Hi Chris, Thanks for this information. =20 A Folded Pendulum seismometer by David H. Youden.......I found this = article, is there other articles, which show the construction details of = the folded pendulum? I will go over all your points in detail. Thanks, Ted ----- Original Message -----=20 From: ChrisAtUpw@.......... To: psn-l@................. Sent: Sunday, June 24, 2007 3:46 PM Subject: Re: Lehman advantages In a message dated 2007/06/24, tchannel@.............. writes: Subj:Lehman advantages=20 Hi Folks, I am thinking about building another Lehman style Horz = Pendulum sensor. I have some construction ideas I wanted to try. = Before I start, could you describe the benefits of these points. 1 FIRMLY ATTACHED THE SENSOR THE EARTH. I wish to make the contact = between the earth, and the sensor as firm as possible, in this case the = concrete slab setting on the earth and the sensor. Presently the sensor = has three feet which just set on the concrete slab. I know some people = use adhesive to the concrete. If I found a way to bolt all three feet = into the concrete, and a new way to make the necessary adjustments, what = benefits would be derived? I understand that even the concrete floor floats on the earth. I am = just talking about the benefits of a tighter connection between the = sensor and the floor. Hi Ted, Bolting the seismometer mounts to the floor may give problems when = the seismometer expands with temperature at a different rate or at a = different time to the floor. They are very unlikely to match.=20 I use three 2" square x 1/8" SS squares glued to the concrete = floor. You can also use >5mm glass or even glazed tiles. The benefit is = that you have a grit free, dead flat surface. Your level settings should = not show drift either with temperature or over time, or be effected by = large quakes. You can use pool cement to glue the plates. The mounting bolts need to be rigidly attached to the frame. To = avoid thermal drift, I glue SS nuts to the underside of the arm with = acrylic glue. On top of the arm I glue a 1/2" SS tube pillar and add a = wavy washer. The SS set bolt has a SS ball bearing glued to a V in the = end, to provide a central rotating contact with mounting plate. The set = bolt also has a nut at the top end. After setting the correct height, I = partly compress the wavy washer with the top nut. This keeps the thread = in tension. The vertical alignment / side slop is controlled by the SS = pillar and the tension. 2 A RIGID VERTICAL SUPPORT FOR THE UPRIGHT. I know that on a = typical Lehman the vertical needs to be rigid and minimise the flex = between the vertical and the horz members. If I found a way to minimise = this flex, what benefits might I see? The one I have has no flex that I = can see, but If I added addition braces so the vertical was at 90 to the = horz with the minimum of flex, What benefit would there be? The vertical and horizontal arms need to be connected quite = rigidly. This can be done conveniently with large triangular reinforcing = plates at the T joint or ~ 45 deg bracing members to both the main beam = and to the cross beam. This will minimise any cross alignment drift and = tend to suppress arm oscillations, due to the vertical + arm + mass = flexing. Unless you do this you are likely to pick up spurious resonant = signals. The original Lehman design was inadequate in this respect. Thump the mass vertically and what do you see on the output? = You need to eliminate any oscillations. 3 USING A LONGER ARM. I used a normal length arm, and I understand = if space was not an issue a very long 100 meters arm would result in a = longer period. I am just asking if space was avail would a 5 foot arm = result in any benefits, over a 3 foot arm? You can provide reasonable temperature and air motion control = for a 2 to 3 ft arm, but not for anything much larger. A 1 m long = pendulum has a period of about 2 sec. To get a 4 sec period you need a 4 = m pendulum. A 20 sec period would require a 100m pendulum.=20 The main factor you need to consider is the ratio between the = natural period of an arm of length L and the desired seismometer period = - the 1/sinA factor. If you try to get greater than x10 period = extension, A becomes a very small angle. You may need fine thread = adjustment screws or a slow motion drive. =20 A folded pendulum design is likely to be more satisfactory / = easier to construct for mechanical periods over about 30 sec. An alternative method is to provide position and velocity force = feedback to stabilise the position of the arm, but the electronics gets = more complicated. Using electronic feedback control can run into noise = and stability problems, but you can turn a 20 sec pendulum into a 200 = sec sensor. See http://www.keckec.com/seismo/ What period do you want? The Rayleigh and Love surface waves = tend to have periods of about 20 seconds and few are over 40 sec. For = very long extension periods you need to measure the position of the arm, = not it's velocity, or you just see noise. The last question is, if I had a sensor which was firmly attached to = the floor, with a very ridged vertical, and a longer arm. (with all the other important factors aside) What kind of = improvements might I expect? I think I could build a new and improved = sensor, addressing these three issues. But would these three issues = make much different. If I would, see improvements would they only be = for teleseismic events, or would the improvements be evident in = recording regional as well. You should see the true ground motion. There should