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Hi All,<br><br><div> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div><font class="Apple-style-span" color="#000000">I largely agree with what went before but will engage in a bit of e-mail diarrhea anyway.</font></div><div><br></div><div><font class="Apple-style-span" color="#000000">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.</font></div><div><br></div><div><font class="Apple-style-span" color="#000000">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).</font></div><div><br></div><div><font class="Apple-style-span" color="#000000">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.</font></div><div><br></div><div>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.</div><div><br></div><div><font class="Apple-style-span" color="#000000">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.</font></div><div><br></div><div><font class="Apple-style-span" color="#000000">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.</font></div><div><br></div><div><font class="Apple-style-span" color="#000000">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.</font></div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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".</div><div><br></div><div>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.</div><div><br></div><div>At present, a basic ASL test sequence for a DC-responsive accel is:</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>(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.)</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>(2) Perform a tilt and/or centrifuge to verify DC linearity over as much of the range as you can reach.</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>(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.</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>(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.</div><div><br></div><div>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!</div><div><font class="Apple-style-span" color="#000000"><br></font></div><div><font class="Apple-style-span" color="#000000">Cheers,</font></div><div><font class="Apple-style-span" color="#000000">John</font></div><div><font class="Apple-style-span" color="#000000"><br></font></div><div><font class="Apple-style-span" color="#000000">John R Evans</font></div><div><font class="Apple-style-span" color="#000000"><a href="mailto:jrevans@usgs.gov">jrevans@usgs.gov</a></font></div><div><font class="Apple-style-span" color="#000000"><br></font></div><div><font class="Apple-style-span" color="#000000">Usually in Menlo Park:</font></div><div><div class="MsoPlainText"><font class="Apple-style-span" color="#000000">U.S. Geological Survey</font></div><div class="MsoPlainText"><font class="Apple-style-span" color="#000000">345 Middlefield Rd, MS-977</font></div><div class="MsoPlainText"><font class="Apple-style-span" color="#000000">Menlo Park CA 94025-3591</font></div></div><div><font class="Apple-style-span" color="#000000">650-329-4753</font></div><div><font class="Apple-style-span" color="#000000"><br></font></div><div><font class="Apple-style-span" color="#000000">Intermittently in Albuquerque:</font></div><div><font class="Apple-style-span" color="#000000">Albuquerque Seismological Laboratory</font></div><div><font class="Apple-style-span" color="#000000">U.S. Geological Survey</font></div><div><font class="Apple-style-span" color="#000000">P.O. Box 82010</font></div><div><font class="Apple-style-span" color="#000000">Albuquerque NM 87198-2010</font></div><div><font class="Apple-style-span" color="#000000">505-846-1793</font></div><div><br></div></span></span></span><div><font class="Apple-style-span" color="#000000"><br></font></div><div><font class="Apple-style-span" color="#000000"><br></font></div></span><br class="Apple-interchange-newline"><div><br></div></span><br class="Apple-interchange-newline"></span><br class="Apple-interchange-newline"></span> </div><br></body></html>