Shallow Thoughts : : Feb

Akkana's Musings on Open Source Computing, Science, and Nature.

Sun, 19 Feb 2012

How a bogus Wikipedia entry changed nationwide tire company policy

We had to get two tires recently, after the Civic got a flat. Naturally, we wanted the new tires on the front. That's where steering and braking happens, as well as the drive wheels and most of the car's weight ... so that's where we wanted the newer tires.

The shop (America's Tire) refused. They said it's a company policy that a new pair of tires must always go on the rear.

[What America's Tire claims will happen] They've even printed up glossy signs explaining their reasoning -- a fancy poster image that is, unfortunately, wrong.

They show two scenarios. In the one on the left, the rear tires are losing traction, and the rear end of the car is sliding out. That's called "oversteer". The car might spin, especially if the driver has never experienced it before.

That part's all true. The problem with their diagram is the scenario on the right, where the presumably better tires are on the rear. In their diagram, magically all four tires are holding -- nothing ever loses traction. Good deal!

[What America's Tire claims will happen]

But what really happens if you put the bad tires on the front is that if something slips, it'll be the front. That's called "understeer".

Understeer can be just as dangerous as oversteer. With practice (I recommend autocross!) a driver can learn to detect oversteer and steer out of it before it gets to be a problem. There's an old saying among racers and performance drivers: "Oversteer is when the passenger is scared. Understeer is when the driver is scared."

Most passenger cars, especially front-wheel-drive cars like our Civic, are designed to understeer severely to begin with. Putting the poorer tires on the front makes that even worse.

And don't forget the importance of braking. Most of a car's braking ability comes from the front tires. Don't you want your best rubber working for you in a panic stop?

While I do understand why the default might be to put new tires on the rear -- it's better for inexperienced or panicky drivers -- to insist on it in all cases is just silly.

We drove the Civic home and rotated the tires ourselves.

How did the policy get started?

Dave and I first encountered this policy a couple of years ago. In the intervening years, it's become pervasive -- just about every tire shop insists on it now. How did that happen?

If you ask at the tire shop, they may tell you that it's a federal policy -- DOT or some such agency -- or even that it's a state law. Neither is true. It's merely company policy.

Some will also tell you that it arose from a lawsuit in which a tire company was sued after a customer spun out. So two years ago, we went looking to see if that was really true.

Back then, googling either "oversteer" or "understeer" led inexorably to a Wikipedia page with a reference to "San Luis Obispo County Court Case CV078853". Unfortunately, Wikipedia's link next to the court case reference actually led to a general page for a law firm that appears to specialize in vehicular personal injury lawsuits. (Nice advertising, that.) There was no information about any such case.

Nor did there seem to be any official records online of such a case; and the SLO courthouse didn't respond to an email request for more information.

Googling the court case, though, got lots of hits -- nearly all of them pasted verbatim from the Wikipedia page, then using that as "proof" of the supposed safety argument.

The test of time

Now, a few years later, it seems that nearly all tire manufacturers have adopted this as a firm, non-negotiable policy. Some shops are even using it as a reason to refuse to rotate tires! (See, the front tires wear faster on most cars, so if you rotate tires between front and rear, now you're putting the more worn tires on the rear ... which is dangerous! Better to just let those front tires wear out and make the customer buy a new pair.)

The news is better on the Wikipedia end. Someone eventually heeded Dave's attempt to fix the Wikipedia page, removed the bogus advertising link to the ambulance-chasing law firm, and added "citation needed". Subsequently, several people rewrote the page in stages, with comments like "This is a complete replacement. The existing version was wrong from the 1st sentence and has little relationship to the standard terminology." The page is much better now.

What isn't better is that the sentence from the old Wikipedia page is still all over the net, word for word. Google for the court case and you'll find lots of examples. Many of them are content mills copying random Wikipedia content onto pages that bear no relation to cars at all. But unfortunately, you'll also find lots of cases of people using this phantom court case to argue the safety point.

Sadly, it seems that once something gets onto Wikipedia, it becomes part of the zeitgeist forever ... and however wrong it might be, you'll never be able to convince people of that.

