[Mark Rehorst] tells us about a tragic incident involving an untimely demise of $200 worth of motor driving hardware, and shares a simple circuit so that we can prevent such tragedies in the future. His Arrakis sand table project has quite a few motors involved, and having forgotten to add limits into the software, he slammed a motor-driven mechanism into a well-fixed part of the table. The back EMF of the motor created a burst of energy, taking out the motor driver, the controller board, and the power supply.
With the postmortem done, he had to prevent this from happening again – preferably, in hardware. Based on a small appnote from Gecko Drives, he designed a simple PCB that shunts the motor with a high-power resistor, as soon as the current starts flowing into a direction it’s not supposed to flow into. He goes in depth about the way that the circuit works and the reasoning behind parts selection, as well as shows an LTSpice simulation and shares the PCB files. This was his first time designing PCBs in KiCad, and we believe he’s done a great job! This worklog is certainly worth reading if you’d like to understand how such circuits work and what goes into building one.
He dubs this a “bank account protection” circuit, and we can absolutely relate. It’s not just CNC tables that need such protections of course – we’ve seen a solution for small hacky makeshift electric vehicles, for instance. A motor’s generative properties aren’t always a problem, however – here’s just one example of a hacker trying to put them to good use.
Personal head-up displays are a technology whose time ought by now to have come, but which notwithstanding attempts such as the Google Glass, have steadfastly refused to catch on. There’s an intriguing possibility in [Basel Saleh]’s CaptionIt project though, a head-up display that provides captions for everyday situations.
The hardware is a tiny I²C OLED screen with a reflector and a 3D-printed mount attached to a pair of glasses, and it’s claimed that it will work with almost any ARM v7 SBC, including more recent Raspberry Pi boards. It uses the vosc speech recognition toolkit to read audio from a USP audio device, with the resulting text being displayed on the screen.
The device is shown in action in the video below the break, and without trying it ourselves we can’t comment on its utility, but aside from the novelty we can see it could have a significant impact as an accessibility aid. But it’s as an electronic Babel fish coupled with translation software that we’d like to see it develop, so that inadvertent but hilarious international misunderstandings can be shared by all.
Regular readers will know that we’ve brought you plenty of HUD tomfoolery in the past.
Competition sure brings out the brute in people, doesn’t it? So what do you do when you need a bunch of switches you can let people fist-pound or stomp on repeatedly without them taking damage? You could look to the guitar pedal industry and their tough latching switches, or you could simply build your own smash-resistant buttons as [wannabemadsci] has done.
The main thing about these switches is that they aren’t easily destroyed by shoes or angry fists. That’s because the shiny red push-me part of the button is made by cutting a foam ball in half.
Not easily crush-able Styrofoam, mind you — squishy, coated foam like an indoor football. This is mounted to the top of a sandwich made of hardboard and a couple pieces of easily-compressible foam from craft paintbrushes.
A brass washer is mounted to the middle of both pieces of hardboard, and these have wires soldered to them to read button presses. Then it’s just a matter of hooking it to a microcontroller like any other momentary.
There are all kinds of things you could cut in half for the top, like maybe tennis balls. Or, do what [Sprite_TM] did and use inverted plastic bowls.
I hope last week’s introduction to bulk material handling got you all thinking up amazing hacks, and we’ll soon be reporting on DIY Cap’n Crunch Robots galore. This week we’ll look at how to measure particle sizes, separate particles, and even grind them up when you need to.
Measuring Material Properties
Last week we talked about cohesive strength. Bulk material behaves somewhere between a solid and a liquid — if you’ve done your homework, it flows down the funnel just fine. But if you haven’t, it sticks together and holds up the rest of the material. Cohesive strength is the measure of how much weight the material at the bottom of the funnel can hold up.
You can get a rough measurement by packing material in a box with a square hole at the bottom. One side of the hole should have a retractable slide. Slowly withdraw the slide, making the hole rectangular. Material will bridge over, and then at some point a larger chunk will fall out. This is about the size of the minimum opening that will not arch, and a practical measure of the material’s cohesive strength.
Many materials cohere better when wet. Dry a sample in a microwave to determine the percent moisture by weighing it before and after.
Cohesive strength is closely allied to shear strength. If you want to measure shear strength, cut two 1 cm wide rings of 5 cm diameter PVC pipe, stack them, pack with material, put a disk atop the material and load it, then drag the top ring off the bottom with a spring scale. The force per unit area is the shear strength at that pressure. If it starts packing you’ll see it in the curve.
