PSN-L Email List Message

Subject: Re: Video of a experimental pivot
From: ChrisAtUpw@.......
Date: Sat, 25 Jul 2009 14:11:07 EDT

```In a message dated 25/07/2009, brett3nt@............. writes:

>     You CAN'T cancel out tilt variations in  vertical sensors by just
using two identical instruments!
>   You CAN do it using a seismometer AND a pure  tiltmeter.

Why  not? Can't you just orient them in opposite directions? Vertical
motions should add, while tilt accelerations will subtract. That's what we do
when we want to evaluate the magnitude of tilt-rotation noise on Dave's
verticals.
Hi Brett,

That won't work!

The vertical force change due to a tilt theta is  mgCos(theta) = 1 -
theta^3 / !3 +

This is HIGHLY non linear with angle. Moreover, you get  the same
negative polarity force change for tilts in ANY orientation in the  horizontal
plane.

Obviously DC tilts don't necessarily cancel, but I wouldn't expect  that to
be the primary problem.  Come to think of it, wouldn't you even  be able to
off-center in one direction and the opposing one the same amount in  the
opposite direction?

Not in general. If you set up two sensors with  identical offsets in
opposite directions, they will both still produce a tilt  error if the tilt
direction is at right angles to the line joining the  centres of the two
instruments.
You need triaxial sensors to resolve this, but you still won't know if  an
effect is due to a tilt, or to a horizontal acceleration. You need a pure
tiltmeter to resolve this hoary ancient problem - a liquid level meter WON'T
do!

Regards,

Chris

In a message dated 25/07/2009, brett3nt@............. writes:

>     You CAN'T cancel out tilt variation=
s in
vertical sensors by just using two identical instruments!>&n=
bsp;
You CAN do it using a seismometer AND a pure
tiltmeter.
Why
not? Can't you just orient them in opposite directions? Vertic=
al
motions should add, while tilt accelerations will subtract. That's what=
we do
when we want to evaluate the magnitude of tilt-rotation noise on Dave's=

verticals.
Hi Brett,

That won't work!

The vertical force change due to a tilt theta=
is
mgCos(theta) =3D 1 - theta^3 / !3 +

This is HIGHLY non linear with angle. Moreover, yo=
u get
the same negative polarity force change for tilts in ANY orientation=
in the
horizontal plane.

You might want cross check your assumptions=
tilt rotation noise?
Obviously DC tilts don't necessarily cancel, but I wouldn't=
expect
that to be the primary problem.  Come to think of it, wouldn't you=
even
be able to cancel steady-state tilts if you adjusted one vertical instru=
ment
slightly off-center in one direction and the opposing one the same amoun=
t in
the opposite direction?

Not in general. If you set up two sensors wit=
h
identical offsets in opposite directions, they will both still produce a=
tilt
error if the tilt direction is at right angles to the line joining th=
e
centres of the two instruments.
You need triaxial sensors to resolve this, but you still won't=
know if
an effect is due to a tilt, or to a horizontal acceleration. You need a pu=
re
tiltmeter to resolve this hoary ancient problem - a liquid level meter WON=
'T
do!

Regards,

Chris
```