Hi Matt
I am not able to get into the math but regarding the second question, I use=
d 0.002"=A0 SS cross foil flexures about 0.5" wide. They were hard to tight=
en because of their delicate nature.=A0 The consensus is that the sensor's =
free period has little to do with the sensors response in a force/balance s=
ystem (within reason). STM has a Mathcad program for evaluating the electri=
cal variables, but Brett as redone it in Excel. I've built two of them. The=
y work well but aren't as environmentally stable as the Inyo FBV but I thin=
k they cost me less :) . They are more sensitive to the elements because of=
 their size limiting the ability to protect for pressure variations. You ca=
n protect for thermal and provide electrical shielding though. I built one =
as per STM and one ~12" long. I have pictures on my website.=A0 The leaf sp=
ring does tend to drift a little.=A0 It may have to do with the stress leve=
l. Brett has a lot of discussions about this and the STM- 8. The other
 drawback I see to the STM-8 is it has a fairly high gain so you are limite=
d to a high gain output only unless you redesign the electronics a little. =
I think this could be fairly easily modified. All in all I am happy with th=
eir performance and don't regret constructing them. - - - I really don't re=
gret constructing any of my sensors I guess :) =A0=20

Regards
Barry
http://www.seismicvault.com



--- On Sat, 3/13/10, Gary Lindgren  wrote:

From: Gary Lindgren 
Subject: RE: Vertical BB derived from STM-8
To: psn-l@..............
Date: Saturday, March 13, 2010, 11:35 AM

Matt,
I suggest you check out the Inyo FBV (Force Balance Vertical) seismometer
designed by Dave Nelson and Brett Nordgren. The seismometer is very
sensitive and picks up M5's half way around the world. Karl Cunningham,
Barry Lotz, and I have built one of these units. Check out
http://sites.google.com/site/seismicsensorinfo/ This site chronicles the
design process I went through to build this amazing instrument. Brett's
loop7 spreadsheet guides you through the design details. You enter the
bandwidth you want, go through a few steps and out pops capacitor and
resistor values you need to fill the circuit board. Let me know if you any
questions.
Gary






Gary Lindgren
585 Lincoln Ave
Palo Alto CA 94301
=A0
650-326-0655
=A0
www.blue-eagle-technologies.com=A0=A0 Check out Lastest Seismometer Reading
cymonsplace.blogspot.com=A0
sites.google.com/site/seismicsensorinfo/Home=A0 =A0 Design Details for New
Vertical Seismometer=20
=A0


=A0=20

-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of Matt Zieleman
Sent: Saturday, March 13, 2010 10:36 AM
To: psn-l@..............
Subject: Vertical BB derived from STM-8

I've been doing some conceptual design work on a vertical broadband
sensor, and I have a few questions. By reading past archives of the
PSN-L I noticed that Sean-Thomas Morrissey was a contributor to this
list, and there was some discussion about his STM-8 vertical
leaf-spring design.

1.) My first question is about deriving the transfer function. I tried
to derive it myself, but from first principles, but what I got isn't
what is shown in Sean-Thomas' work. I think the problem is my
understanding of what the input and output of the transfer function
are. Here's my logic for the transfer function:

You can't=A0measure the position of the Earth's surface from a
stationary reference frame, and you can't measure the position of the
mass from a stationary reference frame. Only the relative position
between the two can be measured. My first postulate is that the force
acting on the mass only depends on the relative position of the mass
and Earth's surface. Combining this postulate with Newton's F=3D ma, I
get:

=A0 =A0 =A0 =A0 =A0 s^2 * M * X(s) =3D F(s) * (Y(s) - X(s))

Where M is the mass, X(s) is the position of the mass in a stationary
frame, Y(s) is the position of the ground in a stationary frame. The
only thing we can directly measure is Y(s) - X(s), so I believe the
transfer function is from Y(s) to Y(s) - X(s)/

=A0 =A0 =A0 =A0 =A0 Now X(s) =3D Y(s) - (Y(s) - X(s)), substituting in the =
left
hand side of my above expression:

=A0 =A0 =A0 =A0 =A0 s^2 * M * (Y(s) - (Y(s) - X(s))) =3D F(s) * (Y(s) - X(s=
))

=A0 =A0 =A0 =A0 =A0 s^s * M * Y(s) =3D (F(s) + s^2 * M)(Y(s) - X(s))

=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0=
 s^2 * M
=A0 =A0 =A0 =A0 =A0 (Y(s) - X(s)) =3D ----------------------- Y(s)
=A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 F(s) + s^2 =
* M

Then F(s) would be k + s*eta + K_p=A0 + s * K_d + K_i /s where K_p, K_i,
K_d are the coefficients of a PID controller, k is the mechanical
spring constant, and eta is the mechanical damping, if any.

When I plug this all in I get a somewhat simpler expression.that what
Sean-Thomas had. So I went back on including that fact that the
integrator pole is not zero, and the back EMF generated in the
feedback coil, and wound up with something way more complicated that I
am not going to type out here. Does anybody have any hints?

