PSN-L Email List Message
Subject: Re: 15 to 20 second free swing
From: "Connie and Jim Lehman" lehmancj@...........
Date: Sat, 22 Oct 2005 18:49:06 -0400
HELLO AGAIN-- Yes the 15 to 20 second free swing is dampened. When set up
& stabilized, a horizontal system free swing would oscillate for hours and
give no definition to an event should one pass. It is a bit of a challenge
to get damping right--but looks easy once you've been through it. Somewhere
between free swing and critical damping(ie. no motion at all) is a
satisfactory zone. Think of it as a ratio--let's say 5 to l. When the
system has stabilized with a natural swing desired--there should be an
equilibrum position or a stable zero point. Displace the pendulum 10 mm,
release it and the pendulum should (with proper damping) overshoot the zero
point by 2 mm. --that is 5 to l. I have usually tuned my sensors to 8:l
ratio.--there is some leeway here. As Chris suggests, use magnetic damping
rather than liquid/vane if possible. Magnetic damping adjusts so
easily--like a piece of cake.
Best Wishes, Jim
----- Original Message -----
From: "KATHRYN ROBERTS"
Sent: Saturday, October 22, 2005 1:21 PM
Subject: 15 to 20 second free swing
> Hi, is the 15 to 20 second free swing dampened? Or does 'free' mean
> undampener? Please remember i am new to all this. Thanks, geofff<><
> ----- Original Message -----
> From: "Connie and Jim Lehman"
> Sent: Saturday, October 22, 2005 10:43 AM
> Subject: Re: Horizontal Pendulum]
> > Geoffery--Thanks for your interest in seismology and the "Lehman
> > horizontal"
> > design. I am puzzled a bit by your use of "shims"-- my idea of shims
> > been only in work demanding small changes in distance--like leveling a
> > table, or a piece of scientific apparatus. The usual shims in precision
> > work are sheets of metal with so many l/1000ths of an inch in
> > thickness--otherwise known as mills} Since the seismo base is a tripod--
> > (for stability) shim material can be used to balance and level the
> > system.
> > I am not sure what version of sensor plans you are following, but I
> > suggest that you keep construction simple with a wire hinge at the top,
> > and
> > a knife edge on the boom end. After you get that working with a 15-20
> > free swing, properly damped, you can refine and modify your design as
> > like. To get a good "garden gate" swing the upper wire hinge
> > should be about 1/4th inch off the vertical with the compressional knife
> > edge.
> > Best wishes, Jim
> > ----- Original Message -----
> > From: "Larry Cochrane"
> > To:
> > Sent: Saturday, October 22, 2005 2:36 AM
> > Subject: [Fwd: Horizontal Pendulum]
> >> -------- Original Message --------
> >> Subject: Horizontal Pendulum
> >> Date: Fri, 21 Oct 2005 20:33:15 -0400
> >> From: KATHRYN ROBERTS
> >> To: Larry Cochrane
> >> Larry, thank you for your reply. The PSN-L@............... did not
> > (i don't
> >> have good computer skills.) Would you post this on the list? i'm new to
> > the list and
> >> seismology. i've enjoying reading the posts and have been thinking
> >> a
> > horizontal
> >> pendulum Lehman type seismo using metal shims (what is the best
> > for shims
> >> and best thickness?) Anyway, the horizontal pendulum would be
> >> supported
> > by shims
> >> above and below the pendulum that are ninety degrees perpendicular to
> > horizontal
> >> axis of the pendulum. The upper shim would be attached to the 'mast'
> >> and
> > then to a
> >> tube or rod that is attached to the pendulum, again with the linear
> > will be
> >> perpendicular to the pendulum's horizontal plane. This would place the
> > upper shim in
> >> tension. The lower shim is also placed in tension by attaching the end
> > away from the
> >> coil to the above mentioned tube or pipe that is attached to the
> >> pendulum.
> > The lower
> >> shim will run away from the mast towards the coil. One thing that will
> > have to be
> >> determined is the position were the upper and lower tubes join the
> > pendulum. The
> >> tubes' positions would determine the flex positon of the shims,
> > to be able
> >> to find the point of greatest flexion of the shims with the lowest
> > needed to
> >> cause the flex or bending of the shim. Have many people tried this? How
> > did it work?
> >> Thanks, geofffrey roberts, Crystal River, Florida
> >> __________________________________________________________
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