Ballistic Chronograph

Projects General Guidance
1

Hello!

I'm new to this forum, and fairly new to electronic projects. I have been playing a little bit with Raspberry Pi and sensors in drone relatated projects and some IoT projects.

But now I want to try something else. I want to construct and build i ballistic chronograph that can detect a bullet in flight at 900m/s +.

The main idea is to have two sensors placed at a known distance apart that the bullet flies over. This starts and stops a timer and the speed can be calculated. This is usually achieved by using two phototransistors, like in this project:

https://hackaday.io/project/9378-diy-ballistic-chronograph

In my project I want to use two magnetic fields that the bullet flies over to trigger the start/stop of the timer. This is where I get into problems. Is it possible to use a permanent magnet and a coil and use the change in the PM's magnetic field as the bullet flies over to trigger the timer part of my idea?

There is a commercial product that utilizes this:

Any tought on the topic are welcome!

Alex
Norway

2

For a ferrous projectile try a couple of $7 pickups from an electric guitar. You'll probably have to lower the number of windings to get the required frequency response.
No idea if it will work with with non-ferrous projectiles.

My suspicion is that your electronics and wiring will be shaken to pieces over time unless potted in epoxy.

3

900 m/sec
distance between sensors lets say 20 cm?
So the Arduino must recogize 2 pulses rougly 200uSec apart

As the standard micros() counts in steps of 4 usec you would get this lookup table.

uSec   M/sec
200    1000,00
204    980,39
208    961,54
212    943,40
216    925,93
220    909,09
224    892,86
228    877,19
232    862,07
236    847,46
240    833,33
244    819,67
248    806,45

Note the stepsize is in the order of 2%

To improve the timing you should use

  • use interrupts to detect the pulses.
  • a hardware timer

The Arduino Uno has 2 interrupt pins on pin 2 and 3
read - attachInterrupt() - Arduino Reference

(to be continued)

4

Using interrupts and a hardware timer you can get precision up to 1/16 uSec.

This will give the following table (assuming the distance between sensors is 20cm)

221,0000	905,0
221,0625	904,7
221,1250	904,5
221,1875	904,2
221,2500	904,0
221,3125	903,7
221,3750	903,4
221,4375	903,2
221,5000	902,9
221,5625	902,7
221,6250	902,4
221,6875	902,2
221,7500	901,9
221,8125	901,7
221,8750	901,4
221,9375	901,2
222,0000	900,9
222,0625	900,6
222,1250	900,4
222,1875	900,1
222,2500	899,9

So that is 0.3m on 900m/sec is in the order of 0.5‰. This is very precise in theory but in practice there are a few possible problems.

  1. if the distance between the sensors is 0.5% off (20.1 cm iso 20.0 cm) the calculated speed will also be 0.5% off
  2. if one sensor reacts slower than the other you will get an additional penalty
  3. if the clock of the Arduino is not 16.000.000 Hz but 1‰ off, the calculated speed will also be 1‰ off.

So although in theory you can get an accuracy of 0.5‰, in practice it will be around 1% at best.
Calibrating the numbers used in the math can help to maximize the accuracy.

5

Wrote a timeMeasurement sketch that uses rising pulses on pin 2 and 3 as resp start and stop signal.
Might help to build your system

//
//    FILE: timeMeasurement.ino
//  AUTHOR: Rob dot Tillaart at gmail dot com
// VERSION: 0.0.1
// PURPOSE: timeMeasurement Arduino UNO between 2 interrupts on pin 2 and 3 using timer1
//     URL: http://forum.arduino.cc/index.php?topic=473669
//
// HISTORY:
// 0.0.1 - 2017-04-28 initial version
//

volatile bool inprogress = false;
volatile bool dataReady = false;
volatile uint32_t ticks = 0;
volatile uint32_t overflows = 0;

const float ticksPerSecond = 16000000.0;
const float sensorDistance = 0.2;


void setup()
{
  Serial.begin(115200);
  Serial.println(__FILE__);
  Serial.println();

  pinMode(2, INPUT);
  pinMode(3, INPUT);

  attachInterrupt(digitalPinToInterrupt(2), startTimer, RISING);
  attachInterrupt(digitalPinToInterrupt(3), stopTimer, RISING);
}

void loop()
{
  if (dataReady && inprogress)
  {
    // MATH
    float duration = ticks / ticksPerSecond;  // seconds
    float distance = sensorDistance;          // meter
    float speed = distance / duration;        // meter / second

    // OUTPUT
    Serial.print("time: ");
    Serial.print(ticks / 16.0, 2);    // 2 decimals
    Serial.print(" usec ");
    Serial.print("\tspeed: ");
    Serial.print(speed, 3);           // 2 decimals
    Serial.println(" m/s");

    // PREPARE NEXT MEASUREMENT
    inprogress = false;
  }
}

ISR (TIMER1_OVF_vect)
{
  overflows++;
}

// note startTimer code itself takes
void startTimer()
{
  // prevent start interrupt when in progress
  if (inprogress) return;
  inprogress = true;
  dataReady = false;

  TCCR1A = 0;             // 
  TCCR1B = 0;             // stop
  TCNT1 = 0;              // reset counter, overflows at 65536
  overflows = 0;          // reset overflowCount
  OCR1A = 1;              // clock pulses per increment TCNT1
  TIMSK1 = bit(TOIE1);    // set overflow interrupt
  TCCR1B = bit(CS10);     // start
}

void stopTimer()
{
  if (dataReady) return;  // prevent additional stop interrupt
  dataReady = true;

  TCCR1A = 0;
  TCCR1B = 0;
  TIMSK1 = 0;
  ticks = overflows * 65536UL + TCNT1;    // calculate clockticks.
}

// ---END OF FILE ---
1 Like
6

In my project I want to use two magnetic fields that the bullet flies over to trigger the start/stop of the timer.

