Subject: Bridge system
Date: Wed, 25 Jul 2001 08:12:51 -0500
From: Terry Nelson <terry.nelson@mts.com>
To: rod.larson@mts.com

Hi Rod,

I have been out of e-mail touch for about 3 days.
I will tell you about it when I see you.

As for the problem. When a system has wiffle tree
fixturing in load or strain control the system must
have a small preload to assure that the actuators
maintain a snug position. The valves drift enough to
allow the actuator to loose contact if zero setpoint
is programmed. The bigger the valve drift the bigger
the preload must be. I hope this makes sense, it is
my best guess from the info you sent. I don't think
you need a cascade loop, just a little strain preload
should do. You mentioned something about 3 microstrain
out of 3000. I don't think the system can resolve that
kind of precision. This problem does not really show
it self on an aircraft as the wings always have a
preload in flight. I hope I helped some. If I were to
design the fixturing I think I would connect the actuators
solid to the bridge and keep the tensil load off with relief
valves and limit detectors. The actuators either require a
compressive preload or solid fixturing to return to neutral
when the simulated tank passes.

Thing are going pretty well here. Hope to talk with you soon.

Best regards,
Terry
