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Nelson-Nordgren, Astatic Spring, Broadband Force Balance, Vertical (FBV) Seismometers

Experiments and Prototypes


Lenape

The Lenape is an attempt to build an instrument which duplicated in many aspects, Dave's Napa, but which can be built with modest equipment. Dave's uni-body construction, ultra low noise Napa requires a large drill press or milling machine to drill the required 1.25 inch hole in the 1.25 inch thick instrument body. My attempt was to use the Yuma design scaled down to fit the small Hammond box used by Dave.

The Lenape shares the exact astatic spring geometry as the Napa, and most of published parameters found in the FBV Forum archives.

  1. Flat velocity response from 50 seconds to 30 Hz
  2. M0 = 65 grams
  3. Gn = 6.6 N/A
  4. Ag(hi+) = 32,400
  5. 750 T, #34 AWG, Rc = 47 Ohms
  6. 0.003", 1.0" wide, 17-7 Stainless Spring
  7. 0.5" diameter x 0.25" N52 magnet
  8. 0.002" Kapton Flexure, 0.25" wide
  9. Clamp to clamp spacing 1.6"
  10. 0.27" centered pivot height above clamps
  11. 0.020" sensor plate spacing

This project is still evolving and several issues exist, but I can see glimpses of good performance. This instrument is highly experimental and no match for the proven performance of the Yuma design! Thanks to Dave and Brett for introducing me to FBVsolve and and for the Yuma design, which this project is based on.

Lenape Photos

Front View. The outside dimensions of the Hammond case is 4.75" x 4.75".

Top of boom

Bottom

Universal Circuit Board mounted to top of Hammond case



Seneca

The Seneca is an early prototype to test the limits of size for the Nelson-Nordgren type astatic spring seismometers. This unit has outline dimensions of appromimately 2.69" x 2.375" x 2.25" high. It's small, but balances out with a 3 second natural period with a little care.

The specs (as of 11/10/2015) are as follows:

  1. Flat velocity response from 50 seconds to 30 Hz
  2. M0 = 31 grams
  3. Gn = 3.6 N/A
  4. Ag(hi+) = 28,110
  5. 750 T, #36 AWG, Rc = 62 Ohms
  6. 0.003", 0.75" wide, 17-7 Stainless Spring
  7. 0.375" diameter x 0.25" N52 magnet
  8. 0.001" Kapton Flexure, 0.125" wide
  9. Clamp to clamp spacing 1.0"
  10. 0.17" centered pivot height above clamps
  11. 0.020" sensor plate spacing
  12. Ca, Cb, balanced range = 30pF to 75pF, +/-10% or better

Balancing Ca and Cb was key in obtaining reasonable performance and consistancy from this instrument. Also the wide sensor gap (0.020"), I believe, contributed to a reduction of large spikes ~3000nm/s to 5000nm/s. The Nelson-Nordgren type astatic spring seismometer successfully scales to this size.