This won't work. Most people use photogates.

7

A lot of bullets are not ferrous. There are plenty of lead projectiles, many copper, and some actually made of a plastic composite with metal particles. If you try to do this using magnetic fields, you'll be limited to a small subset of all ammunition.

8

I believe the magneto speed works with any metal.
Not sure how though.

9

What is "magneto speed"?

A nonferrous, electrically neutral bullet would have no effect on a static magnetic field. Basic physics.

The National High Field Magnet lab has a tutorial article on how to measure bullet speed. They suggest to use foil strips, which are broken by the bullet, and (strangely enough for people who spend their working days using magnets) make no mention at all of magnets.

10

jremington:
What is "magneto speed"?

A ballistic chronograph.

Patent mentions magneto resistive sensors.

11

The default bullet setting is copper with lead core.
There is also a setting for lead slugs.

Instructions mention the signal is weaker with these, there is a setting for steel core which gives a much stronger signal.

EDIT

The instructions mention that with a steel core the signal is inverted so maybe there is a clue there.

I certainly cannot figure it out.

Bullet speed is important too , less than 1000 FPS and it does not work as well.
Thicker copper jacket also improves things.

My best guess is that the bullet travelling through a magnetic field generates current in the bullet which causes a detectable change in the field.

12

Thanks for your replys!

I will do som sensor benchtesting over the weekend when I get home.

robtillaart, thank you so much for your code example. I will look into that aswell :slight_smile:

Any other suggestions regarding sensor. Will a gituar pickup magnet and coil style sensor also pickup a non ferrous metal? Or is better to go with a doubble coil, metal detector style sensor?

Alex

13

robtillaart:
Using interrupts and a hardware timer you can get precision up to 1/16 uSec.

Completely agree with the accuracy. Assuming manufacturing tolerances. Accuracy is the closeness to the "true value". On the assumption the OP uses all equipment to the manufacturing standards, the method will be faily accurate. They could use the "propagation of errors" formulae for a more tangible idea if they want.

Precision on the other hand: Accuracy and precision - Wikipedia

Precision is pretty much the deviation between measurements of the same unknown indepedent. If you have the exact same scenario, any measurement WILL vary somewhat. Precision is the "closeness" of individual measurements of a similar unknown.

Being able to measure with intervals of 1/16uS is resolution. Resolution is the smallest measurable change of an independent variable.

14

JAR4x4:
Any other suggestions regarding sensor. Will a gituar pickup magnet and coil style sensor also pickup a non ferrous metal? Or is better to go with a doubble coil, metal detector style sensor?

Alex

Do not know.
I do not think anyone here knows how this device functions.
Arduino is unlikely to be enough. Analogue electronics is probably needed and it would be better to use hardware for timing accuracy.

15

Hello again!

I've done some more internet research... What about using a pulse induction metal detector circuit as the sensors?

A pulse induction metal detector can detect non ferrous metal.

Any toughts on that?

16

Before you build a metal detector, consider the following.

From a theoretical point of view, a non-ferrous electrically conductive bullet moving in a magnetic field will generate electrical currents within the body of the bullet. The magnetic field generated by those currents will leak outside the bullet perturbing the magnetic field it is moving though which should be detectable by the pickup coil surrounding the magnet.

For less than $10 you could do a test to see if the signal from the pickup coil is visible on an oscilloscope. If so then it shouldn't be too difficult to make an amplifier that will boost the signal level high enough for an Arduino.
If you don't have an oscilloscope then this whole project is in doubt.

17

mikb55:
Before you build a metal detector, consider the following.

From a theoretical point of view, a non-ferrous electrically conductive bullet moving in a magnetic field will generate electrical currents within the body of the bullet. The magnetic field generated by those currents will leak outside the bullet perturbing the magnetic field it is moving though which should be detectable by the pickup coil surrounding the magnet.

For less than $10 you could do a test to see if the signal from the pickup coil is visible on an oscilloscope. If so then it shouldn't be too difficult to make an amplifier that will boost the signal level high enough for an Arduino.
If you don't have an oscilloscope then this whole project is in doubt.

Thanks mikb55,

I have an oscilloscope and i will test this as soon as possible, I will post my results here =)

Alex

18

If you read jremingtons comments , we do not know how that chronograph works.
It seems a theoretical impossibility.

You are likely to require analog electronics skills and a lot of experimentation.

According to the patent magneto resistive sensors are used.
They can be had from electronics suppliers.

19

It seems a theoretical impossibility.

I did not say that. To elaborate:

When an isolated, moving conductor ENTERS or LEAVES a region of homogenous magnetic field strength, a small current flows across the conductor until the charges at the ends accumulate to balance the qvB force. That current flow does work and will create a very small perturbation in the applied magnetic field.

While in the region of homogenous magnetic field, no current flows and no net force is exerted on the conductor, the charges and/or the magnetic field.

From the extremely vague descriptions available, the manufacturer of Magnetospeed has apparently developed a method to detect that initial or final current flow.

I doubt that an inexperienced experimenter would be able to detect such an effect, and for that reason it seemed pointless to recommend trying the method.

On the other hand, it is very easy to detect a moving magnet or ferrous object.

20

Is a somewhat set up like in NMR not a possibility?

2 coils. One that generates say a ~MHz square wave pulsed magnetic field.
The inner coil is the "detector" coil that could detect the induced current from the, what will now be magnetised, moving ferrite object?

Of course, in NMR a Fourier transform is then applied over many millions of iterations to give a spectrum, but in this case, you just want to "detect" a change in signal...