Robots in physics

I just watched the Global Physics Department of Matt Greenwolfe showing his use of Scribbler 2 robots as physics education tools.  See also Matt Greenwolfe’s blog, starting with The Robot Lab (formerly known as the Buggy Lab). It may be easier to get the content from the Matt’s blog posts than from the Global Physics Department recording, which had a lot of technical difficulties.

Scribbler 2 robot. Picture copied from the Parallax web site that sells the robot.

He did some modifications to the code for the Scribbler 2 to provide precise control of the robots (1 mm/sec velocity and 1 mm/sec/sec acceleration accuracy) and made some nice graphical interfaces for students to control the robots with x vs. t, v vs. t, and a vs.t plots (plus one interface that does 2D motion).

One big problem with the Scribbler 2 was the limitation to about 18.5 cm/s velocity, which is pretty slow. The cool thing about them is that they have wheel encoders that allow 0.491 mm resolution with 507.4 counts per revolution. One limitation that is a complete deal killer for me is that the Scribbler 2 library is only available for Windows machines, so porting to a Mac platform would be a major effort.

I was looking to see whether one could easily build such a robot from easily available parts. One cool new part is an integrated wheel, motor, controller, and shaft encoder called the HUB-ee (available for $35 from SparkFun):

The HUB-ee is a type of robot servo but designed for wheels, in fact it is a wheel, but it is also a motor, a sensor and a motor controller. What’s that? Did we just blow your mind?
When you want to add wheels to your robot you would normally start with a whole collection of parts: The motor and gearbox, a motor driver board, and maybe some sensors for measuring wheel speed and a controller to count revolutions or provide closed loop speed control. Well, the folks over at Creative Robotics thought it would be handy if you could just buy a wheel that had all of those things built in, so they designed HUB-ee – just bolt it onto a chassis, apply power and away you go!
The HUB-ee is easy to mount, too! There are two threaded inserts for M3 bolts built in, there’s also a right angle bracket included for situations when you can’t go horizontal into the chassis. The mounting holes are even LEGO® lug compatible!! HUB-ee uses Micro-MaTch connectors to keep electrical connections tight and easily changed, check out the related items for mating connectors.

HUB-ee wheel picture copied from Sparkfun web page, which says that images are licensed by CC BY-NC-SA 3.0.

The HUB-ee has a resolution of 128 counts per revolution of the wheel (1.473 mm resolution, 3× the step size of the Scribbler 2). The HUB-ee runs at 120rpm no load at 7v, which would be 37.7 cm/s, about twice the speed Matt reports for the Scribbler 2.

Although Matt reports 18.5 cm/s, the Parallax spec for the Scribbler 2 claims up to 80RPM, which would be 33.2 cm/sec, but that is probably with a 12V power source, rather than 6AA batteries. I suspect that Matt’s need for very precise control and operation with batteries limited the top speed he could use.  He does say that he would have liked a voltage controller (which would have added a $3–15 part cost to the robot, so a $7–40 increase in retail cost, based on the designs by TI’s WebBench tool or the PTN78000W module from TI) in order to have better speed control without having to worry so much about keeping the batteries fully charged.

The Hub-EE takes up several pins on a microcontroller (1 PWM pin, 2 output pins to control direction, 2 input pins for the quadrature encoder feedback) in addition to power and ground.  Two HUB-ee wheels would cost $70 and use 10 pins on a microcontroller—doable with an Arduino, but not leaving a lot of pins for other functions like sensor inputs.  There aren’t enough interrupt pins on the standard Arduinos to have all 4 wheel encoder pins triggering interrupts (which would be the highest-precision way to use the feedback information to get precise motor control).

Internally, the HUB-ee wheels use a Toshiba TB6593FNG motor controller, an H-bridge designed to work with 1.0A average current, with an on-resistance of about 0.35Ω for the output low.  The Toshiba data sheet doesn’t give the on-resistance of for high voltages directly, but if I’m interpreting their “Vsat” parameter directly, the on-resistance for each leg is about 0.5Ω, about a sixth that of the popular L293D H-bridges.  At under $3 a H-bridge (in single units), the TB6593FNG does not look like a bad choice for a small H-bridge.

