Oya Mercan

Implementation and verification of real-time hybrid simulation (RTHS) using a shake table for research and education

In this study, as a state of the art testing method, real-time hybrid simulation (RTHS) is implemented and verified with a shake table for education and research. As an application example, the dynamic behavior of a tuned liquid damper (TLD)-structure system is investigated. RTHS is a practical and economical experimental technique which complements the strengths of computer simulation with physical testing. It separates the test structure into two substructures where part of the structure for which a reliable analytical model is not available is tested physically (experimental substructure) and coupled together with the analytical model of the remaining structure (analytical substructure). The implementation of RTHS involves challenges in accurate control of the experimental substructure as well as the synchronization of the signals. The details of the hardware and the software developed and the steps taken to improve the controller are discussed in this paper so that the implementation of RTHS is properly introduced. The accuracy has been verified using tracking indicators as well as using the response obtained from a spring-mass oscillator and TLD system. The shake table used in this study is available in over 100 universities around the world. In this paper, the implementation of RTHS is provided with sufficient details to enable easy introduction of this testing method wherever a similar shake table is available. This additional functionality will not only provide a new research tool, but it will also facilitate classroom demonstrations to improve how students understand new concepts in structural dynamics and earthquake engineering.

Experimental and numerical investigations of the dynamic interaction of tuned liquid damper–structure systems

Tuned liquid dampers (TLDs) are dynamic vibration absorbers used in suppressing structural vibration under wind and seismic loads. They are easy to design and implement with low cost and low maintenance. However, due to their highly nonlinear behavior, it is difficult to establish representative models for TLDs that are accurate for a wide range of operations. In this paper, a new numerical model (finite volume method/finite element method (FVM/FEM method)) is introduced by simultaneously using finite volume and finite element approaches to represent fluid and solid domains, respectively. In order to assess the accuracy of the FVM/FEM results a state of the art experimental technique, namely real-time hybrid simulation (RTHS), is used. During the RTHS the response from the TLD is obtained experimentally while the structure is modeled in a computer, thus capturing the TLD–structure interaction in real-time. By keeping the structure as the analytical model, RTHS offers a unique flexibility in that a wide range of influential parameters are investigated without modifications to the experimental setup. This is not possible in traditional shake-table dynamic tests where a physical model of the structure needs to be built and tested together with the TLD. As a result, the verification of the numerical models for TLD–structure interaction available in the literature only consider a smaller, restricted dataset. In this study three numerical models from the literature are selected and together with the FVM/FEM developed here, the accuracies of these four models are assessed in comparison with RTHS results that consider a wide range of influential parameters. Results show that the proposed FVM/FEM model can accurately predict TLD behavior in both sinusoidal and ground motion forces and Yu’s model is the most accurate among the investigated simplified models.

A new tracking error-based adaptive controller for servo-hydraulic actuator control

Real-time hybrid simulation (RTHS) is a testing method which combines the response from an experiment (i.e., experimental substructure) with that of a computer model (i.e., analytical substructure) in real-time. The accuracy and stability of the RTHS are prone to the propagation of error in the measured signals. Thus, critical developments in servo-hydraulic actuator control are needed to enable a wide application of this testing technique. In this study, a new tracking error compensation strategy for servo-hydraulic actuator control is developed, and numerically and experimentally evaluated. This compensation procedure is formulated based on a new set of tracking error indicators, namely, frequency domain-based (FDB) error indicators, which uncouple phase (lag and lead) and amplitude (overshoot and undershoot) errors and quantify them. These indicators are then incorporated into a two degree-of-freedom (d.f.) controller to develop closed-form equations to design an adaptive servo-hydraulic controller with improved tracking performance. The FDB indicators and the new adaptive controller are studied through numerical simulations in Simulink and LabVIEW and also verified experimentally. The proposed controller is computationally efficient, it can be implemented in real-time and it does not require any user-defined (i.e., predetermined) coefficients. As a result of its two d.f. formulation, this adaptive controller can be introduced to any closed-loop servo-controller through a digital or analog interface depending on the experimental setup properties. As such, it can be used to improve the tracking capability of any single hydraulic actuator system, which is essential in RTHS.

Phase and Amplitude Error Indices (PAEI) to Assess the Success of Displacement Based Real-Time Testing

Real-time Pseudodynamic (PSD) and hybrid PSD testing methods are essential tools in understanding the dynamic behaviour of load-rate sensitive structures. In these testing methods, the response of the structure is obtained by combining computer simulation with physical testing. Since the measured signals are used in the command generation, these methods are prone to propagation of error; which, if not handled properly, may render the test results inaccurate and sometimes unstable. For that reason, there is a pressing need to develop measures by which the degree of accuracy of the real-time test results is to be assessed. The scope of this paper is to present a general, simple and invariant method for deriving improved error indices which are able to estimate the amount of phase and amplitude errors independently and through closed-form equations. These indices are also compared to previous error indicators to investigate their capability of assessing the success of real-time PSD tests.

