In a message dated 2008/02/19, Brett3mr@............. writes:

> The rolling foil design is the one I like the best, but I would feel better 
> 
> if there were more experimental results to prove it's as good as I think it 
> will be.  see:   http://bnordgren.org/seismo/zerohng2.pdf

Hi Brett,

       In the rundown tests, the rolling foil performed the best. I enclosed 
a preference list last time I wrote to Charles.

> > But a concurrent question is do I really need a very low amount of loss?  
> I 
> >>know recent discussions have experimented with crossed pivots of extremely 
> 
> >> low loss.  Why?  The immediate next step will be to add a damper to get 
> to
> >> >something close to critical damping.   My understanding is that the only
> >> >reason to have low loss is to be able to use lots of feedback to 
> lengthen
> >> >the period.  But if the period can be achieved directly, and it includes
> >> >some damping, so what?  In my mind, the important item is
> >> >hysteresis/stiction.   As bearings and bearing surfaces can easily be
> >> >ground to a ten-thousandth or even better, 10 or 20 second period
> >> >structures should be in reach.
> >>
> >        Again yes. You need to measure movements down to nano metres, so 
> > you need extremely low hysteresis / stiction -.whatever system you use. 
> > Feedback will not compensate for this.
> 
> Don't agree with Chris here.  Without feedback, mechanical issues are 
> important, but if you have reasonably strong feedback (loop gain), which 
> should be possible at all frequencies in the mid and low region, any 
> *small* effects, linear or non-linear will be made insignificant by the 
> feedback.  

    Sorry but it doesn't. This is one of the known limitations of feedback. 
It may well alter the scale.
    
> >        Don't know where you get this from. The STS-1 used crossed foils. 
> > The problems of making the STS-1 eventually lead to it's replacement!
> 
> See http://www.c-flex.com/technicaldata.pdf  which shows that the 
> crossed-foil bearings take a "set" each time they are rotated which I 
> consider to be a pretty good indication of significant hysteresis.  This is 
> consistent with the observation that the foils must undergo considerable 
> bending stress near their points of connection with the sleeves.  I believe 
> that C-Flex is the successor to a series of companies which made these 
> bearings and am assuming that Streckeisen used either them, or something 
> very similar in the STS-1.  Crossed foils are not necessarily low 
> hysteresis.  That's why I'm partial to the rolling foil design.

       This relates to C-Flex bearings. They are not quite the same 
construction as crossed foils and I would expect there to be some hysteresis. As far as 
I am aware, Streckeisen were making their own flexures.

> >        Let's define out objectives. We don't want extreme periods, just 
> > maybe 10 seconds instead of 1 second. Trying to get very long periods 
> > makes the task increasingly difficult and the small anelastic effects 
> > become major problems, as do thermal variations / expansions.
> >
> >        I am fairly confident that you could extend the period by using 
> > feedback to SOFTEN the suspension forces of a standard vertical pendulum. 
> > Randall can then keep his 1 mm WC low loss bearings - no problem.
> 
> Chris, can you give a bit more detail about what you're thinking here. 
> (block diagram or such)  Almost by definition, any significant positive 
> feedback is going to oscillate.  Are you possibly thinking of using 'feed 
> forward', sometimes called 'open loop compensation' here.  That's not 
> feedback but is a technique for reducing error effects, usually used to 
> moderately improve performance in combination with the usual negative 
> feedback.

       Sure. Let's take the garden gate system as an analogy. You alter the 
suspension angle to alter the centring force relationship and so set the 
period. The centring force involved is a direct function of the deflection angle for 
small angles. Therefore you can alter the period either by changing the 
suspension angle or by reducing the centring force by force feedback. Note that 
force feedback can be positive or negative.
       If you overdo the suspension angle variation, the mass falls up 
against the stops. If you overdo the force feedback, the system may oscillate.

> I won't say that positive feedback can *never* be of use, but it has to be 
> combined with an even stronger dose of negative feedback in order not to 
> oscillate, and even then you have to be careful.

       The term feedforward to me indicates a phase advanced signal?

>   Yes, one approach that has been used is to place a spring at the bottom 
> to 'soften' the restoring force of gravity acting on the pendulum.  Although 
> in principle o.k., in fact it has been shown to be unacceptable, due to the 
> dastardly properties of springs.

       The Willmore IIIs had a period which could be set from 1 to 3 seconds. 
The Willmore IIICs had an additional de-centring spring which allowed the 
period to be extended to ~20 seconds. It has been made to work in the UK ! The 
alternative of providing magnetic repulsion should be a practical possibility 
for the light Volksmeter suspension.
       
       Regards,

       Chris   
In a me=
ssage dated 2008/02/19, Brett3mr@............. writes:



The rolling foil design is the=20=
one I like the best, but I would feel better 

if there were more experimental results to prove it's as good as I think it=20=


will be.  see:   http://bnordgren.org/seismo/zerohng2.pdf



Hi Brett,



       In the rundown tests, the rolling foil=20=
performed the best. I enclosed a preference list last time I wrote to Charle=
s.