Of course, to use the HUB-ee, one would have to build the rest of the robot (chassis, microcontroller, battery, … ). The Hub-EE is designed to mount to Lego beams, which could make chassis building easy, at least for prototyping.

I wonder whether, in a couple of years, we’ll be seeing integrated wheel units like the HUB-ee with an SPI interface, with registers that say how many steps to make, with specified velocity and acceleration curves.  That would provide very simple interfacing with fewer wires and could allow much tighter servo loops, at the price of putting a microcontroller at each wheel (probably adding $10 to the retail price of the wheels).

Sex and the single student

I subscribe to the Sociological Images blog, which often has interesting things to say about American society (and, more rarely, other parts of the world).  One of the two founders of the blog, Lisa Wade, studies hookup culture in colleges.  Some of her observations in The Banal, Mundane Sex Lives of College Students contradict the media-fueled perception of college students as sexual libertines:

Laird makes the common mistake of assuming that casual sex is rampant on college campuses. It’s true that more than 90% of students say that their campus is characterized by a hookup culture. But in fact, no more than 20% of students hook up very often; one-third of them abstain from hooking up altogether, and the remainder are occasional participators.

If you do the math, this is what you get: The median number of college hookups for a graduating senior is seven. This includes instances in which there was intercourse, but also times when two people just made out with their clothes on. The typical student acquires only two new sexual partners during college. Half of all hookups are with someone the person has hooked up with before. A quarter of students will be virgins when they graduate.

I think that a lot of college students would be reassured to know that they are not alone in getting little or no sex in college, but that doesn’t sell newspapers and magazines, so the information is quietly buried in scholarly papers that no one reads, or put into ham-handed university orientation handouts that even fewer people read (and that no one believes).

SMD tools and custom plastic parts

I did a little printed-circuit board design over the last couple of years, designing some motor controllers for Arduinos, some sensor boards, and some boards for my Applied Circuits class.  Almost all my designs have been for through-hole parts, so that I could assemble them with a soldering iron.  (That was an essential constraint for the boards for the course, as the students had never done any soldering before.)  Some of the sensor boards had surface-mount parts, but relatively easy ones to hand solder (0.05″ gull-wing leads).

Almost all electronic parts are cheaper and smaller in surface-mount packages, though, so I’ve been wondering whether I should get a hot-air rework station to be able to work more easily with surface mount components.  Sparkfun has a hot-air rework station for about $100 and Amazon has several, including a Kendal 898D 2-in-1 with a soldering iron for about $82.  I’m curious about how good these tools are and whether they are worth the price (my soldering iron, which works fine for through-hole parts, is only $15 from Parts Express).

One reason I’m considering getting a hot-air rework station is that my son is getting interested in hardware.  He’s been interesting in programming for years, and last year he did some fairly low-level programming involving interrupts for the data logger, but he’s not been particularly interested in the hardware. This summer he’s been teaching himself to use Eagle to do a surface-mount printed-circuit board design for a product design he and a friend of his came up with.  I don’t know whether they will ever finish the design and actually manufacture it, but their goal is to get a completed design by the end of summer and do a Kickstarter project to get enough funding to do a small run of 50.

My son has been learning a lot about product design, computer engineering, small-scale manufacturing, product pricing, design for upgrade, wear leveling in flash memory, power supply components, placement, … .  This project is easily the equivalent of many senior engineering capstone projects I’ve seen.

I’ve been learning some stuff along with him, as he asks me for advice and we have to search the web for information.  For example, we found that there are now small-run production services like Seeedstudio that could manufacture, populate, and test 100 small PC boards for about $14 a board (plus the price of the components).  One gotcha is that they need an engineering prototype, so you have to make one by hand first, hence the usefulness of a hot-air rework station.  I even looked into getting a reflow oven (which would allow doing more than one-at-a-time prototypes), but I don’t think I want to spend $390 (for the cheapest reflow oven I could find) when it is not clear that either of us will ever get beyond doing one-at-a-time prototyping.