Numerical modeling of dynamic behavior of annular tuned liquid dampers for the application in wind towers under seismic loading

In this study, an innovative technique is introduced for application of annular tuned liquid dampers (ATLD) in wind turbines subjected to seismic ground motions. The performance of ATLD in mitigating the vibration of wind turbines is investigated using numerical simulations. The wind tower is modeled using finite element method while the fluid domain is simulated by finite volume method. The numerical study considers the dynamic behavior of ATLD under different seismic records. Also, the effects of earthquake amplitude and frequency content, structural damping and detuning on the interaction between the tower and ATLD are investigated. The results of time-history analysis show that the ATLD is effective in mitigating the response of a wind turbine when subjected to large-amplitude seismic loading. The wind tower equipped with the proposed ATLD also behaves in the elastic range of response during the considered earthquake records which is critical for the integrity and safety of these structures.

Real-time hybrid simulation of structures equipped with viscoelastic-plastic dampers using a user-programmable computational platform

A user-programmable computational/control platform was developed at the University of Toronto that offers real-time hybrid simulation (RTHS) capabilities. The platform was verified previously using several linear physical substructures. The study presented in this paper is focused on further validating the RTHS platform using a nonlinear viscoelastic-plastic damper that has displacement, frequency and temperature-dependent properties. The validation study includes damper component characterization tests, as well as RTHS of a series of single-degree-of-freedom (SDOF) systems equipped with viscoelastic-plastic dampers that represent different structural designs. From the component characterization tests, it was found that for a wide range of excitation frequencies and friction slip loads, the tracking errors are comparable to the errors in RTHS of linear spring systems. The hybrid SDOF results are compared to an independently validated thermalmechanical viscoelastic model to further validate the ability for the platform to test nonlinear systems. After the validation, as an application study, nonlinear SDOF hybrid tests were used to develop performance spectra to predict the response of structures equipped with damping systems that are more challenging to model analytically. The use of the experimental performance spectra is illustrated by comparing the predicted response to the hybrid test response of 2DOF systems equipped with viscoelastic-plastic dampers.

Application of Gyro-Mass Dampers to Mitigate the Seismic Failure in Soft First Story Buildings

Sudden changes in stiffness and strength of the structures present a structural vulnerability leading to damage and collapse during strong earthquakes. This paper investigates the possibility of improving the seismic performance of soft first story structures by incorporating gyro-mass dampers (GMDs) as an alternative and effective novel solution to this problem. GMDs are newly developed passive energy dissipation devices for which the generated force by the element is proportional to the relative acceleration applied at the end nodes. These mechanical devices use a gear assembly to counteract dynamic forces by transforming the transitional motion into rotational one. Dynamic properties of GMDs are adjustable by changing the dimension of the physical components. Moreover, the application of compound gears makes it possible to amplify the small rotational inertial forces and make the GMDs (with/without additional viscous damping mechanism) a viable passive control strategy. Here, GMDs are used to control the drift at the first floor and to correct the mode shapes of the unbraced structure. These dampers are envisioned to be installed in each floor at the bottom of Chevron braces in the associated frame. Consequently, the brace-damper system acts like a Maxwell-element, which modifies the stiffness and equivalent mass of the original frame. Utilizing this concept, the governing equations including the equivalent mass and lateral stiffness contributed by a damper-brace system are formulated; used for design and an application example is presented.

COMPARISON OF INTEGRATION ALGORITHMS FOR REAL-TIME HYBRID SIMULATION USING FREQUENCY DOMAIN-BASED ERROR INDICATORS

Real-time hybrid simulation (RTHS) is a testing method that combines computer simulation (i.e., analytical substructure) with physical testing (i.e., experimental substructure). This enables investigation of dynamic characteristics of load-rate dependent complex structures. To ensure reliable experimental results, even in the presence of sophisticated controllers and compensation methods, it is necessary to evaluate the accuracy of the success of the hydraulic actuators in imposing the command displacements. With recent developments in tracking error monitoring, new indicators have been proposed that are based on frequency domain analysis and can successfully uncouple phase (lag and lead) and amplitude (overshoot and undershoot) errors and quantify them. These new frequency-based error indicators are not structure-specific and can be reliably used to examine the effectiveness of each component of RTHS inner (i.e., servo-control) and outer (i.e., integration algorithm) loops. In this paper, frequency domain-based (FDB) error indicators are employed to evaluate the effectiveness of five commonly-used integration algorithms (i.e. central difference method, explicit Newmark method, discrete state space formulation with a zero-order hold and first-order hold, the state-space formulation by Zhang et al., and the alpha method) in different test scenarios. The applicability of the FDB error indicators to nonlinear systems is first verified through numerical simulations performed on a nonlinear system with predefined parameters. Then these indicators are used to post-process the experimental results obtained from RTHS’s with various integration algorithms. The experiments are carried out on a system with a nonlinear analytical substructure and a linear experimental substructure at the University of Toronto. Findings from numerical simulation and experimental results demonstrate that the FDB method is an efficient approach to compare performances of different integration algorithms.