Details on Banksy's self-shredding painting setup are sparse at best, but let's take a stab at the design process.
As some of you may already be aware, a painting done by Banksy, a well-known (not to mention still anonymous despite being in the public eye for several decades now) British street artist, activist, and director, partially shredded itself immediately after receiving a winning bid for more than $1.3M U.S. dollars (1 million Pound sterling) at Sotheby's auction on Friday night, October 5.
So how did Banksy pull off this widespread attention-getting stunt, squeezing all the necessary electrical and mechanical components inside the frame ... and how can you replicate (if not one-up) it on your own? Details on Banksy's setup are sparse at best, but in combination with my own engineering chops, I'm going to take a stab at answering these questions. And of course, please sound off with your own thoughts in the comments!
The shredding mechanism
Let's start with the most noticeable aspect of the design, the hardware that sliced (a carefully chosen word ... read on for the details) up the painting. I'm guessing that the first thing you think of here would be a paper shredder. For mental calibration's sake, therefore, I'll begin with a few photos from the upper portion of my InfoGuard NX80P, which contains the motor and rollers and fits into a 2.5-gallon trash can-reminiscent plastic receptacle below it:
The upside of this approach is multi-fold: for one thing, the InfoGuard NX80P will (supposedly, at least) shred up to eight sheets of paper at once, along with making short work of even more resilient plastic credit cards, optical discs, and the like. Also, by virtue of its robust design (along with gravity), it will grab onto and pull material to be shredded into the mechanism.
But here's the mental calibration bit: it weighs more than six pounds, according to my kitchen scale, and has dimensions of approximately 13" long by 7" wide by 4" deep. Not exactly easily stowed within a painting frame, eh? Not to mention the fact that it is AC-powered (again, not exactly nondescript). DC-fueled paper shredders do exist; check out, for example, dual-AA-powered Shreddie or any of its similar looking clone-competitors on Amazon. But Shreddie only handles paper up to 4.5" in width and, to put it kindly, online reviews of reliability and battery life for such products are unkind.
And here's the thing: cross-cut shredding, which is intended for dealing with confidential information and thereby turns a piece of paper into something akin to a pile of confetti, is overkill here. In fact, if you've already watched Banksy's video at the beginning of this article, you may have noted that what was done to Balloon Girl was more akin to slicing the painting into a bunch of strips. Conceptually, at least, you can accomplish this more sensible goal by passing the paper through a row of fixed-position, non-moving blades; a much more compact, not to mention battery-friendly, approach. And if you watched the video, you've also already noted that this is exactly what he did, using a row of 38 cutting elements reminiscent of the X-Acto knife blades I used to create my plastic and wood scale-model masterpieces as a wee lad.
There's only one potential hiccup here. By pushing paper into a row of blades below it, versus pulling it into rollers as with a shredder, you've incurred a non-zero probability that the all-important initial pierces of the paper won't actually succeed (especially with thicker-than-average-paper painting canvas material), leading to only crumpled paper jams ahead of the slicing mechanism. Blades can only be so sharp, after all, and atmospheric moisture-induced rust and other long-term environmental effects also don't work on your favor; recall that Banksy's video claims that he assembled the frame "a few years" prior to the auction.
Your likelihood of success is far greater, however, if the initial pierces are already in place and the paper has already been partially fed through the slicer. And, I'd wager, that's exactly what happened in this case. Look closely at the post-sliced state, and you'll see a distinct "bend" in the canvas partway through. Note, too, that the bottom part of the sliced portion of the painting wasn't visible when it was in the frame, only after the slicing was complete. That's because, I'm postulating, it was pre-sliced and loaded up ready to continue the procedure, once the stunt began.
The battery pack
Even if the cutting mechanism is passive, there's still plenty of other electricity-fueled equipment still necessary in the design; the motor (and connected roller) that pushes the painting down and onto the blades, for example, along with the remote control circuitry that activates the assemblage (both of which I'll discuss next). With an AC power cord an obvious non-starter, that leaves an integrated battery pack as the only feasible alternative (wireless long-distance power transmission and solar cells aren't credible options, sorry).
The top two (IMHO, at least) battery technology alternative candidates are alkaline and lithium-metal. Alkaline is less expensive on a per-cell basis, but in a one-off design like this, that's not a particularly critical selection criterion. More important here is lithium's significantly higher comparative stored charge energy for a given-sized cell, which is beneficial in several key respects; higher instantaneous current output (key when overcoming initial static coefficients of friction in the motor, when initially cutting through the painting's canvas, etc.) and longer shelf life. And, if Banksy had decided to tear apart a smartphone, tablet, or laptop in order to obtain the battery pack, for example, he'd also have benefitted from lithium polymer's form factor-moldable characteristics; its rechargeability is, conversely, a don't-care in this particular application.
Speaking of shelf life (I'll remind you again that Banksy's video claims that he assembled the frame "a few years" prior to the auction), I'm going to take this opportunity to toss out an admittedly controversial theory (full disclosure: my wife thinks I'm off-base). Watch that video again. Notice how the Sotheby's auctioneer and other staff remain unnaturally calm when the ear-piercing beeping noise (easily accomplished via a diminutive and inexpensive piezo transducer, located either at the remote control transmitter or, more likely, within the picture frame) kicks off and the sliced-up painting begins slipping out the bottom of the frame? Doesn't that seem odd to you, too? Perhaps it's just a reflection of the perpetually serene British mentality, but I'm skeptical.
My theory, public disavowals from Sotheby's aside, is that the auction firm was in on the stunt in advance. Specifically, someone on staff flipped on a power switch on the back of the picture frame right before this particular auction began. Think about it; Banksy got tons of publicity out of this, as did Sotheby's. And unsurprisingly, the winning bidder decided to keep the sliced-up painting, rightly guessing that it was worth much more now than before. Everyone wins!
Even without the motor(s) operating, and regardless of the battery technology chosen, motor leakage current would still slowly sap the connected battery pack, not to mention the steady electron drain induced by the regularly "sniffing" remote control receiver circuitry. The only reliable way to ensure you had a fully charged battery pack available when you needed it would be to cut power completely until you needed it. In this respect, Sotheby's would be a convenient co-conspirator.
[Continue reading on EDN US: Remote control and motor]
—Brian Dipert is Editor-in-Chief of the Embedded Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company's online newsletter.