Electronic target question

[nemo]

Greetings all,

I was wondering if anyone knows how the electronic targets work? Do the microphones sense the impact of the pellet on paper or the shock-wave the pellet produces? What frequency do the microphones record? Is there any technical description?

Kindest regards,

N

[Rutty]

The manufacturers are very coy about the method of operation of their equipment. The acoustic systems are fairly limited in the way that they can operate and whatever they do will be based upon a hyperbolic fixing method. In simple terms, take 4 microphones and a timer. Start the timer when the Sound Pressure Level (SPL) reaches a pre-determined value on any microphone; that mic becomes the "master" for the fixing algorithm; and and store the elapsed times when the same SPL occurs at the other 3; the "slaves". If you were plotting it manually you could now apply the Local Speed of Sound (LSS) to produce a hyperbolic lattice of lines of equal time difference relative to the Master and Slaves. You would then be able to determine the position of the acoustic event that generated the change in SPL. Alternatively you could do a bit of complicated arithmetic to arrive at the same conclusion. Note that the microphones are not in any sense directional although the may be shielded to avoid them picking up stray signals from outside their area interest.

There's nothing new there, hyperbolic fixing was the basis of most of the pre-gps large area naviagtion aids, whether based upon, time, frequency or phase difference. It also worked rather well for finding submarines, unique sonic event detected on a number of sonobouys, measure the difference in time of arrival, do the sums and make best speed towards the datum.

The 2nd generation EST employ some form of lattice generated within the frame by a LED array and sensed by suitable detectors. Again, there are a limited number of options about how you achieve this but I will let someone with more knowledge than me explain it.

Rutty

[nemo]

Hi Rutty,

thank you for the reply. Based on your explanation of the principle using hyperbolic location determination (not unlike LORAN), I believe that I have much better approach. However, the method of signal processing was not really what I asked.

I am more concerned about the sensor part. It is my understanding, that for rim-fire/center-fire, the bullet's shock-wave is very distinct to be captured by the sensors. I am not quite sure about the - much slower - pellet. I also do not have any feeling which part of the spectrum should the sensors be attuned to. I guess, I will just rig something with a regular microphone. However, should you have any insight into this, I would appreciate it.

Kindest regards,

N

[Rutty]

Loran, Gee, Decca take your pick, they all utilise the same fundamental principles. Remember that you have the paper or rubber roll and or the target mask that will generate sound on penetration. The system will not function without it, try without and you will get some very strange results.

I am sorry, but I cannot help you with the response characteristics of the arrays. I was a user of the systems I described;and ESTs ; not a techie. In the end it all comes down to basic physics, the higher up the spectrum you go the better the response but the greater the propagation loss. As I said previously, I feel that the detection is more to do with SPL than discrete frequencies.

Of course the fun comes in the confidence test to establish the relative positions of the sensors, that's a whole different ball game.

Rutty

[nemo]

Hi Rutty,

thank you once again. If I understand your sentence:

Remember that you have the paper or rubber roll and or the target mask that will generate sound on penetration.

the sound for the sensors is generated by the pellet impact on the paper and not by the shock-wave. Correct?

Kindest regards,

N

[Rutty]

the sound for the sensors is generated by the pellet impact on the paper and not by the shock-wave. Correct?

Logically that would have to be the case with low speed projectiles, you use acoustic event created by penetration. The best comparison I can suggest is the one of a diesel electric submarine snorting to recharge its batteries with the acoustic signature of the diesel engine present on a number of sonobouys (hydrophones\microphones). The moment the diesel engine is shut down there is a significant change in the detected signature; an abrupt stop. As the sonobouys are at differing distances from the source and the sudden change in signature will be detected on them at different times. By measuring the time difference a hyfix may be obtained. The reverse situation was an abrupt start, which is what we are utilising with the EST.

It used to work rather well when diesel submarines were noisier and detection opportunities greater.

Rutty

[GOVTMODEL]

Hi Rutty,

thank you once again. If I understand your sentence:

Remember that you have the paper or rubber roll and or the target mask that will generate sound on penetration.

the sound for the sensors is generated by the pellet impact on the paper and not by the shock-wave. Correct?

