In a message dated 2007/06/28, gmvoeth@........... writes:

> Surely there are new high tech sensors we amateurs may not be aware of that 
> possibly we may obtain. It seems there has been no advancements in the 
> sensor area. I had great troubles with the piezo idea because it has such a high 
> impedance. 

Hi Geoff,

       I have no significant problems with piezo devices, but I build them 
into the screened sensor unit itself. You can realistically get periods out to 
about 3 seconds. This is OK for making 0.5 / 1 Hz and up sensors to replace 1 
Hz geophones, which cost >$1,500.

Whatever I do I must use > a balanced preamp system with most of the gain
> in the front end or the drifting of the class "A" baseline is unacceptable.
> Isn't there a variable resistor that might work over a range of
> a few mm for full scale?

       Not that I know of. The only type that I can think of are the 
tunnelling mode pressure pads / coax cable. These are fine for making intruder alarms 
or pressure controls for joysticks, but they do not have the temperature 
stability suitable for a seismometer.   

       It is usual with amateur sensors to provide a 30 to 60 second high 
pass filter to remove 1/f noise and baseline drift.

 The key is most probably the > range of operation and these special kind of 
> devices
> are just not normally made available to the general public.

       Just what are these mysterious devices and how do they work?

> At the moment only a magnet coil seems to fill the billit
> since I am unwilling to use home built AC devices like capacitive (moving 
> plates) or
> inductive (moving cores) sensors. 

       If you are unwilling to use either of the two most common distance 
measuring techniques, you have severely restricted yourself. One of the sensors 
that I use has a 10 cm ferrite aerial rod and a resolution of about 7 nano 
metres. It cost about $60 for the kit. Capacitor systems are not difficult to make 
or to use and amateur designs are available. I can see no logical reason to 
reject these.

If I had an fcc license I might try > those AC devices but since I do not I 
> will not.

       So, what sort of device needs a FCC Licence? 
       Certainly NOT LCDT or LVDT sensors!!

> Right now only baseband and DC are my options.
> Low impedance front ends have always been best for me.
> 
> Is there any way for the layman to realistically research the new sensors 
> that might be out there ?

       I know of three other options. 

       One is a twin Silicon photodiode sensor with a moving shutter and a 
filament bulb. You can design these for deflections up to +/-10 mm at quite low 
cost and you may get the 15 nano metre resolution that I measured.

       Another is a Hall effect magnetic detector with a quad NdFeB magnet 
block to drive it. These only have a working range of +/-1/2 mm at maybe 20 nano 
metres resolution, but they are OK for simple pendulums, e.g. SG type.

       The third is to use a conducting water manometer / tiltmeter type of 
device. It is not too difficult to make these horizontal sensors work out to 
over 20 seconds. See Gile, WW, Geophys. J. Roy. Astro. Soc. vol 36 1974 pp 
153-165 "A Mercury Pendulum Seismometer" very good but a bit too large for 
amateurs. (You don't have to make them anything like so large! - you just have to be 
more clever!)

       If you know of any other sensor technologies, do tell me! I am aware 
of some of the MEMs devices, but the only ones that I have seen had a lousy 
noise performance.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/06/28, gmvoeth@........... writes:



Surely there are new high tech=20=
sensors we amateurs may not be aware of that possibly we may obtain. It seem=
s there has been no advancements in the sensor area. I had great troubles wi=
th the piezo idea because it has such a high impedance. 




Hi Geoff,



       I have no significant problems with pie=
zo devices, but I build them into the screened sensor unit itself. You can r=
ealistically get periods out to about 3 seconds. This is OK for making 0.5 /=
 1 Hz and up sensors to replace 1 Hz geophones, which cost >$1,500.



Whatever I do I must use 
a bala=
nced preamp system with most of the gain

in the front end or the drifting of the class "A" baseline is unacceptable.<=
BR>
Isn't there a variable resistor that might work over a range of

a few mm for full scale?




       Not that I know of. The only type that=
 I can think of are the tunnelling mode pressure pads / coax cable. These ar=
e fine for making intruder alarms or pressure controls for joysticks, but th=
ey do not have the temperature stability suitable for a seismometer. &n=
bsp; 



       It is usual with amateur sensors to pro=
vide a 30 to 60 second high pass filter to remove 1/f noise and baseline dri=
ft.



 The key is most probably the 
r=
ange of operation and these special kind of devices

are just not normally made available to the general public.



       Just what are these mysterious devices=
 and how do they work?



At the moment only a magnet coi=
l seems to fill the billit

since I am unwilling to use home built AC devices like capacitive (moving pl=
ates) or

inductive (moving cores) sensors. 




       If you are unwilling to use either of=20=
the two most common distance measuring techniques, you have severely restric=
ted yourself. One of the sensors that I use has a 10 cm ferrite aerial rod a=
nd a resolution of about 7 nano metres. It cost about $60 for the kit. Capac=
itor systems are not difficult to make or to use and amateur designs are ava=
ilable. I can see no logical reason to reject these.



If I had an fcc license I might try 
those AC devices but since I do not I will not.




       So, what sort of device needs a FCC Li=
cence? 

       Certainly NOT LCDT or LVDT sensors!!


Right now only baseband and DC=20=
are my options.

Low impedance front ends have always been best for me.



Is there any way for the layman to realistically research the new sensors th=
at might be out there ?




       I know of three other options. 



       One is a twin Silicon photodiode sensor=
 with a moving shutter and a filament bulb. You can design these for deflect=
ions up to +/-10 mm at quite low cost and you may get the 15 nano metre reso=
lution that I measured.



       Another is a Hall effect magnetic detec=
tor with a quad NdFeB magnet block to drive it. These only have a working ra=
nge of +/-1/2 mm at maybe 20 nano metres resolution, but they are OK for sim=
ple pendulums, e.g. SG type.



       The third is to use a conducting water=20=
manometer / tiltmeter type of device. It is not too difficult to make these=20=
horizontal sensors work out to over 20 seconds. See Gile, WW, Geophys. J. Ro=
y. Astro. Soc. vol 36 1974 pp 153-165 "A Mercury Pendulum Seismometer=
" very good but a bit too large for amateurs. (You don't have to make them a=
nything like so large! - you just have to be more clever!)



       If you know of any other sensor t=
echnologies, do tell me! I am aware of some of the MEMs devices, but=
 the only ones that I have seen had a lousy noise performance.



       Regards,



       Chris Chapman