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Unread postAuthor: Technician1002 » Wed Sep 02, 2009 6:43 pm

POLAND_SPUD wrote:that slowmo casio cam that everyone is talking about would make a pretty good chronograph, don't you think ?


Yes. Allow for parallax error and include a measure in the shot like this.

Video was used to confirm the chronograph results to eliminate possible muzzle blast errors for close to the muzzle clocking of projectiles that rapidly slow upon exit. Measured results were confirmed.
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Attachments
WaterTest.jpg
Measuring the speed of the water spray with a slow motion camera. Initial speed is about 1 foot/sec.
WaterTest.jpg (36.82 KiB) Viewed 297 times
Tshirt.jpg
Slow motion video clocking a t shirt in flight.
Tshirt.jpg (28.95 KiB) Viewed 296 times
Chronograph.jpg
Chronograph results for a launched apple.
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Unread postAuthor: jimmy101 » Thu Sep 03, 2009 3:19 pm

Technician1002 wrote:So what's easier and cheaper to obtain? Microphones, analog hall sensors with battery boxes and batteries, or broken earbud headphones and a 1/8th inch stereo headphone cord and surplus cat 5, telephone cord, or other cable for the wire.

Or a couple cheapo piezos from RadioShack for a $1 each and a couple resistors (or diodes) to make sure you don't over-voltage the soundcard. If the sound level is "reasonable" then the resistors/diodes can be omitted. The piezo's aren't great mics but they don't need to be. Since they suck as mic's background noise isn't much of problem If they are actually taped to the target(s) the signal will be huge and S/N shouldn't be an issue.

Piezo's can be used as very sensitive mic (as in recording the sound of ants walking about) but of course in this case that is hardly needed.
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Unread postAuthor: Technician1002 » Thu Sep 03, 2009 5:37 pm

jimmy101 wrote:
Technician1002 wrote:So what's easier and cheaper to obtain? Microphones, analog hall sensors with battery boxes and batteries, or broken earbud headphones and a 1/8th inch stereo headphone cord and surplus cat 5, telephone cord, or other cable for the wire.

Or a couple cheapo piezos from RadioShack for a $1 each and a couple resistors (or diodes) to make sure you don't over-voltage the soundcard. If the sound level is "reasonable" then the resistors/diodes can be omitted. The piezo's aren't great mics but they don't need to be. Since they suck as mic's background noise isn't much of problem If they are actually taped to the target(s) the signal will be huge and S/N shouldn't be an issue.

Piezo's can be used as very sensitive mic (as in recording the sound of ants walking about) but of course in this case that is hardly needed.


Pizeo elements are relatively high impedance. They work best with a high impedance amp such as a guitar amp. The low impedance of sound cards or pro 3 pin XLR mixers tend to overload them and attenuate the signal badly. Magnetic is low impedance and matches better to a sound card. Most sound cards have an input impedance of 2 to 5 K ohm.
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Unread postAuthor: jimmy101 » Fri Sep 04, 2009 1:18 pm

Technician1002 wrote:Pizeo elements are relatively high impedance. They work best with a high impedance amp such as a guitar amp. The low impedance of sound cards or pro 3 pin XLR mixers tend to overload them and attenuate the signal badly. Magnetic is low impedance and matches better to a sound card. Most sound cards have an input impedance of 2 to 5 K ohm.

True, but impedance matching is really only about maximizing the power transfer. But since you really don't care all that much about power transfer poor impedance matching isn't a big deal. You just need a signal, you don't care how big that signal is as long as it is up out of the noise. Indeed since the signal amplitude occurs nowhere in the analysis of the data that suggests that impedance matching really isn't all that critical. When the signal amplitude is actually part of the data stream then impedance matching becomes much more critical.

