The Little Things I Didn’t Know About Small DC Motors

We’ve all taken apart a small toy and pulled out one of those little can motors. “With this! I can do anything!” we proclaim as we hold it aloft. Ten minutes later, after we’ve made it spin a few times, it goes into the drawer never to be seen again.

It’s all their fault

It always seems like they are in everything but getting them to function usefully in a project is a fool’s errand. What the heck are they for? Where do people learn the black magic needed to make them function? It’s easy enough to pull out the specification sheet for them. Most of them are made by or are made to imitate motors from the Mabuchi Motor Corporation of Japan. That company alone is responsible for over 1.5 billion tiny motors a year.

More than Just the Specs

In the specs, you’ll find things like running speed, voltage, stall current, and stall torque. But they offer anything but a convincing application guide, or a basic set of assumptions an engineer should make before using one. This is by no means a complete list, and a skip over the electrics nearly completely as that aspect of DC motors in unreasonably well documented.

The paint mixers high running speed and infrequent use make it a decent candidate for hooking directly to the motor.
The paint mixers high running speed and infrequent use make it a decent candidate for hooking directly to the motor.

The first thing to note is that they really aren’t meant to drive anything directly. They are meant to be isolated from the actual driving by a gear train. This is for a lot of reasons. The first is that they typically spin very fast, 6,000 – 15,000 rpm is not atypical for even the tiniest motor. So even though the datasheet may throw out something impressive like it being a 3 watt motor, it’s not exactly true. Rather, it’s 3 N*m/s per 15,000 rotations per minute motor. Or a mere 1.2 milliwatt per rotation, which is an odd sort of unit that I’m just using for demonstration, but it gives you the feeling that there’s not a ton of “oomph” available. However, if you start to combine lots of rotations together using a gear train, you can start to get some real power out of it, even with the friction losses.

The only consumer items I can think of that regularly break this rule are very cheap children’s toys, which aren’t designed to last long anyway, and those powered erasers and coffee stirrers. Both of these are taking for granted that their torque needs are low and their speed needs are high, or that the motor burning out is no real loss for the world (at least in the short term).

This is because the motors derate nearly instantly. Most of these motors are hundreds of loops of very thin enameled wire wrapped around some silicon steel plates spot welded or otherwise coerced together. This means that even a small heat event of a few milliseconds could be enough to burn through the 10 micrometer thick coating insulating the coils from each other. Practically speaking, if you stall a little motor a few times in a row you might as well throw it away, because there’s no guessing what its actual performance rating is anymore. Likewise, consistently difficult start-ups, over voltage, over current, and other abuse can quickly ruin the motor. Because the energy it produces is meant to spread over lots of rotations, the motor is simply not designed (nor could it be reasonably built) to produce it all in one dramatic push.

Making Contact

Pololu has the clearest picture of the different kind of brushes inside these small motors.
Pololu has the clearest picture of the different kind of brushes inside these small motors.

This brings me to another small note about these tiny motors. Most of them don’t have the carbon brushes one begins to expect from the more powerful motors. Mostly they have a strip of copper that’s been stamped to have a few fingers pressing against the commutator. There’s lots of pros to these metal contacts and it’s not all cost cutting, but unless you have managed to read “Electrical Contacts” by Ragnar Holm and actually understood it, they’re hard to explain. There’s all sorts of magic. For example, just forming the right kind of oxide film on the surface of the commutator is a battle all on its own.

It’s a weird trade off. You can make the motor cheaper with the metal contacts, for one. Metal contacts also have much lower friction than carbon or graphite brushes. They’re quieter, and they also transfer less current, which may seem like a bad thing, but if you have a stalled motor with hairlike strands transferring the pixies around the last thing you’d want to do is transfer as much current as possible through them. However, a paper thin sheet of copper is not going to last very long either.

So it comes down to this, at least as I understand it: if bursts of very fast, low energy, high efficiency motion is all that’s required of the motor over its operational life then the metal strip brushes are perfect. If you need to run the motor for a long stretches at a time and noise isn’t an issue then the carbon brush version will work, just don’t stall it. It will cost a little bit more.