Kindest regards,

N

Does a sub-sonic projectile even have an associated shock wave?

[Rutty]

Does a sub-sonic projectile even have an associated shock wave?

You have to be a little careful with the terminology here. The "Shock" wave is normally associated with the compression wave that occurs when a projectile (or aerofoil) is travelling in excess of the Local Speed of Sound (LSS). However air is compressible and a compression wave will occur at speeds below LSS. For convenience, up until 0.4M air may be treated as incompressible but above that compressibility effects increase. However they are surprisingly small. I shall have to quote the correction in Nautical Miles\Hour (Knots) as that is what I was taught and remember. The correction to True Airspeed (TAS) for compressibilty effect is TAS/100 -4.

However, the thing to remember is that anything moving through any fluid will generate a disturbance that results in noise. All sorts of factors determine how much noise and the speed determines whether or not you hear it before it hits you!

Rutty

[David Levene]

the sound for the sensors is generated by the pellet impact on the paper and not by the shock-wave.

Anyone who has worked ranges fitted with the older style of Sius targets will tell you that if the paper jams you develop a large hole in it.

As it gets larger then, eventually, a shot will pass through without touching the paper resulting in a "failure to register".

That's why experienced range staff will scan the targets with a pair of binoculars every few minutes so that they can move the shooter to a hospital lane before a scoring dispute occurs.

[nemo]

Hi GOVTMODEL,

Yes, in aerodynamics book I have the subsonic noise is called a "normal shock wave" and the supersonic noise a "bow shock wave."

Hi David,

As it gets larger then, eventually, a shot will pass through without touching the paper resulting in a "failure to register".

As I understand your quote, it is the impact of the pellet on paper generating the sensor's signal.

Kindest regards,

N

[David Levene]

As I understand your quote, it is the impact of the pellet on paper generating the sensor's signal.

With the older Sius systems, yes.

The newer systems do not use microphones and do not therefore need the black paper roll (other than for "backtracking" after the match).

Systems such as Megalink use a light grid to sense the pellet.

In short, there are several different methods for sensing the shot position.

[jhmartin]

Systems such as Megalink use a light grid to sense the pellet.

Think you meant Meyton .... as far as I know Megalink targets still use a paper or rubber roll for impact location detection.

[nemo]

Hi David, jhmartin,

do you know the principle behind the light grid?

Kindest regards,

N

[jhmartin]

P5 of: http://www.meyton.info/uploads/media/bro_en.pdf

More: http://www.meyton.info/uploads/media/Bl ... _EN_02.pdf

[David Levene]

Systems such as Megalink use a light grid to sense the pellet.

Think you meant Meyton .... as far as I know Megalink targets still use a paper or rubber roll for impact location detection.

You're right, brain fade ;-)

[nemo]

Hi jhmartin,

thank you very much for the links. I think DIY is out of question. ;-(

Kindest regards,

N

[Rutty]

Nemo - with a username like that acoustics should be your strong point!

wrt first generation ESTs and hyfixing I would stress that approach is only my personal opinion on the techniques that are used. Should anyone else have a plausible explanation I would be very interested to hear it.

Rutty

[m1963]

The SIUS Hybridscore, which we use, has no paper roll. It uses, "double IR beam and acoustic measurement."

http://www.sius.com/downloads/Flyer/B04 ... dscore.pdf

It works well.

[Rutty]

The SIUS Hybridscore, which we use, has no paper roll. It uses, "double IR beam and acoustic measurement."

That's true, but from the description they provide the IR and Acoustic processes are employed separately, not in coijunction with each other.

In the outer zone of the target plane (mainly the white area of the airpistol target) the projectile is located by five independent sound transducers

So it would appear to be the penetration of the mask that is used as the acoustic event whilst a shot that goes into the "black hole" is measured by the IR array. Anyone up for a bit of interferometry?

Rutty

[nemo]

Hi m1963,

thank you for the link.

. . ., but from the description they provide the IR and Acoustic processes are employed separately, not in coijunction with each other.

This appears to be correct. Here is the patent: http://www.google.com.ar/patents/US8570499

Kindest regards,

N