Wedo physics

I'm really excited about getting the Wedo working with Sniff - it seems to be a missing piece of the puzzle that makes it easier to get in on the fun of physical computing without having to wire stuff up, or worry about class 6B shorting out the GPIO on all your Pi's!

It's been ages since we've done any science here on the Sniff blog, so lets do some basic physics with the Wedo...

One of the really useful parts - perhaps one of the most culturally significant of its day is the discovery of the pendulum. A pendulums period depends on its length, and NOT its mass. That let people build clocks, and measure time accurately - without the pendulum there'd be not time keeping, which rules out accurate navigation (remember the original Longitude prize?), pretty much any kind of science (cause we're always measuring how long things take), no school time tables, and no iWatch. I'm sure you could integrate this into a wider cross curriculum activity about time and timekeeping, but for now lets get back to the experiment.

This is the easiest Lego build ever:At one end we've got the Wedo, and an axle. On the axle is a long brick, which is connected to other long bricks to make the arm. At the end of the arm are a couple of tires to make a weight. A little way down the arm I 've attached the wedo tilt sensor, so that its horizontal when the arm is vertical. Strictly for this to work the arm should be much lighter than the weight - we'll see how that works out...

make wedo device

make wedoConnector number

make wedoValue number

make startTime number

when start

.set wedoConnector to 1

.forever

..repeat until wedoValue=1

...tell wedo to "readTilt"

..

..repeat until not wedoValue=1

...tell wedo to "readTilt"

..

..say [timer - startTime]

..set startTime to timer

The code is equally trivial. If we ignore the timer code for now, we wait until the sensor is reading one indicating the sensor is tipped forwards, then wait until it leaves that region. At the moment the sensor stops reading 1 we print out the time since we started, then go back ground, and wait for it to happen again.

It turns out to be pretty accurate - our makeshift pendulum is accurate to within a few hundredths of a second. Using the internationally recognised standard of unit of measurement : the Lego Stud, out arm is 49studs long and has a period of 1.2 seconds (give or take about 2 hundredths of a second). 

Taking out 2 of the bars gives us a length of 27studs, and a pretty reliable period of 0.9s.

So what does the maths tell us to expect? Well assuming a "perfect" pendulum, swinging over a small angle, then:

[Cite Wikipedia]

Where T is the time period, l is the pendulum length, and g is the gravitational force which on earth is 1226studs per second squared (you may be more familiar with the metric version of 9.81 m/s/s).

Plugging 49 studs into this equation gives 1.25s, while 27studs give 0.93s

WOW!!!!! That's pretty much spot on...

But there's another way to spin this - I knew T and l, and I looked up g (then converted it to Lego). I used that to check my experiment. My experiment was how accurate is a lego Wedo pendulum at keeping time (pretty good!).

But there's something in that equation that we actually might not have known: g. It's been pretty well measured and documented, but when did you last check?

We can rearrange that equation, and using 49studs and 1.2 seconds we get g=1343studs/s/s  (or 10.7m/s/s). That's about a 10% error, which considering the pendulum equation is only an approximation, and we build it out of Lego, that's pretty amazing.

WE JUST MEASURED GRAVITY WITH LEGO!

We do WEDO!

Unless you're a primary school teacher you've probably never heard of the Lego Wedo. We hadn't either, which is a shame because they're actually really cool. Everyone loves Lego, but as you've  probably noted in the last few posts, Lego EV3 is expensive and quite hard work to get started with. I know of one local high school where the design and technology team went with regular Lego Technic's Power Functions motors, as the PF components are about half the price of the equivalent EV3 components. They can build machines, and power them much more cheaply than an EV3 setup.

The downside is of course there are no PF sensors, and you can't hook them up to a computer.

EXCEPT THAT LEGO 

DO MAKE  PF SENSORS 

AND A COMPUTER INTERACE!!!

Wow!! Every PF forum has people asking how do I make "smart" power functions, and the replies are all about hacking cables, when Lego sell a set to to exactly that

Lego 9580 WeDo Construction Set

There it is Lego 9580. Look at those bricks at the front... They're Power Functions, but not as we know them.  There's a PF "M" motor, but on the right is the WeDo USB hub... That's right - it plugs into any computer, and replaces the PF battery box, so you can drive motors from the computer. But look what else - that's a tilt sensor, and an IR range sensor!!! All PF compatable.