Take Care of Your Tiny Motors

Here is one of these can motors being restrained properly. Only torque on the case itself is restrained. The motor is otherwise free to move.
Here is one of these motors being restrained properly. Only torque on the case is restrained. The motor is otherwise free to move.

To touch one other small mechanical consideration. They are not designed to take any axial load at all, or really even any radial load either. Most of them have a plastic or aluminum bronze bushing, press-fit into a simple stamped steel body. So if you design a gearbox for one of these be sure to put as little force as possible on the bearing surfaces. If you’ve ever taken apart a small toy you’ve likely noticed that the motor can slide back and forth a bit in its mounting. This is why.

Lastly, because most of these motors are just not intended to run anywhere near their written maximum specifications it is best to assume that their specifications are a well intentioned but complete lie. Most designs work with the bottom 25% of the max number written on the spreadsheet. Running the motor anywhere near the top is usually guaranteed to brick it over time.

These are useful and ubiquitous motors, but unlike their more powerful cousins they have their own set of challenges to work with. However, considering you can buy them by the pound for cheaper than candy, there’s a good reason to get familiar with them.

Hackaday Links: October 16, 2016

You need only look at the weekly user account leak from a popular web service or platform to know there’s a problem with security. Reusing passwords is the dumbest thing you can do right now, and the Mooltipass Mini is the answer to that problem. The Mooltipass originally began as a Developed on Hackaday series, and we log frequent sightings of the Multipass (maxi?) at security cons. The Mini is smaller, has exactly the same capability, and is completely unrepairable. It’s very cool, and if your email password is the same as your banking account passwords, you kind of need this yesterday.

Last weekend was the Open Hardware Summit in Portland. All the talks were worth watching, but editing the talks down into something sensible takes time. In lieu of this, OSHPark has gone through the livestream and timestamped everything

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Program Your Brain, Hack Your Way to Productivity

Most people wish they were more productive. Some buckle down and leverage some rare facet of their personality to force the work out. Some of them talk with friends. Some go on vision quests. There are lots of methods for lots of types of people. Most hackers, I’ve noticed, look for a datasheet. An engineer’s reference. We want to solve the problem like we solve technical problems.

It's got the cover equivalent of click-bait, but the centimeter thick bibliography listing research sources at the back won me over.
It’s got the cover equivalent of click-bait, but the centimeter thick bibliography listing research sources at the back won me over.

There were three books that gave me the first hints at how to look objectively at my brain and start to hack on it a little. These were The Power of Habit by Charles Duhigg, Flow By Mihaly Csikszentmihalyi, and Getting Things Done By David Allen.

I sort of wandered into these books in a haphazard path. The first I encountered was The Power of Habit which I found to be a bit of a revelation. It presented the idea of habits as functions in the great computer program that makes up a person. The brain sees that you’re doing a task over and over again and just learns to do it. It keeps optimizing and optimizing this program over time. All a person needs to do is trigger the habit loop and then it will run.

For example: Typing. At first you either take a course or, if your parents left you alone with a computer for hours on end, hunt-and-peck your way to a decent typing speed. It involves a lot of looking down at the keyboard. Eventually you notice that you don’t actually need to look at the keyboard at all. Depending on your stage you may still be “t-h-i-n-k-i-n-g”, mentally placing each letter as you type. However, eventually your brain begins to abstract this away until it has stored, somewhere, a combination of hand movements for every single word or key combination you typically use. It’s only when you have to spell a new word that you fall back on older programs.

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Get Hands-On: Workshop Tickets Now Available

Get together with awesome hackers and build something cool. That’s the exact description for the workshops of the Hackaday SuperConference. Previously we announced all of the talks and some of the workshop presenters, but starting right now you can reserve your space in these inspiring hands-on sessions.

You must have a SuperCon ticket in order to purchase a workshop ticket. We anticipate SuperCon to be sold out before the end of this week so buy your ticket now! This is the ultimate hardware conference, held in Pasadena California on November 5th and 6th.

Workshops start at $5. This is a “skin in the game” rate to help encourage everyone who registers to show up. Space is limited and will surely sell out (last year the waiting list for some of the workshops was far bigger than the actual workshop). Any tickets above the $5 price are to cover the material expense for that workshop.

