Before the Athens week I started studying the Kalman filter. At the begining of this week I decided to work along with Scott who has been studying the mechanics of Rezero and BallIP as they were presented in the reports we found online. As two heads are better than one, since Monday we have been focusing on the mechanics, control and filters as a whole. The control system as well as the model are different for Rezero from the ones used by BallIP. Certainly Rezero performs better but BallIP seemed more comprehensible at this step. In most of the papers that we read the authors first implemented what was previously achieved with ballbot technology and then built upon those concepts. Therefore today we focused on understanding the planar model (forces, energies, inertia calculations, torque conversion).
Certainly, in practice this model has limitations: the effect of the wheels is not decoupled – this highly influences the angle of tilt (before Rezero the tilt angle achieved was maximum 5 degrees whilst Rezero can recover from a 20 degree tilt). However, we considered we had more chances to understand the planar model before starting directly with the 3D model of Rezero. One of our goals today was to actually implement a Simulink simulation based on the description given in the Rezero report for the planar model.
We discovered that that there is a significant learning curve to designing a Simulink simulation model. We had a nice surprise stumbling upon a 2D model of an inverted pendulum through the Simulink Demos. Though it is not exactly what we need, we intend to use it as a basis for learning and simultaneoulsy adapting it to the planar model.
More on this tomorrow:)
For BallE – Otilia & Scott
P.S. on Monday I have studied the robots’ parameters (mass of the ball, mass of the body, radius of the ball, moment of inertia…). Rezero and BallIP are have significant mechanical differencies: the center of mass of Rezero is significantly higher than the center of mass of BallIP; the ball used for Rezero is a hollow aluminium one of 2.29kg coated with soft rubber whilst the ball of BallIP is a bowling 3.8kg ball coated with Plasti Dip. We could try Plasti Dip as well: http://www.plastidip.com/home_solutions/Plasti_Dip
Today, while Otilia and Scott tried to learn things about Kalman filter and from Rezero’s final report, and almost commited suicide after :p (don’t worry they’re ok), Pierre-Hugues and I continued the mechanical calculations.
Unfortunately, it appears that our calculations must be false as the result with numerical values didn’t make any sense.
So we decided to make tries with a real ballbot. We asked the Telecom ParisTech Robotics’ Club if we could borrow them some omniwheels and DC motor in order to estimate physically the power that need for each motor. We would like to thank them for accepting and we would like to introduce you our new friend : Baby Ball-E
Last friday, we had the communication challenge which put an interesting end to the STM32 practical work (but we know that more should come soon ^^).
The Hexapod team pointed out an interesting software named V-Rep, so I took a look and it seems a bit complicated to simulate a ballbot with it. Therefore I tried to use robotSim and robotBuilder from Cogmation but the program wouldn’t stop crashing on my computer…
As we had found some Matlab simulation code from the existing ballbot project (such as Rezero), I thought the best solution should be to use Matlab and Simulink which seems to be really powerful. My goal for the next few days is to learn how to use it and try to make simulations with (as soon as we’ve finished some mechanical calculation with Pierre-Hugues).
We also made a kind of summary of our meeting with Alexis an Samuel on our Ball-E Google group in order to settle a better communication inside the team.
I hope that next post will come with complicated calculations and some screenshot of our work !
As this project might be quite tough mechanically speaking, we began our work by taking a piece of paper and a pencil to sketch our first ideas.
We also took a look at the existing projects in order to learn more about ballbots and to improve our sketchs. Thus, we discussed some points such as the use of 3 omniwheels (ex : Rezero) instead of 2 (ex : Lego Ballbot).
Finally, we tried to divide the conception of our robot into several parts, and assigned each part to a member of the group. We also estimated a schedule for the realisation of each part. All of this helped us to make the presentation for tomorrow.
Otilia, Phh, Scott, Matthieu