be NO = artefacts from the apparatus. You are more likely to be bothered by = short period signals, but the P and S waves that you want to detect are = above 0.5 Hz, often 1 to 5 Hz. Remember that IT IS THE EARTH WHICH MOVES ---> NOT THE SEISMOMETER = ARM !! You have missed out some important considerations. You need to = suppress, damp, or be insensitive to the natural oscillations / modes of = the apparatus. Earthquakes are transient pulse type signals and can = excite any natural oscillation modes. The arm and the suspension need to be rigid. The arm should be = prevented from rotating around it's long axis. There will inevitably be = some vertical bounce at the end of the arm, but the frequency should be = above that of the low pass electronic filter and the sensor should be = designed to have a low sensitivity to vertical motion. You also need to = design the sensor to have a constant and linear sensor voltage output = for mass position drifts of ~ +/-1/2". You ALWAYS get some position = drift with a Lehman. They are very sensitive to tiny shifts in the local = ground plane due to temperature, rain and seasonal changes. You need = NdFeB bar magnets and rectangular coils to do this, or alternatively = long cylindrical coils with many turns + magnets, similar to a = loudspeaker, but with clearance gaps and coil lengths which allow for a = 1/2" mass drift. The damping force should act ~on the line between the centre of = mass and the lower bearing, otherwise it will try to rotate the arm = about it's long axis. Also, place the centre of the pickup coil close to = this axis.=20 Have a look at = http://jclahr.com/science/psn/chapman/school/MKII/index.html The top = wire suspension was changed to either a V cable or to a 1/2" tube. Both = worked OK. Both hinges were altered to be crossed rods, although a ball = on a plane works equally well. You mount the vertical rods or balls on = the vertical support column, NOT on the arm. The horizontal rods or the = flats are mounted on the moving arm. Also have a look at the Sprengnether at = http://www.geocities.com/meredithlamb/ Have a look at 416 SS 'shoulder screws' 93985A205 or similar = from www.mcmaster.com.=20 Alternatively, buy solid tungsten carbide drills and use the = shank. They are sold for drilling fibreglass circuit board and other = hard materials. See www.DigiKey.com or www.smallparts.com. Smallparts = also sell bearings. You can buy flat triangular Tungsten Carbide tips = for lathe tools quite cheaply with ~ 0.3" sides. Alternatively, you can = use a bit of a SS knife blade glued to the end of the arm. Regards,Hi Chris, Thanks for this=20 information.A Folded Pendulum seismometer by = David H. Youden.......I found = this=20 article, is there other articles, which show the construction details of = the=20 folded pendulum?
I will go over = all your=20 points in detail. Thanks, Ted
----- Original Message -----From:=20 ChrisAtUpw@.......Sent: Sunday, June 24, 2007 = 3:46 PMSubject: Re: Lehman = advantagesIn a=20 message dated 2007/06/24, tchannel@..............=20 writes:
Subj:Lehman advantages
Hi Folks, I am = thinking=20 about building another Lehman style Horz Pendulum sensor. I = have some=20 construction ideas I wanted to try. Before I start, could you = describe=20 the benefits of these points.
1 FIRMLY = ATTACHED THE SENSOR=20 THE EARTH. I wish to make the contact between the earth, and = the=20 sensor as firm as possible, in this case the concrete slab setting = on the=20 earth and the sensor. Presently the sensor has three feet = which just=20 set on the concrete slab. I know some people use adhesive to the=20 concrete. If I found a way to bolt all three feet into the = concrete,=20 and a new way to make the necessary adjustments, what benefits would = be=20 derived?
I understand that even the concrete floor floats on = the=20 earth. I am just talking about the benefits of a tighter = connection=20 between the sensor and the floor.
Hi Ted,
Bolting the = seismometer=20 mounts to the floor may give problems when the seismometer expands = with=20 temperature at a different rate or at a different time to the floor. = They are=20 very unlikely to match.
I = use=20 three 2" square x 1/8" SS squares glued to the concrete floor. You can = also=20 use >5mm glass or even glazed tiles. The benefit is that you have a = grit=20 free, dead flat surface. Your level settings should not show drift = either with=20 temperature or over time, or be effected by large quakes. You can use = pool=20 cement to glue the plates.
= The=20 mounting bolts need to be rigidly attached to the frame. To avoid = thermal=20 drift, I glue SS nuts to the underside of the arm with acrylic glue. = On top of=20 the arm I glue a 1/2" SS tube pillar and add a wavy washer. The SS set = bolt=20 has a SS ball bearing glued to a V in the end, to provide a central = rotating=20 contact with mounting plate. The set bolt also has a nut at the top = end. After=20 setting the correct height, I partly compress the wavy washer with the = top=20 nut. This keeps the thread in tension. The vertical alignment / side = slop is=20 controlled by the SS pillar and the tension.
2 A RIGID=20 VERTICAL SUPPORT FOR THE UPRIGHT. I know that on a typical = Lehman the=20 vertical needs to be rigid and minimise the flex between the = vertical and=20 the horz members. If I found a way to minimise this flex, what = benefits might I see? The one I have has no flex that I can = see, but=20 If I added addition braces so the vertical was at 90 to the horz = with the=20 minimum of flex, What benefit would there be?