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[ 18:38 Feb 19, 2012    More misc | permalink to this entry | comments ]

Thu, 16 Feb 2012

Arduino Nano -- cute, but sensitive about USB cables

[Arduino Nano] I just got an Arduino Nano. Cute little thing -- I'm looking forward to using it in portable projects. But I had one problem when first plugging it in. It was getting power just fine, and blinking its LED -- but it wasn't showing up as a USB serial port in Linux. dmesg said things like:

usb 1-3.4: new full speed USB device number 7 using ehci_hcd
usb 1-3.4: device descriptor read/64, error -32
usb 1-3.4: device descriptor read/64, error -32
with several different device numbers each time, and an occasional unable to enumerate USB device on port 4 thrown in.

A web search found a few other people seeing this problem on Linux or Linux-based devices, with some people saying that pressing the RESET button multiple times helps. It didn't for me. What solved the problem for me was switching cables. The mini-USB cable I'd been using -- which has worked fine for other purposes, including programming other Arduinos through an FTDI Friend -- apparently was missing something the Nano needs for downloading. With a different cable, dmesg showed a much more civilized

usb 1-3.4: new full speed USB device number 20 using ehci_hcd
ftdi_sio 1-3.4:1.0: FTDI USB Serial Device converter detected
usb 1-3.4: Detected FT232RL
usb 1-3.4: Number of endpoints 2
usb 1-3.4: Endpoint 1 MaxPacketSize 64
usb 1-3.4: Endpoint 2 MaxPacketSize 64
usb 1-3.4: Setting MaxPacketSize 64
usb 1-3.4: FTDI USB Serial Device converter now attached to ttyUSB0

What was wrong with the cable? I did some testing with a multimeter versus a pinout diagram. Didn't get a definitive answer, but I did find that on the cable that doesn't work for the Nano, it was hard to get a solid connection on the D- (#2) pin inside the Type A connector. But since that's the connector that goes to the computer end (in my case, a powered hub), if it wasn't making good contact, I would expect it to show up everywhere, not just with the Nano. Maybe the Nano is more sensitive to a good solid D- connection than other devices.

I'm not really convinced. But Arduino's Troubleshooting Guide suggests: "Try a different USB cable; sometimes they don't work." So I guess they don't know what's special about some cables either.

So if your Arduino Nano doesn't initially connect properly, don't panic. Try a few different cables (everybody has about a zillion mini-USB cables lying around, right? If not, here, have five of mine). The Nano is happily composing random chiptunes as I write this.

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[ 15:24 Feb 16, 2012    More hardware | permalink to this entry | comments ]

Sat, 11 Feb 2012

Using motors with an Arduino

[Arduino-powered truck]

This is the story of my adventures learning to drive a little toy truck from an Arduino: specifically, how to drive the motors. Motor control turned out to be trickier than I expected, and I don't see a lot of "Arduino motor control for dummies" pages on the web, so I'm writing one.

My truck is from a thrift shop. It has two brushed motors (about 280-350 size, in R/C plane parlance). It was originally radio controlled. It has space for 4 AA batteries, nominal 6v, which I thought should be perfect for the Arduino.

Connecting directly to the Arduino (don't)

First, you can drive a small motor directly by plugging one lead into ground and the other into an Arduino digital or analog output line. (Analog output isn't real analog output -- it uses PWM, pulse width modulation.) Don't do this. You risk damaging your Arduino, either by putting too much current through it (the Arduino maxes out at 40ma per pin, 200ma total; a small motor can pull several amps), or from back-EMF when the motor stops.

Motor shields

[Freeduino motor shield] Lots of Arduino-oriented shops sell "motor shields". I bought a Freeduino motor shield because I could get it from Amazon and it was cheap. It's a kit you have to solder together, but it's a very easy soldering job. The demo code is easy, too. I wired it up to the Arduino, loaded the demo code, hooked up my Arduino to the truck's onboard batteries, and ... nothing. Sometimes the motor would twitch a bit, or hum, but the truck didn't move.

I wondered if maybe it was something about the batteries (though they were brand new). I tried plugging the Arduino in to the universal AC power supply I use for testing. No improvement.

At first I suspected that the motor shield was junk because its 1 amp maximum wasn't enough. But I was wrong -- the problem was the batteries. Neither the truck's 4xAA batteries nor the (supposedly) 1 amp AC adaptor could put out enough current to drive motors.

When it finally occurred to me to try a lithium-polymer model airplane battery (2 cells, 7.4 volts, 500 mAh), the truck immediately zipped across the floor and smashed into a chair leg.

So motor shields work fine, and they're very easy to use -- but don't underestimate the importance of your power supply. You need a battery capable of supplying a fairly beefy current.

