vibration dampening test setup
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vibration dampening test setup
So I am playing with the idea of doing a vibration dampening test of a variety of core and facing material combos. Got plenty of combo ideas, but not sure how to collect the data. Anyone know how to cheaply collect data from a strain gauge? I am assuming a strain gauge and software like LabView is the best way to go (hoping I can find a torrent for LabView). How do I connect one to the other?
Hi Mattman,
I've done a considerable amount of vibration tests, on various structures as well as skis, to look at damping properties and other characteristics.
Strain gauges are okay, but require a bridge circuit and a gain stage. There are plenty of basic designs available on the Internet, and even IC's that will do the trick. One disadvantage is the gage is glued to a ski and it's pretty much stuck to that ski and can't be reused. For consistency, it would be better to use the same sensor for multiple tests on multiple skis.
Instead, have you thought about a small MEMS accelerometer? You can get small ones capable of measuring in three directions for a few bucks, and usually they just require supply voltage, ground, and provide a simple analog output proportional to the g's. You can attach these sensors to a ski using wax, and remove/attach as you please.
LabView is easy to use and offers a lot of flexibility. Some post processing of the data will have to be done to get information about the damping characteristics as the measured time history gives a limited amount of information.
Measuring the damping (loss modulus) can be tricky. A simple pluck test will only give you a little information, and it also depends on where along the length you put the sensor, too. Anyway, a frequency sweep would be better and multiple sensors placed along the length of the ski will give you spatial variation. For this, you'll need some sort of shaker platform that can vibrate the ski. Lucky for us, each mode of vibration has its own damping value, and the way the ski is constructed affects the different modes.....
I'm usually concerned with the first two cantilever modes, and also pay a little attention to a couple of the earlier torsion modes when a ski is excited.
I'll try to dig up the part number for the accelerometer if you're interested, as well as the strain gage IC.
I've done a considerable amount of vibration tests, on various structures as well as skis, to look at damping properties and other characteristics.
Strain gauges are okay, but require a bridge circuit and a gain stage. There are plenty of basic designs available on the Internet, and even IC's that will do the trick. One disadvantage is the gage is glued to a ski and it's pretty much stuck to that ski and can't be reused. For consistency, it would be better to use the same sensor for multiple tests on multiple skis.
Instead, have you thought about a small MEMS accelerometer? You can get small ones capable of measuring in three directions for a few bucks, and usually they just require supply voltage, ground, and provide a simple analog output proportional to the g's. You can attach these sensors to a ski using wax, and remove/attach as you please.
LabView is easy to use and offers a lot of flexibility. Some post processing of the data will have to be done to get information about the damping characteristics as the measured time history gives a limited amount of information.
Measuring the damping (loss modulus) can be tricky. A simple pluck test will only give you a little information, and it also depends on where along the length you put the sensor, too. Anyway, a frequency sweep would be better and multiple sensors placed along the length of the ski will give you spatial variation. For this, you'll need some sort of shaker platform that can vibrate the ski. Lucky for us, each mode of vibration has its own damping value, and the way the ski is constructed affects the different modes.....
I'm usually concerned with the first two cantilever modes, and also pay a little attention to a couple of the earlier torsion modes when a ski is excited.
I'll try to dig up the part number for the accelerometer if you're interested, as well as the strain gage IC.
Last edited by bigKam on Fri Sep 30, 2011 10:49 pm, edited 1 time in total.
wow Kam, I am going to have to reread that a couple of times to get up to speed on everything that I dont yet know about vibration. The setup sounds great, what hardware do I need to get the analog output into excel or labview? I'd be interested in any info on the accelerometer setup (may as well do it the right way). So do you use a shaker table? My intention was to recreate this:
That demonstration basically shows just the behavior of the first cantilever mode (like the graph below). I've built a similar setup. True that the left ski appears to damp out that vibration mode faster than the ski without the gizmo, but the real question is: does damping out that mode enhances the ski's performance? Different modes can affect different characteristics of a ski. One has to be careful how we 'damp' out the ski, in my opinion.
Anyway, I didn't hear the audio in that video and just looked at it quickly, but it appears that the damping gizmo is located very near the area where the strain is significant for the first cantilever mode. So that damping gizmo dissipates the energy of the tip vibration very quickly. The only thing I don't like about those gizmos is added weight and complexity. It would be better to change the material of the ski and pick the best combination of materials for enhanced damping. This way it's all integrated into the ski itself. I've played with a few different "external" gizmos (like the one in the video), and here's a simple plot that compares one technology. The technology is very very simple -- kind of like duct tape -- but as you can see, the vibration of the first mode decays rather quickly (in red) compared to the control ski (in black). I'll dig up the details of the electronics and share the info shortly..... Fun stuff!!!
