jmhannon@.................... wrote:

> I did a little design calculation based on the 25 Hz upper frequency response of
> the sensor.
> The resonant frequency of the sensor needs to be above 25 Hz. So to see what
> ball park we are in I calculated the 1 g deflection of the mass with a resonant
> frequency of 30 Hz. This works out to be 0.275 mm.  This gives a total
> deflection of 0.55 mm at 2 g. This is not much. We could get away with a simple
> cantilever beam design or thin solid metal disks for the spring.

The idea of using a simple spring/mass sounds good if the sensing system has high
resolution.  The natural
frequency of any mass on a vertical spring system is directly related to the 1 G
deflection.  In fact the
natural frequency is the same as that of a pendulum of the same length as the
deflection.  That is to get a
spring mass system to have a 1 second natural period requires a system which will
deflect about 12 inches
in 1 G.

The most efficient simple spring system is a flat spring having a triangular shape.
That is, the mass is attached to one point of the spring which is clamped along the
opposite side.  Having a voice coil driver and an optical pickup on the mass would
seem to make a simple rugged feedback system.  If low frequencies are to be sensed,
the deflection should be as much as possible to make the resonant frequency low so
that the sensor does not need as much gain.  The feedback corner frequency must be
well above the natural frequency to make the feedback system simple.

I have worked with berrylium copper springs.  An ideal way to make them is to form
the spring in the soft state.  Hardening is then done by heating to about 600
degrees F. for an extended period.  This process makes about the best springs which
can be fabricated.  An etched spring can be made by drawing the shape with the
resist pen used for printed circuits on one side and applying a sheet of adhesive
resist on the back.

I would be glad to work with anyone in the bay area to see if we can come up with a
simple construction.

George Harris - Benicia, CA - gjharris@.............




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