We why have't we heard of it? Well as the ad's used to say "remember, Lego Wedo isn't available in the shops!". It's Lego education, which means its expensive and hard to find, but its out there (and its on Amazon!). Expect to pay around £120 for the set. Alternativly if you shop around, you might find the components available separately - the HUB costs about £35, while the sensors are £15-£20. £70 for the "cool parts" isn't cheap, but its cheaper than the equivalent EV3 kit.

The Wedo is targeted at primary school children, with some simple to use software (not included and another £70!) that lets you build, and control mini-machines. While its expensive, a lot of work has gone into supporting lesson plans and kids will go crazy for it, so its probably good value (ironically my daughter came home from school the same day my Wedo arrived, raving about how they were doing a new exciting project at school - they were using the Wedo!).

But given how obviously useful, and in demand the functionality of Wedo is in secondary schools, and among "older" Lego users, why isn't it a standard PF brick that's in every toy store? I guess the answer is Software - Lego have got software for KS2 children, built and ready to go. To sell Wedo to a wider audience it requires development and support of more general tools, which simply don't exist yet...

The good news is that the Wedo works in Scratch, so you can introduce kids to the Wedo there, and get them building. When they move to Sniff, its probably a good idea to revisit some of their Wedo projects again, and rework them in Sniff.

To get started you'll need to hook your Wedo up to a Pi, or generic Linux machine [Update: It works great on Windows and Mac now too, though wedoScan is Linux only]. Then run the Sniff setup file, and you should be able to type "wedoScan" to find and interrogate your Wedo.

Found Wedo on HIDraw 2

plug 1: Motor

plug 2: Tilt

It should report something like the above. It also turns the motors off which can be handy! If it doesn't report the connections correctly, run it again after a couple of seconds - the hub can report incorrectly if the motors are/have been running, and a recscan usually fixes it.

Now we know its all hooked up we can program it:

make wedo device

make wedoConnector number

make wedoValue number

Start by making a wedo device. If you like you can use the HIDraw number that wedoScan found (usefull if you have several hubs!), or simply let the device find the hub for itself.

when start

.forever

..set wedoConnector to 2

..tell wedo to "readTilt"

..

..say [wedoValue]

To get things going we specify the connector we're interested in, and in this case (as it has a Tilt sensor attached, we call "readTilt". The values returned in wedoValue are:

#0 Flat

#1 Forward (away from wire!)

#2 Back

#3 Left

#4 Right

Having read the Tilt sensor, we can use that to control the motor speed:

..if wedoValue=1

...change speed by 0.1

..if wedoValue=2

...change speed by -0.1

..if speed >1

...set speed to 1

..if speed <-1

...set speed to -1

..

..set wedoConnector to 1

..set wedoValue to speed

..tell wedo to "setMotorSpeed"

We adjust the speed variable, and then select the connector, and value we want to send. For a motor wedoValue should be between -1 and +1, with 0 being stopped.

There's also a readDistance command, which tries to return a value in metres, though that might be a bit inaccurate. Try running:

when start

.forever

..set wedoConnector to 2

..tell wedo to "readDistance"

..

..say [wedoValue]

and pick values appropriately.

And that's it! Lego Wedo control. You could also hook up the PF lights, and Servo (just use the motor commands) and build something really cool. If you're luck enough to have more than one Wedo then you should be able to hook up as many as you like (up to about 90 or so!). Just make a device for each, and give it a name:

make leftHub wedo device 2

make rightHub wedo device 2

Now for the really exciting bit... This works on Linux...The Lego EV3 runs linux... In principle it should be possible to run this on the EV3, and connect the EV3 to Power Functions using official lego blocks!!!! This is something of a holy grail in the EV3 forums. However there's a catch - the standard EV3 linux distro is quite old, so you'll need to update it to something that supports "HIDraw" (a way of accessing USB devices). Lego may provide this in a future update, but if you're prepared to go a little "of grid", you could try installing the ev3dev, or LeJos firmware, and try it!

