That said, [Kamal Carter]’s build is pretty darn cool. He wisely chose to use just about the weakest bows you can get, the kind with strings that are basically big, floppy elastic bands that shoot arrows with suction-cup tips and are so harmless that they’re intended for children to play with and you just know they’re going to shoot each other the minute you turn your back no matter what you told them. Target acquisition is the job of an Intel RealSense depth camera, which was used to find targets and calculate the distance to them. An aluminum extrusion frame holds the bow and adjusts its elevation, while a long leadscrew and a servo draw and release the string.
With the running gear sorted, [Kamal] turned to high school physics for calculations such as the spring constant of the bow to determine the arrow’s initial velocity, and the ballistics formula to determine the angle needed to hit the target. And hit it he does — mostly. We’re actually surprised how many on-target shots he got. And yes, he did eventually get it to pull a [William Tell] apple trick — although we couldn’t help but notice from his, ahem, hand posture that he wasn’t exactly filled with self-confidence about where the arrow would end up.
In the study of ballistics, you can do very little without knowing the velocity of a projectile. Whether you need to hit a target at over a mile, check if a paintball gun is safe for opposing players, or photograph high-velocity objects, you need a way to measure that velocity. [td0g] enjoys the challenge of photographing bullets impacts, and has created an open-source ballistic chronograph to help achieve this.
[td0g]’s design makes use of two light gates spaced some distance apart, and the time that an object takes to travel between the two is measured and used to calculate velocity. Most commercial ballistic chronographs also work in this way. [td0g] created the light gates using pairs of infrared photodiodes and LEDs. When there is a sudden dip in the amount of light received by the photodiode, the Arduino control circuit knows that an object has passed between the photodiode and LEDs and triggers the timer. An LCD shield on the Arduino is used to control the software and display velocity. As you probably guessed, clock accuracy is very important for such time measurements, and [td0g] demonstrates a simple technique using a smartphone metronome app to manually calibrate the clock to acceptable accuracy for his purposes. Continue reading “Measure The Speed Of A Speeding Bullet”→
Summer is here (at least in the Northern Hemisphere) and World’s Greatest Uncle [Mark Rober] is at it again with his nieces and nephews. This time he’s all about skipping stones, that shoreline pastime that kids sometimes find frustrating and adults find humiliating when trying to demonstrate the technique.
But what exactly is the proper technique? [Mark] didn’t know, so he built a robot to find out. Yes, we know it’s not a robot – it’s just a commercial clay pigeon launcher with a few modifications — but work with us here. His idea is to build a rig that can eliminate as many variables as possible when a human tries to skip a stone, and work back one variable at a time to find the perfect set of factors. The prototype in the video below did a respectable job skipping stones, but it was nowhere near optimal. [Mark] then engaged the kids on a careful exploration of the mechanics of rock skipping using the rig, eventually going so far as to eliminate variability in the rocks by making clay pigeons of his own. The results are fantastic; at a 20° approach angle and a 20° tilt of the rock relative to the water, those artificial stones just seem to go on forever. Even skipping natural stones was much improved by what they learned, which is completely counter to the age-old advice to release as low and as parallel to the water as possible.