Subject: RE: B600 Natural Frequency Tests In-Air
Date: Thu, 21 Oct 1999 17:16:03 -0500
From: "Campbell, Craig L" <Craig.L.Campbell@mts.com>
To: "Thomas Coudray (E-mail)" <thomas.coudray@laseyne.principia.fr>
CC: "Zappia, Tim" <Tim.Zappia@mts.com>, "Larson, Rod" <Rod.Larson@mts.com>,
     "Thoen, Brad" <Brad.Thoen@mts.com>

Hello Thomas,

Tim talked to me before sending this message out, so I have had a chance
to consider the questions.

1) The accelerometers will be in several different locations.  The
regular upper and lower flap accelerometers will be in their normal
position.  We also have three more accelerometers that we will glued
(the paint will be repaired after removal of the accelerometers) to the
top of the front face of the waveboard.  They will be attached as close
as possible to the same radius as the system accelerometers to try to
get the same feedback for the same angular acceleration.

The upper flap system accelerometers are already close to the front face
of the waveboard (the attachment is shown on sheet 3 of the MTS Upper
Waveboard drawing, in two detail views at the top left of the page).
The additional accelerometers will be mounted a few inches below the
system accelerometers, on the front face of the waveboard.

The lower flap system accelerometers are mounted on the back face of the
upper bearing mounting plate (the attachment is shown on sheet 2 of the
MTS Lower Waveboard drawing, in a view at the top middle of the page).
The additional accelerometers will be mounted at the top of the front
face of the lower waveboard, which will be almost directly in line with
the system accelerometers.

The three additional accelerometers will be placed directly in line with
the system accelerometers along the length of the basin.  These three
positions are the best to identify the oil column, and torsional modes
of vibration.

2) The final design report mentions three vibration modes without water:
        Mono Flap Oil Column = 3.64 hz
        Upper Flap Oil Column = 8.76 hz
        Lower Flap Only Oil Column = 10.92 hz
It will be very difficult to identify the Lower Flap Only Oil Column,
since it assumes no Upper Flap is attached.

We should be able to identify the Mono Flap Oil Column and Upper Flap
Oil Column frequencies without too much difficulty.  For the Upper Flap
Oil Column measurement, we will leave the lower flap maintenance struts
installed.  The lower flap can then be preloaded against the struts to
achieve a significantly stiffer system.  The "bowstring"  frequency of
the lower strut could cause a problem, but we should be able to identify
its reasonant frequency if required.

The lower flap oil column measurement will be done with the upper struts
installed.  The upper flap can then be preloaded against the upper
struts to achieve a significantly stiffer connection between the two
flaps.  Due to the short length and thick cross section of the upper
strut, it will be very difficult to identify the upper strut's resonant
frequency.

Note that these measurements give us an indication of the accuracy of
the FEA model.  If we get results different than the FEA model
predicted, it will not be too meaningful to investigate why they are
different, especially if the difference is small.  Modal analyses of
resonant frequencies in structures is a science unto itself.  We could
invest significant time without learning any additional information.
Therefore, our goal is to develop reasonable confidence that the
frequencies we identify are the frequencies of interest.  We will not
spend time identifying modes of vibration beyond about 15 hz, or lower
if we have already identified the modes shown above.

3) These measurements will be done using normal control of the flaps.
You are correct that most of the advanced control methods will not be
used.  The gains for the system will be adjusted to achieve control that
is robust enough to gather sufficiently accurate data to confidently
determine the frequencies of interest.

In general, there are different methods for measuring natural
frequencies.  Some measurements rely on random noise as the program,
while other methods use square wave programs.  Different methods are
effective on different systems.  The FFT meter in the controller can be
used to monitor DeltaP or acceleration to determine oil column
frequencies on some systems.  On other systems, transfer functions must
be used.  We will use a mixture of these methods, but transfer functions
of the accelerations will be used to identify the torsional frequency.
A transfer function of the two outside accelerometers will show a phase
difference of 180 degrees for the torsional frequency.  A transfer
function of the middle versus the outer accel will show a difference of
180 degrees for the "warp" frequency.

You have also expressed interest in measuring panel frequencies.  Since
there are many different sizes of panels, we will only measure the
natural frequency of the largest panels on the front and back of each
waveboard, which should be a total of four panels.  An accelerometer
will be glued to the center of the panel of interest, and we will
collect some data.  This measurement should also be relatively
straightforward.

I hope this helps.  Any other input from anyone is welcome.

Regards,
Craig
> -----Original Message-----
> From: Zappia, Tim
> Sent: Thursday, October 21, 1999 11:31 AM
> To:   Campbell, Craig L
> Cc:   'COUDRAY,Thomas'; Larson, Rod; Thoen, Brad
> Subject:      B600 Natural Frequency Tests In-Air
>
> Hello Craig,
>
> At the Coordination Meeting last week, Thomas wanted to know more
> details about how we plan on performing the natural frequency tests.
> In particular, he want to know about:
>
> 1) Where we plan on placing the accelerometers?  He was thinking it
> would be good to have one on the center actuator web, one on one of
> the side actuator webs, and one on the other side panel.
>
> 2)  How can we isolate one of the flaps from the other when running
> the tests?
>
> 3) Is it difficult to isolate tests with and without the advanced
> controller being used?  To me, this should be able to be accomplished
> by turning the gains off on the higher level controller and then
> running the test with the controller only using  low level input and
> output features.
>
> I think it's best if you have this discussion with Thomas.  However,
> please be sure to copy everyone on the mailing list so we are all
> going into the tests with the same background information.
>
> Thanks - Tim
>
> PS: Thomas has a new e-mail address so be sure to update your list.
