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..
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.