How To Build a Cardboard Quadcopter

Keeping with our recent all-multirotor all-the-time theme, it’s time for another how-to post! Plans are afoot, and scheming has been schemed. The flying robot skeletons have been piling up in a corner of the workshop, and after several revisions we’ve narrowed down the design to something worth sharing.

"Eventually, we'll get it right."

Maybe you want to build your own? Maybe you want to take this design and mod it for agility, weight, or style. Awesome. First, here’s the base pattern:

PaperQuadRev5BPattern.pdf
(updated on 04/15/2012)

Getting Started

First off, you’ll need some tools:

  • CNC laser cutter. In theory, you could cut these parts out with an x-acto knife, which is madness. You’ll want to borrow a laser cutter. Honestly, you should just buy one. They’re the absolute best thing in the world, and the prices are dropping very fast. Check out Hurricane Laser, for example. Or TechShop.
  • Scissors, for cutting tape.
  • Soldering iron, and solder.
  • A can of Super77 spray glue.
  • 60degree hole chamfer. Handheld is fine.

You’ll need the following build materials. For my examples, I use cardboard sheeting from ULINE.

  • Several sheets of 4mm cardboard. The thickness matters, if you change the thickness, make sure you update the tab cutouts to match. They’re 3x the thickness, or 12mm.
  • Brown paper packing tape for sealing the edges. The clear stuff doesn’t stick very well. You can also use fiber reinforced tape.
  • 4×4″x1/8″ black ABS plastic sheet. You can also use heavy card stock, sheet metal, acrylic, or aluminum bar stock.
  • No. 127 Black ESD or similar. 7”x1/8″
  • One 14oz ZipLock plastic container, or other lightweight 5″ diameter bowl.
  • Double sided copper clad PCB board, you’ll need about a 0.5×0.5″ square piece.
  • 4 paperclips.

For electronic components, you’ll need the following:

  • 4ea 22mm brushless outrunner motors. I’ve used both Cobra 1300kV and DiyDrones 850kV motors.
  • 4ea matching prop adapters for your motors and propellers.
  • 4ea regular propellers. GWS 8×3, GemFan 10×45, etc. Yes, 4ea. You’ll want extras, lots of extras. You’ll break a lot of props at first.
  • 4ea reverse propellers.
  • 4ea ESC controllers for your motors, with an on board BEC. I use 20A NextLevel controllers.
  • 20mm heatshrink, for the covering the copper clad power board. Electrical tape works too.
  • 6mm heatshink for covering connectors and wires.
  • 0.1″ spacing jumper wires, female socket. For the battery power sense line.
  • Controller board. I use the Quadrino Zoom.
  • Cable assembly for Quadrino Zoom.
  • Spread spectrum 2.4ghz transmitter and receiver. 6 channel or better. Spektrum DX6i, etc. There are 4 control channels, and 2 mode channels. You’ll need another two channels if you want to add head tracking later.
  • 2-4ea, 2000-1300mAh 3S LiPo battery. Trust me, you’ll want more than one. Your motors must match the battery voltage. I use Turnigy batteries.
  • Lipo battery charger.
  • Battery connector plug and wires. I use XT60 plugs.
  • Sparkfun Blutooth module, if you want wireless telemetry. Totally optional.
  • Nylon mesh wire sleeve. I use this to protect the motor leads from prop strikes. Also optional.

(GoogleDocs spreadsheet.)

It’s a lot of parts and pieces, it’s true. Depending on where you source things from, and how fast your shipping times are, it can take up to a month for all the parts and pieces to arrive. HobbyKing has notoriously long wait times, for example. If you care about customer service and speed, order domestic. I recommend Innov8tive Designs.

Got all your parts and pieces? Great! Let’s get started…

(**Hey, what’s with those holes in the picture? How come they’re not in the plans? Turns out the tend to cause frame failure for really hard landings, so I took ’em out. Amazingly, it still flew after one of the arms bent…)

24 Mar 2012, 1:45am
robotics:
by

Comments Off on Cardboard Tricopter Build Plans

Cardboard Tricopter Build Plans

Joachim here,

This tricopter frame design is decently ridged, it’s not extremely crash resistant, but it does fine with a few hard landings. This is the first flying revision, so there is plenty of room for design improvements. Improve and share! Available under a creative commons ShareAlike-NonComm-Attrib license.

