Final Pong Ball

Group members participating:
Morten, Lars and Thomas.


Duration of activity:
15 hours.


Goal:
To improve the Lego construction and the SW, in order to make the Ball move more precise.


Plan:

1. To improve the Lego construction by substituting with smaller wheels and raise the CompassSensor further up and away from the NXT-brick, wires and motors.
2. To construct a P-controller for when rotating to a new angle.
3. To find a way to distinguish between being hit on the front alone, or on either side.

Activities:

1. Initially we constructed the Ball with the big 8.16 cm diameter wheels, this together with a dusty linoleum floor gave problems with spinning upon acceleration. Because of this, we instead constructed the ball with the small 5.6 cm diameter wheels. This seem to reduce the spinning, but choosing the small wheels over the bigger wheels meant we also had to content our selfs with lesser speed.
   With the previous construction, the CompassSensor received errors of the magnetic interference from the NXT electronics. A solution could be to make a table of deviation, but this can only be done for a specific Lego construction, then for every small modification to the Lego one would have to make a new table. Instead the compass was raised further up and away from the NXT. We found that if the compass was in a distance of about 20 cm away from the rest of the NXT, the compass measurements would be trustworthy.
   
   
   Raised compass
   
   
   The ball is based on a construction of two main wheels connected to two separate motors, and a third wheel, that can rotate around its own axis, to balance the weight of the ball. The problem of compensating for the third wheel being in different positions upon rotating to a new direction initially gave a lot of headache. The problem was solved with the new CompassPilot described in the next subsection.



2. Our experience convinced us that the rotateTo method of lejos.navigation.CompassPilot was useless. In preceding tests, use of the navigation.CompassPilot.rotateTo made the rotating movement of the NXT overshoot the target direction several times - bouncing back and forth - never finding the correct angle.
   This of cause might have been due to a misunderstanding of the original CompassPilot on our part, nonetheless we chose to construct our own version of the CompassPilot.rotateTo(int i).
   We felt the need for a more controlled rotation and implemented a sort of P-controlled rotational movement. The closer the actual heading is to the target heading, the less power is supplied to the motors.
   
   power = (int)(60F - (((float)delta-180F)*40F)/(-180F));
   
   This normalization was inpired by Tom Deans notes on constructing Braitenberg vehicles. We changed it to use our own constants. 180 is the maximum angle to turn and we want a power in the range between 20 and 60. A power value below 20 is not enough to turn the wheels around and a value above 60 makes the wheels spin.
   When the power is calculated we make a test to see which is the smaller angle distance to turn - left or right - making the coresponding calls to MotorPort.controlMotor() for a left turn or right turn respectively.



3. With only two RCX touch sensors attached on either side of the ball, it was impossible to detect a collision with the boundary wall on the front. The idea here was that if both sensors was activated simultaneously, this would imply a collision directly on the front of the ball. When running tests, we never managed to see this state being detected.
   One solution, might be to include a time delay when checking the state of the touch sensors. It was estimated to require some debugging before some magic constant could be established. Instead we chose to attach a third NXT touch sensor on the front, on one of the still available input ports, and then adapt the program to account for the new sensor.

Explicit front collision detection with third touch sensor (NXT)

Conclusion:
The implementation of a new rotateTo method and the P-controlled rotation in our own CompassPilot was of great help in increasing the precision of the rotational movement of the ball.

Moving the compass further away removed interference from the electronics. In some initial testing we made the error of using metal shelves as boundary for the ball, which of cause sometimes gave rise to some unexpected behavior. One could argue that a form of average on the compass readings may resolve this issue, but since we are able to decide on the working environment of the ball this is unnecessary.
At the moment when the ball hits a wall it backs up, then change to the new course and resumes driving forward ( see diagram below).

Reversing before finding angle of reflection

In theory this makes the ball imprecise, at least comparing it with a natural round ball. Our estimation on this is that it is possible via software to make the ball find a more correct angle of reflection, but what this adds to the deviation in the long term would have no greater effect, because of the nature of the Lego construction. There is a further discussion on this issue in the final conclusion.