing stability analysis methods involve the parameter identification and numerical modeling of physical test systems. In contrast, an RTHT involves conducting a physical test on the hard-to-model part. Therefore, it is difficult to accurately model the physical substructure as well as the loading and acquisition systems. In this study, an RTHT stability prediction method based on the measured dynamics of the physical test system was developed, which avoid the tedious numerical modeling of the physical test system. The effectiveness of the stability prediction method was verified through numerical simulation and the experimental testing of a multi-degree-of-freedom shaking table RTHT. The experimental results showed that the accuracy of the developed stability prediction method was higher than that of the model-based stability prediction method by 35.1 %. The stability prediction method developed in this study simplifies the stability analysis of RTHT and will help determine the feasibility of test in advance.

Real-Time Hybrid Test Control Research Based on Improved Electro-Hydraulic Servo Displacement Algorithm

Real-time hybrid testing (RTH) is a test method for dynamic loading performance evaluation of structures, which is divided into digital simulation and physical testing, but the integration of the two may lead to problems such as time lag, large errors, and slow response time. The electro-hydraulic servo displacement system, as the transmission system of the physical test structure, directly affects the operational performance of RTH. Improving the performance of the electro-hydraulic servo displacement control system has become the key to solving the problem of RTH. In this paper, the FF-PSO-PID algorithm is proposed to control the electro-hydraulic servo system in real-time hybrid testing (RTH), which uses the PSO algorithm to operate the optimized PID parameters and the feed-forward compensation algorithm to compensate the displacement. First, the mathematical model of the electro-hydraulic displacement servo system in RTH is presented and the actual parameters are determined. Then, the objective evaluation function of the PSO algorithm is proposed to optimize the PID parameters in the context of RTH operation, and a displacement feed-forward compensation algorithm is added for theoretical study. To verify the effectiveness of the method, joint simulations were performed in Matlab/Simulink to compare and test FF-PSO-PID, PSO-PID, and conventional PID (PID) under different input signals. The results show that the proposed FF-PSO-PID algorithm effectively improves the accuracy and response speed of the electro-hydraulic servo displacement system and solves the problems of RTH time lag, large error, and slow response.

Pounding Mitigation Design and Real-Time Hybrid Simulation for a Novel Viscous Damper Applied in Bridges

When the actual displacement of viscous damper exceeds the stroke limit, excessive pounding force may be generated, resulting in damage to the damper. To address this issue, a novel viscous damper with variable stiffness, simple processing, and good economy is proposed. First, the load-displacement relation of the novel viscous damper is established through theoretical analysis and experimental studies. Second, numerical simulations and a series of parametric analyses are conducted to analyze the pounding mitigation effect of the proposed damper. Third, real-time hybrid simulation (RTHS) for considering the pounding effect of the novel viscous damper is presented. Finally, design recommendations for the pounding mitigation design of the novel viscous damper are given. The results show that the novel viscous damper effectively reduces the amplitudes of the pounding force and acceleration by 25% and 24%, respectively. The Fourier power of acceleration decreases in the region from 0.6 Hz to 3.3 Hz. The results of RTHS demonstrate that the real-time hybrid testing system can effectively simulate the pounding effect of the proposed damper. However, the normalized error peak value of velocity between RTHS and the reference result exceeds 10%, indicating that the proposed damper considering the pounding effect imposes new requirements to the real-time hybrid testing method.

Shake table real-time hybrid testing for shear buildings based on sliding mode acceleration control method

Existing studies involving shake table real-time hybrid testing (ST-RTHT) mainly use displacement-based control methods, which generally have good tracking for displacement but their acceleration tracking may be unsatisfactory. Although three-variable control has been introduced in some cases to improve the tracking performance, this approach cannot consider the influence of system nonlinearities and uncertainties. To expand the applicability of sliding mode control theory in shake table, in this study, a sliding mode acceleration control (SMAC) method is presented to overcome the drawback of traditional sliding mode force control (SMFC), and the information of the force applied by structure to shake table and the unmodeled complex nonlinear force is not required. Based on the proposed control method, the framework of ST-RTHT for shear buildings is developed. In the test system, an inner-loop controller composed of feedforward and feedback controls is designed based on the system transfer function, while the proposed SMAC that has asymptotic stability is integrated as an outer-loop controller. Finally, ST-RTHT for a two-story structure is successfully implemented to demonstrate the effectiveness of SMAC. Experimental results show that SMAC can achieve better synchronization of the boundary conditions between the physical and numerical substructures than SMFC, and thus improves the stability of ST-RTHT and accuracy of structural response reproduction.

