PSN-L Email List Message

Subject: Re: Integrating in WinQuake
From: Barry Lotz barry_lotz@.............
Date: Mon, 23 Nov 2009 19:58:26 -0800 (PST)


All
This seems to be a good one. I think we are having issues with semantics. Y=
ou have the movement of the sensor and movement of the ground. One would li=
ke the movement of the sensor ( which we can measure) to relate to movement=
 of the earth. One can only measure displacement (distortion of a material =
- capacitive,LVDT, etc) or velocity(magnet/coil)=A0 of the sensor relative =
to it's base (excluding GPS). Assuming the proper damping ,=A0 for short pe=
riod sensors, the displacement of the sensor is linear with acceleration of=
 the earth. With long period sensors, the movement of the sensor relates to=
 the movement of the earth. If one is measuring displacement of the sensor =
with say a Lehman, the output would be related to the displacement of the e=
arth. The velocity of the sensor (magnet/coil) would relate to the velocity=
 of the earth. With feedback systems I'm not so sure. With STM's feedback h=
e claimed that the displacement measured of the sensor related to the
 velocity of the earth.
Regards
Barry


--- On Mon, 11/23/09, ChrisAtUpw@.......  wrote:

From: ChrisAtUpw@....... 
Subject: Re: Integrating in WinQuake
To: psn-l@..............
Date: Monday, November 23, 2009, 6:09 PM

=0A=0A =0A=0AIn a message dated 23/11/2009, rsparks@.......... writes:=0ATh=
ere =0A  seems to be an ongoing difference of opinion of what constitutes a=
 =0A =20
velocity detector or an acceleration detector.=A0 To my mind, if a =0A =20
seismometer automatically returns to a zero position, then it must be =0A =
=20
recording acceleration.=A0 All vertical seismometers do this because =0A  t=
hey=20
all are measuring against gravity, so they would all be acceleration =0A =
=20
devices.=A0 Excepting the sensitivity to tilt, all horizontal =0A  seismome=
ters=20
also return to a zero position so they could also be called =0A  accelerati=
on=20
sensitive devices.=A0 But maybe this description =0A  oversimplifies the si=
tuation.=0AHi Roger,=0A=A0=0A=A0=A0=A0=A0Sorry, but no. You seem to be=A0co=
nfusing =0Adevices which have AC and DC characteristics.=0ASome =0A  sensor=
s (capacitive and optical for example) clearly record=20
displacement =0A  (neither acceleration or velocity).=A0 =0A=A0=A0=A0=A0Agr=
eed.=0AOn the =0A  other hand, with magnetic/coil devices, velocity is alwa=
ys observed when =0A  electrical=20
output is observed so magnetic/coil devices are velocity =0A  detectors.=0A=
=A0=A0=A0=A0No. They may be either velocity or acceleration =0Adetectors. I=
f the coil is attached to a ~freely suspended, but damped, mass, you =0Aget=
 a velocity detector - eg a Lehman. If it is attached to a mass suspended o=
n =0Aa spring, you get an acceleration detector. The length of the spring i=
s constant =0Aif the velocity is constant. It only changes in length and ca=
uses the coil to =0Amove if there is an acceleration.=0AA =0A  displacement=
 sensor would record the relative distance from a some zero =0A =20
point at the instant of data read, the velocity sensor would record the =0A=
 =20
relative velocity at the instant of data read,=A0 and acceleration =0A  sen=
sor would =0A=A0=A0=A0=A0detect the acceleration! eg a MEMs =0Aacclerometer=
..=0Abe a =0A  calculated number found by using the data from any two veloci=
ty=20
data =0A  points and any three displacement data points.=0A=A0=A0=A0=A0Agre=
ed.=0AFinally, =0A  if two seismometers, identical except for detectors, we=
re=20
placed side by =0A  side, they would both plot the identical earthquake wav=
e=20
form, assuming =0A  that the frequency characteristics were the same.=A0=0A=
=A0=A0=A0=A0No, they wouldn't. A velocity output is =0Aproportional to the =
differential (slope) of a position output plot with =0Atime.=0A=A0However, =
when it is recognized that displacement position is =0A  not time=20
sensitive but velocity is, the builder can expect dramatic =0A  frequency=
=20
response differences between displacement detectors and velocity =0A =20
detectors .=A0 Simply put, distance is distance, but velocity is the =0A =
=20
distance divided by the time needed to travel between two =0A  points.=A0=
=A0 As a=20
result, for velocity detectors, the longer the =0A  wave length, the less=
=20
energy for each instant resulting in decreased =0A  voltage (and current)=
=20
detected at each instant (for any defined magnetic =0A  field).=0A=A0=A0=A0=
=A0Regards,=0A=A0=0A=A0=A0=A0=A0Chris Chapman
All
This seems to be a good one. I think w= e are having issues with semantics. You have the movement of the sensor and movement of the ground. One would like the move= ment of the sensor ( which we can measure) to relate to movement of the ear= th. One can only measure displacement (distortion of a material - capacitiv= e,LVDT, etc) or velocity(magnet/coil)  of the sensor relative to it's = base (excluding GPS). Assuming the proper damping ,  for short period = sensors, the displacement of the earth. With long period sensors, the movement of= the sensor relates to the movement of the earth. If one is measuring displ= acement of the sensor with say a Lehman, the output would be related to the displacem= ent of the earth. The velocity of the sensor (magnet/coil) would relate to = the velocity of the earth. With feedback systems I'm not so sure. With STM'= s feedback he claimed that the displacement measured of the sensor related to the velocity of the <= span style=3D"text-decoration: underline;">earth.
Regards
Barr= y