Source files here: 24cmTrV2.zip

You’ll also need a few build materials:

  • Lasers, or a lot of time and a sharp knife.
  • Glue: less is more!
  • Itoya- O’Glue, school glue, etc.. some kind of water based glue
  • Hot melt glue – hot glue gun
  • CA glue – super glue
  • 3.2mm rod, carbon fiber, fiberglass, wood, or a glue-able plastic

Arms are each made from 4 laminations of 3mm cardboard. To better resist bending, cut the outer two arm laminations parallel to the corrugation, and the inner two perpendicular.

Cut 5 base plates and 1 top plate. Three of the base plates will go on the bottom part of the frame, and the other two will combine with the top plate to make the upper part. For maximum durability, rotate each plate lamination 120 degrees so that the corrugations criss-cross when you glue them. You may need to adjust the motor mounts and servo profile in the tail arm to match your parts.

Total cut list:

  • 4 arms parallel to corrugation
  • 4 arms perpendicular to corrugations
  • 2 tail arms parallel to corrugation
  • 2 tail arms perpendicular to corrugations
  • 5 base plates
  • 1 top plate
  • 1 set servo retainers – 1.4mm cardstock
  • 2 motor mounts – 1.4mm cardstock
  • 1 tail assembly top and bottom – 1.4mm cardstock

For the laminations use a moderate amount of water based glue – Itoya-O’Glue is nice. You could use spray adhesive too, but it’s $18/can.

Press:
Get two sheets of something really flat, i.e. some scrap acrylic sheet you have laying around. Sandwich your newly glued arms and plates between two of these flat sheets and put a good number of books and heavy things on top (~15kg). This will press the glue into any voids and keep your structural members straight as they dry. If you used the water-based glue, leave the arms to dry for about 24hrs, no peeking!

Frame:
Now that you’ve waited 24hrs for your parts to dry, retrieve them! As long as you were somewhat careful when you aligned the laminations, you should be able dry assemble your tricopter frame plates and arms. I start gluing the base plate and arms together with about 3mm gap between them- I squirt some hot glue into that gap and press the parts together. Work fast, hot glue works a lot better when it is hot!

Motor mounts:
Adjust the mounts to fit your motors. Mount your motors to the motor mounts, glue the mounts to the arms with hot melt glue.

Tail yaw assembly, the hard part:
Cut the rod to about 4.5cm or so and slide on your hinge parts, rotating them 180 degrees from each other. The hinge elements should rotate relatively easily but not be loose, or really stiff. Line up the rod flush with the first top tail plate hinge element. Line up your hinge elements with the tail hinge plate top and bottom. Make sure all the hinge elements are straight. Use CA to carefully glue hinge elements to the top and bottom, don’t get glue on the rod! Double check that everything is lined up and let the glue fully set. Now that the glue is set, double check that the rod is aligned with the first top plate hinge element. Carefully glue *only* the top hinge elements to the rod. Trim your servo horn so that its splined hub can fit on the front top hinge element co-axial with the rod, it might help if you sand away any surface features from the servo horn. Double check your alignment and glue the servo horn to the front plate. Allow the glue to set.

Make sure that the servo is centered by connecting it to a powered neutral RX channel.

Carefully partially test fit the hinge on the servo, don’t push it on all the way, I doubt it would survive removal from a snug shaft. Make sure that your hinge is co-axial with the servo splined shaft. You might need to add a shim to the bottom of the hinge or trim the arm a bit to make sure the hinge and servo are co-axial. Carefully fit the hinge hub onto the shaft. I use hot melt glue to glue hinge bottom to the tail arm.

Attach your electronics and go flying!

Additional Resources
Some tricopter fundamentals by David Windestål – http://www.rcexplorer.se

  • http://rcexplorer.se/Educational/kkguide/kkguide/tri.html
  • http://www.rcexplorer.se/projects/tricopterv25/tricopterv25.html
  • http://www.rcexplorer.se/Educational/HKKK/HKKK.html

Joachimp’s parts:

Thing

weight(g)

quan

total weight(g)

sub price

price

Notes

Links
Orange 6 chan RX

9.9

1

9.9

5.99

5.99

de-case, and use heat shrink to save weight

HobbyKing
servo metalGear

14.5

1

14.5

4.67

4.67

HobbyKing
servo Plastic

9.9

0

0

plastic might work.

kk-board

14.6

1

14.6

14.99

14.99

You will also need a programmer to reflash this with the tricopter firmware. Most any iscp 6 pin will do.

