TopChicksDigEngineeringGuys

Sunday, May 12, 2013

Spring Gun CAD

Springs have gotten strong. There are springs out there that can store 500J of energy and deliver that energy very quickly and efficiently. In fact, there are guns that already use springs and shoot with 22lr speed. Also, airsoft rifles often use springs; while their power is low they can be fully automatic because they are not considered firearms. If you've ever fire a full-auto airsoft, it's extremely fun.

After doing some simulation of springs and air pistons using pypy and multi-processing I found that actually the most power transmission happens when the air piston and the projectile have essentially the same diameter. This implied that probably the best design would be a projectile directly driven from the spring.

As with any simulation result we should ask 'does this make sense?' Well, if the spring pushed the projectile itself and was massless and had no internal heating then all the energy would be put into the projectile. Of course there will be internal heating, but that's also a problem with an air piston. And the spring is not massless, but it seems pretty light compared to the projectiles. So it's not a crazy conclusion to think the air part is not necessary. Assuming I could get the weight of the spring and any associated hardware to a minimum I should get good efficiency. If that turns out not to be the case in real-world testing I'll try testing air pistons with a different design.

In this design, we're using a grabber (red) and pulling back a plunger (blue) in the barrel (green). There will be a spring inside the barrel that pushes back. Once the puller gets to a certain point it releases. In this way, we can turn osculating linear movement to continuously load and fire the gun.

Saturday, May 11, 2013

Franken Scooter

I crashed the electric scooter last winter. In the process, the fragile lithium-ion battery flew out and tumbled in the street. I put it back in. Since then it has run for only a few minutes at a time before some part of the scooter turns itself off automatically. Turning off and back on again doesn't seem to fix this. I have only been able to get it back on by pulling and re-adding the fuse that protects the battery from overloading.

I didn't want to spend the $500 for a similar li-ion so I scavenged some sealed lead-acid batteries instead. I needed to run all four of them in series to get the 48V the engine was expecting but I didn't want to charge them in series. Instead I installed a switch network that allowed me to charge them in parallel with a single 12V charger. Since the switches are in the circuit during drive and thus pulling a lot of amps, I doubled them up for each connection.

I used holes already cut in the sides of the scooter to run the wiring and a basic home-depot switch carrier. Of course, not all the switches I managed to find were the same or even fit in the switch carrier. A lot of hot glue keeps them together.

In practice, it doesn't have the acceleration of the lithium and it almost always shows 'battery low' on the console. But it was able to run up a 20deg hill for many blocks on end and still hits at least 20mph.

All in all, I consider it a win. :)

Saturday, March 2, 2013

Robot Targets

Fail Train Automation Robotic Target Stand Set (RTS) - V1.0

Summary: The RTS is designed to teach reactive shooting, vigilance in searching for targets, and rapid target assessment for anyone involved in shooting sports or professions. The shoot-though design of the target stands makes the compatible with anything from airsoft to AP 50 BMG.

Detailed Description: Each of the four RTS targets can rotate to show either an enemy side, a friendly side, or the edge of the target (indicating no-shoot). The targets are shifted to the right so it is obvious whether the shot went to the enemy or the friendly. All four targets are driven from a single control box, connected by ethernet cables. The control box has four algorithms for the targets which are selected by a knob on the top of the box. A remote control is then used to turn the targets off or on. The control box has a self-contained battery which can last for several days of continuous use.

1 vs. 1 Algorithm (click for video) : The 1:1 algorithm is the most basic, showing a single friendly and a single enemy. After several rounds, the targets face you so you can score your work.
3 vs. 1 Algorithm (click for video) : The 3:1 algorithm is the more difficult, showing a single friendly and a three enemy. This is a challenging ratio and timing on purpose; it will help you find and improve the limit of your speed and duration.
Chaos Algorithm (click for video) : The chaos algorithm shows an unknown number of friendly and enemy with an unknown amount of time between. The lack of predictability of this algorithm will help you break any patterns you might be learning from the 1:1 or 3:1 algorithms.
Vigilance Algorithm : This algorithm waits an unspecified but long amount of time between events and launches attacks of unspecified strength. Only shooters who are most committed and patient will feel confident in their shooting against this algorithm.

FAQ:

Q: How do I add/modify algorithms?
A: The control box is powered by an Arduino micro-controller. Opening the cover and removing the black side panels will expose the Arduino's USB port. You can download and modify the existing programming by going to topchicksdigengineeringguys.blogger.com/codeforthetargetstands

Q: The remote control seem to only go about 50 feet, how can I shoot at longer ranges?
A: RTS can be connected to an external switch in addition to the internal remote control. Your RTS came with an additional cable connector that plugs into the front of the control box, right next to the batter charger. Closing the circuit on the two wires of that cable connector will turn the machine on in exactly the same way as the remote control does.

Q: Workmanship on the RTS seems kind of poor, why is that?
A: I've designed every piece possible of the RTS to not need exact tolerances. For example, the welds might look bad, but the targets are mostly made out of 1/8" thick, 1/2" wide mild steel and even a messy weld on this is very strong. In the end, this design means I can make them on the deck of my apartment on weekends instead of having to spin up and manage some factory overseas like everyone else does.

Q: What if I shoot the RTS motors or controller box? They don't seem to be armored.
A: If shot, they will be destroyed. Don't shoot them. I recommend practicing such that you never let bullets go significantly off-target. For example, shoot only within a small area on a given target and treat shots outside that area as 'misses' even if they hit the paper somewhere else. If you find yourself hitting outside of that area regularly, get closer. If you're already at point-blank range and still at risk of hitting the robotics perhaps it's time to practice on static targets for a while instead.

Q: Can I plug the targets or the controller box into my computer? They seem to use ethernet cables.
A: Do not ever plug the ethernet/network-cables from the RTS into a computer; they will likely destroy the computer. RTS does not actually follow the ethernet standards or protocols. Instead, ethernet cables were chosen here because they were cheap and readily available in many lengths. This means that owners like you can buy your own longer or shorter cables depending on your needs.

Q: What's up with the see-through pipe section that sticks out the side of each target stand?
A: This is the position sensor. Notice that when the target turns it moves the slider. That slider tells the controller where the target is. Though less durable than a shaft-encoder, this system does not require calibration or limit switches and saved you about $300 off the price of the RTS.

Q: Why can't I select the target algorithm by remote control?
A: Multi-select remote controls are more expensive. You can hack this by removing the cover and re-wiring the selector switch yourself.

Q: What kind of deal is this; you keep talking about things you did to make it cheaper but it's not exactly pocket change?
A: You should have seen what it would have cost with all the extras that 90% of people wouldn't use. Robots are just expensive that way. Unless they're made by children in China, which this one wasn't.