That had worried me as well. MEMS devices may also not have the needed bandwidth. For accelerations as high as needed a piezoelectric device might be useful. Would need a microcontroller with a passably high sample rate ADC, and calibration may take a while, but it would probably be bulletproof. Something like this.
Ultracap is a good idea.
Good one. A fast enough microcontroller shouldn’t be a problem; for instance, this one comes in a 4x4 mm UQFN package, and does 300 ksps @ 12-bit, along with a 64x gain stage.
If it were a production thing I’d get it fabbed on a flexible substrate; for a one-off, “dead bugging” it would work fine (if a bit delicate considering the sizes).
Piezo units are great, but they don’t have a DC response. In other words, they can’t measure regular ol’ “constant” acceleration (e.g. an object falling with an acceleration of 9.8 ms[sup]-2[/sup]). Because of this, piezos are primarily used to measure shock and vibration.
Having said this, some piezos have a very impressive low frequency response. So good, in fact, that they can measure a constant acceleration for a short period of time. Check the specs carefully.
For studying the power phase it’s make a lot more sense to use (very) high speed photography. Your instrumentation won’t interfere with the test objects, isn’t subject to size / mass restrictions, etc.
Data reduction would be a PITA, manually scoring the WAG thousand frames of a single launch. But you’d have great data when you were done.
I figured that for the flight of an arrow integrating the samples should not be too hard, and should get back sensible and stable acceleration values. As a raw device piezos really measure jerk rather than acceleration.
You mean something likethis?
Actually on second thought I agree with this point.
If the arrow is in free fall (cancelling gravity) but subject to drag, then the only force left is the drag. So all the output of the accelerometer can be attributed to drag.
The arrow doesn’t generate lift, does it ?
This accelerometer seem to measure up to 16g although it is rated to survive 10000g. It is a 3 axes system, and doesn’t measure rotations (specs here).
The website indicates that it can measure drag as well as arrow orientation (before it is shot, I presume). So it is unclear to me what it can do (in the context of archery) beyond measuring drag. You wouldn’t get much information about the arrow’s trajectory with that, nor would you be able to measure the acceleration generated by the bow, that seems to exceed by far the accelerometer’s range.
It would be cool to see some reports of what the system can actually do.
By the way, there is a nicely written technical paper (AN-1057: Using an Accelerometer for Inclination Sensing) in the documentation.
I found a nice description of the system here.
Thats the same article that I had read several months ago. It might take awhile for the price to go down on these. I don’t see any kinfd of huge demand for them in the fore seeable future.
No accelerometer can measure this. A DC accelerometer can, however, measure the acceleration of something sitting at rest on the ground.
Just some random thoughts but you could test your bow limbs by building a draw board.
A couple of 2x4s or 2x6, a boat winch, a digital or spring scale, turnbuckle, carabineer, and maybe a large graph paper to make limb movement easier to see and measure. All limbs are not create equal.
I’ve seen high speed videos and graphs that prove that compound bow limbs do not return exactly the same way as they were drawn, explaining some of the energy loss.
Extremely light weight arrows have been know to overstress bow limbs, especially to those bows with training wheels (hehehe). Unequal movement between the limbs could cause up/down nock movement which could lead to unwanted arrow shaft vibration and an increase in drag.
Years ago I considered adding a fine internal wire between the point and the nock. Increasing tension to the wire would stiffen the arrow shaft. Compound bow manufactures recommend no less than 5 grains per inch (zero gpi is a dry-fire) and I was able to find off the shelf arrow components that met my requirements without resorting to internal bracing. Maybe you can do something with the idea?
An accelerometer that weighs the same as your arrow could be used to test the force of launching an arrow without using an arrow. (Trying to catch the accelerometer would be a problem.)
Good luck.
One of the unique problems with all wood primitive bows is the hysterisis factor which is very small and consistent with synthetic materials. Wood has internal friction that is time sensitive. An arrow shooting at 240 fps will experience a lot more hysterisis than an arrow shot at 120 fps.
In flight shooting we shoot very low arrow weights, often low enough to cause bow failure. 3 grains per pound of draw weight for an all wood bow is not unusual. Some of the composite modern bows are shooting arrows as light as 1/2 grain per pound of draw weight.
Very light arrows are fast but also very inefficient in that they use a much smaller amount of the available energy. One particualr area of bow dynamics that we like to study actually occurs in the final few inches of the power stroke. If a bow had no vibration or distorsion in the limbs it would have to be 100% efficient even with very light arrows. At the very last instant all the energy and momentum left in the limbs would instantly have to be transferred to the arrows as it would have no where else to go.
It is surprising how many small changes can be made to limbs that will have an effect on the final few inches of power stroke. Most of that now is based on experience and trial and error, high speed photography has been somewhat helpful but the small sizes of the samplings we get limit it's usefulness.
I have come up with a few ways of using a series of tests using a wide range of extreme arrow weights to establish and show linear patterns of hysterisis losses. My engineer and scientist friends are pretty impressed with it. ( one of my few prouder moments). The process is long drawn out and cumbersome and not practical for standard testing but it does clearly demonstrate the existence of a phenomina I have been argueing existed for some time now.
A versatile accelerometer that also measured deacceleration as well as shock and sudden changes would be invaluable…
If you are trying to measure properties of the bow, rather than the acceleration of the arrow, wouldn’t putting strain gauges at various points on the bow be helpful, so for various line pulls you can monitor the stress state of the bow and see how the various tweeks you make affect the stress state. If you can sample the strain gauges quickly enough you should be able to see what happens in the bow during release.
One further note if you do want to put strain gages or accelerometers on the bow, you have to be fairly careful about where you put them, and strain gages measure dynamic changes almost 180deg out of phase with acceleration. Basically put the strain gages on parts that don’t move and accelerometers on parts that do move. Combining accelerometer and strain data , when suitably sampled, can give you some really interesting insights into dynamic mechanical systems.
You’re correct; my bad. This explains it very nicely.