HobbyKing
esc

9.8

3

29.4

9.47

28.41

Maybe buy an extra

HobbyKing
motor + hardware

20

3

60

10

30

Get at least one extra

HobbyKing
5030Prop (ccw)

1.5

6

9

1

6

Get a few extra, especially if you don't have experence flying dynamically unstable rotor craft, ie, collective pitch helicopters

HobbyKing
male-male JR

1.8

4

7.2

1

4

HobbyKing
tur 1300

123.2

1

123.2

10

10

Order from domestic HK warehouse, need at least 25C rate, 1000mAh – 1600mAh

HobbyKing
EstAirframe3ply

50

1

0

Actual4plyFrame

60

1

60

1

1

standard corrugated cardboard, nominal thickness 3mm, density 166.7 kg/m^3 (50mg/cm^2 – one 3mm sheet)

Frame assembly structural glue

5

1

5

0

Low-mid temp hot melt glue

Composit layer glue

0

Itoya- O'Glue, might switch to contact adhesive, $18

CA – Glue

0

Servo retainer/ tail tilt hinge assembly glue

0

0

0

0

HK EMS shipping

1

30

30

Total est Weight

332.8

Total Price($)

135.06

Actual Weight

357

Cardboard TriCopter

Hot damn!

My friend Joachim P., taking some inspiration from the cardboard quad, decided to build a paper tri-copter. Using a pretty ingenious tail tilt mount, his Tri-copter comes in at 357g fully loaded. It also costs around 130$ (sans transmitter.)

Of course, he’s using a gyro only board (the KKmicrocopter controller), so one could probably get a lot more stability with 6DOF or 9DOF board, but I’m impressed none the less.

UPDATE: Build plans are are in the next post. Thank’s Joachim!

7 Mar 2012, 6:38am
robotics Uncategorized:
by

Comments Off on Cardboard Quadcopter

Cardboard Quadcopter

I finished another frame this afternoon. Now that I’ve made a few of these, the turn around time is getting shorter.

A new set of shoes.

This time with rubber-band shock landing gear, and card-stock motor mounts. They work better than the old aluminum ones from the previous frame. A friend with a bit more flying experience than I came over to help me grab this video.

The paper shocks aren’t holding up very well, but I think I can fix that.

Cardboard. It’s awesome.

Update: We took it outside and got some test video, this little guy really performs!

6 Mar 2012, 8:16pm
robotics Uncategorized:
by

Comments Off on Paper Robots, Part 4

Paper Robots, Part 4

It took a few round, and I went down a few dead ends, but today I finally managed to make a flying paper robot. Originally, I started out with a flying sphere design copied wholesale from the JDM Flying Sphere. Eventually I figured out that I didn’t know the first damn thing about making a flying robot and that it would probably make a bit of sense to try and build one that had been successfully flown by more than one other person working for the Japanese military.

So, I decided to build a quadcopter. A few people have built those, and it looked like a simpler problem.

The hardest part about getting started with quadcopters is choosing which of the half-dozen or so control platforms best describes you as a quadcopter enthusiast. I settled on the ArduPilot Mega v2. Then I settled on the MultiWii based Quadrino. Both are perfectly capable of loitering around like a lazy robot, but the Quadrino has a certain simplicity and easy-of-use that I found attractive, so I’ve been using that one the most.

Too big to fail?

As I had a few 750W motors lying around from the flying sphere experiments I decided to build my first quad out of those. This created a few unexpected problems, and a few very dangerous close calls. With 3kW of motor connected to the frame it was very much a robot, in the kill all humans sense of the word. While I did manage to make it hover, I eventually learned my lesson and decided to go with something a little more petite.

It seems that the point of all these paper robot exercises has been to try and find interesting design patterns that one can use to build cheap, reproducible robots with. With that in mind, the hard part about building a paper quadcopter frame was going to be getting it rigid enough to fly with some semblance of control.

Folded triangle failure.

