PSN-L Email List Message

Subject: Re: Digest from 10/02/2012 00:00:07
From: chrisatupw@.......
Date: Mon, 8 Oct 2012 18:26:23 -0400 (EDT)










From: Bob Holmstrom 

To: psnlist 
Sent: Wed, 3 Oct 2012 23:47
Subject: Re: Digest from 10/02/2012 00:00:07


    I have seen silicon wafers etched thin enough that they could be rolled=
 up like cigarette paper - perhaps silicon in that form could be used as a =
flexure.  Yes, metal construction is the norm, but it seems to be in the sp=
irit of this list to consider alternative methods and materials.

Hi Bob,=20

    I don't have a figure for the tensile strength of Silicon, but I doubt =
that any sheet thin enough to bend easily would be able to support a pendul=
um weighing several kilograms. Sheet silicon is also liable to brittle frac=
ture under shock loads.      =20


The book by R. V. Jones - "Instruments and Experiences" is an organized col=
lection of many of his articles on instrumentation.  Each section has overv=
iew material that I have not seen published elsewhere.  Unfortunately it is=
 not easy to find considering that it was published in 1988.  bookfinder.co=
m has a few copies available from $170 to $850.  Worldcat.com shows 12 libr=
aries in the UK that hold a copy.  A similar number of libraries have Bob M=
atthys' book.

    Thanks. Er... But 'Worldcat.com' is about boats - catamarans ! Could yo=
u check your lists for the World Library Web Site reference, please ?


I mentioned Aluminium Silicon Bronze because Bob's experiments seem to show=
 that it comes back to the same size after temperature cycling than most ot=
her materials suitable for pendulum bobs.=20

    Sorry, I was thinking about metal rods suitable for pendulum arms !=20



Invar is not a panacea as a pendulum rod material.  It requires elaborate t=
hermal/shock treatment to achieve good performance, and if mechanically sho=
cked or machined, needs to be retreated.  Invar is also infamous for unpred=
ictable micro jumps in dimension.  See Dieter Riefler's book for details.  =
Zerodur does not come in thin rod form of sufficient length for 1 second pe=
ndulums.  Carbon rods have very good thermal performance and machine abilit=
y, but the binder epoxy is humidity sensitive (e.g. the rod gains and losse=
s weight which effects the pendulum period) - attempts to slow the process =
by coating the rod with something just slows down the process - no coating =
is perfect.  Fused quartz is a very good material, note tubing is far stron=
ger than rod, but end fittings must be attached with adhesive, or with pins=
 in cross drilled holes.=20

    How about using Super Invar, which has 5% Cobalt added ? It has greatly=
 improved properties.
    I appreciate that the epoxy binder in carbon fibre rods will have a sma=
ll water absorption, but this should be baked out for any vacuum applicatio=
n. If you want to use the rod in damp air, it can be coated with Epoxy varn=
ish and then rolled in the thicker Aluminium foil used for cooking Turkeys,=
 to give a permanently impervious coating - this will not significantly eff=
ect the linear expansion coefficient and the coating can be expected to be =
'perfect'. It should be painted after assembly to prevent corrosion.=20
    In my last Email, I mentioned the use of two part Acrylic glues for sti=
cking glass, quartz, carbon fiber and most metals. It has a tensile strengt=
h of about 3,500 psi and it cures very slightly flexible, unlike Epoxies wh=
ich mostly cure brittle hard. Eg. Holdtite ST3295 has the consistency of Go=
lden Syrup and it can be used for about 5 minutes after being mixed. It set=
s in under 10 minutes at room temperature and cures completely in 24 hours.=
 Several other brand names are available. It has an extremely high resistan=
ce to shock loads. I suggest that you avoid ST3298 type Acrylic adhesive, w=
hich is a thick paste.

=20

Yes, I am very familiar with the Littlemore clock. I believe that Teddy Hal=
l's first published presentation on the clock was at a NAWCC Symposium in C=
leveland, USA. The clock is currently in storage at the NAWCC Museum - it a=
rrived in not very good shape.  Mice had moved in to the computer and had m=
ade a real mess.  Documentation of the software that controlled the clock s=
eems missing.  If anyone knows someone that worked on the project for Teddy=
 - I would love to get the contact information.  The hard drive from the co=
mputer is currently in the process of trying to extract any information tha=
t may still be on it.  Slow process, because of the components that made up=
 the computer. =20

    Which make and model of computer was it, please  ? Is it just the progr=
am documentation which is missing, or the software as well ? Could you adve=
rtise for an identical old computer and then plug in the old hard drive ?=
=20

There is some controversy about the performance of the clock.  Teddy used a=
 quartz controlled oscillator to measure the amplitude of the pendulum and =
adjusted the drive force to compensate.  Some people are concerned that the=
 quartz oscillator drove the performance of the clock.  It is not a simple =
issue - Tom Van Baak covers the problem very well in his article > http://l=
eapsecond.com/hsn2006/hybrid-pendulum-1.pdf=20

