Hybrid Testing Simulating Dynamic Structures in the Laboratory

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Hybrid TestingSimulating Dynamic Structures in
the Laboratory

• Tony Blakeborough and Martin Williams
• SECED Evening Meeting
• 28 January 2009

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Outline

• Introduction
• Dynamic test methods why do we need new ones?
• The real-time hybrid method
• Displacement-controlled tests
• Testing strategy and equipment
• Numerical integration schemes
• Compensation for transfer system dynamics
• Recent developments and applications
• Tests under force control
• Crowd-structure interaction
• Distributed hybrid testing in the UK-NEES project
• Conclusions

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Acknowledgements

• Numerous colleagues contributed to the work
  described here, particularly
• Current researchers Mobin Ojaghi, Ignacio Lamata
• Past researchers Antony Darby, Paul Bonnet,
  Kashif Saleem, Javier Parra
• Collaborators at Bristol, Cambridge, Berkeley,
  JRC Ispra
• We have received financial support from
• EPSRC
• The Leverhulme Trust
• The European Commission
• Royal Academy of Engineering
• Instron

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Testing methods in earthquake engineering

• Shaking tables apply prescribed base motion to
  models
• Can accurately reproduce earthquake input
• Normally limited to small-scale models
  expensive at large scale
• Scaling problems (physical and time)
• Control problems

SUNY Buffalo
Bristol University
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Testing methods (cont.)

• Pseudo-dynamic test facilities
• Slow test, with inertia and damping components
  modelled numerically, stiffness forces fed back
  from test specimen
• Can be conducted at large scale
• Best suited to flexible structures with
  concentrated masses
• Expanded timescale cant capture rate effects
• Feedback loop can cause errors to accumulate

Lehigh University
JRC Ispra
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Future trends

• Major upgrading initiatives, e.g. NEES (USA),
  E-Defense (Japan)
• Very large shaking tables
• Enhancements to pseudo-dynamic methods
• Effective force testing
• Real-time hybrid testing
• Distributed hybrid testing

Minnesota EFT facility
San Diego outdoor shaking table
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E-Defense, Japan

• 1200 tonne payload
• amax 1.5 g, vmax 2 m/s, umax 1 m
• 24 x 450 tonne actuators
• 15,000 l/min oil flow rates

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Real-time hybrid testing
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Real-time hybrid testing

• Advantages
• Avoids physical scaling problems
• Avoids time scaling problems
• Ideal for testing rate-dependent systems
• Economical only the key parts need to be
  modelled physically
• Now being strongly pursued by NSF NEES programme
• Needs
• High-performance hardware and communications
• Fast solution of numerical substructure
• Compensation of transfer system dynamics

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Typical test set-up
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Structural Dynamics Lab
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Structural Dynamics Lab _at_ Oxford
Hydraulic installation
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The Flight Deck
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Typical real-time control loop

• Dual time-stepping implementation
• Numerical model runs at main steps 10 ms
• Controller runs at sub-steps 0.2 ms
• Imperfect transfer system dynamics cause
• Errors in timing and amplitude of applied loads
• Inaccuracy and/or instability of test

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Typical test strategy

• Solve numerical substructure to give desired
  actuator displacement at the next main step,
• Curve fit to the current and the past few
  displacement points.
• Use curve fit to extrapolate forward by a time
  equal to the estimated actuator delay, to give
  the command displacement,
• Use same curve fit to interpolate dcom values at
  sub-steps. Send to the inner loop controller,
  together with the current actuator position dact
• Repeat step 4 at sub-steps, until the next main
  step.

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Numerical integration schemes

• We require
• Very fast solution of numerical substructure (10
  ms)
• Accuracy, stability, ability to model non-linear
  response
• Explicit integration (e.g. Newmarks method)
• All required data known at start of timestep
• Quick, sufficiently accurate
• Need short timestep for stability
• Implicit integration (e.g. constant average
  acceleration method)
• Requires knowledge of states at end of timestep,
  therefore iteration (or sub-step feedback)
• Unconditionally stable
• Two-step methods (e.g. operator-splitting)
• Explicit predictor step, implicit corrector

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Test system

• Simple mass-spring system
• All springs in numerical model have bi-linear
  properties
• Increase DOFs in numerical model to test
  algorithms