A real-time hybrid testing based on shaking table and actuator for cable tray systems

For precisely disclosing the seismic performance of cable tray systems, one novel Real-Time Hybrid Testing based on Shaking Table and Actuator (RTHT-STA) is proposed in this paper. In the proposed method, one shaking table is used to simulate the ground motion, and one servo-hydraulic actuator controlled with displacement mode is set up on the shaking table for truly simulating the transverse shear boundary conditions between the numerical substructure and the experimental substructure of cable tray systems. The advantage of the proposed method is accurately realizing the loading requirements of high-precision large deformation on cable tray systems. The effectiveness and accuracy of the proposed method are validated by numerical simulations and experimental tests. The experimental results show that the real-time hybrid testing system is stable and reliable. The correlation coefficients between the results of the proposed method and those of the traditional shaking table tests are higher up to 91.78%. The results indicate the effectiveness and the high accuracy of the proposed RTHT-STA for cable tray systems. The RTHT-STA provides an economical and precise investigation strategy for disclosing the seismic performance of cable tray systems and may have broad application prospects in the field of communication engineering and civil engineering.

Real-time dynamic hybrid testing method for the dynamic characteristics of soil-structure interaction system

The radiation damping of the elastic half-space significantly affects the modal damping ratio of the superstructure. However, conventionally, it is hard for the traditional shaking table test to exhibit the radiation damping effect of the site due to the boundary effects of the model box. In this paper, a new impulsive-force-based real-time dynamic hybrid testing (RTDHT) method is developed to investigate the soil-structure interaction (SSI) effect on the structural dynamic characteristics. In the new method, the superstructure is considered to be a physical substructure and the semi-infinite soil deposits is as assumed to be a numerical substructure. The free vibration of the superstructure in the SSI system is excited by applying impulsive force at the interface of the soil and superstructure. Then, a time-domain method combining empirical wavelet transform and Hilbert transform is developed to identify structural modal parameters, which significantly improves the efficiency and accuracy of the experimental modal analysis for the shaking table test of the SSI system. A series of the RTDHT of a two-story steel frame structure on elastic half-space site is carried out, which validated the effectiveness and accuracy of the test method by comparisons between the experimental and numerical results.

Real-Time Hybrid Testing of a Floating Offshore Wind Turbine Using a Surrogate-Based Aerodynamic Emulator

AbstractPhysical modeling of floating offshore wind turbines (FOWTs) is challenging due to the complexities associated with the simultaneous application of two different scaling laws, governing the hydrodynamic and aerodynamic loading on the structure. To avoid these issues, this paper presents a real-time hybrid testing (RTHT) strategy in which a feedback loop, consisting of an on-board fan and control algorithm, is utilized to emulate the aerodynamic forces acting on the FOWT system. Here, we apply this strategy to a 70th-scale IEA Wind 15 MW reference wind turbine mounted on a version of the VolturnUS-S platform. Unlike other similar methods, which directly simulate the aerodynamic loads for the fan’s control using an aerodynamic code running in parallel with the experiment, this example utilizes a surrogate model trained on numerical model data calculated in advance. This strategy enables high-fidelity numerical model data, or even physical data, to be included in the aerodynamic emulation, by removing the requirement for real-time simulation, and, therefore, potentially enables more accurate loading predictions to be used in the experiments. This paper documents the development of the real-time hybrid testing system in the Coastal Ocean And Sediment Transport (COAST) Laboratory at the University of Plymouth in the UK, including the hardware, software, and instrumentation setup, and demonstrates the power of the surrogate-based aerodynamic emulator based on numerical data calculated using OpenFAST.

Implementation of shaking table based offline hybrid testing through neural networks

This study explores the feasibility of using offline hybrid testing to evaluate the seismic performance of an engineering structure. The challenges concerning system control, which include system instability and real-time loading accuracy, restrict the application of real-time hybrid testing (RTHT). To solve this problem, this paper proposes an offline shaking table hybrid test, which uses a combination of RTHT and neural networks to dispense with real-time data interaction between the physical testing and numerical solution in RTHT. As such, the challenges of system instability, accurate online loading, and real-time simulation in RTHT can be overcome. Using a system with two degrees of freedom, the feasibility of the proposed offline hybrid testing is discussed numerically and experimentally. The numerical simulation and experimental testing results both demonstrate that the offline hybrid testing has sufficient accuracy and robustness for seismic testing.

FRF-based non-simultaneous real-time hybrid testing of multiple subsystems with moderate nonlinearities

The existing cyber physical testing methods have been extended in a recent publication by non-simultaneous real-time hybrid testing, see Witteveen et al. [1]. There, an overall system is decomposed into two subsystems, one experimental and one numerical. The subsystems are loaded by certain inputs and simulated or tested separately in loops until compatibility conditions are fulfilled. If not, an input update based on the error of the kinematic compatibility in the frequency domain is computed for the next loop run. Each single test run is independent from the numerical model and therefore both is possible: Slow speed and real-time testing. In this work, the generalization to a systematic approach for an arbitrary number of subsystems is presented. The subsystems can be of numerical or experimental nature. While in the last publication the subsystems had to be tested one after the other within a loop run, they can be processed totally independently from each other in this generalized method. Further, both compatibility conditions (kinematic and cutting force) are considered for the update of the input signals. Finally, approximate models of the tested components can be considered in the numerical subsystems to improve convergence. The update for the inputs is again computed in the frequency domain based on FRFs of the isolated subsystems. The theory section is followed by two examples. The first one is of academic nature, and it is demonstrated that in case of convergence, all subsystems behave as they were part of one assembled overall system. The second one is a full vehicle hybrid testing where two shock absorbers are considered as real hardware while the vehicle is a multibody simulation model on a virtual four-poster test rig.

