PSN-L Message - Subject: Re: Vertical BB derived from STM-8

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The STM design was cutting edge at the time but is=20 outdated.

The Inyo design does not require a low temperature coefficient = spring . The=20 design of the feedback loop is such that the integrator path = will=20 compensate for spring temperature coefficient over at least = 40C and=20 produce a long period corner of 50 seconds. The only effect of = temperature is to=20 produce an offset in the instrument output proportional to the = temperature=20 rate of change. That offset is very slowly = changing=20 compared the the long period corner so can be easily filtered = either=20 in the instrument output electronics or in the filtering in = Winsdr.

I use a high pass of 1 pole and 500 seconds to eliminate the offset = .. All=20 force balance sensors have that effect. My Trillium is much worse = than=20 the Inyo but when well insulated from the environment is is = of no=20 consequence.

Attempting any sort of long period vertical without = feedback is=20 an invitation to poor performance ,frustration and probably failure. = The=20 nice thing about the feedback method is it cures a multitude of = ills=20 without excessive complexity or frustrating setup. They just work. =

Regarding flexures : I use 2 mil stainless = with ~=20 ..01 spacing 1/2 inch wide. The critical design factor is that the = spring=20 keeps the pivot flexure in tension.

I have used negative impedance terminations on a Geophone = with good=20 results but at the expense of much increased noise at = the long=20 periods. There is definitely a limit on that technique as can be clearly = seen in=20 the Lennartz noise curves.

Regards , Dave Nelson

From: ChrisAtUpw@.......

Sent: Sunday, March 14, 2010 1:42 AM

To: psn-l@..............

Subject: Re: Vertical BB derived from = STM-8

In a message dated 13/03/2010, matthew.zieleman@.........= =20 writes:

I've=20 been doing some conceptual design work on a vertical = broadband
sensor, and=20 I have a few questions. By reading past archives of the
PSN-L I = noticed=20 that Sean-Thomas Morrissey was a contributor to this
list, and = there was=20 some discussion about his STM-8 vertical
leaf-spring=20 design.

Hi Matthew,

Go to http://psn.quake.net/infoequ= ip.html and=20 download Sean's file on the STM-8. There are also files on the St Louis = Website=20 http://www.= eas.slu.edu/People/STMorrissey/index.html

Also look at Erhard Wielandt's downloads on = seismometers. And

http://www.geophys.uni-stuttgart.de/~erha= rd/downloads/textfiles/Seismometry/BroadbandDesign.doc

When I=20 plug this all in I get a somewhat simpler expression.that = what
Sean-Thomas=20 had. So I went back on including that fact that the
integrator pole = is not=20 zero, and the back EMF generated in the
feedback coil, and wound up = with=20 something way more complicated that I
am not going to type out = here. Does=20 anybody have any hints?

2.) My second question is about the = hinge. Is=20 foil suitable or should
I look at something like a knife edge or = ball=20 bearing hinge? I'm
leaning strongly towards foil. How much of an = impact=20 does the
flexibility of the foil really have on the period? Given = that=20 the
spring constant of the main spring is probably much much=20 larger.

Foil is perfectly satisfactory, but avoid = brass.=20 The spring in the foil has very little effect on the performance = provided it is=20 flat and has a single flex.

Avoid knife edge bearings like the plague = !!

The main problem in constructing a vertical = seismometer is how to compensate for the large temperature coefficient = of the=20 spring. You are limited to periods of less than 6 seconds in practice = with steel=20 springs. Otherwise the mechanism simply collapse as the instrument=20 temperature changes. You can use Elinvar of Ni-SpanC low coefficient = alloys, but=20 obtaining / forming / heat treating these are likely to give problems. =

The second alternative is to use three = component=20 force feedback to stabilise the operation.

The third option is to use a powerful coil = + magnet=20 sensor and put this into a negative input impedance amplifier. This = prevents the=20 arm from moving and you measure the feedback voltage required to=20 compensate. You get a voltage output proportional to frequency. =

See http://www.lennartz-electronic.de/PDF_documents/Seismometers.pdf=20

The fourth option is to make a vertical = with ~2=20 second period and use a low frequency boost amplifier to compensate for = the=20 sensitivity decrease at greater periods. You can get a x10 period = increase this=20 way with a coil + magnet sensor, but you need low noise electronics. It = may be=20 easier to use a position sensor followed by an integrator, but much more = electronics is required. This can be analogue, or digital as in the = Volksmeter=20 on psn and the CMG-EDU from Guralp.

Atmospheric noise tends to dominate = the other=20 input noises of unshielded vertical sensors by a factor of up to = x100.=20 Current commercial sensor technologies mostly seal small sensors inside = metal or=20 glass containers. It is also possible to use a float to provide=20 compensation for a 'bare' sensor.

I hope that this is of some help.

Regards,

Chris Chapman

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