Tuesday evening, after nearly two days of fine-tuning our testing procedures and security measures, we finally did the first tests on our own vehicle. In order to make sure we didn’t damage the equipment (nor hurt somebody), we followed a simple and strict protocol:
1. First of all, we checked that we could control manually the motors via our interpreter;
2. Then, we verified that the limitations we applied to the variation of the voltage of each motor were adequate (as mentioned in a previous post, an abrupt increase or decrease on the voltage could severely damage the motors);
3. Finally, we determined a maximum speed that seemed reasonable, so that we could stop the vehicle manually should the worst happen.
After this initial procedure, we could start the actual tests of our system. After some iterations of tuning the coefficients of our controller, we realized that it was actually much easier to equilibrate the vehicle with a human being on top of it than without any load, since:
(i) the person enhances the performance of the control system by balancing his/her weight in a way that maximizes stability;
(ii) the presence of the human being makes the test conditions closer to the physical model we used to compute and simulate the parameters.
After several adjustments, we were able to take our first ride on the RoseWheel -- with basic steering capabilities via the integrated potentiometer implemented as a plus.
Nevertheless, by wednesday morning we had not yet found a way of balancing the RoseWheel alone -- a must for our next major goal, remote control. Then, we realized that the chassis we used had a very interesting property: by tilting it to a specific angle, we could align the center of gravity of the steering bar with the axis of the wheels, reaching a metastable equilibrium. We used this fact to offset the error variable of our PID controller, and that proved to be very effective, since we obtained as a result a pretty good stability. But all this comes at a price: since we now have two different equilibrium positions, we must use the safety button located on the chassis to determine if we are carrying a person or not; nothing too difficult to implement, though.
On the other hand, the PID coefficients may be trickier to master. We are still working on them to solve the issues we are having, notably oscillations and vibrations that sometimes occur. To help us with the debugging, we set up a solution to plot angle, angular velocity, and output voltage command for each one of the motors. Our next actions on this front will be to use all these data to better tune Kp, Ki and Kd, so as to have a more responsive and stable control algorithm.
Meanwhile, we started to move on to our next goals: we began to study the Bluetooth controller, and are already able to compile and run a « hello world » Android application. Next steps include implementing drivers for our distance sensors to implement obstacle detection.
To summarize all our work, we prepared a special video showing some of the most interesting (and/or funny) demos we made so far. Enjoy!
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