Dynamic ice loads for offshore wind support structure design

For offshore wind farms which are planned in sub-arctic regions like the Baltic Sea and Bohai Bay, support structure design has to account for load effects from dynamic ice-structure interaction. There is relatively high uncertainty related to dynamic ice loads as little to no load- and response data of offshore wind turbines exposed to drifting ice exists. In the present study the potential for the development of ice-induced vibrations for an offshore wind turbine on monopile foundation is experimentally investigated. The experiments aimed to reproduce at scale the interaction of an idling and operational 14 MW turbine with ice representative of 50-year return period Southern Baltic Sea conditions. A real-time hybrid test setup was used to allow the incorporation of the specific modal properties of an offshore wind turbine at the ice action point, as well as virtual wind loading. The experiments showed that all known regimes of ice-induced vibrations develop depending on the magnitude of the ice drift speed. At low speed this is intermittent crushing and at intermediate speeds is ‘frequency lock-in’ in the second global bending mode of the turbine. For high ice speeds continuous brittle crushing was found. A new finding is the development of an interaction regime with a strongly amplified non-harmonic first-mode response of the structure, combined with higher modes after moments of global ice failure. The regime develops between speeds where intermittent crushing and frequency lock-in in the second global bending mode develop. The development of this regime can be related to the specific modal properties of the wind turbine, for which the second and third global bending mode can be easily excited at the ice action point. Preliminary numerical simulations with a phenomenological ice model coupled to a full wind turbine model show that intermittent crushing and the new regime result in the largest bending moments for a large part of the support structure. Frequency lock-in and continuous brittle crushing result in significantly smaller bending moments throughout the structure.

Real-time hybrid testing for active mass driver system based on an improved control strategy against control-structure-interaction effects

General real-time hybrid testing (RTHT) for active mass driver (AMD) system requires shake table to achieve boundary conditions between the numerical and experimental substructures, which may be unavailable in some situations. Simplified RTHT is free of shake table, but it cannot consider control-structure-interaction (CSI) effects, and may result in an inaccurate performance evaluation of AMD system due to a significant influence of CSI on control performance. In this study, to eliminate the CSI effects, an improved control strategy is proposed, by incorporating the acceleration of control floor into the AMD controller and transforming all the control commands and feedbacks from the geocentric coordinate system to the relative coordinate system. Based on this strategy, the implementations of general and simplified RTHTs for AMD system are described. Experimental investigations are conducted on a two-story structure equipped with an AMD. It is confirmed that the general RTHT can be used to evaluate the actual performances of different control strategies. The CSI effects are essentially eliminated by the improved control strategy, but adversely affect the performance of traditional control strategy. In addition, the simplified RTHT is inappropriate for the performance evaluation of the traditional control strategy, but is available to that of the improved control strategy. Finally, an AMD control scheme is presented for a seismically excited 20-story benchmark building, and is evaluated through the simplified RTHT. Experimental results demonstrate the effectiveness of the proposed AMD control scheme. The maximum roof displacement and acceleration decrease from 0.175 m and 3.059 m/s2 to 0.116 m and 1.356 m/s2, respectively, when subjected to the El Centro earthquake.

Active vibration control for high‐rise buildings using displacement measurements by image processing

This study aims to develop a high-performance active mass damper (AMD) for super high-rise buildings with a low natural frequency. Conventional AMDs utilize accelerometers to control vibration. However, those AMDs cannot sufficiently damp the vibration because the accelerometer sensitivity is low in a low-frequency band (e.g., the natural frequency of high-rise buildings). Under these circumstances, this study proposes the measurement of vibration displacements using cameras and an image processing technique, called template matching, instead of accelerometers. The measurement accuracy in the low-frequency band was evaluated by comparison to that of the accelerometer. The results indicate that the proposed method achieved a higher measurement accuracy compared to the accelerometers in the low-frequency band. One problem when applying the displacement measurement using image processing to the vibration control is time delay with calculation. We solved this problem using the Padé approximation. Finally, real-time hybrid tests attained with a combination of real-time simulation and experiment were conducted. The results confirmed that the proposed method reduced the first resonance peak value by 24% in the low-frequency band compared with the accelerometers. In addition, it can prevent the degradation of the vibration damping performance of the higher vibration modes (i.e., spillover phenomena) caused by acceleration control. The proposed method contributes to the development of a novel AMD that suppresses the shaking of super high-rise buildings.