Real-time Hybrid Simulation Research Articles

Restoring force correction based on online discrete tangent stiffness estimation method for real-time hybrid simulation

In real-time hybrid simulation (RTHS), it is difficult if not impossible to completely erase the error in restoring force due to actuator response delay using existing displacement-based compensation methods. This paper proposes a new force correction method based on online discrete tangent stiffness estimation (online DTSE) to provide accurate online estimation of the instantaneous stiffness of the physical substructure. Following the discrete curve parameter recognition theory, the online DTSE method estimates the instantaneous stiffness mainly through adaptively building a fuzzy segment with the latest measurements, constructing several strict bounding lines of the segment and calculating the slope of the strict bounding lines, which significantly improves the calculation efficiency and accuracy for the instantaneous stiffness estimation. The results of both computational simulation and real-time hybrid simulation show that: (1) the online DTSE method has high calculation efficiency, of which the relatively short computation time will not interrupt RTHS; and (2) the online DTSE method provides better estimation for the instantaneous stiffness, compared with other existing estimation methods. Due to the quick and accurate estimation of instantaneous stiffness, the online DTSE method therefore provides a promising technique to correct restoring forces in RTHS.

Real-Time Manufacturing Machine and System Performance Monitoring Using Internet of Things

This paper introduces a framework to assess the performance of manufacturing systems using hybrid simulation in real time. Continuous and discrete variables of different machines are monitored to analyze performance using a virtual environment running synchronous to plant floor equipment as a reference. Data are extracted from machines using industrial Internet of Things solutions. Productivity and reliability of a physical system are compared in real time with data from a hybrid simulation. The simulation uses discrete-event systems to estimate performance metrics at a system level, and continuous dynamics at a machine level to monitor input and output variables. Simulation outputs are used as a reference to detect abnormal conditions based on deviations of real outputs in different stages of the process. This monitoring method is implemented in a fully automated manufacturing system testbed with robots and CNC machines. Machines are integrated on an Ethernet/IP control network using a programmable logic controller to coordinate actions and transfer data. Results demonstrated the capacity to perform real-time monitoring and capture performance errors within confidence intervals. Note to Practitioners —Estimating expected performance of a manufacturing system processing different parts across multiple machines is a complex problem due to the lack of closed-form equations. Existing solutions focus on monitoring stochastic variables such as production or failure rate, or machine dynamics in separate environments often running asynchronous to the real system. This paper addresses the problem of monitoring and assessing the performance of complex manufacturing systems in real time. The proposed framework uses a real-time hybrid simulation of manufacturing at a machine and system level. The hybrid approach is based on a discrete and continuous model of manufacturing equipment integrated to run synchronously with the real plant floor operation. Data from both the virtual and real environments are merged to assess performance. Deviations from expected values represent an error that can trigger a warning signal to production, maintenance, and/or manufacturing personnel at the plant regarding health and productivity of plant operations.

Analytical and experimental investigations of Modified Tuned Liquid Dampers (MTLDs)

Modified Tuned Liquid Dampers (MTLDs) are a variation of a traditional Tuned Liquid Damper (TLD) which utilize a rotational spring system at the base, and therefore experience combined horizontal and rotational motions as the structure sways. To date, there have been very few studies conducted on MTLDs with experimental validation. In this research study, an MTLD-structure system is investigated using Lu's analytical model. Using Real-Time Hybrid Simulation (RTHS), a state-of-the-art testing method, the capabilities of the analytical model are experimentally verified. Important MTLD parameters such as the dimensionless rotational stiffness parameter, frequency ratio, and damping ratio are studied in detail. RTHS is used to study the effectiveness of the MTLD in mitigating structural response under sinusoidal inputs, as well as a range of ground motion records. It is shown both analytically and experimentally that when properly designed, the MTLD can exhibit more efficiency than a traditional TLD in mitigating vibrations.