The=20 vertical and horizontal arms need to be connected quite rigidly. This = can be=20 done conveniently with large triangular reinforcing plates at the T = joint or ~=20 45 deg bracing members to both the main beam and to the cross beam. = This will=20 minimise any cross alignment drift and tend to suppress arm = oscillations, due=20 to the vertical + arm + mass flexing. Unless you do this you are = likely to=20 pick up spurious resonant signals. The original Lehman design was = inadequate=20 in this respect.
Thump the = mass=20 vertically and what do you see on the output? You need to eliminate = any=20 oscillations.
3 USING A LONGER ARM. I used a normal length arm, = and I=20 understand if space was not an issue a very long 100 meters arm = would result=20 in a longer period. I am just asking if space was avail would = a 5 foot=20 arm result in any benefits, over a 3 foot arm?
You can=20 provide reasonable temperature and air motion control for a 2 to 3 ft = arm, but=20 not for anything much larger. A 1 m long pendulum has a period of = about 2 sec.=20 To get a 4 sec period you need a 4 m pendulum. A 20 sec period would = require a=20 100m pendulum.
The main = factor you=20 need to consider is the ratio between the natural period of an arm of = length L=20 and the desired seismometer period - the 1/sinA factor. If you try to = get=20 greater than x10 period extension, A becomes a very small angle. You = may need=20 fine thread adjustment screws or a slow motion drive. =20
A folded pendulum design is = likely to=20 be more satisfactory / easier to construct for mechanical periods over = about=20 30 sec.
An alternative method = is to=20 provide position and velocity force feedback to stabilise the position = of the=20 arm, but the electronics gets more complicated. Using electronic = feedback=20 control can run into noise and stability problems, but you can turn a = 20 sec=20 pendulum into a 200 sec sensor. See=20 = http://www.keckec.com/seismo/
= What=20 period do you want? The Rayleigh and Love surface waves tend to = have=20 periods of about 20 seconds and few are over 40 sec. For very long = extension=20 periods you need to measure the position of the arm, not it's = velocity, or you=20 just see noise.
The last question is, if I had a sensor which was firmly = attached to the floor, with a very ridged vertical, and a longer=20 arm.
(with all the other important factors aside) What = kind of=20 improvements might I expect? I think I could build a new and = improved=20 sensor, addressing these three issues. But would these three = issues=20 make much different. If I would, see improvements would they = only be=20 for teleseismic events, or would the improvements be evident in = recording=20 regional as well.
= You should see the true ground = motion. There=20 should be NO artefacts from the apparatus. You are more likely to be = bothered=20 by short period signals, but the P and S waves that you want to detect = are=20 above 0.5 Hz, often 1 to 5 Hz.
Remember that = IT IS=20 THE EARTH WHICH MOVES ---> NOT THE SEISMOMETER ARM = !!
You have = missed out some=20 important considerations. You need to suppress, damp, or be = insensitive to the=20 natural oscillations / modes of the apparatus. Earthquakes are = transient pulse=20 type signals and can excite any natural oscillation=20 modes.
The arm and the suspension need to be = rigid. The=20 arm should be prevented from rotating around it's long axis. There = will=20 inevitably be some vertical bounce at the end of the arm, but the = frequency=20 should be above that of the low pass electronic filter and the sensor = should=20 be designed to have a low sensitivity to vertical motion. You also = need to=20 design the sensor to have a constant and linear sensor voltage = output for=20 mass position drifts of ~ +/-1/2". You ALWAYS get some position = drift with=20 a Lehman. They are very sensitive to tiny shifts in the local ground = plane due=20 to temperature, rain and seasonal changes. You need NdFeB bar magnets = and=20 rectangular coils to do this, or alternatively long cylindrical coils = with=20 many turns + magnets, similar to a loudspeaker, but with clearance = gaps and=20 coil lengths which allow for a 1/2" mass drift.
= The=20 damping force should act ~on the line between the centre of mass and = the lower=20 bearing, otherwise it will try to rotate the arm about it's long axis. = Also,=20 place the centre of the pickup coil close to this axis. =
=20 Have a look at = http://jclahr.com/science/psn/chapman/school/MKII/index.html=20 The top wire suspension was changed to either a V cable or to a 1/2" = tube.=20 Both worked OK. Both hinges were altered to be crossed rods, although = a ball=20 on a plane works equally well. You mount the vertical rods or balls = on the=20 vertical support column, NOT on the arm. The horizontal rods = or the=20 flats are mounted on the moving arm.
Also have a = look at=20 the Sprengnether at = http://www.geocities.com/meredithlamb/
Have a=20 look at 416 SS 'shoulder screws' 93985A205= A>=20 or similar from www.mcmaster.com. =
=20 Alternatively, buy solid tungsten carbide drills and use the shank. = They are=20 sold for drilling fibreglass circuit board and other hard materials. = See=20 www.DigiKey.com or www.smallparts.com. Smallparts also sell bearings. = You can=20 buy flat triangular Tungsten Carbide tips for lathe tools quite = cheaply with ~=20 0.3" sides. Alternatively, you can use a bit of a SS knife blade glued = to the=20 end of the arm.
Regards,
Chris=20 Chapman