--- On Mon, 11/23/09, ChrisAtUpw@....... <ChrisAtUpw@= aol.com> wrote:

From: ChrisAtUpw= @....... <ChrisAtUpw@.......>
Subject: Re: Integrating in WinQuake=
To: psn-l@..............
Date: Monday, November 23, 2009, 6:09 PM
=0A=0A =0A=0A
In a message dated 23/11/2009, = rsparks@.......... writes:
=0A
There =0A = seems to be an ongoing difference of opinion of what constitutes a =0A velocity detector or an acceleration detector.  To my mind, if a =0A =
seismometer automatically returns to a zero position, then it must be = =0A
recording acceleration.  All vertical seismometers do this be= cause =0A they
all are measuring against gravity, so they would all be= acceleration =0A
devices.  Excepting the sensitivity to tilt, al= l horizontal =0A seismometers
also return to a zero position so they c= ould also be called =0A acceleration
sensitive devices.  But mayb= e this description =0A oversimplifies the situation.
= =0A
Hi Roger,
=0A
 
=0A
    = Sorry, but no. You seem to be confusing =0Adevices which have AC and D= C characteristics.
=0A
Some =0A sensors (= capacitive and optical for example) clearly record
displacement =0A (n= either acceleration or velocity). 
=0A
 &= nbsp;  Agreed.
=0A
On the =0A o= ther hand, with magnetic/coil devices, velocity is always observed when =0A= electrical
output is observed so magnetic/coil devices are velocity = =0A detectors.
=0A
    No. They= may be either velocity or acceleration =0Adetectors. If the coil is attach= ed to a ~freely suspended, but damped, mass, you =0Aget a velocity detector= - eg a Lehman. If it is attached to a mass suspended on =0Aa spring, you g= et an acceleration detector. The length of the spring is constant =0Aif the= velocity is constant. It only changes in length and causes the coil to =0A= move if there is an acceleration.
=0A
A = =0A displacement sensor would record the relative distance from a some zer= o =0A
point at the instant of data read, the velocity sensor would rec= ord the =0A
relative velocity at the instant of data read,  and a= cceleration =0A sensor would
=0A
   = ; detect the acceleration! eg a MEMs =0Aacclerometer.
=0Abe a =0A calculated number found by using the data fr= om any two velocity
data =0A points and any three displacement data po= ints.
=0A
    Agreed.
=0AFinally, =0A if two seismometers, identical ex= cept for detectors, were
placed side by =0A side, they would both plot= the identical earthquake wave
form, assuming =0A that the frequency c= haracteristics were the same. 
=0A
  =   No, they wouldn't. A velocity output is =0Aproportional to the = differential (slope) of a position output plot with =0Atime.
=0A However, when it is recognized that displaceme= nt position is =0A not time
sensitive but velocity is, the builder can= expect dramatic =0A frequency
response differences between displaceme= nt detectors and velocity =0A
detectors .  Simply put, distance i= s distance, but velocity is the =0A
distance divided by the time neede= d to travel between two =0A points.   As a
result, for veloc= ity detectors, the longer the =0A wave length, the less
energy for eac= h instant resulting in decreased =0A voltage (and current)
detected at= each instant (for any defined magnetic =0A field).
=0A=
    Regards,
=0A
 
=0A
&nbs= p;   Chris Chapman
=0A
<=
[ Top ] [ Back ] [ Home Page ]