Adaptive causal realization of rate-independent linear damping

This paper proposes an adaptive approach to achieve an accurate causal realization of rate-independent linear damping (RILD). RILD provides direct control over displacement, a benefit to low-frequency structures subjected to earthquake ground motion. The damping force generated by RILD is higher (lower) than that of linear viscous damping at lower (higher) frequencies, resulting in effective reduction of displacement and acceleration. The RILD force is proportional to displacement that is advanced in phase by π/2 radians, a noncausality that has limited its practical application. Adaptive controllers are proposed to approximate ideal (noncausal) RILD based on the dominant response frequency estimated in real-time and a filter-based causal model for RILD. By estimating the dominant response frequency, the displacement phase advance is more accurately applied.The adaptive control approach is demonstrated through the real-time hybrid simulation (RTHS) of a 5-story base-isolated building and a 14-story inter-story isolated building. A magnetorheological (MR) damper is added to the isolation layer of each structure to provide supplemental control mimicking ideal RILD. The MR damper is experimentally represented while the remainder of the structure is numerically simulated in the RTHS loop. The desired damping force is tracked by the semi-active damper, which is naturally in phase with velocity and has a controllable magnitude. The results compare well to noncausal numerical simulations in both damping forces and structural responses. Results also show clear improved seismic performance of the adaptive algorithms as compared to non-adaptive causal approximations of RILD and passive-on and off damper controllers (i.e., nonlinear hysteretic damping).

Improvement of Real-Time Hybrid Simulation Using Parallel Finite-Element Program

ABSTRACT This paper proposes a novel framework to efficiently calculate a large-scale finite element (FE) numerical substructure in real-time hybrid simulation (RTHS). It is composed of a non-real-time Windows computer and a real-time Target Computer. The Windows computer is used to solve the FE numerical substructure by parallel computing in soft real-time, while the real-time Target Computer generates displacement signals for the controller in real time. Based on the proposed framework, a RTHS with numerical substructure simulated in Windows environment is developed. It is demonstrated that the computational efficiency of the RTHS could be greatly improved by parallel programming.

Real‐time hybrid simulation for an RC bridge pier subjected to both horizontal and vertical ground motions

SummaryIt is well known that real‐time hybrid simulation (RTHS) is an effective and viable dynamic testing method. Numerous studies have been conducted for RTHS during the last 2 decades; however, the application of RTHS toward practical civil infrastructure is fairly limited. One of the major technical barriers preventing RTHS from being widely accepted in the testing community is the difficulty of accurate displacement control for axially stiff members. For such structures, a servo‐hydraulic actuator can generate a large force error due to the stiff oil column in the actuator even if there is a small axial displacement error. This difficulty significantly restricts the implementation of RTHS for structures such as columns, walls, bridge piers, and base isolators. Recently, a flexible loading frame system was developed, enabling a large‐capacity real‐time axial force application to axially stiff members. With the aid of the flexible loading frame system, this paper demonstrates an RTHS for a bridge structure with an experimental reinforced concrete pier, which is subjected to both horizontal and vertical ground motions. This type of RTHS has been a challenging task due to the lack of knowledge for satisfying the time‐varying axial force boundary condition, but the newly developed technology for real‐time force control and its incorporation into RTHS enabled a successful implementation of the RTHS for the reinforced concrete pier of this study.

Computational Challenges in Real-Time Hybrid Simulation of Tall Buildings under Multiple Natural Hazards

The essence of real-time hybrid simulation (RTHS) is its ability to combine the benefits ofphysical testing with those of computational simulations. Therefore, an understanding of the real-timecomputational issues and challenges is important, especially for RTHS of large systems, in advancingthe state of the art. To this end, RTHS of a 40-story (plus 4 basement stories) tall building havingnonlinear energy dissipation devices for mitigation of multiple natural hazards, including earthquakeand wind events, were conducted at the NHERI Lehigh Experimental Facility. An efficient implementationprocedure of the recently proposed explicit modified KR-a (MKR-a) method was developedfor performing the RTHS. This paper discusses this implementation procedure and the real-time computationalissues and challenges with regard to this implementation procedure. Some results from theRTHS involving earthquake loading are presented to highlight the need for and application of RTHSin performance based design of tall buildings under earthquake hazard.

Seismic Performance of Steel MRF Structures with Nonlinear Viscous Dampers from Real-Time Hybrid Simulations

Real-time hybrid earthquake simulations (RTHS) were performed on steel moment-resisting frame (MRF) structures with nonlinear viscous dampers. The test structures for the RTHS contain a moment-resisting frame (MRF), a frame with nonlinear viscous dampers (DBF), and a gravity load system with associated seismic mass and gravity loads. The MRFs have reduced beam section beam-to-column connections and are designed for 100%, 75%, and 60%, respectively, of the base shear strength required by ASCE 7-10. RTHS were performed to evaluate the seismic performance of these MRF structures. Two phases of RTHS were conducted: (Phase-1) the DBF is the experimental substructure in the laboratory; and (Phase-2) the DBF with the MRF is the experimental substructure. Results from the two phases of RTHS are evaluated. The evaluation shows that the RTHS provide a realistic and accurate simulation of the seismic response of the test structures. The evaluation also shows that steel MRF structures designed with reduced strength and with nonlinear viscous dampers can have excellent seismic performance.