But why is that, when the truck was designed for 4xAA batteries?

Well, the 4xAAs can drive the motors, but they can't drive the motors, the Arduino and the shield all at the same time. If I power the Arduino separately off a 9v battery, the truck will move. It doesn't zip across the room like with the li-po battery, but at least it moves.

Motor Driver

So I had a solution. Except I wanted something a little cheaper. A $20-30 motor shield is fine for a one-time project, but I was also shopping for parts to teach a summer camp class in robotics. We're on a shoestring budget, and an additional $20 per team is a little too much.


[Pololu TB6612FNG Dual Motor Driver] On a recommendation from Eugene at Linux Astronomy, who's been teaching wonderful robotics classes for several years, I discovered Pololu as a source of robotics equipment. Poking around their site, I found the TB6612FNG Dual Motor Driver Carrier, which is under $8 in quantity. Sounded like a much better deal, so I ordered one to try it out.

The TB6612FNG comes with headers not yet soldered. I substituted female headers, so it would be easier to plug in jumper wires to the Arduino and the male leads from the motors.

Writing an Arduino program for the TB6612FNG is a little more complicated than for the motor shield. It has two direction pins for each motor, plus a STDBY pin you have to keep high. So there are a lot more pins to manage, and when you change motor direction you have to toggle two pins, not just one. That'll make it more confusing for the students (who are beginning programmers), but I've written wrappers like drive(int whichmotor, int direc, int speed) to make it simpler.

The motor driver has the same power supply issue as the motor shield did: I can't power it, the Arduino and the motors all from the 4xAA batteries. Like the shield, it works okay with the Arduino on 9v, and great with one li-po powering everything.

Update: use a power transistor

It's been pointed out to me that an even cheaper way to run small motors, suitable for classrooms on a shoestring budged, is to use a transistor. Here's one circuit tutorial I found for that: Using a transistor to control high current loads with an Arduino.

Electronic Speed Controllers

[electronic speed controller]

I also tried using ESCs, the electronic speed controllers I've used with radio controlled airplanes. You can talk to them using the Arduino Servo library (there are lots of examples online). That works, but there are two issues:

  1. ESCs all have wierd proprietary arming sequences, so you have to figure out what they are (e.g. run the voltage up to maximum, hold there for two seconds, then down to zero, then hold for two seconds, then you're ready to drive) and write that into your code. If you switch ESCs, you may have to rewrite the arming code.
  2. ESCs only go in one direction -- fine for driving a truck forward, not so good if you need to drive a steering motor both ways.

I'm sure ESCs have the same battery issue as the other two options, but I didn't even try running one off the AAs. Anyone who has ESCs sitting around probably has beefy batteries too.

Custom H-bridges

All the cool robotics hipsters (cHipsters?) buy H-bridge chips and build their own circuits around them, rather than using things like motor shields or motor drivers.

This H-bridge circuit by Bob Blick is one popular example. (Those double-transistor-in-a-circle things are Darlington transistors.) But a web search like arduino h-bridge circuit turns up other options.

For driving big motors, you definitely need your own H-bridge circuit (or an ESC), since all the available motor shields and drivers are limited to 2 amps or less. For small motors like my toy truck, I'm not sure what the advantage is. Except being one of the cool cats.

Summary

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[ 12:45 Feb 11, 2012    More hardware | permalink to this entry | comments ]

Tue, 07 Feb 2012

Tips on building an Ardweeny

[Ardweeny] I've wanted an Ardweeny for a long time. It's just so cute -- it's an Arduino compatible that's barely bigger than the Atmega 328 chip driving it. Plus, it's cheap: $10 including the chip.

Like most small or low-cost Arduino clones, the Ardweeny doesn't have its own USB connection; instead, you use an adaptor such as an FTDI Friend, which slides onto a 6-pin header on the Ardweeny. I knew that ahead of time.

One thing I hadn't realized was that the Ardweeny gets its only power from the USB adaptor. So if you want to use an Ardweeny by itself with no computer connection, you need a regulated 5v power supply. Those are easy enough to build (see the Breadboard Arduino), but don't forget to allow for that when designing projects.

The Ardweeny comes as a kit that needs soldering -- something that isn't made clear in the sales literature, though for the price, it didn't surprise me. I downloaded the Ardweeny soldering steps (PDF) and got to work.

