In a message dated 2007/07/01 11:19:30 GMT Daylight Time, Tangazazen@....... 
writes:


> Subj:Re: Seismic sensors 
> Date:2007/07/01 11:19:30 GMT Daylight Time
> From:Tangazazen@.......
> Reply-to:psn-l@..............
> To:psn-l@..............
> Sent from the Internet 
> 
> 
> 
> Hi Chris,
>  
>      The seismometer is not a Lehman but a single pendulum with a natural 
> period of 1 second. It is supported by two pairs of leaf suspensions arranged 
> in a gimble form and give the pendulum two degrees of freedom but no rotation 
> about it's axis. The LED and projection optics are mounted on the lower end 
> and projects a 4mm spot of light modulated at 10KHz onto the quadrant photo 
> cell. The quadrant detector produces the X & Y components of motion which in 
> turn, via phase-sensitive detectors and appropriate feedback to orthogonal 
> force transducers, hold the pendulum in a fixed position. A two axis broadband in 
> fact but with a period of 60 seconds. The two axis output signals are of 
> course the restraining force in volts/meter/second. Unlike the vertical 
> broadband, temperature correction is unnecessary and tilt errors are compensated by 
> giving the integrator a large dynamic range. 
>  
> The capacitance does not directly generate noise but modifies the source 
> impedance which in turn change the ratio of the noise voltage to noise current 
> contributions to the total noise. The photo current of 0.7 micro amps is large 
> compared with the FET gate leakage current and is the dominate noise source.
>  
> Narrower bandwidths of lock-in systems will improve the S/N ratio but may 
> compromise the transient response, it's a question of optimisation.
>  
> The important characteristics of the LED are uniformity of the light spot 
> and linearity. The LED control feedback is dynamic i.e. both amplitude and wave 
> form are controlled, but there is a limit to how much the loop can correct 
> non-linearity before the onset of instability.
>  
> The theoretical analysis of Force Balance Feedback is covered in Thomas 
> Forbriger and Erhard Wielandt papers, the former is in German but the more 
> practical aspects are well documented by Sean-Thomas Morrissey and Allen Coleman.
>  
>  I should perhaps point out that large quadrant silicon photo detectors are 
> a little expensive to say the least and I would not have use this approach if 
> I had not been given a handful.
>  
>  
>  
>  Regards Martin
> 

   
In a me=
ssage dated 2007/07/01 11:19:30 GMT Daylight Time, Tangazazen@....... writes=
:





Subj:Re: Seismic sensors 

Date:2007/07/01 11:19:30 GMT Daylight Time

From:Tangazazen@.......

Reply-to:psn-l@..............
To:psn-l@..............

Sent from the Internet 







Hi Chris,

 

     The seismometer is not a Lehman but a single pendul=
um with a natural period of 1 second. It is supported by two pairs of leaf s=
uspensions arranged in a gimble form and give the pendulum two degrees of fr=
eedom but no rotation about it's axis. The LED and projection optics are mou=
nted on the lower end and projects a 4mm spot of light modulated at 10KHz on=
to the quadrant photo cell. The quadrant detector produces the X & Y com=
ponents of motion which in turn, via phase-sensitive detectors and appropria=
te feedback to orthogonal force transducers, hold the pendulum in a fixed po=
sition. A two axis broadband in fact but with a period of 60 seconds. The tw=
o axis output signals are of course the restraining force in volts/meter/sec=
ond. Unlike the vertical broadband, temperature correction is unnecessary an=
d tilt errors are compensated by giving the integrator a large dynamic range=
.. 

 

The capacitance does not directly generate noise but modifies the source imp=
edance which in turn change the ratio of the noise voltage to noise current=20=
contributions to the total noise. The photo current of 0.7 micro amps is lar=
ge compared with the FET gate leakage current and is the dominate noise sour=
ce.

 

Narrower bandwidths of lock-in systems will improve the S/N ratio but may co=
mpromise the transient response, it's a question of optimisation.

 

The important characteristics of the LED are uniformity of the light spot an=
d linearity. The LED control feedback is dynamic i.e. both amplitude and wav=
e form are controlled, but there is a limit to how much the loop can correct=
 non-linearity before the onset of instability.

 

The theoretical analysis of Force Balance Feedback is covered in Thomas Forb=
riger and Erhard Wielandt papers, the former is in German but the more pract=
ical aspects are well documented by Sean-Thomas Morrissey and Allen Coleman.=


 

 I should perhaps point out that large quadrant silicon photo detectors are=20=
a little expensive to say the least and I would not have use this approach i=
f I had not been given a handful.

 

 

 

 Regards Martin