Earthquake Engineering Research - HKU Libraries - The University ...

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environment constraints and the configuration of the system to be tracked, the required accuracy can
range anywhere between millimeters to fractions thereof. Perhaps the most difficult problem faced in
earthquake engineering experimental research is the cumbersome physical attachment and associated
lengthy set-up times of most conventional motion sensors. For reduced scale experiments, these
sensors generally add substantial mass or stiffness and therefore change the response characteristics of
the system.
In this paper, different motion tracking technologies are reviewed and a vision-based approach is
selected and evaluated for its applicability as a new sensing technology for measuring earthquake
induced motions. The approach is advantageous since very limited physical attachment to the structure
of interest is needed, is high-speed, high-resolution and does not introduce additional mass or
otherwise modify the properties of the structure. An exploratory study is conducted using four digital,
high-speed, high-resolution cameras outfitted with red-light emitters to track reflective (nearly) mass
less spherical elements discretely mounted on a scale 5-story steel frame structure. The structure is
mounted on the University of California, Irvine's (UCIs) bi-axial shake table and subjected to different
earthquake motions. The primarily uni-axial dominated motion of the frame provides a simplified
reference expenment with reduced degrees of movement for evaluating this approach. The structure is
also instrumented with a total of eleven conventional (wired) transducers [linear variable displacement
transducers (LVDTs) and accelerometers]. A review of motion tracking technologies and specifically
vision-based technologies is provided, followed by a description and select results from the exploratory
studv conducted.
TYPES OF MOTION TRACKING TECHNOLOGIES
Specific types of motion tracking technologies include: (1) mechanical, (2) magnetic, (3) acoustic, (4)
optical, (5) and hybrid solutions combining two or more of the previously mentioned techniques to
achieve higher accuracy or sampling rates. Mechanical tracking techniques such as linear displacement
transducers and linked boom structures (in VR) provide excellent results for linear and rotational
movement measurements at the cost of limited working volumes, degrees-of-freedom, added mass and
friction. They also generally require physical attachment or point-wise anchorage to elements of
interest. Electro-magnetic techniques are less constraining and are based on generation of a magnetic
field by an emitter and collection by a receiver. Used prevalently in VR, the transmitter system is
generally statically mounted while the receivers can be freely moving within the field. While these
systems can operate at nearly 150Hz sampling rates at a static accuracy of less than two millimeters
and 0.5 degrees under ideal conditions, they are sensitive to metal components and other magnetic
fields located in the workspace. This noise introduced by the environment can greatly distort the
electromagnetic reference signal making it difficult to use in earthquake testing facilities. While these
systems remove the mechanical constraint mentioned earlier and reduce sensor mass at the same time,
currently a wired connection is still required. Acoustic or ultrasound techniques use time of flight
information to triangulate the position of the target object, which is instrumented with active sensors.
Generally, multiple emitters are mounted in a known pattern on the target and broadcast a signal in
sequential order. The time of flight from the target to receivers mounted throughout the environment is
then used to triangulate its position. Potential problems include ambient ultrasonic noise produced by a
variety of electronic devices and slow update rates.
Optical tracking techniques come in different forms and are often termed vision-based systems. Early
and very extensive usage of optical tracking technologies can be traced to robotics and subsequently to
the medical field for biornechanics and human motion studies with a strong focus on gait analysis [e.g.