Easy initial build

The PDF estimates 15 minutes for the construction. The first part, soldering the 10 components, was a snap, and took maybe 10 minutes. At this point you can take the Ardweeny and nestle it down over the Atmega chip, and test it to check your soldering work.

I plugged in my FTDI Friend and the LED immediately started blinking. Success! (It's nice of them to pre-program the chip with blink code, so it's easy to tell it's working.) Downloading my own version of the blink sketch (use the Duemilanove w/Atmega 238 setting, or atmega328 if you use my Makefile) also worked fine.

The last step: soldering the legs

Except that I wasn't done. The next step of the build is to solder all 28 legs of the Ardweeny directly to the Atmega chip's legs. Scary idea -- won't all that heat kill the chip? But the instructions didn't have any warnings about that. I took a deep breath and forged ahead.

This part put me way over the 15-minute estimate -- 28 pins is a lot of pins, and I took it slowly, careful to be sparing with heat and solder.

When I was finally done, I plugged the FTDI Friend back in and ... nothing. Not a blink. And when I checked voltage on one of the V+ pins versus the ground pin, it measured around 1.5v, not the 5v I expected to see.

So I'd messed something up. Somehow, even though it worked without soldering the legs, that additional soldering had broken it. I went through all the pins with a voltmeter checking for shorts, and tested everything I could. Nothing obviously wrong.

It might have been nice to inspect my solder joints on the Ardweeny -- but once the Ardweeny is soldered to the chip, the solder is all inside and you can't see it. But anyway, I'd tested it and it had worked fine.

Detaching the backpack from the chip

So I figured I must have destroyed the chip with heat or static during that soldering process. My Ardweeny was a brick. Nothing salvageable at all. Unless -- if I could somehow de-solder the legs and pull the two apart, I could use the Ardweeny with another chip.

But how do you de-solder 28 legs? I tried a solder sucker (a pen-shaped vacuum pump) and de-soldering braid, but neither one broke the bond between the two sets of legs. I tried sliding an X-acto knife in between the Ardweeny's legs and the chip's while heating the leg with solder; no luck, the knife blade was too good a heat sink and the solder never melted.

Dave finally found a solution. With my assurance that the chip was probably dead anyway, he rolled the Ardweeny on its back, and used the tip of the heated soldering iron to bend each chip leg inward away from the Ardweeny leg. When he was done, the chip looked bent and sad, like a squashed millipede -- but the pieces were separated.

Testing to find the problem

And now I could take the Ardweeny and stick it on an Atmega 328 I pulled out of another Arduino. Plugged in the FTDI Friend and -- nothing.

Wait, it was the backpack that was bad? But I tested it before doing that last soldering phase!

I took the sad squashed-millipede Atmega and carefully bent all the pins back into shape, or at least close enough that I could plug it into a socket in my Diecimila. And, amazingly -- that poor abused overheated squashed bent 328 still worked just fine.

Okay, so the problem is definitely in the Ardweeny backpack. Now that the solder joints were exposed again, I examined them all and found two that looked questionable. I re-soldered them -- and everything worked.

Lessons for the Ardweeny

I still don't know why my board worked the first time, then failed after the step of soldering the legs. But it makes me nervous about repeating that leg-soldering step. What if something else, something that's working now, stops working?

For now, I'll probably solder just a few pins -- maybe the four outermost ones -- and rely on pressure for the other contacts. Of course, in a real environment where the Ardweeny might be subject to vibration and temperature changes, that might not be good enough. But until then, it seems the safest option.

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[ 16:26 Feb 07, 2012    More hardware | permalink to this entry | comments ]

Thu, 02 Feb 2012

Watch out, or you'll end up in Pleasant Valley

[Pleasant Valley State Prison sign] I love this sign, along Interstate 5 near Coalinga. Pleasant Valley State Prison.

I guess if you have to be locked away, Pleasant Valley doesn't sound like the worst place to be.

Do Pleasant Valley ex-cons have a hard time getting respect when people find out where they did their hard time?

I love picturing the local parents trying to strike fear into their kids' hearts. "If you don't straighten up, Junior, you're going to end up in ... PLEASANT VALLEY!"

Or the local judges -- "You're not getting off this time. No, I'm giving you ten years in ... PLEASANT VALLEY!"

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[ 17:07 Feb 02, 2012    More humor | permalink to this entry | comments ]

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