One advantage of piezo over magnetic is that though the magnet+coil has low electrical impedance it often has a fairly high mechanical impedance. That is, it has mass and there is a limit to how it will be accelerated for a given input pulse and pulse duration. Even if what is actually being moved is a piece of sheet metal (or foil) that piece has mechanical impedance. For a single pulse input (where there is no chance of resonance in the detector) mechanical devices often have problems. IIRC, devices for measuring things like shock waves are generally piezo based and not magnetic based. The very short (though possible insanely high magnitude) pressure pulse just won't read on a mechanical device because the moving parts simply cannot accelerate fast enough to actually move during the very short duration of the pulse.
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Unread postAuthor: Technician1002 » Fri Sep 04, 2009 3:08 pm

jimmy101 wrote:True, but impedance matching is really only about maximizing the power transfer.


The impedance is more than power matching. A Pizeo element does not have a DC path. It has an effective series capacitance. A low impedance load will change the fidelity so only high frequency components pass. This distortion can cause the low frequency components of impact to be removed masking the impact movement from muzzle blast noise.

The very short (though possible insanely high magnitude) pressure pulse just won't read on a mechanical device because the moving parts simply cannot accelerate fast enough to actually move during the very short duration of the pulse.


This is a non contact sensor. No component of the coil or magnet move. Only the movement of the foil target is detected. Please review the style of sensor mentioned. There is no added mass. Movement of the foil is detected without any contact with the sensor.

Movement of the foil in a magnetic field creates current in the foil. This current creates an opposing magnetic field. This new combined field is a change in field strength. This change in strength creates current in the pick up coil. There is no delays in any of these events. The signal is real time and directly proportional to the change in flux. The coil and magnet do not move. Only the foil from impact. This insanely fast mechanical motion is the movement of the foil from the projectile passing through. Detection speed due to the acceleration of mechanical parts is not an issue.

Attaching a pizeo does add mass to the target membrane, unless you are planning on shooting the pizeo sensor directly.. :D
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Unread postAuthor: jimmy101 » Fri Sep 04, 2009 3:40 pm

Technician1002 wrote:This is a non contact sensor. No component of the coil or magnet move. Only the movement of the foil target is detected. Please review the style of sensor mentioned. There is no added mass. Movement of the foil is detected without any contact with the sensor.

Movement of the foil in a magnetic field creates current in the foil. This current creates an opposing magnetic field.

Read my post. Something is moving (the foil, like I said) and that something has mass hence impedance. Unless you can come up with a zero mass conductor you have mass impedance. Doesn't matter if the coil and magnet are stationary, either a conductor or a magnet must move.

Attaching a pizeo does add mass to the target membrane, unless you are planning on shooting the pizeo sensor directly.. :D

Yes it does add mass. But that mass doesn't have to move to create the signal (unlike the conductive foil). The effective mass of a piezo sensor is very small, all you have to do is get the crystal to compress by a distance that is probably less than a micron. So a much smaller mass moving by a distance that is perhaps three to six orders of magnitude less than with an inductive system. That gives much lass mass impedance.

Many things other than electronc devices have impedance. Many wind instruments function because the movemnt of sound through air exhibits impedance. That is why a tube open at both ends, like a flute, will still resonate. The end of the tube has impedance and behaves as if there is a solid end on the tube. So, a sheet of conductive material will show impedance affects when it is excited by either direct contact with a projectile or exicited by sound waves. A short enough burst of energy, regardless of it magnitude, will be absorbed (and not passed) by the impedance of the sheet. The sheet is basically a low pass filter to sufficiently high frequency inputs.

Yes, piezo do have very high internal resistance (though it typically isn't infinite) and yes loading can be a problem but I can't see anything that would be a big enough problem in this application. Again, impedance really doesn't matter since signal amplitude is fairly irrelevant. The only time impedance affects would be a problem would be with a slowly changing signal. Since that isn't what we are discussing here it isn't a problem. If the signal change is so slow that it would be a problem for a piezo then it is likely to also be a problem for a microphone or any other type of pickup since the signal is going through the high pass filter on the input to the soundcard. If the piezo can't get a suitable signal then even if a different detector did get a signal the soundcard would ignore it anyway.
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