I work closely with a non-profit group in Des Moines, called the Justice Corps of Iowa (http://www.justicecorpsofiowa.com/) that does superhero appearances for kids events and donates the proceeds to charity. They're evolving their costumes and I've been able to help out by 3D Printing quite a few parts (Hawkeye's Quiver & Arrow heads, Wolverine's claws, Spiderman's webshooters, soles, and face shell, and more). I mentioned to the guy who plays Thor in the group that he should have a hammer that only he can pick up. He agreed, so now it's up to me to make it happen!
Just a quick post about the recent progress I've been making on Mjolnir. I've been learning a lot about tapping aluminum! I went through 3 drill bits and 4 taps, but I finally got all of the various brackets, standoffs, and mounting holes done. Additionally, I created a custom designed and 3D printed bracket to hold my batteries and RFID securely in place, while also allowing for wires to pass through it where needed. I'm now starting the final wiring. Pretty pleased with how this is looking (even though it will all be hidden when completed). Still quite a bit of work to do before it's done, but the end is in sight!
My biggest concern at this point is about whether the magnets will be as strong as I need them to be once everything is wired up. They were pretty weak when I breadboarded them, which is to be expected, but still makes me nervous. I'll find out soon!
Ok, it has been a while since I posted any updates on this project. That's because I was waiting for a crucial component to be completed by a third party. I finally got it and...it was wrong. So I got another one made and I'm moving forward with the build again! The part I was waiting for was the main chassis. This is made of welded aluminum because it's lightweight and strong. See below for pics.
The major challenge with this chassis design is that there is virtually zero room for the screws to go into the insides of the 2 magnet brackets! It took me a while (and a lot of overly complex designs) before I figured out the simple solution. I can screw in one set of inside screws while the other side is empty, and then I simply filed down the threads on the second one to form perfect pins! To hold those pins in place I used a bit of packing foam I had lying around (you can see it the background). Now it holds perfectly!
With that hurdle overcome, the next step was to do a quick test of the breadboarded version. This version uses a breadboard and small jumper wires, plus it's only pulling current from a single LiPo battery to run the whole system. Yet it still performed admirably!
Below is a picture of this less-than-ideal setup holding up the weight of the 1/4" steel plate. I'm not sure how much it actually weighs, but it's heavy!
Then, just to see how it fits, I put a rough version of the 3D printed enclosure around it. Definitely going to need to make some tweaks, but not bad overall!
The next step is getting the final layout completed and replacing some of the parts with final versions. One thing that needed to be replaced was the relays. The ones I was using with the breadboard were standard mechanical relays. This won't work because the illusion of Thor being magical will be shattered when people hear it CLICK as it turns off the magnets. DC-DC Solid State Relays are surprisingly hard to find, but I found a board with a couple of them on it at Sainsmart.com. Here's an approximation of the final layout. It turns out that these 2 LiPo battery packs are a perfect fit between the top of the aluminum chassis and the inside of the 3D Printed enclosure. Sometimes things just work out beautifully (but most times they don't).
I have started drilling and tapping the holes in the Aluminum, but I must be doing something wrong because I've only got 5 holes drilled so far and I've broken 4 drill bits. Additionally, when I've attempted to tap one of them I have only succeeded in breaking 2 different M3 tap bits. Once I get this figured out I'll mount the main electronics and start wiring them up for a test since I'm using slightly different parts now. I'm also going to design a bracket to hold the LiPo battery packs and RFID reader in place, then 3D Print it along with the rest of the final enclosure.
I slapped together the necessary components to make the RFID reader work with the MCU and toggle the relays, wrote the code, fired it up and...IT WORKS!
I've decided to add a third RFID tag that can be placed somewhere on Thor's costume so that it can be read by the hammer and re-magnetize without being too obvious. The other 2 RFID tags in his boots will demagnetize the hammer, but there wasn't a good way to turn the magnets back on. This additional tag takes care of that.
The hammer's handle (aluminum rod) and main chassis (aluminum plate) are being welded together now, so I can start kitting it as soon as I get that back.
I still have to move the electronics to something a bit more permanent than a breadboard, but otherwise this project is progressing nicely. My primary concern at this point is the 3D Printing. The hammer is massive and has intricate detail. I'll have to spend some time figuring out how to make that work.
The inspiration for this build comes from "Sufficiently Advanced" who did a YouTube video showing his build for Thor's Hammer:However, I'd like to change it up a bit. For one thing, it's a bit too heavy. You can see how he tries to handle it and has trouble holding it upright. If Thor is going to carry this thing around for hours at a time during an appearance, it can't be that heavy.
Second, the release is too slow and too obvious. There's a big square window in the side of the handle. He has to put his finger on it in a very specific place, then wait for it to recognize his fingerprint before it finally releases. I've got experience using those readers and I can also add that it will often not recognize the fingerprint. Both things ruin the illusion.
I propose using a pair of security door magnets for the build, each rated in excess of 600 pounds holding force. They weigh less than 2 pounds each, and are already intended to be mounted to something (typically a door frame). They also run on 12v and fairly low current draw (~400mA) Also, rather than 4 sealed lead-acid batteries, I'd like to use Lithium-Ion batteries. They're more expensive, but they're much lighter for the same mAh rating. I'll also use an aluminum frame to hold it all together.
Lastly, my plan for the release system is to use a small arduino microcontroller such as an arduino pro micro which is reading from a compact RFID reader such as the ID20LA. This way, rather than it being tied to a specific fingerprint, it's tied to an RFID tag. The RFID tag can be in Thor's boots such that when he steps up to pick up the hammer, his foot is naturally right next to the side of the hammer where the reader is. In my experience, this is much faster and more reliable than fingerprint readers are. My only concern is about the interference from the magnetic coils. But we'll see if that poses an issue or not.