Welcome back to the final chapter in our journey exploring two-stage tentacle mechanisms. This is where we arm you with the tools and techniques to get one of these cretins alive-and-kicking in your livingroom. In this last installment, I’ll guide us through the steps of building our very own tentacle and controller identical to one we’ve been discussing in the last few weeks. As promised, this post comes with a few bonuses:
Depending on your situation, some design files may be more important than others. If you just want to get parts made, odds are good that you can simply cut the pre-offset DXFs from the right plate thicknesses and get rolling. Of course, if you need to tune the files for a laser with a slightly different beam diameter, I’ve included the original DXFs for good measure. For the heavy-hitters, I’ve also included the original files if there’s something about this design that just deserves a tweak or two. Have at it! (And, of course, let us know how you improve it!)
Ok, now that we’ve got the parts on-hand in a pile of pieces,let’s walk through the last-mile tweaks to making this puppet work: assembly and tuning. At this point, we’ve got a collection of parts, some laser-cut, some off the shelf. Now it’s time to string them together.
This week, we’re continuing our Creating A PCB In Everything series, where we go through the steps to create a simple, barebones PCB in different EDA suites. We’re done with Eagle, and now it’s time to move onto Fritzing.
Fritzing came out of the Interaction Design Lab at the University of Applied Sciences of Potsdam in 2007 as a project initiated by Professor Reto Wettach, André Knörig and Zach Eveland. It is frequently compared to Processing, Wiring, or Arduino in that it provides an easy way for artists, creatives, or ‘makers’ to dip their toes into the waters of PCB design.
I feel it is necessary to contextualize Fritzing in the space of ‘maker movement’, DIY electronics, and the last decade of Hackaday. Simply by virtue of being an editor for Hackaday, I have seen thousands of homebrew PCBs, and tens of thousands of amateur and hobbyist electronics projects. Despite what the Fritzing’s Wikipedia talk page claims, Fritzing is an important piece of software. The story of the ‘maker movement’ – however ill-defined that phrase is – cannot be told without mentioning Fritzing. It was the inspiration for CircuitLab, and the Fritzing influence can easily be seen in Autodesk’s 123D Circuits.
Just because a piece of software is important doesn’t mean it’s good. I am, perhaps, the world’s leading expert at assessing poorly designed and just plain shitty PCBs. You may scoff at this, but think about it: simply due to my vocation, I look at a lot of PCBs made by amateurs. EE professors, TAs, or Chris Gammell might beat me on volume, but they’re only looking at boards made by students using one tool. I see amateur boards built in every tool, and without exception, the worst are always designed in Fritzing. It should be unacceptable that I can even tell they’re designed in Fritzing.
Fritzing has its place, and that place is building graphical representations for breadboard circuits. Fritzing has no other equal in this respect, and for this purpose, it’s an excellent tool. You can also make a PCB in Fritzing, and here things aren’t as great. I want to do Fritzing for this Creating A PCB In Everything series only to demonstrate how bad PCB design can be.
For the next few thousand words, I am going to combine a tutorial for Fritzing with a review of Fritzing. Fritzing is an important piece of software, if only for being a great way to create graphics of breadboard circuits. As a PCB design tool, it’s lacking; creating parts from scratch is far too hard, and there’s no way to get around the grid snap tool. No one should ever be forced to create a PCB in Fritzing, but it does have its own very limited place.
ESP32 is the hottest new wireless chip out there, offering both WiFi and Bluetooth Low Energy radios rolled up with a dual-core 32-bit processor and packed with peripherals of every kind. We got some review sample dev boards, Adafruit and Seeed Studio had them in stock for a while, and AI-Thinker — the company that makes the most popular ESP8266 modules — is starting up full-scale production on October 1st. This means that some of you have the new hotness in your hands right now, and the rest of you aren’t going to have to wait more than a few more weeks.
As we said in our first-look review of the new chip, many things are in a state of flux on the software side, but the basic process of writing, compiling, and flashing code to the chip is going to remain stable. It’s time to start up some tutorials!
For the next post in the Creating A PCB series, we’re going to continue our explorations of Eagle. In Part 1, I went over how to create a part from scratch in Eagle. In Part 2, we used this part to create the small example board from the Introduction.
