September 1, 2008

GearBob, a LEGO Mindstorm NXT orbital gearbox

Hi Everyone!

I've been willing to do an automatic LEGO-style Gearbox since a long time, but I never managed to do it because i could only think of a linear one ( like those in european cars), and it 's quite difficult to do a LEGO one. Then I learned that there was another type of gearboxes : the orbital one(like in the automatic cars)! In LEGO geometry, it' s often easier to do orbital stuff ( which means "round" stuff) than linear mechanisms.

The principle of the gearbox is simple : whith the same motor, you have to make the wheels turn at different speeds, by changing the gearing. You may wonder why not to choose the fastest at the beginning, and it' s because the car doesn't have enough torque(=strenght) to move : the faster you go, the weaker you go. The car is able to move at high speed if it' s already moving, thanks to the phenomenon of inertia.

There is not much other things to say about gearboxes ( at least lego ones, because a real car gearbox is much more complicated). Still, the difficulty is to make a stable speed-changing mechanism, because the gears have the tendancy to say goodbye to each other, and to make the wheels move, you must absolutely stick them together.

Even if it' s not very complicated, it' s always fun to play with gears, and the gearing is quite beautiful, it makes me think of a watch mechanism :).

Bobly yours.

August 20, 2008

GrabBob, a 4 fingers wiimote controlled grabber

Hi Everyone!

It' s been a long time since I have not built a grabber. And this time, I tried to do an unsusual one:

Most LEGO grabber arms use 2 different axis to open or close the hand. It actually is the most eficcient way to make a good grabber, and I first wanted to do one like it. Still, a friend of mine asked me if I could build a grabber like the one in the movie "Iron Man" : the main character has a small cute fellow grabber, whith 3 fingers on different axises. Considering that it' s very hard to deal with triangles in LEGO geometry, I decided to make it 4 fingers.

I had to use a small trick for the linear actuator to open or close the hand, as you can see on the video below, but building the grabber was not the main difficulty.
I actually had a lot of trouble to make the grabber rotate on its base, because of the high torque that was applied on the axis. I didn' t use the part 48542 because it involved way too much friction. So I decided to tinker something, still the result is pretty lousy.

The wiimote has 3 sensors : an hitechnic accelerometer, for the wii-like control, a light sensor, to control the opening of the fingers, and a touch sensor, only to stop the robot ( I only use the touch sensor to put the robot back in its initial position).

Obviously, it is not a good grabber! Anyway, I had a lot of fun building and programming it, and it' s kind of a pretty robot. ^^

Bobly yours

July 20, 2008

BiBob, a dynamic biped walker.

Hi everyone!

The problem of the biped walker has exicted every robot maker at least once! But one problem that is not often very well known is the one of the "dynamic" biped walker... Let me explain:

Most of lego biped are "static" walkers : their stability criteria is to keep their center of gravity (COG) inside their footprints. It works pretty good, but if you just look at the way you walk, you'll realise that you actually don' t move your own COG( which is approximately located behind your navel) on top of your feet.
That is because your body is embedded in its own movement, it' s the phenomen of inertia. It is the same kind of forces that pushes you when you suddenly slow down whith your car.

A "dynamic" biped is a walker that masters inertia (like any humanoid robot today). When the robot is moving, the point to keep inside the footprints is not exactly the COG, but a point called ZMP ( Zero Moment Point). If some of you are interested, it is the point on the floor where the sum of the torques of the forces of inertia and of the weight is equal to zero.
You can find more iformation about the ZMP here.

I skip all the boring math stuff I had to perform to make sure to keep BiBob's ZMP inside its footprints, just note that I had to modelise BiBob whith the software SolidWorks (my school owned it) to get the properties of mass, as shown on this picture.

I was so able to know what trajctory give to the robot to have a good enough tajectory of the ZMP.

And finally, the robot walks pretty good, here is a video of BiBob walking :

Bobly yours.

July 18, 2008

The Bob Factory is created!

Hi Everyone!

I finally decided to create my own Lego mindstorm NXT Blog. I will here show you my Lego mindstorm NXT projects, and explain them as good as I can!

Every robot I make is part of "The BOB family", and BOB stands for "Bobs Ought to Be".

Have fun exploring the blog, and if you want to criticize some of the robots I will show you, don't hesitate, no robot is perfect, and I will try to enhance them with your precious remarks!

Bobly yours.