2.) My second question is about the hinge. Is foil suitable or should
I look at something like a knife edge or ball bearing hinge? I'm
leaning strongly towards foil. How much of an impact does the
flexibility of the foil really have on the period? Given that the
spring constant of the main spring is probably much much larger.
__________________________________________________________

Public Seismic Network Mailing List (PSN-L)


__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

Hi Matt
I am not able to get into the math=
 but regarding the second question, I used 0.002"  SS cross foil flexu=
res about 0.5" wide. They were hard to tighten because of their delicate na=
ture.  The consensus is that the sensor's free period has little to do=
 with the sensors response in a force/balance system (within reason). STM h=
as a Mathcad program for evaluating the electrical variables, but Brett as =
redone it in Excel. I've built two of them. They work well but aren't as en=
vironmentally stable as the Inyo FBV but I think they cost me less :) . The=
y are more sensitive to the elements because of their size limiting the abi=
lity to protect for pressure variations. You can protect for thermal and pr=
ovide electrical shielding though. I built one as per STM and one ~12" long=
.. I have pictures on my website.  The leaf spring does tend to drift a
 little.  It may have to do with the stress level. Brett has a lot of =
discussions about this and the STM- 8. The other drawback I see to the STM-=
8 is it has a fairly high gain so you are limited to a high gain output onl=
y unless you redesign the electronics a little. I think this could be fairl=
y easily modified. All in all I am happy with their performance and don't r=
egret constructing them. - - - I really don't regret constructing any of my=
 sensors I guess :)   

Regards
Barry
http://www.seismicvault.co=
m



--- On Sat, 3/13/10, Gary Lindgren <gel@th=
econnection.com> wrote:

From: Ga=
ry Lindgren <gel@.................>
Subject: RE: Vertical BB deriv=
ed from STM-8
To: psn-l@..............
Date: Saturday, March 13,
 2010, 11:35 AM

Matt,
I suggest you chec=
k out the Inyo FBV (Force Balance Vertical) seismometer
designed by Dave=
 Nelson and Brett Nordgren. The seismometer is very
sensitive and picks =
up M5's half way around the world. Karl Cunningham,
Barry Lotz, and I ha=
ve built one of these units. Check out
http://sites.google.com/site/s=
eismicsensorinfo/ This site chronicles the
design process I went thr=
ough to build this amazing instrument. Brett's
loop7 spreadsheet guides =
you through the design details. You enter the
bandwidth you want, go thr=
ough a few steps and out pops capacitor and
resistor values you need to =
fill the circuit board. Let me know if you any
questions.
Gary





Gary Lindgren
585 Lincoln Ave
Palo Alto CA
 94301
 
650-326-0655
 
www.blue-eagle-technologies.c=
om   Check out Lastest Seismometer Reading
cymonsplace.blogspo=
t.com 
sites.google.com/site/seismicsensorinfo/Home    De=
sign Details for New
Vertical Seismometer 
 


  <=
br>
-----Original Message-----
From: p=
sn-l-request@.............. [mailto:psn-=
l-request@..............] On
Behalf Of Matt Zieleman
Sent: Saturd=
ay, March 13, 2010 10:36 AM
To: psn-l@..............<=
br>Subject: Vertical BB derived from STM-8

I've been doing some conc=
eptual design work on a vertical broadband
sensor, and I have a few ques=
tions. By
 reading past archives of the
PSN-L I noticed that Sean-Thomas Morrissey=
 was a contributor to this
list, and there was some discussion about his=
 STM-8 vertical
leaf-spring design.

1.) My first question is abou=
t deriving the transfer function. I tried
to derive it myself, but from =
first principles, but what I got isn't
what is shown in Sean-Thomas' wor=
k. I think the problem is my
understanding of what the input and output =
of the transfer function
are. Here's my logic for the transfer function:=


You can't measure the position of the Earth's surface from astationary reference frame, and you can't measure the position of the
=
mass from a stationary reference frame. Only the relative position
betwe=
en the two can be measured. My first postulate is that the force
acting =
on the mass only depends on the relative position of the mass
and Earth'=
s surface. Combining this postulate with Newton's F=3D ma,
 I
get:

          s^2 * M * X(s) =3D F(s=
) * (Y(s) - X(s))

Where M is the mass, X(s) is the position of the m=
ass in a stationary
frame, Y(s) is the position of the ground in a stati=
onary frame. The
only thing we can directly measure is Y(s) - X(s), so I=
 believe the
transfer function is from Y(s) to Y(s) - X(s)/

 =
;         Now X(s) =3D Y(s) - (Y(s) - X(s)), substituti=
ng in the left
hand side of my above expression:

    &n=
bsp;     s^2 * M * (Y(s) - (Y(s) - X(s))) =3D F(s) * (Y(s) - X(s)=
)

          s^s * M * Y(s) =3D (F(s) + s^2 =
* M)(Y(s) - X(s))

              &=
nbsp;                    =
   s^2 * M
          (Y(s) - X(s)) =3D ---=
-------------------- Y(s)
            &nbs=
p;
                   F(s) + s^2 =
* M

Then F(s) would be k + s*eta + K_p  + s * K_d + K_i /s wher=
e K_p, K_i,
K_d are the coefficients of a PID controller, k is the mecha=
nical
spring constant, and eta is the mechanical damping, if any.
When I plug this all in I get a somewhat simpler expression.that what
S=
ean-Thomas had. So I went back on including that fact that the
integrato=
r pole is not zero, and the back EMF generated in the
feedback coil, and=
 wound up with something way more complicated that I
am not going to typ=
e out here. Does anybody have any hints?

2.) My second question is a=
bout the hinge. Is foil suitable or should
I look at something like a kn=
ife edge or ball bearing hinge? I'm
leaning strongly towards foil. How m=
uch of an impact does the
flexibility of the foil really have on the per=
iod? Given that the
spring constant of the main spring is
 probably much much larger.
____________________________________________=
______________

Public Seismic Network Mailing List (PSN-L)

To=
 leave this list email PSN-L-REQUEST@SEISMIC=
NET.COM with 
the body of the message (first line only): unsubscribe=

See http://www.seismicnet.com/maillist.html for more information.
__________________________________________________________

Public S=
eismic Network Mailing List (PSN-L)

To leave this list email PSN-L-REQUEST@.............. with
the body o=
f the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more inf=

[ Top ] 
[ Back ] 
[ Home Page ]