Packing factor is another useful measurement. Gently shake material to fill a rigid container and weigh it. Now empty the container and refill, packing the material as hard as you can with a length of 1” dowel. Reweigh, and the ratio of the two weights tells you how well the material packs.
Real bulk material is almost always made up of particles of varying sizes, shapes, and compositions. Dirt is particles of different kinds of mineral and organic matter varying from outright rocks to sub micron clay particles. If you’re having problems, getting a graph of material size distribution can be helpful.
For particles above about 75 μM, you can measure the sizes with sieves. If you want to be fancy, they sell nice sets of metal sieves with wire mesh in the bottom. Screen assortments are cheaper. Below 75 μM, you have to use a hydrometer. This is messy and takes a while, but does work.
The idea is to mix the material with soapy water and then use a hydrometer from the auto parts store to measure the density. The particles fall out by Stokes law, big ones first. Stokes law is just that the drag force on a sphere is proportional to the square of the radius. Mass will go up as cube of the radius, so large particles fall faster than small ones. As they fall out, the density of the fluid decreases. This page describes how to do it, and this page has a handy calculator for interpreting the results.
Grinding
You can also change the size of particles in your mix. If particles are too large, they can be crushed or ground. You can separate by size and only grind some of the sizes or discard some of the material. There’s a whole science to grinding. The finer you grind, the harder it gets to grind. Cosmetics and pharmaceutical companies are full of grinding experts.
In general, there are three ways to make something smaller – crush it, cut it, or hit it.
Crushing is straightforward. Use rollers or jaws, a rolling pin or a rock crusher. Don’t overlook the vise. A jaw crusher only crushes particles larger than the jaw space, useful to make a certain size. Rock crushers have a complex motion (video) that should nonetheless be easily imitated by a hacker project. Amateur/hobby gold prospectors have an accessible community.
Crushing action in rollers only works until the particle is small enough that the surface of the roller deforms instead of the particle. Stones have been used to crush grain into flour for most of history.
Cutting is best for soft things, like gummy worms, and tough things (video.). Make sure the cut material has an easy path out. Think of an old fashioned kitchen meat grinder. .
If you want small particles, you need an impact grinder. A coffee mill or blender works by striking the particle with a fast moving impactor. This can be a blade – useful if the material first needs to be cut up, as in a coffee mill – or blunt. Many industrial mills use two pivoting weights on a shaft, and this unit just uses chains (video).
Another impact mill is the ball mill. Rotate a drum on it’s side with steel balls and the material. The balls travel up the side, then fall back down, striking the material.
All these work by fracturing the material. What if you’re trying to powder something that doesn’t fracture, say rubber O rings? For that, there’s cryogenic grinding.
Many rubbery materials are really glasses — materials that are a gloppy liquid at a higher temperature, often brittle at a cool temperature, and soft in-between. The glass you’re probably thinking of is a brittle, breakable material at room temperature, but at high temperature is a liquid. The transition point is the ‘glass transition temperature’.
So what about our O rings? If they’re natural rubber their transition point is about -70° C. Below that temperature they’re brittle and can be ground up. Unfortunately, grinding is going to put heat back in. So consider grinding slowly – some labs grind biological materials like skin samples with a special mortar and pestle cooled beforehand with liquid nitrogen. Just be sure everything in contact with the material has been cooled, and use a thick walled container with lots of thermal mass.
Separating Wheat From Chaff
Sometimes you have a mix and need to separate it. Your roommate dumped all the gummy bears and all those weird ginger candies into a bowl or whatever. Last week we introduced particle segregation as a bad thing. But when you want to un-mix a mixture, it can be a good thing. Any of the techniques from last week can be an aid.
Sieves and screens work to separate by size. They clog unless the material keeps moving over them. One simple way to do this is to flow the material over sieves on a slanted board, finest sieve first. Another is to mechanically shake the screen. Paper filters are just fine screens, and do clog.
A trommel is a slowly turning cylinder with walls of different sized screens along it’s length. Material is fed into the fine screen end and slowly moves towards the other.
Stokes law provides another way to separate materials as we saw above. Make an upward air draft in a vertical pipe. Deliver the material into the pipe part way up. Materials with more drag than weight will go up, larger materials will go down. You can use the air speed to control the size of particle. An industrial machine called an air classifier does this with higher velocity air blowing material into the rim of a spinning set of blades.
It could be the air (or another gas) you want to remove. There are a couple ways to do it. The first is the cyclone familiar to wood shops. The second is even simpler – inject the air/material mix into the top of a tall, slender container with a tube that extends about halfway into the container. Let the air out from an outlet pipe in the roof. The air flow expands, slows down, and the material falls out.
