Petter Reinholdtsen

Entries tagged "lego".

My own self balancing Lego Segway
4th November 2016

A while back I received a Gyro sensor for the NXT Mindstorms controller as a birthday present. It had been on my wishlist for a while, because I wanted to build a Segway like balancing lego robot. I had already built a simple balancing robot with the kids, using the light/color sensor included in the NXT kit as the balance sensor, but it was not working very well. It could balance for a while, but was very sensitive to the light condition in the room and the reflective properties of the surface and would fall over after a short while. I wanted something more robust, and had the gyro sensor from HiTechnic I believed would solve it on my wishlist for some years before it suddenly showed up as a gift from my loved ones. :)

Unfortunately I have not had time to sit down and play with it since then. But that changed some days ago, when I was searching for lego segway information and came across a recipe from HiTechnic for building the HTWay, a segway like balancing robot. Build instructions and source code was included, so it was just a question of putting it all together. And thanks to the great work of many Debian developers, the compiler needed to build the source for the NXT is already included in Debian, so I was read to go in less than an hour. The resulting robot do not look very impressive in its simplicity:

Because I lack the infrared sensor used to control the robot in the design from HiTechnic, I had to comment out the last task (taskControl). I simply placed /* and */ around it get the program working without that sensor present. Now it balances just fine until the battery status run low:

Now we would like to teach it how to follow a line and take remote control instructions using the included Bluetooth receiver in the NXT.

If you, like me, love LEGO and want to make sure we find the tools they need to work with LEGO in Debian and all our derivative distributions like Ubuntu, check out the LEGO designers project page and join the Debian LEGO team. Personally I own a RCX and NXT controller (no EV3), and would like to make sure the Debian tools needed to program the systems I own work as they should.

Tags: debian, english, lego, robot.
Isenkram, Appstream and udev make life as a LEGO builder easier
7th October 2016

The Isenkram system provide a practical and easy way to figure out which packages support the hardware in a given machine. The command line tool isenkram-lookup and the tasksel options provide a convenient way to list and install packages relevant for the current hardware during system installation, both user space packages and firmware packages. The GUI background daemon on the other hand provide a pop-up proposing to install packages when a new dongle is inserted while using the computer. For example, if you plug in a smart card reader, the system will ask if you want to install pcscd if that package isn't already installed, and if you plug in a USB video camera the system will ask if you want to install cheese if cheese is currently missing. This already work just fine.

But Isenkram depend on a database mapping from hardware IDs to package names. When I started no such database existed in Debian, so I made my own data set and included it with the isenkram package and made isenkram fetch the latest version of this database from git using http. This way the isenkram users would get updated package proposals as soon as I learned more about hardware related packages.

The hardware is identified using modalias strings. The modalias design is from the Linux kernel where most hardware descriptors are made available as a strings that can be matched using filename style globbing. It handle USB, PCI, DMI and a lot of other hardware related identifiers.

The downside to the Isenkram specific database is that there is no information about relevant distribution / Debian version, making isenkram propose obsolete packages too. But along came AppStream, a cross distribution mechanism to store and collect metadata about software packages. When I heard about the proposal, I contacted the people involved and suggested to add a hardware matching rule using modalias strings in the specification, to be able to use AppStream for mapping hardware to packages. This idea was accepted and AppStream is now a great way for a package to announce the hardware it support in a distribution neutral way. I wrote a recipe on how to add such meta-information in a blog post last December. If you have a hardware related package in Debian, please announce the relevant hardware IDs using AppStream.

In Debian, almost all packages that can talk to a LEGO Mindestorms RCX or NXT unit, announce this support using AppStream. The effect is that when you insert such LEGO robot controller into your Debian machine, Isenkram will propose to install the packages needed to get it working. The intention is that this should allow the local user to start programming his robot controller right away without having to guess what packages to use or which permissions to fix.