    The problems may be fully described, but what it does lack are mathemat=
ical formulae / estimates of the accuracy / errors.=20
    What puzzles me is why pairs of large area Silicon photocells are not u=
sed to monitor the swing angle ? This is dead easy to do and you should be =
able to get an accuracy of better than 0.1 micron ! I can get a detection s=
ensitivity of about 14 nano metres with an optical seismometer detector usi=
ng VTD34 photocells.=20



A 1 second pendulum back and forth more than 31 million times per year over=
 much larger amplitudes than a seismometer.  Wear, particulates, variations=
 in spring constant due to temperature, etc are all problems to be solved. =
 The spring I measured was a standard Syncronome spring - it is not unusual=
ly thick.=20

    I appreciate the difference between the Synchronome suspension, both ty=
pes of 'normal' pendulum clock suspensions and a clamped flex foil - Cardan=
 type. What I am querying is just how do you determine the precise pendulum=
 length with the Synchronome suspension and does this vary with the swing a=
ngle ? =20



The Short and Fedchenko suspensions have a feature that might be worth expl=
oring for seismometers. The springs are not flat stock clamped between "che=
eks" as are most pendulum suspensions instead the springs are ground from t=
hicker stock - i.e. they have thick ends.  Stuart Smith's book on Flexures =
has as section on the issues that arise at the clamping points.  The Fedche=
nko clock is the only production that incorporates a working circular error=
 compensation mechanism.  Most other attempts have failed because the extra=
 mechanism is very difficult to temperature compensate.

    What 'other' methods have been tried to compensate for 'circular errors=
', please ?

    I would be very interested to hear of anyone trying out a clock suspens=
ion using the rolling crossed foil system that I suggested. This uses two p=
arallel circular rods with 2 'S' shaped and 2 'mirror S' shaped foil strips=
 wrapped around them. The foils are stuck to the top and bottom of the rods=
 with two part acrylic adhesive. Since the rolling foils give equal torques=
 in opposite directions, there is NO NET torque. Since the foil bending rad=
ius is large, the intrinsic noise is very low - and there AREN'T ANY clamp =
points to give problems !! You do need to use a long sleeve dust cover. The=
 performance can be expected to be superior to clamped crossed foils.=20

    Another type of suspension which would definitely be worth trying out i=
s rolling crossed cylinders, preferably using circular artificial sapphire =
rods.
 =20

Again - the goal here is cross fertilization - perhaps something from the w=
orld of horology might be of use to seismologists. =20

Bob

    Regards,=20

    Chris Chapman   =20

=20
=20
=20
=20
=20
=20
=20
=20
=20


From: Bob Holmstrom <hol= mstro@..........>
To: psnlist <psnlist@..............>
Sent: Wed, 3 Oct 2012 23:47
Subject: Re: Digest from 10/02/2012 00:00:07

   = I have seen silicon wafers etched thin enough that they could be rolled up= like cigarette paper - perhaps silicon in that form could be used as a fle= xure.  Yes, metal construction is the norm, but it seems to be in the = spirit of this list to consider alternative methods and materials.

Hi Bob,

    I don't have a figure for the tensile strength of Silicon, bu= t I doubt that any sheet thin enough to bend easily would be able to suppor= t a pendulum weighing several kilograms. Sheet silicon is also liable to br= ittle fracture under shock loads.      

The book by R. V. Jones - "Instruments and Experiences" is an organize= d collection of many of his articles on instrumentation.  Each section= has overview material that I have not seen published elsewhere.  Unfo= rtunately it is not easy to find considering that it was published in 1988.=  bookfinder.com<= /a> has a few copies available from $170 to $850.  Worldcat.com shows 12 libraries in = the UK that hold a copy.  A similar number of libraries have Bob Matth= ys' book.

    Thanks. Er... But 'Worldcat.com' is abo= ut boats - catamarans ! Could you check your lists for the World Library We= b Site reference, please ?

I mentioned Aluminium Silicon Bronze because Bob's experiments seem to= show that it comes back to the same size after temperature cycling than mo= st other materials suitable for pendulum bobs.

    Sorry, I was thinking about metal rods = suitable for pendulum arms !

Invar is not a panacea as a pendulum rod material.  It requires e= laborate thermal/shock treatment to achieve good performance, and if mechan= ically shocked or machined, needs to be retreated.  Invar is also infa= mous for unpredictable micro jumps in dimension.  See Dieter Riefler's= book for details.  Zerodur does not come in thin rod form of sufficie= nt length for 1 second pendulums.  Carbon rods have very good thermal = performance and machine ability, but the binder epoxy is humidity sensitive= (e.g. the rod gains and losses weight which effects the pendulum period) -= attempts to slow the process by coating the rod with something just slows = down the process - no coating is perfect.  Fused quartz is a very good= material, note tubing is far stronger than rod, but end fittings must be a= ttached with adhesive, or with pins in cross drilled holes.

