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'[EE:] Gravimeter - was Sensing very small current '
2004\10\26@232316 by Denny Esterline

picon face
> On Tuesday 26 October 2004 11:49 am, Peter L. Peres wrote:
> > On Tue, 26 Oct 2004, Jake Anderson wrote:
> > > possibly easier than an interferomiter is to use the gross position
of
> > > the laser spot on a distant wall. use a CCD camera to look at it.
> > > nowhere near as cool of course ;->
> >
> > This is not so far off, you know. The lacoste-romberg gravimeters use a
> > microscope type of optics to get a mark's position on a scale (thye are
> > null reading instruments, the microscope reads the null - in a way).
Using
> > a student microscope could take you a fair way in the desired
direction,
> > it being able to magnify 1000x . Then that would be so much less gain
to
> > make up for in the electronics (measure uA instead of nA f.ex.). For
the
> > target I'd suggest a pyrite cristal which has the necessary high
contrast
> > features. You can get one at a gem shop, and chip off a tiny piece to
use
> > as a target.
>
> Consider yourself a genius  ;-)
>
> If I built a 10 meter long horizontal arm holding a weight on the end, I
> would be looking for small 0.1 mm movements.... no batteries required.
;-)
>
>

But that's not really practical. It'd be subject to significant tempco
effects, and it's not really practical to put in a test chamber.

-Denny


____________________________________________

2004\10\26@235038 by Denny Esterline

picon face
> To get back "On Topic"...   ;-)
> After reading the solution you suggested in another thread....
> I fully agree with you that resistor accuracy is not relevant considering
you
> have your eye set on trying to propose a time-based weight-drop solution.
> Let's put the resistor stuff aside.... ;-)
>
> However, a weight-drop apparatus is only able to measure one instance at
a
> time. you got to reset the apparatus, and therefore you have less
guarantee
> that you set it up identical to the 1st condition. You can't measure
> steadily over hours of time, therefore you may collect some numbers, but
> those may or may not be enough to correlate/prove/disprove something.

>From what I've read the weight drop apparatus is self resetting with some
kind of motor and belt drive to lift the weight, with test rates in the 10s
per minute. Holding and releasing the mass is definitely something to
consider carefully. But there are ways around it. If you manipulate the
equations some, you can work with velocity at two points and a distance
between them - the initial velocity doesn't have to be zero. It's a little
more complicated sensor wise, but a lot less to worry about for the release
mechanism.

> You can't build the apparatus as part of the vacuum chamber... it has to
be
> free-standing within the chamber, due to air crushing of the chamber.
> You can't discount for vibrations or random events which can be filtered
out
> because it is a one-shot type measurement.

I agree that varying air pressure outside the chamber would have *some*
effect, but considering the likely candidates for a chamber (steel pipe) I
would expect the air pressure variation to be significantly less then the
tempco.

> Finally, you are trading one set of drift problems with another set of
drift
> problems because you are building a 1-of-a-kind apparatus that you can't
> reference against an identical 1-of-a-kind apparatus. You can't discount
> voltage, temp, air pressure,or other drifts because you got nothing to
knock
> your drift values against.
> If you were able to build a 2nd "reference" to discount drifts, then a
> weight-drop solution may be the way to go, but how do you propose that?
> you can't build a 2nd weight-drop apparatus in the same room as the 1st
> because they are both affected by gravity.
>

I think the pendulum apparatus would be the easiest to produce, it's
basically just a high precision frequency counter that can average over
many cycles. I would only need a high stability clock source. Such clocks
are readily available, but out of my price range (10-100 MHz -stability
better than 0.1 PPM, in case you missed the first post) At the moment I'm
looking into rolling my own with a handheld GPS to discipline a crystal
(anybody have any experience with an Earthmate GPS receiver?, looks like
they can be had on eBay for <$40, serial port only, claims a NMEA
DataStream)

The sensitive balance apparatus is probably next easiest. My thinking was a
magnet/electromagnet with optical position feedback and measuring current,
but the consensus of the list seems to be this would require a precision
level beyond realistic. I may try some experiments with the Lacoste &
Romberg zero length spring arrangement - that's back to measuring
displacement.

-Denny


____________________________________________

2004\10\27@045656 by Alan B. Pearce

face picon face
>At the moment I'm looking into rolling my own with a
>handheld GPS to discipline a crystal (anybody have any
>experience with an Earthmate GPS receiver?, looks like
>they can be had on eBay for <$40, serial port only,
>claims a NMEA DataStream)

You don't need the NMEA datastream, you need the 1pps output, and I doubt
you will have that on the connector of a handheld unit. Look for an OEM
receiver, most of which do have the 1pps pulse available, but check the
manufacturers datasheet before you buy. I have a garmin GPS25-LVS which does
this, as well as a Motorola Oncore which also has it.