Anyway, I didn't hear the audio in that video and just looked at it quickly, but it appears that the damping gizmo is located very near the area where the strain is significant for the first cantilever mode. So that damping gizmo dissipates the energy of the tip vibration very quickly. The only thing I don't like about those gizmos is added weight and complexity. It would be better to change the material of the ski and pick the best combination of materials for enhanced damping. This way it's all integrated into the ski itself. I've played with a few different "external" gizmos (like the one in the video), and here's a simple plot that compares one technology. The technology is very very simple -- kind of like duct tape -- but as you can see, the vibration of the first mode decays rather quickly (in red) compared to the control ski (in black). I'll dig up the details of the electronics and share the info shortly..... Fun stuff!!!
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skidesmond
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Thanks, Richuk, for posting that link. I haven't seen it yet, and I just read it over briefly.
Anyway, back in 2007 I first looked more closely at the vibration characteristics of one pair of my own skis, and used a 3D scanning laser vibrometer (same device that Alex used in his post, it looks like). So, the measured few modes of the skis are shown below. I have a bunch of data in my logs from back then when I first started this work, including movies, etc. I've collected more data since then. The torsion modes are not shown for the ski.
The frequency response for the tip movement is shown here:
As one can see, the sharp peaks are the resonances, and the degree of the 'sharpness' relates to the amount of damping in the ski. Also, more or less stiffness will simply shift the peaks around. Having said that, one has to be careful not to confuse damping and stiffness. And, adding more mass doesn't affect the damping, either. Damping is energy dissipation; mass and stiffness affect the resonance frequency. The elements of a mechanical system are inertia, stiffness, and damping. What's more complicated is they can be nonlinear.
The above data only shows a basic pair of skis -- this was back in 2007, so my skis were pretty simple; nothing fancy, just wood, vds, glass, and epoxy. As we all know, the VDS does very little to improve damping; it just acts as a compliant layer to help minimize separation when the skis flex/bend. I looked at this back then and did not see any differences in skis w/ and w/o VDS (similar to the conclusion that Alex pointed out). But, if you had a damped ski with similar mass and stiffness, and its response was plotted simultaneously, you would see that the peaks would not be as 'sharp'.
mattman: Here's the schematic for the strain gage.
I can't find the exact part number for the super small accelerometer I'm using, but here's something similar, except you will need to find one with higher g rating. For example, I'm using a super small one rated up to 50g's. I'll keep digging for the part number....
http://www.mouser.com/ProductDetail/Fre ... xovQ%3d%3d
Anyway, back in 2007 I first looked more closely at the vibration characteristics of one pair of my own skis, and used a 3D scanning laser vibrometer (same device that Alex used in his post, it looks like). So, the measured few modes of the skis are shown below. I have a bunch of data in my logs from back then when I first started this work, including movies, etc. I've collected more data since then. The torsion modes are not shown for the ski.
The frequency response for the tip movement is shown here:
As one can see, the sharp peaks are the resonances, and the degree of the 'sharpness' relates to the amount of damping in the ski. Also, more or less stiffness will simply shift the peaks around. Having said that, one has to be careful not to confuse damping and stiffness. And, adding more mass doesn't affect the damping, either. Damping is energy dissipation; mass and stiffness affect the resonance frequency. The elements of a mechanical system are inertia, stiffness, and damping. What's more complicated is they can be nonlinear.
The above data only shows a basic pair of skis -- this was back in 2007, so my skis were pretty simple; nothing fancy, just wood, vds, glass, and epoxy. As we all know, the VDS does very little to improve damping; it just acts as a compliant layer to help minimize separation when the skis flex/bend. I looked at this back then and did not see any differences in skis w/ and w/o VDS (similar to the conclusion that Alex pointed out). But, if you had a damped ski with similar mass and stiffness, and its response was plotted simultaneously, you would see that the peaks would not be as 'sharp'.
mattman: Here's the schematic for the strain gage.
I can't find the exact part number for the super small accelerometer I'm using, but here's something similar, except you will need to find one with higher g rating. For example, I'm using a super small one rated up to 50g's. I'll keep digging for the part number....
http://www.mouser.com/ProductDetail/Fre ... xovQ%3d%3d
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skidesmond
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- Contact:
but it gets the mind to start exercising.OK, I finally found the super small accelerometer I was taking about:
http://search.digikey.com/scripts/DkSea ... 80-R2CT-ND
It's two-axis, just needs GND, +5V, and provides analog output for x/y directions. Pretty simple. Here's a picture that shows the accelerometer (above the penny) and also the small PCB I made for the stain gages (circuit shown above). Sorry about the poor quality of the picture --- it's the worst I've taken (cell phone photo). I'll talk more about damping later.
http://search.digikey.com/scripts/DkSea ... 80-R2CT-ND
It's two-axis, just needs GND, +5V, and provides analog output for x/y directions. Pretty simple. Here's a picture that shows the accelerometer (above the penny) and also the small PCB I made for the stain gages (circuit shown above). Sorry about the poor quality of the picture --- it's the worst I've taken (cell phone photo). I'll talk more about damping later.
