Sensors to detect high speed object crossing plane

I'm looking to detect a somewhat small (5-20mm) high speed (600m/s) object crossing a stationary 0.5m x 0.5m plane. When the object crosses the plane, I want to know the X, Y.

I apologize for the very broad nature, but I'm just trying to figure out what kind of sensors could be used to construct such a device. I plan on using the arduino for processing, but am clueless wrt sensors. (I'm really a software guy - dont kill me!)

Thanks, Chris

• Is the object emitting any radiation? Light? Sound? Is it traveling in a restricted space? Is it metallic? Is it a duck? I like ducks. – AngryEE Apr 3 '12 at 13:31
• @angryEE, it is metallic, but not a duck. The object is a bullet, so I don't think visible light will be emitted. I imagine a bullet does emit sound as it travels, but not sure how easy that would be to sense. Also, no ducks will be harmed in the development of this project. – chris Apr 3 '12 at 14:02
• Those ducks might not be harmed, but traveling at more mach-2 I bet you will have Angry Birds! But heck, buy the time you hear them complaining they will be long past. – JonnyBoats Apr 3 '12 at 20:02

Do the math.

600 meters per second is 1342 miles/hour. Mach 2.0 is 680 m/sec. We're talking "bullet" here.

Assuming 5 mm for the projectile length, it will cross the plane in 8.33 microseconds. A 20mm projectile will take all of 33.3 microseconds.

The traditional way to measure the accuracy and precision of a bullet trajectory is to shoot it at a piece of paper, and see where the hole shows up.

You COULD use something like black construction paper, lit from behind (light OFF AXIS!) and use a TV camera to detect the sudden appearance of a bright spot where there wasn't one, where the newly-punched bullet hole is now letting light through. An Arduino is going to be kind of wimpy for image processing, though.

• Yep, we are talking bullets. Are you saying, there probably are not any sensors that can detect the object within 8-30 microseconds? Off axis camera is a cool idea, but would require a bit more setup than I was hoping for. – chris Apr 3 '12 at 13:06

The way a bullet chronograph works is it measures the momentary change in light intensity through an aperture with a diffuse bright background (either the sky, or a white plastic diffuser), by means of an infrared or visible-light photodetector such as a phototransistor or photodiode.

If you don't need a super accurate XY coordinate on the plane, you could potentially use the same method with an array of photodetectors along each axis, and some optics or shields to restrict which parts of the plane each photodetector can "see". You would then calculate the position of the projectile as it passes across the plane by looking at which photodetectors triggered (changed intensity) at the same time.

Alternatively, a very high speed CCD (or two) could work. You could calibrate the system using a reference plane printed on a panel, to know which pixel corresponds to which coordinate on the plane. I doubt the Arduino has enough processing power for running computer vision algorithms, so this may not be plausible in your application.

• "Photocell" is a word often used for a photoresistor, aka LDR. I hope you don't mean that, because they're the slowest devices existing. – stevenvh Sep 11 '12 at 15:27
• @stevenvh I figured "photocell" was just a general term for photoelectric components. I changed the wording in my answer. Thanks! – Steven T. Snyder Sep 11 '12 at 15:39

How much development do you want to do?

There might be several ways to do tomography within a 500mm2 window.

At 600m/s, the projectile will move 15mm in one cycle of a 40kHz ultrasonic acoustic signal. If you had several transmitters and receivers around the edge of the detection window, you might get enough signal at the receivers to reconstruct where the extra echo was coming from.

Compared to light, it is of course to all intents and purposes, stationary. Being small, it will not be a good reflector of radio signals until its size approaches a wavelength. Perhaps you could jig some 6GHz doppler door openers (cheaply available) around the periphery and do the same reconstruction on the echoes trick.

You can use the timing of the sonic disturbance from several microphones placed around the periphery of the target. 3 would be the absolute minimum required but I would recommend 5 to 10. The pulses will need to be timestamped with a fast enough master clock to get the resolution you desire. This becomes a relatively simple statistical analysis when the aparatus is operation correctly. The projectile willjust need to be traveling > M=1.0 when it crosses the threshold of the mics.