____________________________________________

2004\10\27@062426 by Russell McMahon

face
flavicon
face
> >At the moment I'm looking into rolling my own with a
>>handheld GPS to discipline a crystal (anybody have any
>>experience with an Earthmate GPS receiver?, looks like
>>they can be had on eBay for <$40, serial port only,
>>claims a NMEA DataStream)
>
> You don't need the NMEA datastream, you need the 1pps output, and I doubt
> you will have that on the connector of a handheld unit. Look for an OEM
> receiver, most of which do have the 1pps pulse available, but check the
> manufacturers datasheet before you buy. I have a garmin GPS25-LVS which
> does
> this, as well as a Motorola Oncore which also has it.

It would be interesting to see how well time locked the edge of the NMEA
sequence was to the 1 pps output - probably woefully inexactly but maybe
not. It is conceivable that it may be possible to use the first start bit of
each NMEA burst to do the same job. If there are asynchronous processes
between 1 pps pulse and data stream this will not work well.


       RM

____________________________________________

2004\10\27@082342 by olin_piclist

face picon face
Denny Esterline wrote:
> I think the pendulum apparatus would be the easiest to produce, it's
> basically just a high precision frequency counter that can average over
> many cycles.

This is a little off topic, but I used this technique to make a very
accurate thermometer once for fun.  I deliberately used about a 1m thin
copper pipe with a mass on the end as the pendulum.  The thermal expansion
of the pipe changed the pendulum length, which made a surprisingly easy to
measure shift in the pendulum period.  In this case I used an ordinary
crystal as the time reference and assumed gravity was constant.  Holding all
this constant and trying to measure gravity instead seems like a few orders
of magnitude more difficult.

This thing was sensitive enough so that you could see the temperature go up
as you stood next to it.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
____________________________________________

2004\10\27@145044 by Peter L. Peres

picon face

On Tue, 26 Oct 2004, Denny Esterline wrote:

{Quote hidden}

One mounts the thing in a basement in a pipe mounted to a wall and
thermostats the whole thing. But a light lever will be much better. I
posted a link to Boys's experiment about a week a go. It is worth reading
(several times) imho.

Peter
____________________________________________

2004\10\27@145044 by Peter L. Peres

picon face

Why don't you simply build a simple pendulum with period ~1 second (~1
meter long) put it in a thermostated pipe and measure its period. It would
have an oscillation amplitude of ~5mm (throw = 10mm) and you need to
detect it to 1/1000 (10um) precision using the microscope over say 100
seconds (periods). That is 1 part in 100,000. So you kick the pendulum
perhaps using a small electromagnet and then you watch the first pass and
count time till the 100th. A heavy pendulum with such a small amplitude
should give you much longer than 100 seconds time between 'kicks'. I
suggest you suspend the pendulum on a steel wire (guitar string etc)
instead of using a bearing. Some old clock pendulums used this method of
suspension.

Peter
____________________________________________

2004\10\27@151552 by Jose Da Silva

flavicon
face
On Wednesday 27 October 2004 03:08 am, Russell McMahon wrote:
> > >At the moment I'm looking into rolling my own with a

This may appear useful.
http://electronics.howstuffworks.com/question461.htm

http://www.sas.org/E-Bulletin/2003-03-28/features2/body.html

http://www.amug.org/~jthomas/wwvb.html
____________________________________________

2004\10\27@193339 by Denny Esterline

picon face
> Why don't you simply build a simple pendulum with period ~1 second (~1
> meter long) put it in a thermostated pipe and measure its period. It would
> have an oscillation amplitude of ~5mm (throw = 10mm) and you need to
> detect it to 1/1000 (10um) precision using the microscope over say 100
> seconds (periods). That is 1 part in 100,000. So you kick the pendulum
> perhaps using a small electromagnet and then you watch the first pass and
> count time till the 100th. A heavy pendulum with such a small amplitude
> should give you much longer than 100 seconds time between 'kicks'. I
> suggest you suspend the pendulum on a steel wire (guitar string etc)
> instead of using a bearing. Some old clock pendulums used this method of
> suspension.
>
> Peter

There's actually quite a few 'details' glossed over by that scenario.
First, the pendulums' period *does* depend on it's amplitude - not a
lot, but way more than the tolerances in this project. But it can be
compensated for, a couple of slot pairs for timing and amplitude
sensing and a electromagnet to give it a kick.

Second (and most relevant) is the precision needed in the reference
oscillator. To produce meaningful results the reference osc needs to
have a very small drift - on the order of 10^-9 per day or better.
This should be achievable with an OCXO, but GPS disciplined would be
better (now to find a way to do it on my budget :o)


As to the suspension system, the best ones I've read about are
"Kater's pendulums" (sp?) they were the first devices to establish 'g'
to about four decimal places. They use a knife edge against a glass
substrate.