The code in this post is in the Sniff/examples/Hosted/wedo folder. You can also find the source for wedoScan (and ev3dude) in Sniff/utils.

Release 10

We've been working crazy hard the last few weeks to get a lot of new features into Sniff.

The new build now supports EV3 over bluetooth. We'd still recommend having a wifi adapter as that lets you telnet into the Brick, and run programs directly but using the ev3dude program we developed you can now transfer programs to the Brick using only bluetooth, and once you've installed the sniffMenu program you can run Sniff programs straight from the Brick's onscreen menu.

We also discovered one of Lego's best kept secret products - the Wedo! This lets you connect Lego power functions bricks to a computer via USB. We've figured out how it works and added support for a Wedo Sniff device - it works great on Raspberry Pi, and we'll post a longer post about it asap.

However the big change is under the hood. Previously devices worked like code libraries - you imported them once and used them. However now they're more or less real objects - each time you creates a new instance, for example you can hook up as many dht11's as you have pins. You can even hook up multiple Wedos!

You can download Release 10 now!

(We've also given up calling them Beta releases - Sniff works. We'll still be evolving it, but its now a stable and fully operations system).

ev3dude: programming the Lego EV3 without WIFI

To get access to the Unix shell on the EV3 you need a WiFi adapter. That's sort of OK, but only 1 adapter is supported, and its quite large. If you don't have one, you don't have access to a WiFi network or perhaps you've simply put the EV3 somewhere a the adapter isn't going to fit, then you can't get a shell, you can't use SSH and you can't transfer files.

However the EV3 has USB and Bluetooth built in. The problem is that they only supports the proprietary protocol used by the Lego Software. A couple of people have built tools to work with it, buts what's needed is a generic, and simple command line tool that lets you transfer files to the EV3. Well here it is and its called ev3dude!

Firstly you need to pair your computer to the EV3 over Bluetooth. As my only bluetooth machine is a Mac, I've built and tested on that, and when you pair it creates a file called /dev/cu.EV3-SerialPort. The name will change if you've renamed your brick to something other than EV3, but it'll be something like that.

Now get the source to ev3dude, and compile it by typing:
make ev3dude

now test it by running
ev3dude /dev/cu.EV3-SerialPort

(or whatever your device is called).

Now you can get a directory listing of your SD card by typing:
ev3dude /dev/cu.EV3-SerialPort -l /media/card

The formatting is a bit funny, as the EV3 sends md5 hashes of the files, along with their size and name, but the info is pretty clear.

To write a file
ev3dude /dev/cu.EV3-SerialPort -w myLocalFile /media/card/filenameOnBrick

and if you change your mind you can delete it again:
ev3dude /dev/cu.EV3-SerialPort -d /media/card/filenameOnBrick

That's it! Simple and clean EV3 file transfer. It should compile without any serious issues on any Unix (let me know if it doesn't!). You're free to use and modify the code as you see fit, but please send back any changes/improvements/bug fixes so we can make this a better tool.

Combined with the Sniff menu system described in the last post we've got a mechanism for installing and running Sniff code on the Brick without WiFi. It'll be fully integrated into Sniff in the next release, but for now you can use ev3-sniff to generate the binary, and transfer it manually with ev3dude.

You can download ev3dude.c here.

Running Sniff on the Lego EV3

In a previous post we discussed how to get a C program running on the Lego EV3, as a working C toolchain is prerequisite for running Sniff. It's quite a hassle to set up, and to get the EV3 running nicely is a bit more of a hassle, but once its set up it's a pretty nice platform for running Sniff on.

The new release (Beta 9) supports the EV3, though so far it only supports the buttons, LED's and the screen. Support for motors and sensors will be forthcoming in the next releases, as and when I can get my hands on the relevant devices.

Once you've installed the C compiler, and the new Sniff release on your host machine, there's a couple of things you need to do to make everything work.

The first thing to do is make sure the bricks WiFi is enabled, and connected. Then on your host machine, initialise Sniff, and run "ev3finder". This prints out the ip address of your Brick. In the shell type:

export EV3HOST=192.168.0.123

or whatever the IP address reported is, to tell the system how to find the Brick.