I think that they may be able to make 50 of their devices for about $3500 for the assembled electronics and they want to sell at around $100–120 retail (direct sales).  Their biggest problems will probably be in the mechanical design: making the custom cases and stuff like that.  Although they have access to a 3D printer, I don’t think it has the precision and strength of materials that they need—they could probably make a prototype, but not 50 units that way.  Small scale manufacturing for mechanical parts seems to be harder to find and more expensive than for electronics.  Companies like Shapeways can do higher quality 3D printing, but the cost would be about $50 for the product—way too high for their $100–120 price point.

Short-run injection molding companies  (like DragonJewel) also exist, but the tooling is expensive: probably about $3000–5000 for what they want, which is too much for a run of only 50–100 units.  (The same tooling could be used for a run of 1000 or even 100,000 units, at which point it becomes cheap, but they’re not thinking of taking their first design to that scale.)  I mentioned DragonJewel by name only because it was one of the few sites we found that had any sort of estimate of costs.  Most of the sites are “quote only”.  Of course, real prices will require a quote, but it is good to know whether you are talking $300, $3000, or $30000 before you even start thinking about a service. Protomold has an automated quoting system, where you just upload a CAD model and they quote the costs, but they do have a minimum of about $1500.

Resin casting seems like a more feasible technique for very small runs.  Companies like Specialty Resin and Chemical claim prices like $50–100 for a mold and $4–8 a part, which is in the right ballpark for the cases for their product, if cast resin is suitable for the cases.  (Again, I picked this company out of many found by Google, because they gave a rough estimate of prices, and not just a “call for a quote” number.) The mold has to be made from a sample, but 3D printing by Shapeways might be a suitable way to make the sample. For that matter, there are companies (like Scicontech) that make molds from 3D printed models in house and do the casting, so there may be ways to go from CAD file to molded part quickly within one company—I’m actually surprised that Shapeways doesn’t provide that service. Perhaps the constraints on what is moldable are too complex to communicate easily.

 

 

Custom printed fabric

I just found out today, from a newspaper article in the Santa Cruz Sentinel, that there are now companies doing custom-printed fabrics.  You design your repeat block using whatever tools you like to get a raster image (like GIMP or Photoshop).

The finished block is then … uploaded to an on-demand digital printing service, which can print on all sorts of fabric in quantities of your choosing. Heather uses Spoonflower, which prints swatches, fat quarters, and yards of fabric, plus wallpaper, peel-and-stick wall decals and wrapping paper. … Another site, Fabric on Demand, specializes in custom fabric printing, and offers some fabrics, such as lycra/spandex, not available through Spoonflower.

Both websites are user-friendly, offering several design layout options, step-by-step instructions, and examples. The process is not inexpensive, though. On Spoonflower, for example, cotton fabric starts at $17.50 a yard, and their most expensive option, silk crepe de chine, is $38 a yard.

The prices are pretty high, but it sounds like a cool way to get custom-printed fabrics. (My interests in fiber arts were in weaving, not sewing, so I don’t really have any use for custom-printed fabrics, but the idea is cool.)

Santa Cruz as patent center?

I’ve always thought that Santa Cruz was somewhat underrated as a high-tech place, because of its proximity to the much larger Silicon Valley, but I was surprised to see that it is one of the top 10 places in patents per capita:

Almost 2/3 of U.S. patents are developed by people living in just 20 metro areas, which are home to 1 in 3 Americans. From 2007 to 2011, the places with the highest number of patents per capita are: San Jose (computer hardware and peripherals), Burlington, Vt. (semiconductor devices), Rochester, Minn. (computer hardware), Corvallis, Ore. (semiconductors), Boulder, Colo. (communications), Poughkeepsie, N.Y. (semiconductors), Ann Arbor, Mich. (motors, engines and parts), San Francisco and Oakland (biotechnology), Austin, Texas (computer hardware) and Santa Cruz, Calif. (computer hardware).

via 10 Surprising Facts About Patents : Discovery News.