In a message dated 2006/10/05, tchannel@.............. writes:

> 1 Does your coil and damper assemblies just set on the aluminum base, free 
> to move for adjustment, or are they fixed to the base?

Hi Ted,

       My arm has a support wire fixture and then a mass right at the end of 
the arm.
       The support fixture carries a damping tongue in the direction of the 
hinge and a coil on the mass side.
       Both the sensor and damper magnets slide on the base strip, for 
mounting and adjustment of their position. They have small guide strips underneath 
which hold them parallel.   

> 2 You cleaned the plates, which hold the magnets, and then painted the 
> exterior, but not the surface the magnets touch.  Would the paint interfere with 
> something on that surface?

       I suggest that you paint everything with Hammerite or similar, but try 
to get it smooth and level. I etch coated the magnet position with phosphoric 
acid, allowed it to react and dry overnight and then put CL anticorrosion car 
grease on it. I was not certain that I could get enough damping initially, 
but I found that I could overdamp the system quite easily, so I increased the 
damper magnet spacing.

> 3 This is a hard question. Assuming I have finished my sensor, and levelled 
> it, what is the procedure for adjusting the period? Maybe you could walk me 
> through this procedure?

       You first lower the base plate / strip at the mass end. Then you mount 
the arm and make any necessary solder connections for the coil wiring. Then 
you put the magnet units on the base and adjust the top wire so that both the 
coil and the damping plate are central within the magnet spacings. The first 
time, you may need to make adjustments of the magnet block heights, so that the 
arm is parallel to the base.
       Then you slide the damper free and adjust the cross levels so that the 
arm is stable in the central position. If you can't get a stable central 
position, you need to adjust the suspension so that the top hinge is nearer the 
mass end - or the bottom fitting is further away. You may mark the end of the 
arm with an inkline and also put a central mark on the base. You deflect the arm 
and time a swing. It may be about 5 sec initially. Then you slowly raise the 
base at the mass end of the arm with the adjusting screw, testing to determine 
the period, until you get the period you want. As the period is lengthened, 
you will probably need to trim the cross level slightly.
       Then you slide the damping magnet over edge of the tongue, deflect the 
arm 10 mm and release it. It may help to stick a bit of graph paper to the 
base to be able the measure the position of the damping block. 0.7 damping is 
obtained when the arm swings 0.5 mm past the zero position and then falls back 
again.
       The amount of damping decreases as the set period increases, so it 
needs to be easily adjustable. If the critical position is obtained with the 
tongue less that half covering the magnets, increase the damper magnet spacing. 

> 4 I get confused with many of these terms, so I hope you understand this 
> question.  If the target period is say 10 seconds, would the arm move, when 
> pulled 10mm, very slowly, taking 10 seconds to complete one cycle?

       That is correct. Assuming that the arm is not damped, you deflect it 
and then release it. It takes 2.5 sec to get to zero, overshoots and comes back 
to zero in 5 sec, overshoots again and comes back to zero from the initial 
direction in another 5 sec. Total 5 + 5 = 10 sec. The period is the time for one 
complete oscillation cycle. You want at least twice this.

> 5  After it is leveled and the period is set, Is it then that you can check 
> the damp, by moving the arm 10mm and releasing it?

       Correct, you set up the period first and then you slide the damper 
further over the tongue in small steps until it is just a bit under critical.

>     I know what an underdamped situation would look like.  But is there way 
> to tell if it is over damped?

       As you increase the damping from zero, the time that the arm takes to 
fall back through / to the balance position increases slightly. If you 
overdamp the arm, it never swings through the zero position and the more you damp it, 
the longer it takes to get there. Look for the arm swinging just past the 
zero line, but without another oscillation.

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/10/05, tchannel@.............. writes:



1 Does your coil and damper ass=
emblies just set on the aluminum base, free to move for adjustment, or are t=
hey fixed to the base?




Hi Ted,



       My arm has a support wire fixture and t=
hen a mass right at the end of the arm.

       The support fixture carries a damping t=
ongue in the direction of the hinge and a coil on the mass side.

       Both the sensor and damper magnets slid=
e on the base strip, for mounting and adjustment of their position. They hav=
e small guide strips underneath which hold them parallel.   



2 You cleaned the plates, whic=
h hold the magnets, and then painted the exterior, but not the surface the m=
agnets touch.  Would the paint interfere with something on that surface=
?




       I suggest that you paint everything wi=
th Hammerite or similar, but try to get it smooth and level. I etch coated t=
he magnet position with phosphoric acid, allowed it to react and dry overnig=
ht and then put CL anticorrosion car grease on it. I was not certain that I=20=
could get enough damping initially, but I found that I could overdamp the sy=
stem quite easily, so I increased the damper magnet spacing.



3 This is a hard question. Ass=
uming I have finished my sensor, and levelled it, what is the procedure for=20=
adjusting the period? Maybe you could walk me through this procedure?=




       You first lower the base plate / strip=
 at the mass end. Then you mount the arm and make any necessary solder conne=
ctions for the coil wiring. Then you put the magnet units on the base and ad=
just the top wire so that both the coil and the damping plate are central wi=
thin the magnet spacings. The first time, you may need to make adjustments o=
f the magnet block heights, so that the arm is parallel to the base.

       Then you slide the damper free and adju=
st the cross levels so that the arm is stable in the central position. If yo=
u can't get a stable central position, you need to adjust the suspension so=20=
that the top hinge is nearer the mass end - or the bottom fitting is further=
 away. You may mark the end of the arm with an inkline and also put a centra=
l mark on the base. You deflect the arm and time a swing. It may be about 5=20=
sec initially. Then you slowly raise the base at the mass end of the arm wit=
h the adjusting screw, testing to determine the period, until you get the pe=
riod you want. As the period is lengthened, you will probably need to trim t=
he cross level slightly.

       Then you slide the damping magnet o=
ver edge of the tongue, deflect the arm 10 mm and release it. It may help to=
 stick a bit of graph paper to the base to be able the measure the position=20=
of the damping block. 0.7 damping is obtained when the arm swings 0.5 mm pas=
t the zero position and then falls back again.

       The amount of damping decreases as the=20=
set period increases, so it needs to be easily adjustable. If the critical p=
osition is obtained with the tongue less that half covering the magnets, inc=
rease the damper magnet spacing. 



4 I get confused with many of=20=
these terms, so I hope you understand this question.  If the target per=
iod is say 10 seconds, would the arm move, when pulled 10mm, very slowly, ta=
king 10 seconds to complete one cycle?




       That is correct. Assuming that the arm=
 is not damped, you deflect it and then release it. It takes 2.5 sec to get=20=
to zero, overshoots and comes back to zero in 5 sec, overshoots again and co=
mes back to zero from the initial direction in another 5 sec. Total 5 + 5=20=
=3D 10 sec. The period is the time for one complete oscillation cycle. You w=
ant at least twice this.



5  After it is leveled an=
d the period is set, Is it then that you can check the damp, by moving the a=
rm 10mm and releasing it?




       Correct, you set up the period first a=
nd then you slide the damper further over the tongue in small steps until it=
 is just a bit under critical.



    I know what=
 an underdamped situation would look like.  But is there way to tell if=
 it is over damped?




       As you increase the damping from zero,=20=
the time that the arm takes to fall back through / to the balance position i=
ncreases slightly. If you overdamp the arm, it never swings through the zero=
 position and the more you damp it, the longer it takes to get there. Look f=
or the arm swinging just past the zero line, but without another oscillation=
..



       Regards,



       Chris Chapman