-Denny
____________________________________________

2004\10\27@205354 by Denny Esterline

picon face
What about using WWVB signals? How acurate is the 1 PPS from those recievers?

I found a web site where the guy had hacked up a cheap one :
http://www.webtronics.com/wwvbsdr.html

Looks like I might be able to use it to characterize a much less
accurate clock at the same time the other clock is sampling pendulum
time - I could then 'normalize' the data in post proccessing.

Thoughts?

-Denny
____________________________________________

2004\10\28@172704 by Jose Da Silva

flavicon
face
On Wednesday 27 October 2004 05:53 pm, Denny Esterline wrote:
> What about using WWVB signals? How acurate is the 1 PPS from those
> recievers?

I expect it has jitter.
You will want to PLL to it, not derive a signal from it.

> I found a web site where the guy had hacked up a cheap one :
> http://www.webtronics.com/wwvbsdr.html
>
> Looks like I might be able to use it to characterize a much less
> accurate clock at the same time the other clock is sampling pendulum
> time - I could then 'normalize' the data in post proccessing.
>
> Thoughts?

Probably the way to go.
The less accurate clock can be a TCXO or VCXO phase locked to the 1 PPS WWVB
signal.
Being phase locked means it IS going to oscillate around the 1 PPS signal.

For your accurate clock, it is going to oscillate around some temperature
setting of it's own unless you are going to run it on some sort of forced
temperature ramp.
you will have to post process, either way.

-----
If going for the steady temperature idea, a tub of ice-water may help in
holding temperature control if you make your control temperature = 0 celcius
Stick a radiator in the tub of water to control the ratio of ice to water.

...going back to the 10 meter long pole idea.
Have you thought about changing the pole to a large 10 meter spring idea
mounted in a fridge. large weight with a mirror. Temperature should add
rotation, Gravity should add lean or lift.
____________________________________________

2004\10\28@174400 by Bob Ammerman

picon face
> Probably the way to go.
> The less accurate clock can be a TCXO or VCXO phase locked to the 1 PPS
> WWVB
> signal.
> Being phase locked means it IS going to oscillate around the 1 PPS signal.

Don't lock to the 1PPS signal, lock to the carrier. WWVB carrier is _very_
accurate.

Bob Ammerman
RAm Systems

{Quote hidden}

> ______________________________________________

2004\10\28@181027 by olin_piclist

face picon face
Bob Ammerman wrote:
> Don't lock to the 1PPS signal, lock to the carrier. WWVB carrier is
> _very_ accurate.

It is very accurate when it leaves the transmitter because it is derived
from the NIST atomic clock in Boulder.  However, there is significant phase
noise as the signal takes different paths thru the atmosphere and off the
ionisphere.  If you're close enough so that you get mostly ground wave, this
won't be too bad.  Here in New England 2000 miles from the transmitter we
only get skips.  The long term accuracy is still good, but "long term" is
now several hours to a day.

This sounds like a good PIC project.  Create a highly accurate local
oscillator by phase locking it to the WWVB carrier frequency.  Due to the
very long phase lock loop time constant required, this would need to be done
digitally.

I was thinking of making a WWVB receiver and PIC real time clock that would
be a "NTP server in a box".  Maybe adding a high accuracy frequency output
would be a nice additional feature.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
____________________________________________

2004\10\28@181159 by Peter L. Peres

picon face

On Wed, 27 Oct 2004, Denny Esterline wrote:

>> Why don't you simply build a simple pendulum with period ~1 second (~1
>> meter long) put it in a thermostated pipe and measure its period. It would
>> have an oscillation amplitude of ~5mm (throw = 10mm) and you need to
>> detect it to 1/1000 (10um) precision using the microscope over say 100
>> seconds (periods). That is 1 part in 100,000. So you kick the pendulum
>> perhaps using a small electromagnet and then you watch the first pass and
>> count time till the 100th. A heavy pendulum with such a small amplitude
>> should give you much longer than 100 seconds time between 'kicks'. I
>> suggest you suspend the pendulum on a steel wire (guitar string etc)
>> instead of using a bearing. Some old clock pendulums used this method of
>> suspension.

^^^^ Oops = the period of a 1 meter long mathematical pendulum is not 1
second. The Formula is T= 2*PI*sqrt(l/g) and the correct time for 1 meter
is 2 seconds (nominal).

>> Peter
>
> There's actually quite a few 'details' glossed over by that scenario.
> First, the pendulums' period *does* depend on it's amplitude - not a
> lot, but way more than the tolerances in this project. But it can be
> compensated for, a couple of slot pairs for timing and amplitude
> sensing and a electromagnet to give it a kick.

