We’re pretty familiar with budget resistor-based bend sensors at this point, but this sensor is in a totally different class. Instead of relying on resistive elements, [Useok Jeong] and [Kyu-Jin Cho] devised a bend sensor that relies on geometric properties of the sensor itself. The result is a higher-fidelity measuring device made from a pretty widely available collection of stock parts.
We’ll admit, calling this device a bend sensor might be a bit of a stretch, so let’s dig into some of the operating principles. What we’re actually measuring is the accumulated angle, the sum of all the curvature deformations along the length of the sensing element. The sensor is made of 3 main pieces: an outer extension spring-based wire sheath; a flexible, tensioned inner wire core that’s fixed at one end; and a small displacement sensor that measures the length changes in the wire’s free end. The secret ingredient to making this setup work is a special property of the outer wire sheath or spring guide. Here, the spring guide actually resists being compressed while being bent. Because the inner radius of the bend remains a constant length, the center wire core is forced to elongate. With the excess wire spooled up at the sensor base, we simply measure how much we collected, apply some math, and get a resulting angle! What’s more, the folks behind this trick also demonstrate that the length and angle relationship is linear with an R-square of 0.9969.
One of the best parts about this sensor is how reproducible it seems from from a modest collection of stock parts. Spring guide (aka: extension spring) is available from McMaster-Carr and DR Templeman, and that flexible core might be readily substituted with some wire rope.
It’s not everyday that new topologies for bend sensors pop into the world, let alone linear ones. To learn more, the folks behind the project have kindly made their research paper open access for your afternoon reading enjoyment. (Bring scratch paper!) Finally, if you’re looking for other bend-related sensors, have a look at this multi-bend measurement setup.
Measuring length is a pain, and it’s all the fault of Imperial measurements. Certain industries have standardized around either Imperial or metric, which means that working on projects across multiple industries generally leads to at least one conversion. For everyone outside the last bastion of Imperial units, here’s a primer on how we do it in crazy-land.
Definitions
The basic unit of length measurement in Imperial units is the inch. twelve inches make up one foot, three feet make up one yard, and 5,280 feet (or 1,760 yards) make up a mile. Easy to remember, right?
Ironically, an inch is defined in metric as 25.4 millimeters. You can do the rest of the math for exact lengths, but in general, three feet is just shy of a meter, and a mile is about a kilometer and a half. Generally in Imperial you’ll see lots of mixed units, like a person’s height is 6’2″ (that’s shorthand for six feet, two inches.) But it’s not consistent, it’s English; the only consistency is that it’s always breaking its own rules. You wouldn’t say three yards, two feet, and six inches; you’d say 11 1/2 feet. If it was three yards, one foot, and six inches, though, you’d say 3 1/2 yards. There’s no good rule for this other than try to use nice fractions as often as you can.
Users of Imperial units love fractions, especially when it comes to parts of an inch or mile. You’ll frequently find drill bits in fractions of an inch, which can be extremely frustrating when you are trying to do math in your head and figure out if a 17/64″ bit is bigger than a 1/4″ bit (hint, yes, it’s 1/64″ bigger).
A socket wrench set in Imperial fractions on the left and metric on the right. Metric is so much easier.
If it wasn’t hard enough already, there came the thousandth of an inch. As the machine age was getting better and better, and parts were getting smaller and more precise, there came a need for more accurate measurements than 1/64 inch. Development of appropriate tools for measuring such fine resolution was critical as well. You can call a 1/8″ bit a .125″ bit, and that means 125 thousandths of an inch. People didn’t like to wrap their mouths around that whole word, though, so it was reduced to “thou.” Others used the latin root for thousand, “mil.” To summarize, a mil is the equivalent of a thou, which is one thousandth of an inch. It should not be confused with a millimeter. It takes about 40 mils to make 1 millimeter. Also, the plural of mil is mils, and the plural of thou is thou.
Tools
Outside calipers for measuring the outer dimensionBy Glenn McKechnie (Own work) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia CommonsMeasuring length is done with a variety of tools, from GPS for long distances, to tape measures for feet/meters, and rulers for inches/centimeters. When it comes to very small measurements, the caliper is the tool of choice. This is the kind of tool that should be in everyone’s toolbox. Initially it started with the inside caliper and outside caliper, which were separate tools used to measure lengths. The Vernier caliper combined the two, added a depth meter and a couple other handy features, and gave machinists an all-around useful tool for measuring. Just like the slide rule, though, as soon as digital options became available, they took over. The digital caliper can usually switch modes between decimal inches, fractional inches, and metric.
Every industry has picked a different convention. Plastic sheets are usually measured in mils for thin stuff and millimeters or fractions of an inch for anything greater than 1/32″. Circuit boards combine units in every way imaginable, sometimes combining mils for trace width and metric for board dimensions, with the thickness of the copper expressed in ounces. (That’s not even a unit of length! It represents the amount of copper in one square foot of area and 1 oz is equivalent to 1.4mil.) Most of the time products designed outside of the U.S. are in metric units, while U.S. products are designed in either. When combining different industries, though, the difference in standards gets really annoying. For example, order 1/8″ plexiglass, and you may get 3mm plexiglass instead. Sure the difference is only .175mm (7 thou), but that difference can cause big problems for pieces that are press fit or when making finger joints on boxes, so it’s important that when sourcing components, you not only verify the unit, but if it’s a normal unit for that industry and it’s not just being rounded.
Often you can tell with what primary unit a product is designed with only a few measurements of a caliper. Find a dimension and see if it’s a nice round number in metric. If it’s not, switch it to imperial, and watch how quickly it snaps to a nice number.
Moving forward
Use metric if you can. The vast majority of the world does it. When you are sending designs overseas for production they will convert to metric (though they are used to working in both). It does take time to get used to it (especially when you are dealing with thou/mils), but your temporary discomfort will turn to relief when your design doesn’t crash into the Mars (or more realistically when you don’t have to pull out the Dremel and blade to get your parts to fit together).
Unless you get one of those fancy resistor kits where every value has its own compartment in a case or plastic baggie, you’ll soon rue the day your loose resistors become disorganized. [Kirll] has an interesting solution to hundreds of loose resistors: packaging tape. If you want a resistor, just grab a pair of scissors.
Okay, these Adafruit “totally not Muppets™” are awesome. The latest video in the Circuit Playground series is titled, “C is for Capacitor“. There’s also “B is for Battery“, because when life gives you lemons, light up an LED. Here’s the coloring book.
A few years ago, a couple of people at the LA Hackerspace Crashspace put together an animated flipbook device – something between a zoetrope and the numbers in those old electromechanical clocks – and launched a kickstarter. Now they’re putting on a show, presented by Giant Robot, featuring the animated art of dozens of artists.
Vintage electronics? Yes. Vintage Soviet electronics? Here’s 140 pages of pictures, mostly of old measurement devices.
![Outside calipers for measuring the outer dimensionBy Glenn McKechnie (Own work) [GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0], via Wikimedia Commons](https://hackaday.com/wp-content/uploads/2016/07/outsidecalipers.jpg?w=400)
