by Chris Chapman
Date: Sun, 16 Jul 2006 15:59:43 EDT
A peizo sensor can cover frequencies as low as 0.5 sec and be more sensitive than 4.5 Hz geophones. The disks themselves are maybe ~$1 each, but you also need a quiet FET input opamp. Maplins and Digikey stock suitable types.
Date: Wed, 5 Oct 2005 01:27:10 EDT
I have attached a circuit and a sketch to show what I have been doing with the piezo disks. I can supply component values for lower frequency roll offs. This is the general one for 7 Hz Bessel bandwidth which is similar to a Butterworth 10 Hz.
Further to my pics of the circuit and the sketch of the piezo transducer, I
designed magnetic damping for it, but I am not sure that it is absolutely
necessary. The resonance of the mass + disk is over 150 Hz and this is removed
in the LP filter. The filter is 7 Hz Bessel, which is similar to a 10 Hz
Butterworth. I do have component values for frequencies down to 3.5 Hz.
This is an alternative approach to very cheap seismology! It covers the P & S wave frequencies, which only extended period $1000 geophones respond to. It can be mounted either vertically or horizontally. The OPA604 does give improved performance and you probably won't need the zero adjustment. You do need to put it in a thermally insulated cover - the PZT elements are quite temperature sensitive.
The response is from about 6 sec to 10 Hz, but it can be ~extended down to maybe 12 sec by adding a 68 nF polyester across the PZT disk. This is 'pushing it' and you might need a bit more gain. You can convert to a velocity response by adding a 1/f amplifier roll off with increased LF gain. I designed magnetic damping for it, but I am not sure that it is absolutely necessary. The resonance of the mass + disk is over 150 Hz and this is removed in the LP filter. The filter is 7 Hz Bessel, which is similar to a 10 Hz Butterworth. I do have component values for frequencies down to 3.5 Hz.
The glass DS board is standard. The spacers are sold for use with PCBs. You can buy Brass 10 + 20 mm threaded standoffs with 3 mm screws and nuts (I can buy 30 mm threaded spacers + 3 mm screws), or a 1.125" threaded standoff + a couple of washers from http://dkc3.digikey.com/PDF/T053/1454-1457.pdf The foil and square tube comes from http://www.ksmetals.com/ These standard model making supplies are sold widely across the USA and in Europe. I use acrylic two component glue Holdtite ST3295. It sets very tough and slightly flexible, unlike epoxies which usually set to hard and brittle. Several brands of acrylic are available.
The disk bends in a cylindrical flex mode. You can use the sensor vertically or horizontally. Check that the suspension is on top for horizontal use. This is the 65 nF speaker sensor. The 4 thou brass disk is actually 44 mm od, glued inside a 50 mm plastic ring. The central piezo element is 25 mm OD.
Piezo elements for making seismic sensors
www.digikey.com have the 1N3595 diodes and PZT disks
See also p1421 for piezo elements - 102-1170-ND 44mm OD, 70 nF at $1.62
www.mouser.com also have the 1N3595 diodes and PZT disks
See p1270 for piezo elements 665-KBI-4406 44mm OD, 70nF at $1.69
665-APS-100 50mm OD, 65nF at $2.05
The circuit diagram shows two options for the op amps. The cheap option
is LF351 + LF353 while the lowest noise option is OPA604 + OPA2604. Note
that the zero set potentiometer has different values for the LF351 and the
4/27/2007 Update: Here are a few files on the Piezo sensors that I have built. They seem to work OK down to about 3 seconds.