But when I sat down with my son the other day to program our NXT unit using his Debian Stretch computer, I discovered something annoying. The local console user (ie my son) did not get access to the USB device for programming the unit. This used to work, but no longer in Jessie and Stretch. After some investigation and asking around on #debian-devel, I discovered that this was because udev had changed the mechanism used to grant access to local devices. The ConsoleKit mechanism from /lib/udev/rules.d/70-udev-acl.rules no longer applied, because LDAP users no longer was added to the plugdev group during login. Michael Biebl told me that this method was obsolete and the new method used ACLs instead. This was good news, as the plugdev mechanism is a mess when using a remote user directory like LDAP. Using ACLs would make sure a user lost device access when she logged out, even if the user left behind a background process which would retain the plugdev membership with the ConsoleKit setup. Armed with this knowledge I moved on to fix the access problem for the LEGO Mindstorms related packages.

The new system uses a udev tag, 'uaccess'. It can either be applied directly for a device, or is applied in /lib/udev/rules.d/70-uaccess.rules for classes of devices. As the LEGO Mindstorms udev rules did not have a class, I decided to add the tag directly in the udev rules files included in the packages. Here is one example. For the nqc C compiler for the RCX, the /lib/udev/rules.d/60-nqc.rules file now look like this:

SUBSYSTEM=="usb", ACTION=="add", ATTR{idVendor}=="0694", ATTR{idProduct}=="0001", \
    SYMLINK+="rcx-%k", TAG+="uaccess"

The key part is the 'TAG+="uaccess"' at the end. I suspect all packages using plugdev in their /lib/udev/rules.d/ files should be changed to use this tag (either directly or indirectly via 70-uaccess.rules). Perhaps a lintian check should be created to detect this?

I've been unable to find good documentation on the uaccess feature. It is unclear to me if the uaccess tag is an internal implementation detail like the udev-acl tag used by /lib/udev/rules.d/70-udev-acl.rules. If it is, I guess the indirect method is the preferred way. Michael asked for more documentation from the systemd project and I hope it will make this clearer. For now I use the generic classes when they exist and is already handled by 70-uaccess.rules, and add the tag directly if no such class exist.

To learn more about the isenkram system, please check out my blog posts tagged isenkram.

To help out making life for LEGO constructors in Debian easier, please join us on our IRC channel #debian-lego and join the Debian LEGO team in the Alioth project we created yesterday. A mailing list is not yet created, but we are working on it. :)

As usual, if you use Bitcoin and want to show your support of my activities, please send Bitcoin donations to my address 15oWEoG9dUPovwmUL9KWAnYRtNJEkP1u1b.

Tags: debian, english, isenkram, lego.
Debian, the Linux distribution of choice for LEGO designers?
11th May 2013

In January, I announced a new IRC channel #debian-lego, for those of us in the Debian and Linux community interested in LEGO, the marvellous construction system from Denmark. We also created a wiki page to have a place to take notes and write down our plans and hopes. And several people showed up to help. I was very happy to see the effect of my call. Since the small start, we have a debtags tag hardware::hobby:lego tag for LEGO related packages, and now count 10 packages related to LEGO and Mindstorms:

brickosalternative OS for LEGO Mindstorms RCX. Supports development in C/C++
leocadvirtual brick CAD software
libnxtutility library for talking to the LEGO Mindstorms NX
lnpddaemon for LNP communication with BrickOS
nbccompiler for LEGO Mindstorms NXT bricks
nqcNot Quite C compiler for LEGO Mindstorms RCX
python-nxtpython driver/interface/wrapper for the Lego Mindstorms NXT robot
python-nxt-filersimple GUI to manage files on a LEGO Mindstorms NXT
scratcheasy to use programming environment for ages 8 and up
t2nsimple command-line tool for Lego NXT

Some of these are available in Wheezy, and all but one are currently available in Jessie/testing. leocad is so far only available in experimental.

If you care about LEGO in Debian, please join us on IRC and help adding the rest of the great free software tools available on Linux for LEGO designers.

Tags: debian, english, lego, robot.
New IRC channel for LEGO designers using Debian
2nd January 2013

During Christmas, I have worked a bit on the Debian support for LEGO Mindstorm NXT. My son and I have played a bit with my NXT set, and I discovered I had to build all the tools myself because none were already in Debian Squeeze. If Debian support for LEGO is something you care about, please join me on the IRC channel #debian-lego (server There is a lot that could be done to improve the Debian support for LEGO designers. For example both CAD software and Mindstorm compilers are missing. :)

Update 2012-01-03: A project page including links to Lego related packages is now available.

Tags: debian, english, lego, robot.

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