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Results
Explicit

• 10-DOF numerical substructure
• Sine sweep input through several resonances
• 5 ms main-step
• 0.2 ms sub-step
• Red numerical simulation
• Blue hybrid test

Two-step methods
Implicit
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Results
Explicit

• In frequency domain
• 10-DOF numerical substructure
• Sine sweep input through several resonances
• 5 ms main-step
• 0.2 ms sub-step
• Red numerical simulation
• Blue hybrid test

Two-step methods
Implicit
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Results

• 50-DOF numerical substructure
• Sine sweep input through several resonances
• 25 ms main-step (15 ms Newmark)
• 0.2 ms sub-step
• Implicit schemes unable to compute in real time
• Red numerical simulation
• Blue hybrid test

Explicit
Two-step methods
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Results

• 50-DOF numerical substructure
• Sine sweep input through several resonances
• 25 ms main-step (15 ms Newmark)
• 0.2 ms sub-step
• Implicit schemes unable to compute in real time
• Red numerical simulation
• Blue hybrid test

Explicit
Two-step methods
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Actuator dynamics

• Both timing and amplitude errors exist, and may
  vary during test
• Delay of the order of 5 ms is unavoidable
• Delay has an effect similar to negative damping ?
  instability

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Compensation schemes

• Two components
• Forward prediction scheme
• Aims to compensate for known or estimated errors
  through scaling and extrapolation
• Exact polynomial extrapolation
• Least squares polynomial extrapolation
• Linearly extrapolated acceleration
• Laguerre extrapolator
• Delay estimation
• Delay and amplitude error estimates are updated
  as test proceeds

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Validation experiments Test A

• Linear, 2DOF system, single actuator

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Test B

• Non-linear, 2DOF system, single actuator

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Test C

• Linear, 3DOF system, two actuators
• Asynchronous input motions, stiff coupling

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Effect of forward prediction
Hybrid test

• Test A, with fixed delay estimate, exact
  polynomial extrapolation

Analytical response

• Synchronization plots

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Comparison of forward prediction schemes

• RMS errors () over a test with constant delay
  and amplitude error estimates

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Delay updating results

• Delay estimates produced by updating scheme in
  Test C

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Effect of delay updating

• RMS errors () over a test with with third order
  exact extrapolation
• Tests A and B used 0.5 ms sub-steps
• Test C used 0.2 ms sub-steps

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Developments and applications

• Tests under force control
• Dorka and Jarret Damper
• Crowd-structure interaction
• Grandstand simulation rig
• Distributed hybrid testing
• Oxford-Bristol-Cambridge

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EU NEFOREE project comparison of testing methods
Single storey test building designed by Prof
Bursi at Trento Parallel tests on shaking table,
reaction wall and real time hybrid
substructuring Two dissipative devices to be
tested - Dorka shear device and Jarret
dampers Natural frequency Unbraced 2.6Hz
2 damping Braced 8.6Hz 5 damping
(Dorka)
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Seismic testing of dampers

• NEFOREE EU study

Shaking table set-up (elevation)
Hybrid test of device
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Dorka and Jarret devices
Dorka shear panel shear diaphragm in SHS -
hysteretic damping
Jarret dampers Non-linear visco-elastic devices
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Control problems

• Two actuators equal but opposite forces
• Dorka cell - very stiff specimen
• Significant rig/specimen interaction
• LVDT noise 30mm rms produced significant forces
• Not possible to run under displacement control

Solution

• Run test in force-control
• Two MCS controllers one for magnitude and other
  for force imbalance
• Displacement feedback into numerical model

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Force control loop
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Numerical substructure
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Earthquake records
El Centro
Synthesised EC8 record
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Response of Dorka device (El Centro 0.2g)
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Detail - EC8 synthesised earthquake tests
0.2g pga
1.2g pga
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Specimen hysteresis curves
EC8 0.2g
EC8 0.6g
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Large hysteresis loops
EC8 0.9g
EC8 1.2g
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Conclusions Dorka device

• Real time hybrid tests successful
• Simulated behaviour in 8Hz frame with 5 damping
• Stiff specimen required force feedback loop
• Device robust enough for use

