The STM-8 Leaf Spring Seismometer
Edition as of 02 October 1998
See the Abstract on another page.
This is a collection of figures, schematics, and data, as of 02 October 98.
NOTICE OF ERRORS: As errors are brought to my attention, I will note
them near the link to the figure involved, until I have time to
re-scan a corrected figure.
ALSO: if you see any problems, like the scale cut off at the bottom,
let me know. This is for your information; what do YOU want to see.
See the records of the latest Broadband records
and other data samples on the quakes and data page.
Look here for a photo (64k) of the Beta (January 1998) version of the vertical
seismometer. More photographs of the Beta prototype are on the photos
page.
Do you want to see a store that might be a good
source for getting parts to make this?
A WORD OF CAUTION ABOUT THE LEAF SPRING: WHEN IT IS BENT IN AN ARC
AS SEEN IN THE PHOTOS AND DRAWINGS, IT IS DANGEROUS, AND HAS BEEN
KNOWN TO GET LOOSE AND FLY ACROSS THE ROOM. BE CAREFUL WITH IT.
List of figures and illustrations:
Note that not all of the figures may have been scanned yet.
The best way to get a current set of about 20 pages of this documentation
is to send me your address (or label) and 5 ea 33cent stamps. I
will provide the copies and an envelope.
1. Block diagram
of the leaf spring vertical sensor, showing the basic
configuration of the mechanical sensor and the feedback electronics.
2.The Transfer Function, a Mathcad worksheet showing the parameters of the transfer function,
and the resulting VBB velocity response.
See figure 10 below for a comparison with a calibration step.
3.The Mechanical Drawing of the
Vertical Seismometer and Spring Mounting Detail.
This is the new Beta version of February 6, 1998.
PATENT APPLICATION NOTICE: A patent application has been filed
on various unique details of this broadband leaf spring vertical seismometer
design. This has been done by St. Louis University in order to
protect our rights to all aspects of the instrument, specifically to
prevent any third party from caliming any rights to it and preventing
us from disseminating it under our control.
By placing these drawings on this web site, we are making the
design available for personal and private use, particularly for
use by the members of the PSN, (Public Seismograph Network),
to make a single copy for their personal use.
A higher resolution version
is available HERE (1500 pixels wide; it is a LARGE 350k file)
The drawing now (22 July) includes everything from the hinges
to the mass end of the boom, with the coil and
speaker magnet mounting, and the VRDT mounting.
However, it is as yet only a two dimensional drawing.
An alternate is a high
resolution version of the spring mounting
that is 1900+ pixels wide (a 75k file).
4.Mechanical drawing of the displacement
transducer (VRDT) construction, showing the detail of the mounting.
ERROR NOTICE: The Mouser transformer should be a TL021 4k to 600 ohm
transformer, The windings are connected in series for a total of
about 200 ohms. The central part of a laminate of the larger TM009
is used for the sensing vane.
5.Schematic of the VRDT oscillator.,
(rev of 23 Oct 98 ) which has been adapted from the geodetic tiltmeter.
THIS IS NOW REVISED as of DEc 99. (98k)
6.Schematic of the VRDT Bridge
configuration., (rev of 02 Oct 98 ) showing the connections
between the oscillator and the amplifier/demodulator
as it is used for the VBB seismometer.
(76k)
7. Schematic of the VRDT bridge,
AC amplifier, Demodulator, and Amplifier .(revision of 22 October. 98)
NOTE OF 23 OCTOBER: in studying a problem with an unusual level of
short period noise from this circuit, I found some significant
discrepancies in the trade-offs of the circuit design (and some typos),
The demodulator was originally focused on long term thermal stability
for the tiltmeters; short period noise (10hz to 20 seconds) was
filtered out. But now I have addressed the short period noise in detail,
and shown the changes that reduce the effective displacement noise
(when using the TL021 based VRDT with an output of 250mv/micron)
to 0.1 nanometer in the 100hz to 10 second range. The schematic does
not show the use of OP-27 amplifiers in place of the LM308s at
A1 and A5, which reduces the noise by a factor of ten. I have ordered
some OP-97s, which might be even better. The OP-27 is power hungry.
(46k)
8.Plot of the VRDT output,
showing its' sensitivity and linearity. The two lines are from the
clockwise and counterclockwise rotation of the differential micrometer.
The slope of the line is the calibration: here 102 millivolts/micron.
The cyclic wandering is due to the eccentric error of the micrometer.
9. Schematic of the Instrumentaion
Amplifier (NOT DONE YET)
This WAS used for the integrator. However, the integrator is on the
revised quad amplifier card. A single high quality op-amp can be used
if low-leakage capacitors are used for the integrator.
10. Schematic of the general-purpose two channel
Line Driver Amplifier ,
O>>> THIS HAS BEEN COMPLETELY REVISED <<<< wait or ask for the new circuit
which is used for additional gain for the VRDT,
and for amplifying the high pass of the VBB output for the recorder/ADC.
(along with a table of component selections for various gains)
11.A block diagram the Electronics,
with details of the feedback,
and monitoring system. (some details TBA)
12.Response of a calibration step:
comparing the response recovered from the FFT of the output, (which
is divided through by omega^2) with the response predicted by the
transfer function from 2 to 1000 seconds.
13.Sine Wave Calibration Response:
The response recovered by driving the calibration coil
with sine wave currents compared to the response predicted by the
transfer function from 2 to 1000 seconds.
Two responses are indicated: one is from the digital data, which is
at 1 sample/second, so it falls off at 2 seconds; the other is from
the monitor drum recorder, which has the indicated high-pass and
low-pass filters.
14a.A plot of the data for the
Sine Wave Calibration Response: (scanned as B & W PHOTO)
This shows the output of the VBB system as recorded by the
digitizer for a "constant velocity" calibration, where the driving
current is changed for each period by changing the series resistor
(decade box) to 10k ohms x the period (in seconds).
14b.A plot of the data for the
Sine Wave Calibration Response: (scanned as B & W DRAWING)
By Sean-Thomas Morrissey
Geophysical Instrumentation Consultant
(since 1969 - 2002)
Seismic Instrumentation Engineer at
St. Louis University,
Department of Earth and Atmospheric Sciences
- The views and opinions expressed in this
site are strictly those of the site author.
- The contents of this site have not been
reviewed or approved by Saint Louis University.
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