Feasibility study of an adaptive mount system based on magnetorheological elastomer using real-time hybrid simulation

This article investigates an adaptive mount system based on magnetorheological elastomer in reducing the vibration of an equipment on the isolation table. Incorporating MR elastomers, whose elastic modulus or stiffness can be adjusted depending on the applied magnetic field, the proposed mount system strives to alleviate the limitations of existing passive-type mount systems. The primary goal of this study is to evaluate the vibration reduction performance of the proposed MR elastomer mount using the hybrid simulation technique. For real-time hybrid simulations, the MR elastomer mount and the control system are used as an experimental part, which is installed on the shaking table, and an equipment on the table is used as a numerical part. A suitable control algorithm is designed for the real-time hybrid simulations to avoid the responses of the equipment’s natural frequency by tracking the frequencies of the responses. After performing a series of real-time hybrid simulation on the adaptive mount system and the passive-type mount system under sinusoidal excitations, this study compares the effectiveness of the adaptive mount system over its passive counterpart. The results show that the proposed adaptive elastomer mount system outperforms the passive-type mount system in reducing the responses of the equipment for the excitations considered in this study.

Efficiency analysis of numerical integrations for finite element substructure in real-time hybrid simulation

Finite element (FE) is a powerful tool and has been applied by investigators to real-time hybrid simulations (RTHSs). This study focuses on the computational efficiency, including the computational time and accuracy, of numerical integrations in solving FE numerical substructure in RTHSs. First, sparse matrix storage schemes are adopted to decrease the computational time of FE numerical substructure. In this way, the task execution time (TET) decreases such that the scale of the numerical substructure model increases. Subsequently, several commonly used explicit numerical integration algorithms, including the central difference method (CDM), the Newmark explicit method, the Chang method and the Gui-λ method, are comprehensively compared to evaluate their computational time in solving FE numerical substructure. CDM is better than the other explicit integration algorithms when the damping matrix is diagonal, while the Gui-λ (λ = 4) method is advantageous when the damping matrix is non-diagonal. Finally, the effect of time delay on the computational accuracy of RTHSs is investigated by simulating structure-foundation systems. Simulation results show that the influences of time delay on the displacement response become obvious with the mass ratio increasing, and delay compensation methods may reduce the relative error of the displacement peak value to less than 5% even under the large time-step and large time delay.

Design and Performance Evaluation of an Optimal Discrete-Time Feedforward Controller for Servo-Hydraulic Compensation

AbstractServo-hydraulic actuators exhibit frequency-dependent variations of amplitude and delay during real-time hybrid simulation (RTHS). Effective compensation techniques to overcome these variat...

Real‐time force control for servo‐hydraulic actuator systems using adaptive time series compensator and compliance springs

SummaryServo‐hydraulic actuators have been widely used for experimental studies in engineering. They can be controlled in either displacement or force control mode depending on the purpose of a test. It is necessary to control the actuators in real time when the rate‐dependency effect of a test specimen needs to be accounted for under dynamic loads. Real‐time hybrid simulation (RTHS) and effective force testing (EFT) method, which can consider the rate‐dependency effect, have been known as viable alternatives to the shake table testing method. Due to the lack of knowledge in real‐time force control, however, the structures that can be tested with RTHS and EFT are fairly limited. For instance, satisfying the force boundary condition for axially stiff members is a challenging task in RTHS, while EFT has a difficulty to be implemented for nonlinear structures. In order to resolve these issues, this paper introduces new real‐time force control methods utilizing the adaptive time series (ATS) compensator and compliance springs. Unlike existing methods, the proposed force control methods do not require the structural modeling of a test structure, making it easy to be implemented especially for nonlinear structures. The force tracking performance of the proposed methods is evaluated for a small‐scale steel mass block system with a magneto‐rheological damper subjected to various target forces. Accuracy, time delay, and resonance response of these methods are discussed along with their force control performance for an axially stiff member. Overall, a satisfactory force tracking performance was observed by using the proposed force control methods.

