The EQUALS facility
The EQUALS laboratory is a unique facility that combines strong walls and strong floors allowing a wide range of different types of structures (e.g. intake towers, bridges, buildings etc.) to be tested. The facility also houses the shaking table so that large scale dynamic tests can be performed in addition to more conventional static or pseudostatic testing.
Shaking table / earthquake simulator (ES) platform
The shaking table comprises a 3m by 3m platform supported by 8 hydraulic actuators. The table can carry up to 15 tonnes and (depending on the loading) can reach acceleration levels up to about 5g with peak displacements of ±150mm which makes it idea for testing specimens to destruction. The behaviour of the specimens can be recorded using accelerometers, displacement transducers, load cells and strain gages and other types of sensor. Typically between 50 and 100 sensors are used to measure the performance of the specimens.
The platform sits inside a reinforced concrete seismic block that has a mass of 300 tonnes. The block is located in a pit in the ES laboratory and is isolated from the rest of the laboratory by a 20mm cork filled gap running between the block and the rest of the laboratory.
Hydraulic power for the ES is provided by a set of 6 shared variable volume hydraulic pumps providing up to 900 litres/min at a working pressure of 205 bar. The maximum flow capacity can be increased to around 1200 litres/min for up to 16 seconds at times of peak demand with the addition of extra hydraulic accumulators. The platform is attached to the block by eight hydraulic actuators arranged so as to make best use of the space available. The horizontal actuators are attached to the block by concrete filled steel box sections and to the platform by smaller closed triangular brackets. The vertical actuators are connected directly to the block and platform. Each actuator has a dynamic capacity of 70 kN and has a maximum stroke of 300 mm. The four vertical actuators each have a static section to carry the static loads of the platform plus specimen.
Digital control system and instrumentation
The original analogue control system has been upgraded several times and with the move to the BLADE laboratory has been replaced by a digital control system manufactured and installed by INSTRON in 2004. Built-in instrumentation makes information from three sources available to the DCS: each actuator having an LVDT within the cylinder and an accelerometer at the platform end of the piston. In addition load cells measure the loads applied by each of the actuators.
Required table motion information can be specified in two ways:
- Either an analogue signal can be applied to any combination of the six degrees of freedom of the table with a voltage of –10 to +10 V representing required maximal displacements; or
- A digital representation of displacement of the six components (in mm or degrees as appropriate) can be downloaded directly into the digital control system which transforms from specified 6 degree of freedom displacements into eight required actuator displacements digitally at a rate of 10 kHz.
The digital control system employs a traditional PID control system which currently uses a composite displacement, acceleration and load signal for feedback from the ES. A state-of-the-art real time digital controller (the "Accelerator") can be used to embed custom control algorithms within the standard system to further enhance performance. For example a control system utilizing the Minimal Control Synthesis ("MCS") algorithm has been available since 1998. This method incorporates real time adaption of gains such that there is a strong likelihood that a required displacement time history can be matched in a single iteration independently of the size and damage state of any model on the ES. In addition to controlling the dynamic motion of the ES the DCS controls the static (preload) vertical pressure. The DCS allows limits upon acceleration, displacement and load to be selected at which the drive signal can be frozen, cut or the ES shutdown if safe working parameters are exceeded.
Shaking table specification
|Size||3 m by 3 m|
|Construction||4 piece cast aluminium|
|Max payload||15 tonnes|
|Max payload height||15 m|
|Max payload C of G||5 m|
|Craneage capacity||2 x 10 tonnes|
|Operational frequency:||0 -100 Hz|
|Hydraulic supply:||900 l/min|
|Hydraulic pump:||6 pumps each 450kW|
|Supply pressure (working):||205 bar|
|Supply pressure (max):||230 bar|
|Longitudinal (X) and lateral (Y) actuators:||4 at 70 kN|
|Horizontal acceleration (no payload):||3.7 g*|
|Horizontal acceleration (10 tonnes payload)||1.6 g*|
|Horizontal velocity:||1.2 m/s|
|Horizontal displacement||± 150mm|
|Yaw rotation||± 3.6 degrees|
|Yaw velocity||1.5 rads/sec|
|Vertical (Z) actuators||4 at 70 kN|
|Vertical acceleration (no payload)||5.6 g*|
|Vertical acceleration (10 tonnes payload:||1.2 g*|
|Vertical velocity||1.2 m/s|
|Vertical displacement||± 150mm|
|Pitch/roll rotation||± 5.2 degrees|
|Pitch/roll velocity||1.5 rads/sec|
* Quoted accelerations are harmonic components of sinusoidal acceleration response.