I recently received a comment on my post on controlling NXT robots with Matlab that pointed me to the RWTH – Mindstorms NXT Toolbox for MATLAB®, which is a public domain Matlab package that enables one to interface with and control LEGO mindstorms.
These are designed after Theo Jansen’s amazing walking kinetic sculptures that are powered by wind or solar power. Here are some great videos of these Theo Jansen’s kinetic sculptures.
Here are two computer graphics studies of Theo Jansen’s mechanism.
Some of the limitations of the LEGO NXT system are related to the fact that, without electronic multiplexers, one can only control three motors with the NXT brick. This individual introduces a mechanical NXT multiplexer that uses two motors but provides 12 outputs!
I have thought about linear designs of this sort, but a rotary design is much more practical. Very nice.
An elegant worm-gear multiplexer submitted to nxtasy.org by Guy Ziv provides another solution in the event that your two outputs do not need to change direction.
I have just uploaded three videos of a LEGO NXT rover that uses the rocker-bogie suspension system employed by the current Mars rovers. This design is based heavily on the design presented by Brian Bagnall in his book Maximum Lego NXT: Building Robots with Java Brains
I described the rocker-bogie suspension system in a previous post. The idea originated with the bogie, which is a set of six wheels on a train designed in such a way to keep all the wheels on a curved track. The innovation here is to add rockers allowing the wheels to move up and down independently. This enables the rover to handle extremely rough terrain, and as I demonstrate, climb barriers higher than the wheels themselves. A more detailed description can be found on BrickVista Tech-Notes.
The first demo was filmed in my brother’s backyard in Wisconsin where he had just put in a fence. The terrain is relatively rough with bumps and dips with sizes on the order of the diameter of the rover’s wheels.
The second demo was filmed in my office in the Physics Department at the University at Albany. Here the rover climbs a pile of some of my favorite books. Several of the book heights are on the order of the diameter of the wheels themselves. Watch how the rockers allow the wheels to climb independently.
The third demo was filmed in the access road just outside the Physics Department. A small parking barrier, approximately the height of a curb, is the obstacle to be overcome. The rover is able to climb the barrier, and the rocker-bogie suspension allows its wheels to hug the barrier as it rolls over. The rover then heads off towards a small tree… perhaps in search of life.
We improved on the design by increasing the torque on the tires (decreasing the speed) and by replacing the front drive shafts with a gear system. Long LEGO axles tend to take a good deal of torsion and store this energy like a spring. This leads to oscillatory motions in the wheels. In addition, the coupling was too weak to enable our rover to climb the desired obstacles, and our gear system overcomes this. Another way we found to overcome the torsion of long drive shafts is to construct a shaft out of small axles joined by axle connectors.
There are more improvements to be made. One design flaw is that the front wheels are too powerful and sometimes lift the entire front end of the rover without allowing the rockers to rotate. This is because the back wheels are also progressing a given rate of speed and for the rockers to rotate, these wheels would have to slow down. A properly-placed differential should solve this problem.
In the meantime, this basic rover design is sufficiently robust for outdoor exploration.
I am slowly constructing my first intelligent instrument. It will be an instrument that learns the acoustic radiation pattern emitted by a speaker. It is not a perfect acoustic experiment—nor is it meant to be. Uncertainties and errors abound, especially since I am using the Lego NXT Mindstorms system to construct the instrument.
The design is sufficiently complex that I found that I need to document it using the LDraw system, specifically MLCAD. I have been practicing my animation skills as well. Here you can see a short animation of the central drive shaft for the acoustic platform. The gears are turning at the appropriate rates and everything. However, there is an aliasing effect in this downsampled image (which used to be referred to as the wagon wheel effect). So it may look as if some gears are rotating backwards, or not at all. If you click on the image, you can download a 6 MB version that is much smoother.
The longer animation will appear in two talks I am giving at the University at Albany this week: NTIR 2007 and PASCAL 2006.