This time around I’ll be going over Design Rule Check (DRC) — or making sure your board house can actually fabricate what you’ve designed. I’ll also be covering the creation of Gerber files (so you can get the PCB fabbed anywhere you want), and putting real art into the silkscreen and soldermask layers of your boards.
The idea behind this series is to explore different EDA suites and PCB design tools by designing the same circuit in each. You can check out the rest of the posts in this series right here.
In the last (and first) post in this series, we took a look at Eagle. Specifically, we learned how to create a custom part in Eagle. Our goal isn’t just to make our own parts in Eagle, we want to make schematics, boards, and eventually solder a few PCBs.
The board we’ll be making, like all of the boards made in this Creating A PCB In Everything series, is the Nanite Wesley, a small USB development platform based on the ATtiny85. This board has less than a dozen parts, most of which are through-hole. This is the simplest PCB I can imagine that has sufficient complexity to demonstrate how to make a board.
With that said, let’s get onto the second part of our Eagle tutorial and lay out our circuit board.
For the first in a series of posts describing how to make a PCB, we’re going with Eagle. Eagle CAD has been around since the days of DOS, and has received numerous updates over the years. Until KiCad started getting good a few years ago, Eagle CAD was the de facto standard PCB design software for hobbyist projects. Sparkfun uses it, Adafruit uses it, and Dangerous Prototypes uses it. The reason for Eagle’s dominance in a market where people don’t want to pay for software is the free, non-commercial and educational licenses. These free licenses give you the ability to build a board big enough and complex enough for 90% of hobbyist projects.
Of course, it should be mentioned that Eagle was recently acquired by Autodesk. The free licenses will remain, and right now, it seems obvious Eagle will become Autodesk’s pro-level circuit and board design software.
Personally, I learned PCB design on Eagle. After a few years, I quickly learned how limited even the professional version of Eagle was. At that point, the only option was to learn KiCad. Now that Eagle is in the hands of Autodesk, and I am very confident Eagle is about to get really, really good, I no longer have the desire to learn KiCad.
With the introduction out of the way, let’s get down to making a PCB in Eagle.
Practically any combination of motor and gearbox can be mathematically arranged to fit all sorts of problems. You could lift a crane with a pager motor, it just might take a few hundred years. However, figuring out exactly what ratio you need can feel bit backwards the first time you have to do it.
A gear is nothing more than a clever way to make two circles rotate in concert with each other as if they were perfectly joined at their circumferences. Rather than relying on the friction between two rotating disks in contact, the designer instead relies on the strength of a gear tooth as the factor limiting the amount of torque that can be applied to the gear.
Everything is in gearing is neatly proportional. As long as your point of reference is correct, and some other stuff. Uh, it gets easier with practice.
Now as my physics professors taught me to do, let’s skip the semantics and spare ourselves some pedantics. Let us assume that all gears have a constant velocity when you’ve averaged it all out. Sure there is a perceptible difference between a perfect involute and a primitive lantern gear, but for the sake of discussion it doesn’t matter at all. Especially if you’re just going to 3D print the thing. Let’s say that they’re sitting on perfect bearings and friction isn’t a thing unless we make it so. Also we’ll go ahead and make them perfectly aligned, depthed, and toleranced.
Typically, a gearbox is used for two things. You have a smaller torque that you’d like to make into a bigger one or you have one rotational velocity that you’d like to exchange for another. Typically torque is represented with a capital or lowercase Tau (Ττ) and rotational velocity likes to have a lowercase omega (ω). It also doesn’t matter at all; it just makes your equations look cooler.
Now a lot of tutorials like to start with the idea of rolling a smaller circle against a bigger one. If the smaller circle is a third as large as the big one, it will take three rotations of the small circle to make the big one rotate twice. However, it is my opinion that thinking it in terms of the force applied allows a designer to think about the gearing more effectively.
If the friction between the two surfaces of the circle is perfect, then any force applied tangentially to one of the circles will result in a perfectly perpendicular and equal force to the other circle at the point of contact between the two. Midway through writing the preceding sentence I began to understand why textbooks are so abstruse, so I also drew a picture. This results in two equations.
Now, when you have a force perpendicular to the line drawn to describe the radius, the equation for torque becomes really simple.
Multiply the length of the “lever arm”, “radius”, etc. by the force to get the preceding equations. Make sure to include the units.