I started with folded up triangle beams, as I had been using these to make legs for the walking robots earlier. Unfortunately, they proved to be difficult to anchor in a cross configuration. I could get one beam rigid, but the other would be split in two. The frame was floppy, and had a very wide profile against the prop downwash, and was a fantastic failure.

After that, I abandon triangles. I decided to try and build laminated cardboard beams in the hopes that they would be rigid enough to make a frame. For some reason I was still hung up on folding, and went with a triangular folded stiffener. It too was a dismal failure. Eventually it became clear, that I was going to have to find some way to make the arms cross each other at the center, and to find some other way of attaching the stiffeners.

In retrospect I probably should have started with slotted construction from the beginning, but sometimes you just have to do things the wrong way first. Maybe it makes for a better story. Maybe it makes figuring out the right way that much more enjoyable once you pull your head out of the… bushes.

Closer...

Hot off the laser, I slotted the new frame together. Already it was considerably stiffer than any of my previous attempts. By gluing the layers together, I could make the frame stiffer still. The final piece in the puzzle was when I remembered a bit about applying polyurethane to paper my friend Pete told me about when I first showed him my walking robots. After applying kraft tape to strengthen the edges of the cardboard, I sprayed the frame down 3-4 times with an oil based polyurethane wood finish. At 52 grams, the final frame was incredibly light, water proof, and very, very rigid. Surprisingly so!

Stronger, faster, lighter.

Impatient for my HobbyKing order to arrive, I purchased a set of 2213/34 Cobra motors from Innov8tive Designs, (highly recommended. Thanks again for the advice!), and set about integrating the electronics. This involved finding a way to attach 4 motor controllers, a radio, battery, and the control board onto the paper frame. It needed to be done in a way that wouldn’t compromise frame integrity, be easily serviceable, and survive multiple crash landings from operator and software error.

By notching the stiffener plates I found a novel way to use large black rubber bands to secure various elements to the frame. They have proven to be very versatile in holding ESCs, cables, and even heavy 4S battery packs to the underside of the frame. The 1.75 cup ZipLock plastic container that acts as the crash dome is even secured with rubber bands.

(Photo by Audrey Penven)

You got to get up to get down.

Without the battery, this little guy weighs 600 grams and pushes approximately 2400. With the hulking 4S lipo pack installed it comes in just under 900 grams and takes off at about 1/3 throttle. As the motors are small enough, I can fly indoors. The motor mounts where the only part where I resorted to using sheet metal, but I’m fairly confident they can be replaced with a heavy card stock. I can probably knock another 50-100 grams off the frame by replacing the cable harness as well.

…but that’s for another day. Now, I must go outside and fly.

Update:
If you’d like to make your own, here are the flat patterns. FlyingPaper.pdf

1 Mar 2012, 1:53am
robotics Uncategorized:
by

Comments Off on Dorkbot SF

Dorkbot SF

I’ll be giving a short talk on my paper robots tonight at dorkbotsf. Here are a few of the flat patterns should you wish to try and make some paper hexapods of your own. Have fun!

9 Jan 2012, 6:43am
robotics software
by

Comments Off on SpaceExplorer & Processing

SpaceExplorer & Processing

This might be useful to someone.

I spent a few moments today trying to cobble together an interface for driving multi-DOF robots around. It’s been a long time since I’ve done anything with inverse kinematics, but it seems like a good place to start. First things first though. Having a controller than can come close to expressing the range of motion in a hexapod is going to be useful for moving the IK chain targets around.

A while ago I bought a 3DConnexion SpaceExplorer and spent a few hours trying to learn how to make it useful. It’s pretty hard to use at first. I recommend downloading Google Earth and spending about five hours exploring the virtual planet on a fast internet connection. Highly worth it, the time will fly by.

It occurred to me that a space-ball is the an excellent controller for smoothly directing a symmetrical hexapod. Tilts, directional gait mixing, and rotational turns are all natural gestures. Most of the heavy lifting for this code is handled by the ProControll library. It allows you access to multi-axis controllers. There’s a bug in it that someone named “bud” patched that allows it to see the SpaceExplore and other controllers like it.

Anyway, someone else might find this useful for their own processing sketchs. The working parts and pieces where collected together from this thread on the processing forum. It’s been tested out on MacOSX 10.7.2, and likely works on other platforms as well.

