Implementation of hybrid simulation

The SRMD testing facility was originally designed to conduct dynamic characterization tests of full scale bearing devices and dampers. This facility hosts required infrastructure (hardware and software components) that can be used to adapt the facility to conduct hybrid simulation. Hybrid simulation requires that customizable hardware components communicate with the computational driver, which solves the equations of motion for the hybrid model composed of numerical and experimental subassemblies, and the controller in the laboratory loading the experimental subassembly. For the large scale bearing tests presented here, the hybrid model needs to communicate with the SRMD control system in each integration time step to send command signals and receive measured feedback signals.

For real-time testing, communications need to be very fast and reliable among the various components.

Performance limitations such as actuator tracking, actuator delays, and communication speeds determine the rate of testing that can be achieved.

5.6.1 Hardware configuration

The three-loop architecture shown in Figure 123 was implemented at the SRMD to achieve continuous communication of commands and feedbacks between different components. This architecture includes OpenSees as the computational driver, the SRMD control system, and a real-time digital signal processor (DSP) all communicating through SCRAMNet+ (Shared Common Random Access Memory Network [35]).

FIG. 123. Hybrid simulation hardware configuration at SRMD.

In its original configuration, the SRMD control system could only receive and send signals through analogue input/output channels and this was the approach used in previous tests (Schellenberg et al.

[32]). To enable real-time testing and eliminate previously encountered problems with synchronization, delays, and noise caused by D/A and A/D conversions, the SRMD control system was upgraded with a SCRAMNet+ interface that provides complete digital communication, thus eliminating any D/A and A/D conversions.

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5.6.2 Software configuration

The complex models utilized in the hybrid simulation presented in this publication require integrated software components. Fast solver operated under multi-processor computing system communicates with the hydraulic controller through a middleware, OpenFresco.

OpenSeesSP is used as the computational driver in the hybrid simulation. It provides advanced capabilities for modelling and analysing the nonlinear response of structural systems using a wide range of material models, elements, and solution algorithms.

The OpenFresco (Open-source Framework for Experimental Setup and Control) software framework [14] is the middleware that was deployed to connect the finite element model with the SRMD control and data acquisition systems. A new experimental bearing element was developed in OpenFresco that is able to transfer three translational and three rotational degrees of freedom to the experimental substructure. Furthermore, this new experimental bearing element provides the user-selectable option to either transfer deformation or force in the axial direction. Due to the high axial stiffness of the bearings investigated here, force control is preferred in the vertical direction. This enables 3D testing that can capture the vertical-horizontal coupling of large-scale seismic isolation bearings.

The SRMD control system operates at a rate of 1000 Hz, updating the actuator commands and getting feedback signals from sensors measuring the current displacement and force state of the machine. For smooth control and movement of the platen, the external commands to the actuators need to be updated and synchronized at the same rate. However, the numerical analyses of the hybrid simulation don't complete in the given time step size of 1 ms. The real-time predictor-corrector algorithm applied in the hybrid simulation enables to make smooth command signals by synchronizing the rate of the control system base clock frequency.

5.6.3 Special Settings for Testing Triple Friction Pendulum Bearings

Testing pendulum bearings in horizontal direction requires significant movement of the table platen in the vertical direction due to the geometry of these bearings. In other words, once the slider inside the bearing starts to move up as it slides sideways on the concave surfaces, the table platen needs to move down to accommodate the increase in height of the bearing. In case of poor tracking of the vertical displacement of the system (which is controlled by the inner loop in the cascade control loop), the axial force on the bearing (controlled by the outer loop in the cascade control loop) will likely have poor tracking as well. This may result in a different horizontal response of the bearing due to the coupling between the vertical and horizontal bearing response.

The horizontal response of friction pendulum bearings strongly depends on the axial force of the bearing.

During 1D and 2D hybrid simulations with a constant axial force on the bearing, it was observed that the SRMD controller could not maintain that constant axial force very accurately. Significant axial force fluctuations occurred mostly at horizontal displacement turnaround points during fast and real-time hybrid simulations. To compensate for these erroneous fluctuations of the axial force, the Simulink model was modified such that the horizontal shear forces of the bearing were first normalized by the instantaneous measured axial force on the bearing and then multiplied by the constant design axial force on the bearing.