Hi Richuk,
Good question. I haven't looked into different resins. Most of the resins I've used all have about the same characteristics. So, I would suspect that there wouldn't be much of an affect. In fact, I haven't noticed it in the measurements I've taken over the years. If the resins were significant, I would have seen something. But if one were to incorporate an additive into the resin, it might help. I've seen in the literature some fancy tricks involving small particles added into the epoxy matrix, and even fiber orientation. In fact, I've done similar with some interesting results, but the trick is to get the right particles and too much of it is a bad thing.
Regarding Tg, I would suspect a change in damping properties below and above this value, but again, skis are usually used in fairly cold conditions. It would be interesting to see the data, though.
I'm going to put together a short video soon showing a couple simple techniques to enhance damping for a ski. The goal is to get people thinking. The tricks are not special, just to illustrate what one can do. Stay tuned....
Good question. I haven't looked into different resins. Most of the resins I've used all have about the same characteristics. So, I would suspect that there wouldn't be much of an affect. In fact, I haven't noticed it in the measurements I've taken over the years. If the resins were significant, I would have seen something. But if one were to incorporate an additive into the resin, it might help. I've seen in the literature some fancy tricks involving small particles added into the epoxy matrix, and even fiber orientation. In fact, I've done similar with some interesting results, but the trick is to get the right particles and too much of it is a bad thing.
Regarding Tg, I would suspect a change in damping properties below and above this value, but again, skis are usually used in fairly cold conditions. It would be interesting to see the data, though.
I'm going to put together a short video soon showing a couple simple techniques to enhance damping for a ski. The goal is to get people thinking. The tricks are not special, just to illustrate what one can do. Stay tuned....
A video would be much appreciated 
Nano-particles are quite interesting, but I have not started to test the effects on vibration dampening - I'm still looking at abrasion, adhesion and impact resistance.
The things I have read suggests orientating fibres at 60 degrees offer some value, but that's not the issue, because you want to be in a position to filter the vibration...hmmm, more to do here in the UK.
Thanks for the feedback on the ideas - when I get down to it, I will post.
Nano-particles are quite interesting, but I have not started to test the effects on vibration dampening - I'm still looking at abrasion, adhesion and impact resistance.
The things I have read suggests orientating fibres at 60 degrees offer some value, but that's not the issue, because you want to be in a position to filter the vibration...hmmm, more to do here in the UK.
Thanks for the feedback on the ideas - when I get down to it, I will post.
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skidesmond
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- Joined: Tue Apr 07, 2009 3:26 pm
- Location: Western Mass, USA
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Kam looking forward to seeing your video on this topic... I started doing some very low tech dampening testing. No computers or electronic measuring devices, although I wish I had access to them.
I took one of my skis and clamped it to a table so the tip to the boot center was over hanging. I clamped the tail down to the table as well. I placed a short table underneath the part of the ski that was overhanging. The gap between the ski tip (start of the RL) and table was about 15mm. I flexed the tip down, held it down, and then released and timed how long the ski vibrated. I did this about 20 times. The times were fairly consistent at about 13.5-14 sec. The only way to observe the smallest of vibrations was to watch the shadow of the ski tip on the lower table.
Then I took 5 layers of cork measuring about 1mm thick by 3in wide by 8in long and using double stick tape, taped it to the ski about 8 inches from the tip.
I repeated the flex/vibration testing again, about 20 times. The times were consistently 10.5-11.5 sec.
So it seems the cork provided a small level of dampening.
So the million dollar question is, does reducing these small vibrations really effect the skier?
I've been thinking of using this thin cork on my layup of all wood skis thinking it would help in dampen the ski. I've shown in another set of tests that cork doesn't do much (if anything) in the way of supporting the ski like veneer or FG.
Perhaps I'll conduct low tech tests on the samples I created for POLY-U glue VS Epoxy test I did earlier this year.
Kam would love to see your more scientific tests.
I took one of my skis and clamped it to a table so the tip to the boot center was over hanging. I clamped the tail down to the table as well. I placed a short table underneath the part of the ski that was overhanging. The gap between the ski tip (start of the RL) and table was about 15mm. I flexed the tip down, held it down, and then released and timed how long the ski vibrated. I did this about 20 times. The times were fairly consistent at about 13.5-14 sec. The only way to observe the smallest of vibrations was to watch the shadow of the ski tip on the lower table.
Then I took 5 layers of cork measuring about 1mm thick by 3in wide by 8in long and using double stick tape, taped it to the ski about 8 inches from the tip.
I repeated the flex/vibration testing again, about 20 times. The times were consistently 10.5-11.5 sec.
So it seems the cork provided a small level of dampening.
So the million dollar question is, does reducing these small vibrations really effect the skier?
I've been thinking of using this thin cork on my layup of all wood skis thinking it would help in dampen the ski. I've shown in another set of tests that cork doesn't do much (if anything) in the way of supporting the ski like veneer or FG.
Perhaps I'll conduct low tech tests on the samples I created for POLY-U glue VS Epoxy test I did earlier this year.
Kam would love to see your more scientific tests.