Trebucheting Tennis Balls at 124 MPH

A trebuchet is one of the older machines of war. It’s basically a sling on a frame, with a weight that you can lift up high and which pulls the sling arm over on release. Making one opens up the doors to backyard mayhem, but optimizing one opens up the wonders of physics.

[Tom Stanton] covers just about everything you need to know about trebuchet building in his four-part video series. Indeed, he sums it up in video two: you’ve got some potential energy in the weight, and you want to transfer as much of that as possible to the ball. This implies that the optimal path for the weight would be straight down, but then there’s the axle in the way.  The rest, as they say, is mechanical engineering.

Video three was the most interesting for us. [Tom] already had some strange arm design that intends to get the weight partially around the axle, but he’s still getting low efficiencies, so he builds a trebuchet on wheels — the classic solution. Along the way, he takes a ton of measurements with Physlets Tracker, which does video analysis to extract physical measurements. That tip alone is worth the price of admission, but when the ball tops out at 124 mph, you gotta cheer.

In video four, [Tom] plays around with the weight of the projectile and discovers that he’s putting spin on his tennis ball, making it curve in flight. Who knew?

Anyway, all four videos are embedded below. You can probably skip video one if you already know what a trebuchet is, or aren’t interested in [Tom] learning that paying extra money for a good CNC mill bit is worth it. Video two and three are must-watch trebucheting.

We’re a sad to report that we couldn’t find any good trebuchet links on Hackaday to dish up. You’re going to have to settle for a decade-old catapult post or this sweet beer-pong-playing robotic arm. You can help. Submit your trebuchet tips.

Thanks [DC] for this one!

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A Robot Arm for Virtual Beer Pong

Leave it to engineering students to redefine partying. [Hyun], [Justin], and [Daniel] have done exactly that for their final project by building a virtually-controlled robotic arm that plays beer pong.

There are two main parts to this build: a sleeve worn by the user, and the robotic arm itself. The sleeve has IMUs at the elbow and wrist and a PIC32 that calculates their respective angles. The sleeve sends angle data to a second PIC32 where it is translated it into PWM signals and sent to the arm.

There’s a pressure sensor wired sleeve-side that’s worn between forefinger and thumb and functions as a release mechanism. You don’t actually have to fling your forearm forward to get the robot to throw, but you can if you want to. The arm itself is built from three micro servos and mounted for stability. The spoon was a compromise. They tried for a while to mimic fingers, but didn’t have enough time to implement grasping and releasing on top of everything else.

Initially, the team wanted wireless communication between the sleeve and the arm. They got it to work with a pair of XBees, but found that RF was only good for short periods of use. Communication is much smoother over UART, which you can see in the video below.

You don’t have to have a machine shop or even a 3-D printer to build a robot arm. Here’s another bot made from scrap wood whose sole purpose is to dunk tea bags.

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Hackaday Links: January 24, 2016

The RepRap wiki was spammed this week. Everything is fine now, but I feel I should call attention to the fact that the RepRap wiki needs some people to contribute, organize, and maintain everything. The wikis for obscure anime shows are better than the RepRap wiki, so if you’re looking to contribute to an important open source project, there ‘ya go.

The 200cc, 5.5HP, 4-stroke OHV Honda GX200 engine is found in a whole lot of tools, and is a fantastic power plant to build a go-kart around. It also costs about $350. There are clones of this engine available direct from China for about $100. Here’s how you add a turbo to one of these clone engines.

Freescale makes some pretty cool sensors and [Juan Ignacio Cerrudo] figured they needed breakout boards. He has some boards for a low-power three-axis accelerometer, an accelerometer and magnetometer, and a pressure sensor.

The Tektronix TDS744A is an older but still extremely capable 500MHz, 2Gsps, 4-channel scope. You can upgrade it to the 1GHz TDS784A by desoldering a few resistors. Very cool if you’re looking for a cheap-ish 1GHz scope.

[TheBackyardScientist] hung out with some cub scouts a few weekends ago and launched a high altitude balloon over Florida. The payload included a game camera, APRS tracker, GoPro, and a few other bits and bobs. The balloon reached 106,000 feet and landed only a few miles from Cape Canaveral.

Big RC planes – UAVs especially – are a pain to launch. Flying wings above a certain size are just dangerous to launch by hand, and landing gear is heavy and for the most part unnecessary. What’s the next best solution? A trebuchet, of course. It mounts on a car and is able to give a UAV a little bit of altitude and some speed. A pretty good idea that could be easily implemented with some load-bearing PVC pipe.

Everybody likes the Game of Life, so here’s one built with a 6502. It’s built around a Western Design Center 65c816 board we’ve seen before, nine MAX7219 LED controllers mapped to the VIA, and nine 8×8 LED matrix displays. Here’s a video of it in action.

About a month ago, a search of AliExpress turned up Apple’s A8 CPU. I bought one. Here’s what I got. It’s a stupidly small pitch BGA, and I don’t have a datasheet. What am I going to do with it? Make a non-functioning board with a few ports, resistors, no traces, and the A8 chip planted square in the middle.