Hello All,

My first sensor was made from a device called a "limited angle torquer"
available from surplus. It has a magnet rotating in a stator winding
supported by low friction ball bearings. I built an arm to mount a weight of
about 5 Lbs at a distance of about 7 or 8 inches. It was tilted a few
degrees from vertical and had a period of about 4 seconds. I more than
saturated my chart recorder on the Landers event and aftershocks around a
3.0 (several years ago) at a distance of about 150 miles. The garage could
have fallen on it without breaking it. I also made a smaller version out of
a pen driver from a chart recorder that worked equally as well but you could
hold it in your hand. Using a heavy mass helps to overcome some of the
friction problems with these kinds of devices.

There are pen drivers built like a large version of a meter that could
probably be made to work but I never found one when I was looking for stuff
like that.

There must be a simple and cheap way to get something for strong motion.
____________________________________

Al Allworth           allworth@..............

Gold Beach

On The Beautiful Southern Oregon Coast

____________________________________

----- Original Message -----
From: Larry Cochrane 
To: PSN-L Mailing List 
Sent: Monday, October 18, 1999 8:05 PM
Subject: Re: Strong motion sensor design


> At 09:28 PM 10/17/99 -0500, Jim Hannon wrote:
>
> >Ok Larry now I see where you are coming from.
> >Only covering the strong motion range say +/- 2 g it is not hard to come
> up with a
> >lot of different ideas for sensor. The real challenge as I see it is in
the
> >details of the design. That is picking a design that is easy to
> manufacture the
> >parts or using a design that can be build from common parts such as the
> STM-8.
> >
>
> Exactly!
>
> >The very first sensor I built consisted of a small weight glued to the
> cones of
> >two speakers. It easily detected footsteps across the basement on a
concrete
> >floor.
> >
>
> I like that. Maybe you are on to something here. If one could add some
mass
> and a position loop could a speaker be user as s SM detector? Was two
> speakers used for X and Y motion or were they used together somehow?
>
> >We should try and nail down a few specifications so that the discussion
> don't get
> >too far out in left field. How about +/- 2 g max acceleration with 16
bits of
> >dynamic range. (How does this match the low end of a geophone
> sensitivity?) What
> >about frequency response? ( 0.01 to 10.0 Hz).
>
> The SM sensor should have +- 2 g max acceleration, this should produce a
+-
> 5 or +-10 volt output. It should be accurate to within +- 5% or 10%?? over
> the operating temperature range of the device. I don't think the operating
> temp  should be too extreme. If needed power can be used to either cool or
> heat the device.
>
> I don't think super low power should be a big design criteria. Since the
> data will most likely be collected using a PC, 110 v (or 220 v) should be
> handy. A UPS should be added by the user so that the sensor and computer
> will keep recording if the power goes down.
>
> The dynamic range should be 16 bits or better. If a sensor could be built
> with very good dynamic range then the unit could have two outputs. One at
> +- 2g and another more sensitive channel with maybe .2 or .02 g max
output.
> This maybe asking too much for one sensor? Maybe the same mechanical and
> electronics parts, with a little tweaking, could be made to look like a
> short-period, higher gain device that could be used to record the smaller
> events?
>
> I think the frequency response should be higher, in the 25 hz range. The
> low end of .01 hz sounds good. The FBA-23 I have has a DC output. If I
> rotate one of the horizontal channels vertically I get a constant 1 g
> output, or about 5 volts DC. The sensor I have has a +- 2 g = +- 10 v
> output. I high and low pass the signal out of the FBA-23 at .07 hz (~14
> sec) and 20 hz.
>
> Regards,
> Larry Cochrane
> Redwood City, PSN
>
>
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