ISAAC test bench: what does it consist of?
The Test Bench is a system designed and developed by ISAAC to study the behavior of a scale building, subjected to earthquakes of real intensity, on which Active Mass Dampers (AMD) type active control systems are installed.
Figure 1 - ISAAC Test Bench
The ISAAC Test Bench is made up of four main elements.
1. Scale building
The frame of the structure, entirely in aluminum, is made up of six floors. Each floor is made up of six columns and an aluminum plate, which constitutes a rigid floor.
In real buildings, the stiffness is partly given by the non-structural elements (such as, for example, the infill walls), which are also the first elements of a building that are damaged during a seismic event.
In order to simulate the presence and breakage of the infill panels, the Test Bench is equipped with a system of copper tie rods joined by two clips. The stiffness is calibrated by adjusting the tension of the copper cables, while the break during the seismic event is simulated with the detachment between the clips.
Figure 2 - Copper tie rods that simulate the behavior of the cladding
The figure below shows the increase in stiffness given by the infill panels in terms of the increase in the natural frequencies compared to the frame-only structure. In particular, it is noted that the first mode of vibration of the structure passes from 2 to 4 Hz.
Figure 3 - Comparison between the Frequency Response Functions (FRF) of the structure with and without the infills
2. Base and relative movement in the two different directions of space (x and y)
The base is made up of three aluminum plates connected to each other by linear guides and driven by two rotary motors with worm screw transmission for converting the motion from rotary to linear.
Plate y (shown in the figure) constitutes the base plane of the building, and can slide independently in the x and y directions depending on the motion imposed by the motors.
Figure 4 - Test bench base
The base constitutes a real vibrating table with which it is possible to simulate the motion law of any earthquake or environmental vibration in a combined way along the x and y directions.
3. Active Mass Dampers (AMD)
The Active Mass Dampers adopted for the Test Bench are four linear electric motors (two for each direction). Each AMD moves according to the law of motion it receives from the control algorithm inside the central computer.
An accelerometer is installed on each AMD: the accelerometer sends the acceleration signal to the central computer, which controls the AMD in real time feedback as defined by the control law.
Figure 5 -AMD installed on the roof of the Test Bench building
4. Active control system: central computer (PLC), sensors and control logics
The active control system (composed of central computer, sensors and control logics) represents the heart of the overall system.
Biaxial accelerometers are mounted on the structure, at the roof and at the base, which allow to measure the relative acceleration of the structure with respect to the base. The information coming from the sensors is sent to the central computer which processes the signals and generates the control law of the AMDs, according to the control logic adopted.
Figure 6 - Sensors installed on the Test Bench building (left) and central computer (right)
The goal of the Test Bench is to develop, test and improve the active control system that is implemented on the systems installed on real buildings and to study the mutual interaction between the machines.
Particular attention is paid to the software part, i.e. algorithms, logics and user interfaces implemented within the central computer, such as:
- Control algorithmss: algorithms that, on the basis of the measured building oscillation, generate the control law of the AMDs.
- AMD on / off logics: logics that determine the switching on and off of the AMD based on the acceleration measured by the accelerometers; the AMDs turn on when an earthquake / vibration of a certain intensity is detected, and turn off when the earthquake / vibration stops.
- Accelerometer exclusion logics: logics that allow to detect if a certain accelerometer is broken or is measuring an anomalous signal compared to the measurement of other accelerometers installed in the same point (caused, for example, by a foreign object hitting the accelerometer). When an accelerometer is identified as broken or an abnormal signal is detected, the signal from that accelerometer is discarded.
- User interface: interface that allows the operator to test and monitor the operation of the active control system in a simple, intuitive and safe way.
Author: Andrea Codina