Strong Motion Accelerograph Evaluation
Creators: name: Peters, Rex Bredesen
Publisher: name: California Institute of Technology
Year: 1969
Subjects: subject: Mechanical Engineering
subject: FOS: Mechanical engineering

description: A brief study is made of the effect of common instrument
errors on the accuracy of data obtained from strong motion earthquake
accelerographs. Error sources considered include zero drift,
tilts, nonlinearities, cross-axis sensitivity, lack of initial conditions,
noise, and time base errors. It is concluded that most data of current
engineering interest are not critically affected by the level of errors
found in existing accelerographs. Techniques are suggested for
reducing or eliminating many of these errors by instrument design
changes.


An experimental study is made of a new strong motion
accelerograph during its engineering development. This new accelerograph
is designed to record an FM analog of ground acceleration
on magnetic tape, providing a record which may be rapidly and
automatically converted to digital form. The accuracy limits of the
accelerograph are explored and the design reasons for these limits
investigated. The more significant findings may be briefly
summarized:


(1) Static accuracy. The sensitivity and linearity of the instrument
are found to depend critically on a series of interdependent adjustments.
Reasonable care will bring errors in both of these
quantities to within ±2% of 1/2 g full scale. Higher accuracies
are possible, but require much more time and care, primarily
due to the limiting effect of mechanical drift in the accelerometers.


(2) Zero point drift. Uncertainties in the accelerometer zero
point arise from both mechanical and electronic drifts. Long
term drifts may be related to temperature or relative humidity, or
may be entirely random. Short term drifts of up to 2% of full scale
may occur during the course of a typical record. The total
variation may be as much as ± 30% of full scale for a 100°F range
of temperatures. These variations require adjustment of the data
before processing, but are not sufficient to interfere with operation
of the accelerograph.


(3) Noise. Random noise in the system as tested amounted to
1.4% of full scale, RMS, and was mostly due to the tape recording
system. By comparison with optical accelerographs, this noise
figure is marginal, but acceptable, and can be improved by changes
to the compensation system.


(4) Timing. The advantages of an effectively continuous time base
over discrete time marks were discovered and means devised to
obtain such a base from the test accelerograph. This method of
timing is a qualitative improvement over the best system which is
practical on optical recorders.


The overall performance of the test accelerograph is
adequate to yield acceptably accurate acceleration vs. time records
and Fourier spectra within the range of frequencies which are of
current engineering interest. It is able to produce useful displacement
records only for periods shorter than several seconds. The
reasons for this latter limitation are sufficiently fundamental that
markedly superior instruments are not expected to be available
within the next ten years.