Files here: SpaceExplorer.zip

4 Jan 2012, 5:47pm
robotics
by

10 comments

Paper Robots, Part 3

One of the often overlooked, and rather important steps in discovering a novel solution to a problem, is getting it wrong the first half dozen times. It helps inform the design process later, and allows one to not worry so damn much about the current state of affairs. Other things that help that process along are not worrying about it costing too much, or being too emotionally invested in the outcome because you’ve already spent so much money that you can’t stop now. This is something that organizations and individuals seem to struggle with.

To that end, I’ve been trying to figure out ways to make experimenting with robots cost less. Optimizing for multiple iterations, instead of getting it right the first time, as it where.

As I’ve written about in the past, cost can be measured in many ways. For this exercise I’m primarily interested in cost of materials, cost of manufacturing, and assembly time. Design time isn’t particularly optimized here, but as we build on previous iterations, hopefully we can stay ahead of complexity.

"Manufacturing time is a factor, so please pay attention."

On the cold hard cash front, I think I’m doing pretty well. This little fellow consists of: $1.00 in cardboard, $54.60 in servos, and $29.95 in servo controllers. Plus some odds and ends like a power supply and a laptop. Which, if you’re reading this, you probably already posses. If your budget is especially tight, you could probably even “borrow” the necessary gallon of rubber cement, 24 paper clips, and 6 yards of packing tape from the office. Tell them its “for robots”. Anyway, cost is important to me. Mostly because I’m currently unemployed, and while I enjoy spending money like a drunken sailor, it does tend to cut into my savings more than I like.

So, how well does a cardboard 12-DOF robot work? Pretty well, I’m happy to say. I finished setting it up on the kitchen table last night and some friends and I made some really basic motion tests. The main area for improvement is the hip joint. Instead of making them out of paper, I made them out of tape. This probably would have been okay if I hadn’t run out of structurally sound filament tape mid way through. The clear packing tape I replaced it with is absolutely terrible for hinges, and most of the legs are starting to come off. It’s okay, we can repair them.



There are a bunch of areas where things could be improved. The way the body segments go together is a little hokey. It’s extremely rigid and strong, (holds a large DSLR quiet nicely), but the process of gluing it together is tricky and requires a bit more patience than I want to be known for. Also, toxic glue. I currently use rubber cement because its strong, flexible, and dries fast, but I could do with out the smell.

"You're in the desert, you see a robot lying on its back, struggling, and you're not helping— why is that?"

You can see how the segments fit together here. For the next revision, the top and bottom plates will likely be slotted to help align them. I should probably also make some allowances for controller mounting. We had a good time getting the little bot to walk in circles, tying its umbilical cord up around its feet. Cable management in high servo count machines becomes a problem quickly if you don’t design for it up front.

The other thing that becomes obvious, is that 12 degrees of freedom is not really enough for a six-legged, symmetrically shaped robot. The plane of motion for the front/back sweep is an arc, and the feet drag quite a bit in anything other than circular gaits.

Still, it’s fun to play with, and if you’de like to make your own it shouldn’t be too difficult. You’ll need a printer for patterns and an x-acto knife, or a laser cutter, to cut out the cardboard. Have an irrational fear of x-acto knives & don’t have a lasersaur? Your local hackerspace may be able to help you out. Here in the bay area there is the excellent Noisebridge, and Tech Shop. On the east cost check out NYC Resistor. There are many others.

You’ll need a sizable chunk of cardboard to get started. Here’s the flat-pattern as a PDF file. It’s CC-NonCom-ShareAlike licensed for your remixing pleasure. It’s also half baked, as the hip joints aren’t quite done yet. An exercise left to the reader, as it where. For servos I use the Corona DS 939MG. For the drivers, I can say enough nice things about Pololu. I have the 18 channel version of there driver, as well as some of their other products. They’re all very well thought out and documented. They go the extra mile and even have a cheesy little desktop controller app for bootstrapping the process of animating 18 channels. On that note, you may want to pick up a little step down voltage converter and Arduino if you don’t have a bench supply and you want to go wireless.

After that, it’s software the whole way down. Then it’s turtles.