5.6.4 Test Setup

5.6.4.1 Hybrid Infrastructure

Several hardware and software components as well as correction and compensation procedures are necessary to conduct hybrid simulations with the SRMD, especially when performing rapid or real-time hybrid simulations. Figure 124 shows the relationship and flow of several hardware and software components. OpenSeesSP, the equation of motion solver, calculates new target displacements at time t+t and sends these to all numerical elements as well as the experimental element in OpenFresco which generates the desired horizontal deformations and vertical load on the isolator test specimen.

OpenFresco transforms the target signals from global degrees of freedom to basic element degrees of

129 freedom and then communicates with the xPC-Target machine running the Simulink predictor-corrector model. The command signals generated by the predictor-corrector algorithm are sent as external reference values to the SRMD real-time controller which ultimately moves the machine platen accordingly.

FIG. 124. Schematic of the test setup.

Two horizontal displacements and their corresponding horizontal shear forces are measured in 2D hybrid simulation tests. For a 3D hybrid simulation, the measurements also include the vertical displacement and force. However, the vertical force measurement in the 3D tests is not sent back to the hybrid model and the analytically estimated vertical bearing force (which is identical to the vertical force command sent to the SRMD) is being used instead. Once the numerical simulation receives the restoring forces from the numerical and experimental elements, the analysis proceeds to the next integration time step.

5.6.4.2 Delay Compensation

Delay is one of the most critical parameters in real-time hybrid simulation that needs proper compensation. In general, delay is the time difference between the command signal and its measured response. The rate of testing for hybrid simulation is primarily governed by the computation time of the numerical model and the delay in the actuator system. For these tests, the numerical portion of the response was computed using a high-performance computer with parallel processing capabilities achieving computation times that were sufficient fast to enable real-time testing.

In order to assess system delays in the SRMD for the different platen degrees of freedom during a hybrid simulation, cross correlation functions and minimization of RMS (root mean square) errors were used for system identification. The SRMD was estimated to have about 60 msec of delay in the measured displacement response compared to a command from its internal control system. This delay is mainly due to a lag in the response of the hydraulic actuators driven by the four-stage poppet valve assembly.

To enable real time testing, the adaptive time series (ATS) delay compensator developed by [21] was utilized and implemented in the xPC-Target machine and feed-forward control was turned on and tuned

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on the SRMD controller. It important to note that since SCRAMNet+ shared memory was used for communication; negligible communicational delays were added in terms of signal transmission.

5.6.4.3 Inertia and Friction Correction

A model based on system identification techniques was developed and implemented in the xPC-Target machine to correct for friction and inertia on the fly during a hybrid simulation test.

5.6.4.4 Experimental Setup

Concrete spacer blocks, steel spaces and upper and lower adapter plates were utilized to locate the isolators for tests. An overview of the basis experimental setup is shown in Figure 125.

5.6.4.5 Instrumentation

Four uniaxial load cells recorded horizontal forces. Pressure cells on vertical actuators and outrigger actuators read the sinals of vertical forces. Accelerometers on the platen, within the top surface plate, measured acceleration signals in x, y and z directions.

FIG. 125. Overview of the typical experimental setup.

Linear voltage displacement transducers (LVDTs) were used to obtain more accurate vertical displacement measurements for the bearings (Figure 126).

Video and audio recordings were obtained of all the tests via three colour cameras. Three additional cameras were installed inside the EQSB and TPFB bearings. Video captured from these cameras helped to understand the behaviour of the bearing and in the case of failure helped identify the failure mechanism.

A total of 10 thermocouples (five on the top and five on the bottom) were installed in the LRB bearings and monitored by a separate data acquisition system. Four of the five thermocouples on each side were monitoring lead temperature on each core while the fifth one was placed in the centre of the bearing.

CROSS BEAM

131 Thermocouples on the top side were placed 20 to 25 mm into the lead cores while the bottom ones were placed on the end caps. Synchronization of this system with the primary data acquisition system was achieved by comparison of a common signal recorded on both systems.

FIG. 126. LVDTs to measure average vertical displacement of a bearing.