You should end up force-unit * length-unit. Since I usually work in smaller gears I like to use N * mm. American websites typically use oz-in to rate motors. It is technically ozf-in (ounce-force), but the US customary system has a fetish for obtuseness.
We can make some observations. The smaller gear always sees less torque at its center. This initially seemed a bit counter-intuitive to me. If I’m using a cheater bar to turn a bolt the longer I make the bar the more torque I can put on the bolt. So if I touch the outside of a really large gear I should be seeing a ton of torque at the center of a small gear rotating along with it. However, as we mentioned before, any torque applied on the outside of the larger gear is seen equal and tangential on the smaller. It’s as if you’re touching the outside of the small gear. The torque has to be smaller.
This is why you have to pedal so much harder when the rear sprocket on a bicycle gets smaller. Each time you make the sprocket smaller you shrink the torque input into the wheel. If the perpendicular output where the wheel hits the ground is <input from the small gear> / <radius of the wheel> then it’s obvious why this happens.
It’s also important to note that any time you increase the torque, the speed of the gears slow by the same proportion. If you need 60 N*m out of a motor that can give 20 N*m and you use a 3:1 gearbox to do it. If the motor previously ran at 30 rpm it’s now running at 10 rpm.
Let’s jump right into an example. Let’s say you want to make a device that automatically lifts your window blinds. You’ve got some junk and a 3D printer.
Now you’ve taken a spring scale and pulled until the shutter moves and you know you need 10 lbs. of pull to get the blinds to pull up. To make it easy on yourself you multiply this number by two so you know you need exactly 20 lbs of force to pull the curtain up. Then to make it really easy on yourself convert it all to Newtons. It’s approximately 90 N.
Now you don’t really care how fast the blinds pull up, but you go ahead and pull them up yourself. You get the feeling that the blinds won’t appreciate being lifted faster than the whole range in two seconds. You personally don’t care if takes ten seconds to, but you’d like it not to take too long.
You also measure the length of string pulled out to raise the blinds. It’s 1.2 meters.
Lastly, you only have one spare power supply and a matching motor left in your entire laboratory after you followed the advice in a Hackaday article. Cursing the day the author was born, you sullenly write down the last specifications. You’ve got one of those cheap GM9 gear motors. 5 V, 66 rpm, and 300 N*mm. You damn him as you think fondly of your mountain of windshield washer motors and 80 lb server rack power supplies that you tossed out.
To start with, you do some experiments with a pulley. You arbitrarily pick, 3D print, and find that a 100 mm in diameter pulley seems to wind it up nicely by hand. By the end of the winding the outside diameter of the string is 110 mm. So you use the torque equations above. You find that at the end of the rotation, if you attach the motor directly, there is only 5.45 N of force being applied to the string. Not nearly enough.
So, since you know everything is more or less proportional, you divide 90 N / 5.45 N, and arrive at an answer of 17. So, at a minimum for every turn of the pulley you need 17 turns of the motor to get the torque needed.
That would be okay, but it messes with our other specification. At a 17:1 ratio, it will take our 66 rpm motor pretty close to a minute to wind the blinds up.
This is a moment for some pondering. Make a coffee. Maybe go write a relaxing comment to a Hackaday writer listing their various flaws, perceived and true, in excruciating detail.
What if you wound the string up on a closet rod? Those are only about 30 mm in diameter. You take a bit of rod and wind it up. It seems to work and since it’s wider the string only ends up adding 5 mm to the final diameter. You rework the calculation and find that in this case you only need a ratio of 6! Yes.
Now some of you who have done this before are likely gnashing your teeth, or more likely already down in the comments. Unfortunately it’s all proportional. While you only need a ratio of 6:1 now, nearly a third. You also need to rotate the pulley approximately three times as much to pull the same length of cord.
Sometimes you can’t win. In this case the only solution is to order a new motor. You look online for a bit and realize that one of the 12 V motors you threw away last week would work perfectly for this. You wouldn’t even need a gear box. You could attach it straight to the pulley. You look around your perfectly clean and orderly garage and feel empty.
However, just for fun you build a 6:1 gearbox anyway. It’s a hack after all.
Cover photo of the hilariously complicated Do Nothing Machine credit to the Joe Martin Foundation.