PSN-L Email List Message

Subject: Re: desensitivity due damping vs hinge
From: ChrisAtUpw@.......
Date: Sun, 24 Nov 2002 14:20:02 EST


In a message dated 24/11/2002, rsparks@........... writes:

> Of course the restoring moment which is developed by the hinge in any
> seismometer is important, but it fades in importance compared to the
> restoring moment which is introduced by the damping method.
> 
> A hydraulic damper will use some sort of piston or blade.  Motion of the
> piston or blade through the liquid will involve setting some of the liquid
> into motion. To set anything into motion requires overcoming inertia, which
> will make the initial piston or blade movement "sticky". I think this can
> be minimized by using a liquid with higher viscosity at the same time as 
> the
> minimum blade or piston size is used.

Hi Roger,

     I think that you may not quite have appreciated the difference between 
sliding friction between solids and the resistance to motion of a vane 
through a viscous liquid. A vane in a liquid experiences a resistive force 
due to motion, but there is none of the 'stick / slip' hysteresis that you 
get in friction between solids. One serious problem with liquid damping is 
that the viscosity of most fluids is very strongly temperature dependant. A 
seismometer which is correctly damped at 20 C will be underdamped at 30 C and 
overdamped at 10 C. 

> I am less familiar with magnetic damping so I hesitate to comment.  The
> hysteresis of metal will be important.  Perhaps someone could comment on
> magnetic damping.

     I am not sure what 'metal hysteresis' you mean. A magnetic damping 
system involves moving an electrically conducting vane, usually made of 
Aluminum or Copper, through a very strong magnetic field. These metals are 
'non-magnetic'. Electric currents are induced in the vane proportional to the 
velocity, which slowly die away due to the electrical resistance of the 
metal. Again, there is NO stick / slip hysteresis. The forces are only 
slightly effected by the change in the field strength with temperature of the 
magnet and the change in resistivity with temperature of the vane metal. This 
has become a simple, cheap and practical method with the ready availability 
of very strong NdBFe magnets. The four pole 'wing' shaped magnets recovered 
from old computer hard drive systems are ideal for this.

       Regards,

       Chris Chapman
In a message dated 24/11/2002, rsparks@........... writes:

Of course the restoring moment which is developed by the hinge in any
seismometer is important, but it fades in importance compared to the
restoring moment which is introduced by the damping method.

A hydraulic damper will use some sort of piston or blade.  Motion of the
piston or blade through the liquid will involve setting some of the liquid
into motion. To set anything into motion requires overcoming inertia, which
will make the initial piston or blade movement "sticky". I think this can
be minimized by using a liquid with higher viscosity at the same time as the
minimum blade or piston size is used.


Hi Roger,

     I think that you may not quite have appreciated the difference between sliding friction between solids and the resistance to motion of a vane through a viscous liquid. A vane in a liquid experiences a resistive force due to motion, but there is none of the 'stick / slip' hysteresis that you get in friction between solids. One serious problem with liquid damping is that the viscosity of most fluids is very strongly temperature dependant. A seismometer which is correctly damped at 20 C will be underdamped at 30 C and overdamped at 10 C.

I am less familiar with magnetic damping so I hesitate to comment.  The
hysteresis of metal will be important.  Perhaps someone could comment on
magnetic damping.


     I am not sure what 'metal hysteresis' you mean. A magnetic damping system involves moving an electrically conducting vane, usually made of Aluminum or Copper, through a very strong magnetic field. These metals are 'non-magnetic'. Electric currents are induced in the vane proportional to the velocity, which slowly die away due to the electrical resistance of the metal. Again, there is NO stick / slip hysteresis. The forces are only slightly effected by the change in the field strength with temperature of the magnet and the change in resistivity with temperature of the vane metal. This has become a simple, cheap and practical method with the ready availability of very strong NdBFe magnets. The four pole 'wing' shaped magnets recovered from old computer hard drive systems are ideal for this.

       Regards,

       Chris Chapman

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