Brett et al
I think this discussion has been made before but... I was trying to decide =
the best displacement sensor style. What was the problem with inductive sty=
le sensors? I agree LVDT's can have clearance issues. VRTD's - I like, but =
someone had a problem with it but I can't remember what the reason was. I u=
se it on a small version of STM style sensor. I do get an occasional low fr=
equency oscillation which I can't nail down (~0.01 hz). I tentatively attri=
bute to maybe my triple feedback is slightly off. I would use a capacitive =
sensor but they seem so large to get a nominal capacitance value. Especiall=
y when one is trying to design a small unit.=20
Barry
=A0

--- On Fri, 8/21/09, Brett Nordgren  wrote:

From: Brett Nordgren 
Subject: Re: Real time traces
To: psn-l@..............
Date: Friday, August 21, 2009, 4:01 PM

Hi Karl,

Nothing particular at the ends.=A0 The stress builds from the center of the=
 spring as you move toward the ends by maybe 25 or 30%, but nothing terribl=
y exciting happens at the clamps.=A0 However there is another problem relat=
ed to creep that we did address.=A0 By using relatively thin (0.012") sprin=
g material we kept the max stress level down to roughly 70-75,000 psi, whil=
e using material with a yield strength of 280,000 psi.=A0 By working at 1/4=
 of yield, creep is a non-issue.=A0 If you missed it, I ran through some nu=
mbers in my 8/16 message to Barry Lotz.

In general, I feel that most home designs use springs which are much too th=
ick and hence too highly stressed and prone to creep, etc.=A0 Of course if =
you use a thin spring, the mass has to go down too, but in a feedback instr=
ument, that's all to the good. (up to a point, of course).

Actually, Dave was the one who designed the spring.=A0 He may want to add s=
omething.

Regards,
Brett



At 01:57 PM 8/21/2009 -0700, you wrote:
> Hi Brett,
>=20
> Thanks for the reply.
>=20
> I have a mechanical design question about your sensor. Did you do any thi=
ng special with the spring anchor / clamp mechanism?
>=20
> For some time I've been focused on creep when leaf springs are clamped an=
d stressed. The thought is that when the spring gets bent, the stress in th=
e outer fibers of the spring will be beyond what the clamp can retain. This=
 is especially true right where the spring emerges from the clamp.
>=20
> In some of my experiments this has caused noticeable hysteresis and creep=
.. I surmise the same problem might occur when clamping flexures although th=
e stress is bound to be much lower than in a leaf spring.
>=20
> Possibly some of this can be dealt with by stressing the spring and then =
clamping it, and maybe even artificially aging the assembly after clamping.
>=20
> Ideally the spring would be made with thicker ends for clamping but this =
is obviously impractical for our purposes.
>=20
> I'm curious if you did anything to help mitigate this effect.
>=20
> Karl

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


__________________________________________________________

Public Seismic Network Mailing List (PSN-L)


[ Top ] 
[ Back ] 
[ Home Page ] 
Brett et al
I think this discussion has be=
en made before but... I was trying to decide the best displacement sensor s=
tyle. What was the problem with inductive style sensors? I agree LVDT's can=
 have clearance issues. VRTD's - I like, but someone had a problem with it =
but I can't remember what the reason was. I use it on a small version of ST=
M style sensor. I do get an occasional low frequency oscillation which I ca=
n't nail down (~0.01 hz). I tentatively attribute to maybe my triple feedba=
ck is slightly off. I would use a capacitive sensor but they seem so large =
to get a nominal capacitance value. Especially when one is trying to design=
 a small unit. 
Barry
 

--- On Fri, 8/21/09, Brett Nor=
dgren <brett3nt@.............> wrote:

From: Brett Nordgren <brett3nt@.............>
Subject: =
Re: Real time traces
To: psn-l@..............
Date: Friday, August 21=
, 2009, 4:01 PM

Hi Karl,

Nothing par=
ticular at the ends.  The stress builds from the center of the spring =
as you move toward the ends by maybe 25 or 30%, but nothing terribly exciti=
ng happens at the clamps.  However there is another problem related to=
 creep that we did address.  By using relatively thin (0.012") spring =
material we kept the max stress level down to roughly 70-75,000 psi, while =
using material with a yield strength of 280,000 psi.  By working at 1/=
4 of yield, creep is a non-issue.  If you missed it, I ran through som=
e numbers in my 8/16 message to Barry Lotz.

In general, I feel that =
most home designs use springs which are much too thick and hence too highly=
 stressed and prone to creep, etc.  Of course if you use a thin
 spring, the mass has to go down too, but in a feedback instrument, that's =
all to the good. (up to a point, of course).

Actually, Dave was the =
one who designed the spring.  He may want to add something.

Reg=
ards,
Brett



At 01:57 PM 8/21/2009 -0700, you wrote:
&g=
t; Hi Brett,
> 
> Thanks for the reply.
> 
> I have=
 a mechanical design question about your sensor. Did you do any thing speci=
al with the spring anchor / clamp mechanism?
> 
> For some time=
 I've been focused on creep when leaf springs are clamped and stressed. The=
 thought is that when the spring gets bent, the stress in the outer fibers =
of the spring will be beyond what the clamp can retain. This is especially =
true right where the spring emerges from the clamp.
> 
> In som=
e of my experiments this has caused noticeable hysteresis and creep. I surm=
ise the same problem might occur when clamping flexures although the
 stress is bound to be much lower than in a leaf spring.
> 
> P=
ossibly some of this can be dealt with by stressing the spring and then cla=
mping it, and maybe even artificially aging the assembly after clamping.> 
> Ideally the spring would be made with thicker ends for clamp=
ing but this is obviously impractical for our purposes.
> 
> I'=
m curious if you did anything to help mitigate this effect.
> 
>=
; Karl

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm=


or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html

__________________________________________________________
Public Seismic Network Mailing List (PSN-L)

To leave this list emai=
l
 PSN-L-REQUEST@.............. with the =
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