[ANSS-netops] strong motion calibrations

[John R Evans]

Hi All,

I largely agree with what went before but will engage in a bit of e- 
mail diarrhea anyway.

A (legally) defensible true "calibration" requires NIST-traceable  
laboratory work with tilt, shake, and other facilities.  So be  
careful of that word "calibration", it carries special weight.  To  
date, ANSS "tests" and "characterizes" but does not "calibrate".  The  
distinctions are primarily legal since we use good references and get  
pretty darn close regardless.

ASL is working toward NIST-traceability but is not yet there.  (Would  
love to hear from Jamie more about the NEES facilities.)   
Nevertheless, we can do a lot better than most folks can manage and,  
clearly, more than anyone can manage in the field.  The notion of  
cycling spares through the network to get them back to a lab every  
few years, coupled with basic field sanity checks, is the most viable  
option we have.  It is also as much as is needed since no one should  
be trusting any seismic sensor to better than about 1% in any  
realistic field operation.  Just flying and driving it to the side  
knocks it off by some small amount.  And then there are temperature  
and humidity fluctuations (every sensor ever made is also a  
thermometer) and the drift and aging of every component, from voltage  
references right down to the resistors (yes, they age too).

Those sanity checks have customarily included (A) a 0-90-(-90)-0  
degree tilt test to sanity check DC sensitivity (only works for  
accels that are flat to DC, of course), and (B) a very basic  
functionality test, generally triggered by the DAU (datalogger) at  
intervals of a day to a month and reported back to the lab in some  
way.  (A) is only done during servicing, of course.  (B) will reveal  
most gross failures and some subtle ones.

With regular (several year) lab tests and field sanity checks one can  
be confident to that 1% sanity threshold.  A sensor at the end of  
it's lab tests might be good to 0.1% ... until you move it.

Note that (B) depends on the reliability of at least two things --  
the DAU's voltage reference and the sensor's translation of that into  
a step in acceleration.  Neither of these things is well known and  
they both age with time, field conditions, and certainly temperature.

KMI probably responded as they did because most (not all) feedback  
accels do (B) by bumping the feedback loop, not the proof mass.  You  
need to know at least an effective motor constant and then to believe  
that there is no malfunction in the feedback that would make a sick  
sensor (Fi) look healthy anyway.

Tilt tests ideally should be done at five or more angles equally  
space in acceleration, not angle.  The KMI tilt table and ASL does it  
this way to avoid bias in the requisite regression of output (V)  
against input (g).  Note that most manufacturers do this regression  
L2, a linear regression minimizing squared error.  At ASL I use  
either an L1 regression (minimized sum of absolute errors) or maximum  
likelihood method -- I have MatLab for both.  Nonlinear methods are  
less biased by large, local departures from linearity and produce  
results that focus on the most linear (usually middle) portion of the  
data, as would the human eye.

Getting above 1 g requires a centrifuge (on order for ASL so we can  
reach the 3.5-g ANSS spec and 4-g typical spec of manufacturers).   
Unless one adds a tumble option to the centrifuge (rare and tricky)  
the centrifuge test is also for DC sensitivity.

A shake table is required for any dynamic test worth its salt but  
there are major issues, principally in noise, cross-axis, and tilt  
errors in the shaker.  At ASL we use Anorad precision linear slides  
(0.4 m PTP span) driven by a linear motor below the stage (watch out  
for mag sensitivity in the sensor).  These shaker have been pretty  
good on tilt (maybe even acceptable).  However, note that one  
microradian tilt is a one micro-g error and it only takes a few ug to  
thoroughly bugger a double integration.  If one actually does the  
math, a table operating horizontally would have to be machined to a  
few millionths of an inch to bring it to this precision -- currently  
impossible.  We are working to put one of the two Anorads into  
vertical orientation (supported by constant-force spring motors) to  
reduce this uncertainty -- tilt sensitivity is (1-cos) in vertical  
but sin in horizontal.  Noise (and other cross axis) is not bad but  
these are still roller bearings and they rumble and waver down an  
imperfectly straight track.  We priced our fondest hopes -- a  
precision ground, air-bearing, vertical table ... $250k.  Maybe someday.

At present we use the old Russian shakers at ASL for THD and  
translation-translation cross-axis tests.  This table is driven in a  
very clean sine by a low-distortion oscillator.  It is quiet because  
it moves as a parallelogram supported by eight cross flexures,  
however, that also means it moves in a vertical-plane arc and seems  
to have some rotation as well.  Still working on suitable THD and  
cross-axis test setups.

Note that cross-axis tests require some means of nulling the output  
to the degree possible, by changing the sensor orientation slightly.   
That technique allows us to align the sensor active axis 90 deg to  
the motion axis (axes).  We measure the case-to-active-axis  
orientation errors separately with the "box test".

Given the exquisite sensitivity of accels (and broadbands) to tilt  
errors, I'd wave off using a carpenter's level to set up your  
precision slab -- use a machinist's level and shims on a very stable  
surface and then grout it in.

At present, a basic ASL test sequence for a DC-responsive accel is:

	(1) Carefully case-orient it inside a machined rectilinear box and  
take its output with the box successively placed on each of its six  
faces atop a precision-ground precision-leveled surface.  The +/-1 g  
positions yield a simple measure of sensitivity and offset.  The  
other four allow you to measure the orientation of the active axis  
relative to the case axis.  (Note that local g varies several tenths  
of a percent about the nominal value of 9.80665 m/s/s and more  
precise tests should use the true local value.)

	(2) Perform a tilt and/or centrifuge to verify DC linearity over as  
much of the range as you can reach.

	(3) Use a shaker to input known band-limited displacement steps --  
that is, rounded displacement square waves.  Doubly integrate the  
accel outputs to test the recoverable displacement for amplitude and  
waveform fidelity.  This "step test" is thoroughgoing but  
nonspecific.  If almost anything is wrong you'll get garbage but not  
necessarily know its source.

	(4) Use the cleanest, quietest, lowest-distortion sine-motion  
generator you can come up with at a range of frequencies across the  
seismological and engineering bands of interest -- call it 16 s to 64  
Hz in one- or half-octave steps.  From this one can estimate the  
transfer function, including its normalization frequency and the  
sensitivity there.

To do NIST traceability, you need NIST-calibrated reference sensors  
(displacement, acceleration, rotation, ...) and generators (voltage  
source, shake tables, ...) and need to have these recalibrated  
regularly, keeping all paperwork.  So far, ANSS and USGS have escaped  
a requirement for NIST traceability because courts consider us  
reliable, independent observers.  But times are changing, for  
example, for USNRC and some engineering work.  ASL is well  
disciplined and used to such rules, at least to a degree, so it  
wouldn't be too awful for the lab tests.  Yuck-poo, stand by!

Cheers,
John

John R Evans
jrevans at usgs.gov

Usually in Menlo Park:
U.S. Geological Survey
345 Middlefield Rd, MS-977
Menlo Park  CA  94025-3591
650-329-4753

Intermittently in Albuquerque:
Albuquerque Seismological Laboratory
U.S. Geological Survey
P.O. Box 82010
Albuquerque  NM  87198-2010
505-846-1793








-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://geohazards.usgs.gov/pipermail/anss-netops/attachments/20090625/09611d3e/attachment.html