I am pretty sure that the pendulum will have a repeatable time over 100
periods even if the amplitude changes (decays) if the initial amplitude
(kick) is constant to the nearest 10um. This can be achieved with feedback
control from the microscope. This is not to be an absolute instrument. You
do not want to tamper with the pendulum while it is being timed. You want
to stop it (probably using an electromagnetic brake), then kick it with a
defined amount of current for a defined amount of time, and then measure
without further touching it. The magnetisation is a good point but a
copper ring + ac powered magnet could be used to avoid this. Obviously the
details are glossed over, this is a concept.

> Second (and most relevant) is the precision needed in the reference
> oscillator. To produce meaningful results the reference osc needs to
> have a very small drift - on the order of 10^-9 per day or better.
> This should be achievable with an OCXO, but GPS disciplined would be
> better (now to find a way to do it on my budget :o)

How much sensitivity do you need ? To measure 1x10^-5 you need 2x10^-6
clock stability for the measuring period with a pendulum. That's why they
stopped using pendulums for this I think.

> As to the suspension system, the best ones I've read about are
> "Kater's pendulums" (sp?) they were the first devices to establish 'g'
> to about four decimal places. They use a knife edge against a glass
> substrate.

The flexure suspension is more accurate after it ages and settles. The
Kater pendulum had to be inverted for use and was timed manually afaik.
The Boys, Cavendish-Hill Coulomb etc systems used torsion wires as
suspension. The torsion wire suspension is a type of flexure suspension.
The Askani (sp?) gravimeter uses a horizontal spring to suspend the mass
at the end of a lever. This is also flexure.

Peter
____________________________________________

2004\10\28@182848 by Denny Esterline

picon face
On Thu, 28 Oct 2004 17:43:05 -0400, Bob Ammerman <spam_OUTrammermanTakeThisOuTspamverizon.net> wrote:
> > Probably the way to go.
> > The less accurate clock can be a TCXO or VCXO phase locked to the 1 PPS
> > WWVB
> > signal.
> > Being phase locked means it IS going to oscillate around the 1 PPS signal.
>
> Don't lock to the 1PPS signal, lock to the carrier. WWVB carrier is _very_
> accurate.
>
> Bob Ammerman
> RAm Systems
>

I wasn't actualy thinking of locking my local osc to the WWVB signal,
but using it to create a medium term (~1/2 hour intervals)
characterization of the local osc. Then I could correct for it in post
proccessing.

-Denny
____________________________________________

2004\10\28@185800 by Dave VanHorn

flavicon
face

>
>I was thinking of making a WWVB receiver and PIC real time clock that would
>be a "NTP server in a box".  Maybe adding a high accuracy frequency output
>would be a nice additional feature.

Sounds almost like something Dallas would make! :)
A REAL time clock.  

____________________________________________

2004\10\28@194656 by John J. McDonough

flavicon
face
----- Original Message -----
From: "Olin Lathrop" <.....olin_piclistKILLspamspam@spam@embedinc.com>
Subject: Re: [EE:] Gravimeter - was Sensing very small current changes


> It is very accurate when it leaves the transmitter because it is derived
> from the NIST atomic clock in Boulder.  However, there is significant
phase
> noise as the signal takes different paths thru the atmosphere and off the
> ionisphere.  If you're close enough so that you get mostly ground wave,
this
> won't be too bad.  Here in New England 2000 miles from the transmitter we
> only get skips.  The long term accuracy is still good, but "long term" is
> now several hours to a day.

The WWV signal is affected by skip, but I don't think the WWVB signal is
affected by the same mechanism.  Here the problem, I think, isn't that the
frequency changes due to path length changes like at HF, but rather the
signal simply isn't audible through much of the country for much of the day.

A friend was recently doing the WWV receiver thing to calibrate a DDS, and a
little math shows the doppler effect to amount to several Hertz at 10 MHz.
I don't think the WWVB signal moves around as much, but I have to admit I
don't really understand VLF propagation as well as I understand HF.  I'm
pretty sure, though, that there's not much "skip" going on at that
frequency.

--McD


____________________________________________

2004\10\29@080127 by olin_piclist

face picon face
John J. McDonough wrote:
> The WWV signal is affected by skip, but I don't think the WWVB signal is
> affected by the same mechanism.  Here the problem, I think, isn't that
> the frequency changes due to path length changes like at HF, but rather
> the signal simply isn't audible through much of the country for much of
> the day.

Yes, and why is that?  If the propagation path were always the same, there
would be little reason for the received amplitude to change by time of day.
Why would the position of the sun have much of an effect on ground wave
propagation?  The answer is it doesn't, but it does have a major effect on
the upper atmosphere and its characteristics at radio frequencies.

The WWVB wavelength is 5Km, which is still small enough to not make it
around a 34 degree bend over 2000 miles of ground.  Unfortunately, the
various skip paths can easily differ by 5Km in length, and therefore there
is significant phase noise and even outright drops and additions of whole
cycles.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
____________________________________________

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