Then connect to the brick and enable ssh:

telnet $EV3HOST

login as root (no password!)

and type

dropbear

First time setup:

If this is the first time you've used Sniff on the Brick you'll need to make  a folder for it:

mkdir /media/card/Sniff

Now go back to your Mac or Linux machine and you should be able to compile programs using ev3-sniff, just as you would for any other platform. When you compile a program it will be placed in the Sniff folder on the memory card, and you can run it from the telnet session.

However if you want to use the keypad or the screen your program will "fight" the LMS/EV3 user interface, and it'll be a bit of a mess. It also means you have to have another machine, and a WiFi connection which might not always be possible.

The solution to this is based on a trick by RobotNav. In the Sniff/Examples/EV3/SniffMenu folder is a Lego VM program: the source is Sniff.lms, but you just need the compiled version Sniff.rbf. Place this in the Sniff folder with

scp Sniff.rbf root@$EV3HOST:/media/card/Sniff

You can run this from the Bricks on-screen menus. When you do it will try and run a Sniff program called sniffMenu. You'll find this in the same folder. If you simply compile that it will install on the Brick. sniffMenu lists all of the installed programs in the Sniff folder, and lets you choose one to run, so now you can compile the program in your desktop machine, and run it easily on the Brick. SniffMenu is pretty primitive at the moment, but its written in Sniff, so you can make a better version!

EV3 Devices

Currently there are two EV3 specific devices: ev3lcd and ev3Keypad. There are examples of using them both in the Sniff/examples/EV3 folder. Basically ev3lcd behaves in the same way as all the other display devices. ev3Keypad mimics the Gambuino keypad. However the setBacklight uses values 0-9 to represent different patterns and colours, rather than adjusting brightness. Fortunatly 0 is off and 1 is sold green so the results work fairly consistently cross platform.

To test these there's a version of Bounce Out - our favourite game written in Sniff, running on the EV3. The only changes from the Arduino version is to comment out the Sound player, replace Arduino devices with their EV3 equivalent, and add a scale factor so that the game is sized to fill the EV3's larger screen. Things are a little hard to see on the small screen (and the "standard" 5x7 font is a bit small!), but it generally works great.

The one thing to remember if you're running from the EV3 menu (rather than telnet'ing in from your computer) is that your program needs to end, otherwise it will run forever and hang the EV3 (you can still connect by telnet, use ps to find the offending program and kill it)! There's an example in the EV3 examples folder which shows how you can make your program quit when you hold the back button for 1 second. Also remember that "say" and "ask" will only work from the telnet session.

Release Beta 9

Release B9 is a bit of a special one as it is the first version of Sniff to run on the Lego EV3. We've even changed the banner text at the top of this page to celebrate!

You can compile Sniff programs on a Mac or Linux machine, and run them on the EV3. There's also a little menu program which is written for the Lego VM(lms2012) which allows you to run installed Sniff programs directly from the EV3's onscreen controls. It's especially sweet that a big part of that is written in Sniff, so you can improve it!

There'll be a full blog post later today with a step by step explanation of working with Sniff on the EV3, including installing the onscreen menus.

The EV3 device support in this release only includes the screen and keypad/led's, but we'll be adding support for motors and sensors as soon as possible.

This release also includes the Studuino support we've previously posted about.

You can download then release here.

First Steps onto the Lego EV3

Admit it - you want a Lego EV3 kit. Of course its strictly for serious academic work related reasons... its not that a giant Lego robot is seriously cool.  The only thing that stands between you and world domination is the £300 cost. Lego sell the EV3 brick by itself in their online store, but that's £200 which actually seems even worse value. Your inner child says I WANT, but your outer adult just has to explain to your inner child how credit cards really work...

The EV3 has a lot of potential as a Sniff platform. Lego is instantly accessible, and familiar to pretty much everyone, so building machines from Lego and programming them in Sniff is pretty attractive. It's been on the radar for a while, but the up front cost meant it kept getting put off...

Then a few days ago I found a few EV3 bricks for £75 on Ebay... While the full EV3 robot kit would be nice, I do already have quite a lot of lego (I mean my kids have quite a lot of lego!), and I'm really interested in the programming side of the thing (honest!). I can pick up a few sensors and motors as I need them for about £20 each.