2 Jan 2012, 3:15am
robotics
by

1 comment

Paper Robots, Part 2

Paper linkages are pretty neat, but ultimately if I want these little parts to do anything I need to figure out how to make them move. Not being force based actuators, servos are the wrong answer, but they’re cheap and I have piles of them. Because of their ubiquity, supporting hardware is easy to find as well. For example, these excellent low cost, high channel count PWM servo driver boards from Pololu.

This platform of parts for position based actuators is a large part of what perpetuates their continued use in hobby robotics. There really isn’t a platform for force based actuators yet, and we won’t see real accelerated innovation in force based control systems until they’re a bit more ubiquitous. That’s what platforms do. They lower the cost of entry, and allow a larger number of people to try out new ideas quicker than before. Until then, it’s going to be servos the whole way down.

I tried two designs. The first was an attempt to keep with my self-imposed goal of making linkages fold up from a single flat pattern. There are a bunch of paper folding tricks that can allow one to create arbitrarily complex geometries in paper, but they require back folding which tends to weaken the joints a bit. You can see an example of this folding near the mini grommets:

The back fold creates an extension in the flat pattern that creates more clearance at the expense of increased bulk. This particular design worked out ok, but the back fold was very fragile when loaded from one direction. The distance from the servo arm and the plane of motion was also a bit of a problem. The servo arm should ideally be centered under the upper swing arm linkage. Offset as it was, it still generated a fair amount of force and a good travel distance, but I wouldn’t expect it to last very long before it tore itself apart.

Confident that I could handle up/down reasonably well, I set about tackling the forward/back motion for the leg. I kept the same servo mount pattern for the up/down link, but moved the grommet bearing farther out for more leverage. For the front/back servo I decided that folded single pattern robots, while technically challenging, don’t really add a lot to the experience. So, with that ideal cheerfully abandoned, I was free to come up with a more robust general solution to mounting servos in paper.

The front/back servo is mounted in a little friction fit cup with lip tabs that glue to a hole cut out of the side panel. Sort of like a tiny paper cardboard box. As long as the direction of force runs parallel-ish to the mounting surface the servo will stay put. The mounting tabs also add a bit of rigidity to the side wall. So far so good.

While the paper servo mount cup was a success, the attachment for the arm linkage was a complete failure. I didn’t bother to design the paper-clip servo arm linkage up front, and tacked it on afterwards. As a consequence the lower edge where the paper clip attached started to bend and fall appart almost immediately. Connecting the servo arms to the paper links in a robust and secure method isn’t something I’ve found a good general solution too. The paper-clip is pretty strong, but it presses on the paper with a small surface area which needs to transfer to a larger surface area on the linkage. We’ll try to find some solutions to those ideas next time…

Ultimately, the goal of these objects is to try and find a way to make a really, really low cost force actuated motion system. To that end, I thought I’d play around with force and see how these linkages and construction methods behave under load.

I’ve been making all my previous legs with 140Lb charcoal sketch paper. For this next one, I wanted to see how well cardboard would work. Corrugated cardboard is really strong, and when creased, flexible. It’s an awesome construction material, so long as it never gets wet.

I used 4mm cardboard, which has a much larger bend radius than 0.2mm card stock I had been previously using. This necessitated scaling up the leg quite a bit. I made a double swing arm, with a lower leg. Mostly to try try out two different hinge attachment methods, but also ’cause it looks cool 😉 Important stuff. The larger swing arm uses a pair of grommets, paper clip hooks, and large rubber bands to set the force. It works beautifully, and the paperclip rubber band holders let me adjust the tension easily.

The lower swing arm sets the rubber bands from the outside, and uses a single grommet. It’s too delicate to hold together very well at this scale. The leg itself is pretty fun to play with, and can jump surprisingly high. It’s completely passive of course, but it gives me an idea of how responsive the linkage itself could be. Because the leg weighs almost nothing on its own, it can move really fast.

There are a couple of directions I want to go from here. Maybe some simple force based actuators. In the mean time, maybe I’ll make some jumping coffee tables.

28 Dec 2011, 8:35pm
robotics
by

3 comments

Paper Robots, Part 1

I’ve been trying to make little paper robots. Or rather, I’ve been trying to create a library of mechanical linkages that I could later make robots with. Paper is cheap, easy to work with, and I can try out a bunch of different ideas quickly without breaking my piggy-bank. In some forms, such as cardboard, paper is extremely strong for its weight. This makes it an ideal platform for building robots, and leads to The First Problem with robotics: any sufficiently agile robot will weigh less than its power supply.