> But a concurrent question=20=
is do I really need a very low amount of loss?  I 

>>know recent discussions have experimented with crossed pivots of ext=
remely 

>> low loss.  Why?  The immediate next step will be to add a=
 damper to get to

>> >something close to critical damping.   My understandi=
ng is that the only

>> >reason to have low loss is to be able to use lots of feedback t=
o lengthen

>> >the period.  But if the period can be achieved directly, a=
nd it includes

>> >some damping, so what?  In my mind, the important item is<=
BR>
>> >hysteresis/stiction.   As bearings and bearing surfac=
es can easily be

>> >ground to a ten-thousandth or even better, 10 or 20 second peri=
od

>> >structures should be in reach.

>>

>        Again yes. You need to measur=
e movements down to nano metres, so 

> you need extremely low hysteresis / stiction -.whatever system you use.=
 

> Feedback will not compensate for this.



Don't agree with Chris here.  Without feedback, mechanical issues are <=
BR>
important, but if you have reasonably strong feedback (loop gain), which 
should be possible at all frequencies in the mid and low region, any 

*small* effects, linear or non-linear will be made insignificant by the 

feedback.  




    Sorry but it doesn't. This is one of the known limitation=
s of feedback. It may well alter the scale.

    

>    &n=
bsp;   Don't know where you get this from. The STS-1 used crossed=20=
foils. 

> The problems of making the STS-1 eventually lead to it's replacement!


See http://www.c-flex.com/technicaldata.pdf  which shows that the 

crossed-foil bearings take a "set" each time they are rotated which I 

consider to be a pretty good indication of significant hysteresis.  Thi=
s is 

consistent with the observation that the foils must undergo considerable 
bending stress near their points of connection with the sleeves.  I bel=
ieve 

that C-Flex is the successor to a series of companies which made these 

bearings and am assuming that Streckeisen used either them, or something 
very similar in the STS-1.  Crossed foils are not necessarily low 

hysteresis.  That's why I'm partial to the rolling foil design.<=
FONT  COLOR=3D"#000000" BACK=3D"#ffffff" style=3D"BACKGROUND-COLOR: #ffffff"=
 SIZE=3D2 PTSIZE=3D10 FAMILY=3D"SANSSERIF" FACE=3D"Arial" LANG=3D"0">



       This relates to C-Flex bearings. They=20=
are not quite the same construction as crossed foils and I would expect ther=
e to be some hysteresis. As far as I am aware, Streckeisen were making their=
 own flexures.



>    &nb=
sp;   Let's define out objectives. We don't want extreme periods,=20=
just 

> maybe 10 seconds instead of 1 second. Trying to get very long periods <=
BR>
> makes the task increasingly difficult and the small anelastic effects <=
BR>
> become major problems, as do thermal variations / expansions.

>

>        I am fairly confident that yo=
u could extend the period by using 

> feedback to SOFTEN the suspension forces of a standard vertical pendulu=
m. 

> Randall can then keep his 1 mm WC low loss bearings - no problem.



Chris, can you give a bit more detail about what you're thinking here. 

(block diagram or such)  Almost by definition, any significant positive=
 

feedback is going to oscillate.  Are you possibly thinking of using 'fe=
ed 

forward', sometimes called 'open loop compensation' here.  That's not <=
BR>
feedback but is a technique for reducing error effects, usually used to 

moderately improve performance in combination with the usual negative 

feedback.




       Sure. Let's take the garden gate syste=
m as an analogy. You alter the suspension angle to alter the centring force=20=
relationship and so set the period. The centring force involved is a direct=20=
function of the deflection angle for small angles. Therefore you can alter t=
he period either by changing the suspension angle or by reducing the centrin=
g force by force feedback. Note that force feedback can be positive or negat=
ive.

       If you overdo the suspension angle vari=
ation, the mass falls up against the stops. If you overdo the force feedback=
, the system may oscillate.



I won't say that positive feedb=
ack can *never* be of use, but it has to be 

combined with an even stronger dose of negative feedback in order not to 
oscillate, and even then you have to be careful.




       The term feedforward to me indicates a=20=
phase advanced signal?



  Yes, one approach that h=
as been used is to place a spring at the bottom to 'soften' the restoring fo=
rce of gravity acting on the pendulum.  Although in principle o.k., in=20=
fact it has been shown to be unacceptable, due to the dastardly properties o=
f springs.




       The Willmore IIIs had a period which co=
uld be set from 1 to 3 seconds. The Willmore IIICs had an additional de-cent=
ring spring which allowed the period to be extended to ~20 seconds. It has b=
een made to work in the UK ! The alternative of providing magnetic repulsion=
 should be a practical possibility for the light Volksmeter suspension.

       

       Regards,



       Chris