I'm ready to begin a seismometer project. I'm trying to keep the cost as low as possible while still yielding a reasonable instrument. My expertise in in electronics but the mechanical stuff is more problematic. So far I have only roughed out a hardware approach -- I'm hoping to use some ready-made piece of code for display/analysis purposes. This is the only post in the forum for seismometers, so I don't think I need a new thread until I get more specific.
Here’s some thoughts on my seismometer project:
A short period seismometer would be easier to manage than a long period. I figure a 1 or 2 second period is acceptable. It might be possible with post-processing to extend the period, but I am not sure about that. I also have no immediate plans to make it into a broadband unit, which would require additional electronics and a motor to keep the mass stationary. The sensor interface that I am planning is generally incompatible with a force-balance approach anyway. I want to keep the cost and complexity of the mechanical system to a minimum -- just to prove the concept of the sensor and it's interface. Later, if things go well, I can splurge on a better mechanical system.
In keeping with the low cost easy-to-make philosophy, I think that this link (http://www.instructables.com/id/This-Se ... /?ALLSTEPS
) is an appropriate fist effort. It is a Lacoste pendulum. The post and base are painted wood. the most expensive component is the 1/8” thick aluminum boom and magnets for the damping mechanism.
But the sensor magnet would be replaced with the capacitance displacement sensor - probably hanging the middle plate inline with the boom axis (which might cause other problems…) and a lead mass. Dr. Peters patented this sensor in 1995, so it has most probably expired by now. The middle plate must be grounded - this can be accomplished by using the boom as a conductor and just grounding the plate where the hinge is attached. I figure a capacitive displacement sensor will have much better low frequency performance than a magnetic acceleration sensor.
This is Dr. Peters’ Volksmeter link(http://www.rllinstruments.com
), where you can find a bit about the theory of his product, including the Synchronous Differential Capacitance sensor. I will be using the AD7745 Capacitance to Digital Converter -- it converts a +/- 4pF range into 24-bits of resolution (ENOB=21bits). It uses the I2C interface of the Pi. The conversion rate can be as high as 10ms, but 62ms is enough to yield a Nyquist rate of 8Hz.
Here's the sensor patent link(http://www.google.com/patents/US5461319
). And a link to the AD7745 data sheet (http://www.analog.com/en/products/analo ... d7745.html
I created an spreadsheet that calculates the size of the sensor vs. the required sensitivity. Most of the literature that I’ve read indicates that 1nm displacement resolution is an acceptable goal. I don’t think we can get there with a reasonable size sensor area. My initial calculations show that about 10-20nm displacement is a good first order target.
I can get 6”x6” single sided FR4/1oz copper boards for about $3 each from Tayda Electronics. The middle sensor requires double sided copper - about $4 for 6”x6” board. I plan to cut the outer plates with an exacto knife and route out the sections of the middle plate. No shielding is planned. If shielding is required a simple faraday cage can be mounted around the five open sides of the sensor unit. This ideal method to create the capacitance sensor is to fabricate them as PCBs, but at $5/sq. inch this becomes very expensive.
The capacitance to digital converter circuit is pretty simple.
I will order 2-sided boards from OSH Park - a 0.93”x1.32” board is about $6.14 for 3 copies. Here’s the BOM breakout:
Board - $2
AD7745 - $11.28
LD2981 - $0.66
discretes - $free (I already have all of them in quantity)
2x13 header (stackable) - $0.75
Shipping - $1.18 ($3.54/3)
Total sensor interface cost - $15.87
I am a rank beginner at the seismometer game, so if you see any glaring errors please let me know.