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Jarret devices
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Response to square wave input
0.15g alternating sign (square wave) ground
acceleration of period 2s
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Response of Jarret devices
El Centro record with a pga of 0.2g around the
peak at 3.3s
.... and at end of record
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Response of Jarret devices
Force displacement response of to the EC8
record with a pga of 0.6g
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Response of Jarret devices
Force against displacement and velocity for the
EC8 record with a pga of 0.6g
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Response of Jarret devices
Velocity projection
Displacement projection
EC8 record with a pga of 0.6g
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Conclusions Jarret device

• Tests successfully completed
• Realistic tests at low velocities
• Problems at higher velocities due to extreme
  non-linear response in velocity
• Student just starting work on this possibly use
  velocity feedback with improved displacement
  measurements

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Human-structure interaction in grandstands
EPSRC funded study RA Anthony Comer
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Grandstand rig

• 15-seater grandstand rig
• Standard design typical rake seat distances
• Test crowd coordination
• Effect of grandstand movement on coordination
• Simulate various natural frequencies and mass
  ratios

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Grandstand rig design

• Aluminium alloy fabricated rakers and stretchers
• Light stiff lowest internal natural frequency
  gt30Hz
• Air spring at each corner to take out mean load
• Electro-mechanical actuator at each corner to
  control rig
• Load cell under each spectator

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Control problems

• Force feedback from load cells at actuators
  suffered large levels of interference from e/m
  fields emitted by motors
• Filtering would introduce too much lag for
  stability
• Digital displacement feedback available from
  linear encoders (resolution 3µm) immune from e-m
  interference
• Use force control with displacement feedback

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Control strategy

• Three significant degrees of freedom
• Heave (vertical displacement)
• Roll
• Pitch
• Feedforward
• Measure loads applied by spectators
• Resolve into resultant vertical load and roll
  pitch moments
• Apply equivalent forces at actuators to balance
  force resultants and keep rig stationary
• Numerical model
• Simulate vertical and rotational damped springs
  numerically to control dynamics of grandstand
• Apply a proportion of vertical resultant load to
  excite the rig

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Response to 130kg male jumping
Vertical response only Rotations successfully
tared off
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Conclusions grandstand simulation

• Controlled tests possible on grandstand with
  spectators jumping and bobbing
• Can also be used to wobble seated and standing
  spectators to assess the acceptability of motion
  (main dynamic use in project)
• Can be used to simulate human-structure
  interaction

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Split-site testing hybrid testing over the
internet

• Numerical and physical substructures at separate
  locations
• Possibility of testing very large components
• Possible only over the internet

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Network architecture
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JANET internet route
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Communication interruptions

• JANET delays
• 10ms - OK
• Inconsistency causes problems
• Solution
• Use UK-light a dedicated link

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Oxford-Bristol test
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Results of test on Monday
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Limitations

• Physical substructure
• Limits set by equipment
• Response times of actuators
• Control problems at limits of actuator capacity
• Stiffness of frames
• Reduce uncertainty
• Proof testing (strength/performance guarantee)
• Check individual items
• Assess design under realistic loading
• Validate computer models used in design

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Architecture of 3 site test radial model
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State of work in split site testing

• Ethernet not a problem provided use a dedicated
  link
• Tests possible and seem to work
• Future work
• Increase natural frequencies of systems
  currently 3Hz but up to 10 should be possible
• Investigate different interconnection links
• At moment there is a central numerical model with
  physical sites as servers at end of radial spokes
  other arrangements are possible
• Investigate force control
• Extend to the rest of the world planning links
  with EU in FP7 research

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Conclusions

• Simulation of real time behaviour
• It works for stiffness and rate dependent
  components
• Reproduces rate/time dependent effects
• Useful for more realistic component testing
• Allows devices to be checked in much more arduous
  circumstances
• Copes with non-linear behaviour in both physical
  and numerical substructures

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General conclusions

• What test at all?
• Reduce uncertainty
• Proof testing (strength/performance guarantee)
• Check individual items
• Assess design under more realistic loading
• Validate computer models used in design
• Challenging activity
• Push current control techniques and test
  equipment to limits
• Trickle down effect improved techniques help
  standard testing