You can just blow the material sideways – the age old system of threshing wheat works this way. Wheat comes from the plant with a husk, you beat it with a flail to loosen the husk, giving you wheat grains and chaff mixed. Put the mix on a blanket and have four peasants toss it repeatedly. The chaff blows away in the wind.
Inertial Separation
A very sensitive separation technique is inertial separation. Here’s a mix of gummy colas and jelly beans. We separated them by tilting and gently shaking the sheet. A material moves on a sheet by staying in place until the acceleration is more than some critical value. Then it rolls or slides.
If your material is dirt or such, run a magnet through it. There’s iron ore and bits of human generated iron in a lot of soil. It can get into motors and such. If you need it out run the material past magnets. An eddy current separator uses AC magnetic effects to do the same with nonferrous metals.
You can also segregate materials by dissolving them. A mixture of table salt and white sand would seem impossible, but if you stir it into water, then decant and boil off the water, the salt and sand can be recovered separately. But we think we’re veering into chemistry now, and we should stop.
Next time we’ll finish up by looking at controlling movement: building gates and contraptions that move your bulk material without clogging up.
Around here, we’re always excited about a new actuator design. Linear actuators are particularly hard to make cheap, fast, and good, so it’s even better when something new that we can build ourselves slides onto the scene.
Researchers at U Penn’s Pikul Research Group took inspiration from the cascade of falling dominoes for an innovative take on linear motion. This article on IEEE Spectrum describes the similarity of the sequential tipping-over with the peristaltic motion of biological systems, including you, swallowing right now.
The motion propagation in falling dominoes, called a Soliton Wave, can be harnessed to push an object at the front of the wave, just like a surfer. See the videos after the break for examples of simple setups that any of us could recreate with laser-cut or 3D printed parts. Maybe you won’t be using them to help a robot swallow (a terrifying idea that the article suggests), but you might need a conveyor or a novel way to help a device crawl like a shrimp. The paper is behind a paywall on IEEE, though you readers likely see enough in the videos to get started, and we can’t wait to see where your dominoes will lead us next.
It is with sadness that we note the passing of John Birkett, proprietor of the legendary eponymous surplus radio and electronics store on an unassuming street in the British city of Lincoln, at the age of 93. He has been a fantastic source of esoteric parts and electronic assemblies for many decades, and though many of you from beyond where this is being written may never have heard of him the chances are that if you follow electronics enthusiasts from the UK you will have unwittingly seen parts which passed through his hands.
Gateway to a world of wonders: a typical Birkett advert from 1986.
There was a time when surplus stores were a relatively common sight, given their window of opportunity by the huge quantity of post-war and Cold War military gear at knock-down prices. My town had one when I was a kid, but though it sold its share of electronic goodies it was more of a place for sturdy olive green outdoor wear or all the 1930s British military uniform items you might ever need. J. Birkett was different, as a purely electronics store the shop rapidly became the go-to place for both the most necessary and the most unexpected of parts.
His motto was “Not a piece of junk in sight”, and though as with much surplus equipment there is plenty of junk to be found it was his eye in managing to stock the junk which was most interesting and useful that made his selection special. Such was its reach that most of his customers including me never made it to Lincoln and the store itself, instead we came to him through his mail-order business and attendance at radio rallies. I fondly remember the anticipation of receiving a Birkett parcel, and I still have plenty of parts that came from him. An FM tuner converted for use as a 2 meter receiver is still in a box somewhere, and I’m pretty certain my storage unit still holds a pair of Pye Cambridge VHF transceivers he supplied.
According to Google the shop remains open, and we hope that state of affairs will continue. Surplus may not be what it once was, but we thank John Birkett for what he gave to generations of British hardware hackers. May he rest in peace.
Aviation consists of two major groups. Airplane enthusiasts, and helicopter enthusiasts. The two groups rarely get along, each extolling the virtues of their chosen craft. Somewhere in between are autogyro folks. People who like vehicles that blend the best (or worst) of both airplanes and helicopters. Aviation master [Peter Sripol] has dipped his toes into the autogyro world, but not without some trouble.
Autogyros are propelled by a propeller, like a plane. They also have a tail section that works similar to a fixed-wing aircraft. That’s where the similarities end though. Lift for autogyros comes in the form of a rotating set of blades, much like a helicopter. Autogyro rotors aren’t powered during flight. They utilize autorotation. The blades freewheel, spun by the air as the craft moves forward.
[Peter] recently got his hands on a full-scale autogyro. So it made sense to build a model to help learn to fly. This isn’t [Peter’s] first attempt with autogyro models. He’s built a few in the past, with limited success. This time he started from scratch and ran into even more problems!