At £75 the brick is still pretty expensive compared to an arduino Uno clone, but about the same as a Yun. For your cash you get something that's designed to be dropped, has a nice screen, LEDs, and buttons, includes bluetooth (with iPhone support), runs Linux on a 300MHz ARM, and has LEGO written on it. The whole thing is really nice quality, so on balance (at that price!) its really good value for money if we can get it to play nice...

So having got my hands on an EV3, the first stage of getting Sniff running, is getting C programs running. While you'd think this is pretty easy, the resources out there can be pretty confusing (to be fair the EV3 is still pretty new, so a lot of users are transitioning form NXT, so there's jumble of info on the different systems). The best resource is www.robotnav.com. However as setting this up is going to be a pre-requisite to any Sniff system, here's a walk through of setting up a Mac or Linux box as a dev platform for EV3.

Lets get the ARM tool chain up and running on a generic Linux.

The standard compiler that seems to be used with the EV3 Code Sourcery lite. You could build gcc from source, but CS is a free dowload:

wget -c http://www.codesourcery.com/sgpp/lite/arm/portal/package4571/public/arm-none-linux-gnueabi/arm-2009q1-203-arm-none-linux-gnueabi-i686-pc-linux-gnu.tar.bz2

tar -jxvf arm-2009q1-203-arm-none-linux-gnueabi-i686-pc-linux-gnu.tar.bz2

This puts the executables into a folder called arm-2009q1/bin so add that to your path:

export PATH=$PATH:[wherever you put it]/arm-2009q1/bin

One gotcha is that CS is a 32bit binary, so you might need to install the 32 bit compatabiltiy libs on your machine. On Centos the command to do that is:

sudo yum install compat-libstdc++-296.i686

Similar libs will exist on your preferred Linux. I used Centos for testing, not because its good, but because its so far behind Debian and Ubuntu that if it works on Centos it'll probably work anywhere!

Next we just need to compile a C program:

arm-none-linux-gnueabi-gcc hello.c -o hello

And On Mac!

Unfortunatly Code Sourcery isn't distributed officially for Mac, and its a pain to build yourself. However some nice people have done the work for you and you can download an unofficial package from here:

http://www.carlson-minot.com/available-arm-gnu-linux-g-lite-builds-for-mac-os-x

Install the package, and then add to your path:

export PATH=$PATH:/usr/local/carlson-minot/crosscompilers/bin

Get Networking ...erm... working?

While the EV3 supports communication over bluetooth and usb, the only way to get directly into the Linux side of the EV3 is to use WIFI. You'll need to get hold of a Netgear WNA1100 wifi dongle. Nothing else will work. You may or may not be able to get them officially from Lego (at Lego prices), but ebay will get you one for less than £10 (While you're on Ebay pick up a microSD card too). Plug in the dongle, and turn it on from the bricks settings menu. You'll also need to select the network, and enter any passwords. You can also configure and set it up from the Lego desktop software (having connected the brick via USB), which is probably easier.

From the Brick's "Brick Info" screen you should be able to find the IP address (something like 192.168.1.123). Alternativly you can download my EV3 Finder. Run it on your computer and it should print out the addresses of any EV3's on the local network. This serves as a handy check that everything is working.

 Now from your desktop, you can log into the EV3 by typing:

telnet 192.168.1.123

(or whatever your number was).

 log in as root (no password!).  At this point you'll find a pretty generic, embedded Linux installation. Once you've got over the novelty of poking round Lego Unix(How cool is that!!) Then run "dropbear" which starts ssh services, so we'll be able to copy programs onto the EV3.

Transfer and Run

Back on the desktop machine we can copy the program we compiled earlier onto the EV's SD card by running:

scp hello root@192.168.1.123:/media/card

Then use telnet (or ssh) to access the EV3 again, and you can run the program by typing

/media/card/hello

And that's it - we've got a working development system which lets its compile C code, and run it on the EV3. From there its a few minor tweaks to take Sniff and get it running. The current state is that the Sniff core system is running fine, with support for the buttons, LED's and Screen. Bounce Out is running nicely! I'll post a release "real soon", so you can get started. Motors and Sensor support will follow after that.