Ballbot NXT was born the summer of 2008 as a challenge and a project proposal to Master of the University of Alcala de Henares, Madrid, Spain.
This robot is defined as a two-axis inverted pendulum technology developed with LEGO MINDSTORMS, a prototyping kit technicians, initially thought to make toys.
The challenge at baseline was a problem of imagination:
Using LEGO MINDSTORMS NXT as a development platform was a decision of departure, since there was no solution to the problem with this technology and wanted to push the boundaries of the development environment. While this appeared to be in principle a restriction later proved to be key, because the solution space was also limited.
The central problem is not a control engineering problem, as the inverted pendulum problem is a classic problem that is resolved as required in engineering practice. The central problem was to build a physical model that supports the theoretical solution.
While the development seems complex once completed, I have to note that the progressive construction process was the key to success in this case as a cutting system shows that it is built layer by layer on simpler systems, to reach a basic frame that holds the ball control on the tractor down the servo.
After solving the problem of stability of the platform is an additional problem that occurs in the time it is desired to guided paths and the system in terms vector, provides a measure of position (x, y ), but not a course and therefore not known if there is a rotation central axis of the friction system derived from irregular or a simple air flow.
The solution to this second problem can be obtained through a central rotation measure, but uses gyroscopic technology it is invalid because the speed that can measure the rate gyroscope HiTechnics can not make reliable measurements below 1 ° / s, and it is these small problems but accumulated detectable only those who put a path error. Thus it is necessary to have an external absolute measure. In our case we used a solid state compass, which returns the degree of orientation to the Earth's magnetic north.
Finally, the external guidance process is done through a communication process based on PC-NXT Bluetooth technology that implements the NXT and any external device. The communication protocol is a standard BT NXT series, easily programmable and can develop such a mechanism from Smartphone devices. In our case it was decided to make a Visual Basic application running on a PC and allows us to make use of the mouse, draw complex paths by number of points that NXT Ballbot pursued. The displacement of the robot returns in real time to guide the implementation of the absolute position and thus draw on the screen position.
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