Delve into ultrasonics, try your hand at rapid prototyping connected devices, head out on the town with your robot, or get building with PCBs, FPGAs, conductive ink, and servo motors. These workshops span a range of very interesting skill sets and will send you away inspired to explore that next big hack.

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Ask Hackaday: DIY Handwriting Recognition

Computer handwriting recognition is very cool by itself, and it’s something that we’d like to incorporate into a project. So we went digging for hacker solutions, and along the way came up with an interesting bit of history and some great algorithms. We feel like we’ve got a good start on that front, but we’re stuck on the hardware tablet sensor itself. So in this Ask Hackaday, we’re going to make the case for why you could be using a tablet-like device for capturing user input or doing handwriting recognition, and then we’re going to ask if you know of any good DIY tablet designs to make it work.

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NextThingCo Introduces C.H.I.P. Pro, GR8 System On Module

NextThingCo, makers of the very popular C.H.I.P. single board Linux computer, have released the latest iteration of their hardware. It’s the C.H.I.P. Pro, an SBC designed to be the embedded brains of your next great project, product, or Internet of Things thing.

The C.H.I.P. Pro features an Allwinner R8 ARMv7 Cortex-A8 running at 1 GHz, a MALI-400 GPU, and either 256 MB or 512 MB of NAND Flash. The Pro also features 802.11 b/g/n WiFi, Bluetooth 4.2, and is fully certified by the FCC. This board will be available in December at supposedly any quantity for $16.

The design of the C.H.I.P. Pro is a mix between a module designed to be installed in a product and a single board computer designed for a breadboard. It features castellated edges like hundreds of other modules, but the design means that assembly won’t be as simple as throwing down some paste and reflowing everything. The C.H.I.P. Pro features parts on two sides, making reflow questionable and either 0.1″ headers or a cutout on a PCB necessary. As a single board computer, this thing is small, powerful, and a worthy competitor to the Raspberry Pi Zero. A C.H.I.P. Pro development kit, consisting of two C.H.I.P. Pro units, a ‘debug’ board, and headers for breadboarding, is available for $49, with an estimated ship date in December.

A $16 Linux module with WiFi, Bluetooth, and no NDA is neat, but perhaps a more interesting announcement is that NextThingCo will also be selling the module that powers the C.H.I.P. Pro.

The GR8 module includes an Allwinner R8 ARMv7 Cortex-A8 running at 1 GHz, a MALI-400 GPU, and 256 MB of DDR3 SDRAM. Peripherals include TWI, two UARTS, SPI (SD cards support is hacked onto this), two PWM outputs, a single 6-bit ADC, I2S audio, S/PDIF, one USB 2.0 Host and one USB 2.0 OTG, and a parallel camera interface. This isn’t really a chip meant for video out, but it does support TV out and a parallel LCD interface. A limited datasheet for the GR8 is available on the NextThingCo GitHub.

Putting an entire Linux system on a single BGA module must draw comparisons to the recent release of the Octavo Systems OSD355X family, best known to the Hackaday audiences as the Beaglebone on a chip. Mechanically, the Octavo chip will be a bit easier to solder. Even though it has almost twice as many balls as the GR8, 400 on the Octavo and 252 on the GR8, the Octavo has a much wider pitch between the balls, making escape routing much easier.

Comparing peripherals between the OSD355X and GR8, it’s a bit of a wash, with the OSD coming out slightly ahead with Ethernet, more RAM and fancy TI PRUs. Concerning pricing, the GR8 wins hands down at $6 per chip in any quantity. That’s significantly less than the OSD355X.

The original C.H.I.P. has been exceptionally well received by the community NextThingCo is marketing to, despite the community’s distaste for Allwinner CPUs, cringeworthy PR, and questions concerning the true price of the C.H.I.P.. The C.H.I.P. Pro will surely see more than a few uses, but the GR8 is the real story here. A jellybean part that contains an entire Linux system has been the fevered dream of a madman for years now. The GR8 makes putting the power of open software into any project much easier, and we can’t wait to see the applications it allows.

Creating A PCB In Everything: Friends Don’t Let Friends Use Fritzing

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-logoFritzing 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.

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