An optimal discrete-time feedforward compensator for real-time hybrid simulation

Real-Time Hybrid Simulation (RTHS) is a powerful and cost-effective dynamic experimental technique. To implement a stable and accurate RTHS, time delay present in the experiment loop needs to be compensated. This delay is mostly introduced by servo-hydraulic actuator dynamics and can be reduced by applying appropriate compensators. Existing compensators have demonstrated effective performance in achieving good tracking performance. Most of them have been focused on their application in cases where the structure under investigation is subjected to inputs with relatively low frequency bandwidth such as earthquake excitations. To advance RTHS as an attractive technique for other engineering applications with broader excitation frequency, a discrete-time feedforward compensator is developed herein via various optimization techniques to enhance the performance of RTHS. The proposed compensator is unique as a discrete-time, model-based feedforward compensator. The feedforward control is chosen because it can substantially improve the reference tracking performance and speed when the plant dynamics is well-understood and modeled. The discrete-time formulation enables the use of inherently stable digital filters for compensator development, and avoids the error induced by continuous-time to discrete-time conversion during the compensator implementation in digital computer. This paper discusses the technical challenges in designing a discrete-time compensator, and proposes several optimal solutions to resolve these challenges. The effectiveness of compensators obtained via these optimal solutions is demonstrated through both numerical and experimental studies. Then, the proposed compensators have been successfully applied to RTHS tests. By comparing these results to results obtained using several existing feedforward compensators, the proposed compensator demonstrates superior performance in both time delay and Root-Mean-Square (RMS) error.

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.

Analysis of actuator delay and its effect on uncertainty quantification for real-time hybrid simulation

Uncertainties in structure properties can result in different responses in hybrid simulations. Quantification of the effect of these uncertainties would enable researchers to estimate the variances of structural responses observed from experiments. This poses challenges for real-time hybrid simulation (RTHS) due to the existence of actuator delay. Polynomial chaos expansion (PCE) projects the model outputs on a basis of orthogonal stochastic polynomials to account for influences of model uncertainties. In this paper, PCE is utilized to evaluate effect of actuator delay on the maximum displacement from real-time hybrid simulation of a single degree of freedom (SDOF) structure when accounting for uncertainties in structural properties. The PCE is first applied for RTHS without delay to determine the order of PCE, the number of sample points as well as the method for coefficients calculation. The PCE is then applied to RTHS with actuator delay. The mean, variance and Sobol indices are compared and discussed to evaluate the effects of actuator delay on uncertainty quantification for RTHS. Results show that the mean and the variance of the maximum displacement increase linearly and exponentially with respect to actuator delay, respectively. Sensitivity analysis through Sobol indices also indicates the influence of the single random variable decreases while the coupling effect increases with the increase of actuator delay.

Model-based framework for multi-axial real-time hybrid simulation testing

Real-time hybrid simulation is an efficient and cost-effective dynamic testing technique for performance evaluation of structural systems subjected to earthquake loading with rate-dependent behavior. A loading assembly with multiple actuators is required to impose realistic boundary conditions on physical specimens. However, such a testing system is expected to exhibit significant dynamic coupling of the actuators and suffer from time lags that are associated with the dynamics of the servo-hydraulic system, as well as control-structure interaction (CSI). One approach to reducing experimental errors considers a multi-input, multi-output (MIMO) controller design, yielding accurate reference tracking and noise rejection. In this paper, a framework for multi-axial real-time hybrid simulation (maRTHS) testing is presented. The methodology employs a real-time feedback-feedforward controller for multiple actuators commanded in Cartesian coordinates. Kinematic transformations between actuator space and Cartesian space are derived for all six-degrees-offreedom of the moving platform. Then, a frequency domain identification technique is used to develop an accurate MIMO transfer function of the system. Further, a Cartesian-domain model-based feedforward-feedback controller is implemented for time lag compensation and to increase the robustness of the reference tracking for given model uncertainty. The framework is implemented using the 1/5th-scale Load and Boundary Condition Box (LBCB) located at the University of Illinois at Urbana- Champaign. To demonstrate the efficacy of the proposed methodology, a single-story frame subjected to earthquake loading is tested. One of the columns in the frame is represented physically in the laboratory as a cantilevered steel column. For realtime execution, the numerical substructure, kinematic transformations, and controllers are implemented on a digital signal processor. Results show excellent performance of the maRTHS framework when six-degrees-of-freedom are controlled at the interface between substructures.

