PSN-L Email List Message

Subject: STS-1 info
From: S-T Morrissey sean@...........
Date: Sun, 26 Mar 2000 21:30:15 -0600 (CST)

Brett and Karl,

Interesting work on the STS-1 leaf spring. The complexity of it
reassures my own decision to avoid it and use a simple linear
spring configuration. 

Unfortunately, I have not had the opportunity to dismantle a STS-1
to examine the hidden details. (but there is an interesting thought:
just what major new insight into seismometer design will make those
pricey pups so obsolete that they show up on ebay for $100 ? !!).
I have heard (a rumor?) that they will not be made anymore because
of the cost, and since the STS-2 is suitable for most applications.

The advertised mass of the  STS-1 is 0.6 kg. The mechanical period
of the vertical used in calculating the response is 6 seconds. But 
note that the mechanical period only affects the damping in a VBB 
fedback system. The closed loop period is controlled by Cp, RI, and TI.
As the sensor is tilted over backwards toward the hinge, the period will
become infinite as the restoring force approaches zero. It seems that
the micro-manipulative factory adjustments of the spring are made
to achieve this, along with modifications of the thermal contributions
of the two different spring materials. However, with the feedback turned 
on, long term changes in the mechanical period result in only very small 
changes in the damping and closed loop period. When the seismometers 
are installed, both the vertical and the horizontal are physically adjusted 
by tilting the base so that barely sustained constant amplitude self 
oscillations are achieved at a Tn of 20 seconds by the closed loop with the 
feedback resistor Rp disconnected. This way a specific mechanical period
is achieved so that the factory calibration is replicated in the field.

As for the clamping bars, I can only guess. An old technique used
for clamping leaf springs involves a raised rounded ridge (or even a 
small hardened rod installed in a groove) on one side of the clamps' 
fixed half towards the spring body. The screwed clamp was
outside this, and tighting the screws pivoted the spring over the
ridge, changing the exit angle from the clamp and therefore the torque 
applied to the end of the spring.  If this is the case for the STS-1, 
tightening the screws would change the ellipticity of the springs.


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Larry Cochrane <cochrane@..............>