    How about using Super Invar, which has = 5% Cobalt added ? It has greatly improved properties.
    I appreciate that the epoxy binder in carbon fibre rods = will have a small water absorption, but this should be baked out for any va= cuum application. If you want to use the rod in damp air, it can be coated = with Epoxy varnish and then rolled in the thicker Aluminium foil used for c= ooking Turkeys, to give a permanently impervious coating - this will not si= gnificantly effect the linear expansion coefficient and the coating can be = expected to be 'perfect'. It should be painted after assembly to prevent co= rrosion.
    In my last Email, I mentioned the use of two part Acryli= c glues for sticking glass, quartz, carbon fiber and most metals. It has a = tensile strength of about 3,500 psi and it cures very slightly flexible, un= like Epoxies which mostly cure brittle hard. Eg. Holdtite ST3295 has the co= nsistency of Golden Syrup and it can be used for about 5 minutes after bein= g mixed. It sets in under 10 minutes at room temperature and cures complete= ly in 24 hours. Several other brand names are available. It has an extremel= y high resistance to shock loads. I suggest that you avoid ST3298 type Acry= lic adhesive, which is a thick paste.

 
Yes, I am very familiar with the Littlemore clock. I believe that Tedd= y Hall's first published presentation on the clock was at a NAWCC Symposium= in Cleveland, USA. The clock is currently in storage at the NAWCC Museum -= it arrived in not very good shape.  Mice had moved in to the computer= and had made a real mess.  Documentation of the software that control= led the clock seems missing.  If anyone knows someone that worked on t= he project for Teddy - I would love to get the contact information.  T= he hard drive from the computer is currently in the process of trying to ex= tract any information that may still be on it.  Slow process, because = of the components that made up the computer. 

    Which make and model of computer was it= , please  ? Is it just the program documentation which is missing, or = the software as well ? Could you advertise for an identical old computer an= d then plug in the old hard drive ?

There is some controversy about the performance of the clock.  Teddy u= sed a quartz controlled oscillator to measure the amplitude of the pendulum= and adjusted the drive force to compensate.  Some people are concerne= d that the quartz oscillator drove the performance of the clock.  It i= s not a simple issue - Tom Van Baak covers the problem very well in his art= icle > http://leapsecond.com/hsn2006/hybrid-pendulum-1.pdf <= /a>

    The problems may be fully described, bu= t what it does lack are mathematical formulae / estimates of the accuracy /= errors.
    What puzzles me is why pairs of large area Silicon photo= cells are not used to monitor the swing angle ? This is dead easy to do and= you should be able to get an accuracy of better than 0.1 micron ! I can ge= t a detection sensitivity of about 14 nano metres with an optical seismomet= er detector using VTD34 photocells.


A 1 second pendulum back and forth more than 31 million times per year= over much larger amplitudes than a seismometer.  Wear, particulates, = variations in spring constant due to temperature, etc are all problems to b= e solved.  The spring I measured was a standard Syncronome spring - it= is not unusually thick.

    I appreciate the difference between the= Synchronome suspension, both types of 'normal' pendulum clock suspensions = and a clamped flex foil - Cardan type. What I am querying is just how do yo= u determine the precise pendulum length with the Synchronome suspension and= does this vary with the swing angle ? 

The Short and Fedchenko suspensions have a feature that might be worth= exploring for seismometers. The springs are not flat stock clamped between= "cheeks" as are most pendulum suspensions instead the springs are ground f= rom thicker stock - i.e. they have thick ends.  Stuart Smith's book on= Flexures has as section on the issues that arise at the clamping points. &= nbsp;The Fedchenko clock is the only production that incorporates a working= circular error compensation mechanism.  Most other attempts have fail= ed because the extra mechanism is very difficult to temperature compensate.=

     What 'other' methods have been tried to compensate for 'ci= rcular errors', please ?

    I would be very interested to hear of anyone trying out = a clock suspension using the rolling crossed foil system that I suggested. = This uses two parallel circular rods with 2 'S' shaped and 2 'mirror S' sha= ped foil strips wrapped around them. The foils are stuck to the top and bot= tom of the rods with two part acrylic adhesive. Since the rolling foils giv= e equal torques in opposite directions, there is NO NET torque. Since the f= oil bending radius is large, the intrinsic noise is very low - and there AR= EN'T ANY clamp points to give problems !! You do need to use a long sleeve = dust cover. The performance can be expected to be superior to clamped cross= ed foils.

    Another type of suspension which would definitely be wor= th trying out is rolling crossed cylinders, preferably using circular artif= icial sapphire rods.
 

Again - the goal here is cross fertilization - perhaps something from = the world of horology might be of use to seismologists. 
Bob

    Regards,

    Chris Chapman   


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