Mixed Force and Displacement Control for Testing Base-Isolated Bearings in Real-Time Hybrid Simulation

ABSTRACTThis paper presents a robust mixed force and displacement control strategy for testing of base isolation bearings in real-time hybrid simulation. The mixed-mode control is a critical experimental technique to impose accurate loading conditions on the base isolation bearings. The proposed mixed-mode control strategy consists of loop-shaping and proportional-integral-differential controllers. Following experimental validation, the mixed-mode control was demonstrated through a series of real-time hybrid simulation. The experimental results showed that the developed mixed-mode control enables accurate control of dynamic vertical force on the base isolation bearings during real-time hybrid simulation.

Servo-hydraulic actuator in controllable canonical form: Identification and experimental validation

Abstract Hydraulic actuators have been widely used to experimentally examine structural behavior at multiple scales. Real-time hybrid simulation (RTHS) is one innovative testing method that largely relies on such servo-hydraulic actuators. In RTHS, interface conditions must be enforced in real time, and controllers are often used to achieve tracking of the desired displacements. Thus, neglecting the dynamics of hydraulic transfer system may result either in system instability or sub-optimal performance. Herein, we propose a nonlinear dynamical model for a servo-hydraulic actuator (a.k.a. hydraulic transfer system) coupled with a nonlinear physical specimen. The nonlinear dynamical model is transformed into controllable canonical form for further tracking control design purposes. Through a number of experiments, the controllable canonical model is validated.

Experimental and analytical studies of seismic response of highway bridges isolated by rate-dependent rubber bearings

The mechanical behavior of a majority of rubber bearings used for seismic isolation is known to exhibit rate dependence. Although this feature has been known for years, few studies have attempted to quantitatively evaluate the influence of this characteristic on the seismic response of an isolation system at the structural level. In this study, experimental and analytical investigations were conducted to probe the seismic response of highway bridges isolated by rate-dependent rubber bearings. Three types of such bearings were investigated: natural rubber bearings, high damping rubber bearings (HDRBs), and super high damping rubber bearings (SHDRBs). To achieve the study objective, real-time hybrid simulation (RTHS) tests were performed on a typical multi-span continuous girder highway bridge under various earthquake ground motion intensities. In the RTHS tests, a velocity loading method was adopted and the bearings were physically tested in a real-time scale and larger time scales such that the bearings were exerted under different loading rates. The influence of the rate-dependent behavior of bearings on the seismic response of the bridge was investigated by conducting a series of parametric studies. The test results indicated that the seismic response of the bridge was significantly affected by all three types of rate-dependent rubber bearings, especially HDRBs and SHDRBs. Finally, a newly proposed rate-dependent analytical model and the traditional rate-independent bilinear model for rubber bearings were used in the numerical simulation. A comparison of the simulation results indicated that the proposed model performed better than the bilinear model in the seismic response analysis.

Structural control using a deployable autonomous control system

Structural control devices facilitate the construction of lightweight structures by suppressing excessive vibrations that arise from the reduced self-weight. Most of the current structural control systems are permanent installations designed to control particular structural properties and are hence specific to a particular application. This paper presents a novel concept of a deployable, autonomous control system (DACS) targeting specific applications where short-term vibration mitigation is desired. These applications may include control of existing structures during predictable extreme loading events or temporary structures where the need for vibration mitigation depends on usage characteristics. This control system consists of an electromechanical mass damper (EMD) mounted on an unmanned ground vehicle (UGV) equipped with vision sensors. The mobility of the UGV combined with on-board vision sensors facilitates autonomous positioning of the device at any desired location of the structure. This allows the device to update its position on the structure as required, through a simultaneous localization and mapping (SLAM) solution, to effectively control different structural modes. The performance of the SLAM solution is evaluated using a full-scale pedestrian bridge while the ability of the proposed system to re-position itself to control various modes of vibration is studied through real-time hybrid simulation (RTHS). The experimental results confirm the ability of the proposed system to effectively control large amplitude motion in slender bridges, while being able to position itself at the appropriate locations for multi-modal control. The concept of the overall system presents promising results for applications where temporary control is desired.