Hardware Design

In the early stages of development on RoboPele, the team experimented with many design ideas, however we unanimously decided to have a robot with an external RCX unit. By having the RCX external, we would eliminate all difficulties with respect to changing the batteries by maintaining easy access to the battery access. In previous demos of the Lego Mindstorms system the team had encountered the problem of having batteries run out in the middle of an exercise. In order to change the batteries, we had to spend costly time in dismantling our robot to gain access to the battery panel on the internal RCX and then reassemble the robot again to continue with the exercise. Furthermore, the effects of reducing the cart's weight led to better acceleration and a more consistent performance. It was noticed that the performance and behaviour of RoboPele was dependent on the battery power left. By having the RCX external, RoboPele would have a smaller mass and would therefore use less battery power and accelerate more efficiently. A robot which has a lower rate of power consumption will have a more consistent and predictable behaviour over a longer period. A very important consideration given the amount of testing required to tweek the final behaviour of the robot.

RoboPele has a tricycle design, with one powered wheel in the rear and two wheels in the front for stability. Also, on the the left front wheel is a rotation sensor which was used to count the number of rotations completed by the wheel so that the RCX can keep count on how far the robot has traveled from its original starting point. Initially it was believed that one motor would be sufficent to power RoboPele due to its light mass, but we later found out that we would require two motors. Two motors were needed because in order to catch a fast moving ball, we would need a lot of power to produce an appropriate amount of acceleration from a stand still. Furthermore, having two motors would give RoboPele as much as another 8 levels of power, thereby increasing the output granularity. The two motors can be seen in the following figure, located at the center of the cart.

Also visible in the cart are a set of gears. The purpose of these gears was to provide not only a means of combining the motors into a parallel power source but to also provide greater torque - important for quick acceleration and overall responsiveness. The construction of RoboPele's ramp was a trying one. Initially embedded in the ramp was two touch sensors that measured the speed of the ball as it leaves the trough. Because Lego has "bumps" on their tops, we orientated the ramp upside down, smooth side up. We decided on this orientation because the bumps on the Lego caused the ball to move in an unpredictable fashion. Initially we attempted to use one light sensor and measure the time at which the ball is on top of the light sensor and correlate that time to a given ball velocity, but we found that the light sensor was not sensitive enough for that purpose. Next the team tried a two touch sensor design and measured the speed of the emerging ball as a function of the time between the two touch sensor activations. Unfortuately, at high velocity the ball would not activate the second touch sensor, either because there was not sufficent force to depress the sensor or because the sensor was not depressed long enough.

RoboPele's final design was a touch sensor and light sensor system. This system functions in the same principle as the two touch sensor system but the second touch sensor was replaced by a light sensor. We corrected the problem of insufficient light sensitivity of the light sensor by taping the perimeter of the light sensor to reduce the amount of light reflecting off the ball and back into the sensor. In addition, it was noticed that the black ball performed much better than the green ball in the scoring task.