Of course, people have built robots out of paper before. Some of them have even managed to get up and walk around a bit. What I’m interested in though, is reinventing the wheel. Or leg, as it where. I’m interested in seeing what sort of mechanical linkages one can fold up from a single sheet of paper. No purpose really, other than that I like the patterns left behind, like little shadows on the page.

The best way to experiment with these paper mechanisms, is to place a sheet of card stock on a cutting matt, pick up an x-acto knife, and start folding up parts. This is where I started.

Progression

The first linkage I tried didn’t work very well. It was difficult to control because the string only had leverage for the first joint. After that, there wasn’t a good way to provide an antagonistic return force. The second joint used the side walls to create an arm lever, but I got it backwards, and the control linkage went the wrong direction. The third design worked beautifully. The side walls make up the extension arm levers, and the front face folds out to make an attachment point for contraction.

Not bad for 20 minutes of doodling and a shot of vodka.

What about multiple axis? Is there a way to make a double joint? The obvious solution is to make two separate bends at 90 degrees. This isn’t very strong, structurally speaking. Rectangular sections in paper are inherently unstable, and lateral forces would cause the tube to collapse. Triangular tubes, on the other hand, are very strong and stable. By combining three hinges offset at 120°, I can get a full 360 rotation. A paper universal joint!

Universal Joints

I started out with a simple double bend, but got the number of triangles wrong, the second one faired much better. For the third, I tried to see if the pattern could be repeated indefinitely along an axis. It could, and in this manner, I can make an articulating linkage of indefinite length. I made a CAD model, cut it out on the laser, and assembled a three-link arm.

Paper Snake

Quickly, one notices that there is going to be a rather large number of control lines in short order. This lead to The Second Problem with robotics: any sufficiently agile robot will have more actuators than you have the patience to solder. This is why, outside of universities, you will rarely see a robot with more than 18 actuators. The human body has well over 200, by comparison. The larger the number, the more likely they will use an open-loop position based control system. The reason for this is what I call The Third Problem with robotics: any sufficiently agile robot will have a sensor system twice as complicated as its motor system. This leads to an interesting observation about how robots are built.

I am often struck by how backwards the field of robotics can seem at times. If your goal is to achieve the agility of biological systems, there are some excellent models out there to learn from. One is probably doing the backstroke in your soup right now. Their’s a fair amount of variation in how the major components are arranged, but without exception they all have the following characteristics:

  • A massive collection of sensors.
  • Force based actuators.
  • A neural network to connect the two.

Everything else is details. Invariably, and I’m not immune to this, when someone starts building a robot they usually start with the actuators and skeletal linkage. Why not start with the sensors? There’s a lot more territory to cover there, and a lot more room for improvement. The answer probably has something to do with engineers. We really like making flashy demos, and a clunky pile of servos is inherently more interesting than an inert pile of sensors. Also, sensors are hard. Can’t we just go back to actuators and linkages?

After making the little paper universal joints, I got stuck in an existential loop. Why make robots at all? What’s the point? It’ll just wiggle around a table at best. With so many hard problems to solve ahead of me, why not just give up now and make puppets? Puppets are cool, and really interesting in their own right. There’s plenty of room for experimentation, 100% less ARM/AVR assembly, and no messy wire harnesses to solder.

But… robots!

Sometimes one just has to slog through the existential crisis of purpose in the hopes that the journey will be its own reward. I decided to try and remake a classic: the hexapod. Only, I would make it out of paper. I’m sure it’s been done before, but anything worth doing once, is probably worth doing with style. For extra silliness, we’ll try and make the entire thing fold up from as few parts as possible.

More of These

After a few hours, I came up with the first part. A two axis swing arm linkage. It folds up from a single sheet of paper, and will scale up to 6 legs. Next up is the main body, but first, it’s time for me to use my own sensor network and go find lunch.

Update:

"But will it ever know love?"

Everything is easier when I’ve had lunch. Here’s the completed hexapod skeleton. I still need to figure out how to jam 12 little servos, a battery pack, RF link, and a sandwich in there. In the meantime it’ll just hang out on the desk looking confused.