Duration of activity: 12:00-16:00 --> 4 hours
Group members participating: Ole Rasmussen and Kasper Føns
Group members participating: Ole Rasmussen and Kasper Føns
1. Goal
The goal of this lab sessions is to think about how we should store the sorted bricks, how we should get a sorted brick to its storage and what type of actuators and sensors we need. In one sentence, we need to think about how we will implement the distributor submodule.2. Plan
Our plan is to start figuring out how we want to store the bricks. We will then find out how we get the bricks into the storage unit.
3. Storage
We first needed to figure out how we should store the sorted Lego bricks.
3.1 Wooden construction
One idea was to use a pre-built wooden construction that included compartments at the sides as seen on Figure 1. The distributor should then drive to the designated area and drop off the brick. |
| Figure 1: Wooden construction with storage units for sorted bricks at the side. The distributor would deliver pieces to the correct compartment by using the black lines as guide. |
We believe such a setup would be slow, since we need a robot that can drive on the wooden plate, turn to the correct side and eject the item while making sure that it is following the black lines. The compartments are also very large, too few and when they begin to get full, the bricks could begin to fall out onto the driving area.
3.2 Conveyor belt
Another idea we had, was to have a conveyor belt bringing bricks forward, have storage units at each side of the belt and have motor controlled arms to push bricks into the correct storage units. A sketch can be seen in Figure 2. We also discussed removing the statically placed arms and replacing them with a single car that should drive on a track on top of the conveyor belt. It would then need to drive back and forth to the correct compartments and push bricks down when they were passing the designated compartment. This would remove a lot of motors and would allow us to use a PID controlled car [1].
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| Figure 2: A system where a conveyor belt transport bricks and arms pushes them down into their designated storage units based on their lengths. |
3.3 Driving on top of Lego trays
In the end we settled on a solution where we could use the Lego trays seen on Figure 3 which were already used as storage units in the Lego Lab. The compartments have a perfect size and we could use a tray for each sorted brick type, i.e. one for beams and one for axles. The next step was to find a way to put the pieces into the correct tray compartment. |
| Figure 3: Lego Tray already used in the Lego Lab. This particular tray stores beams. |
4. Distributing bricks to correct tray compartments
An idea for distributing bricks into the correct compartment, was to have a crane-type system, where a brick would be picked up and then dropped into the right compartment by a crane. Axles are very small and slippery and could pose a problem for our pick-up mechanism. We were therefore unsure if we could create a robust crane solution.
We ended up deciding to build a platform around the tray instead. A car driving on top of the platform should be able to move both sideways and back and forth. The car should have a tray mechanism to transport and release the bricks at the correct compartments.
We needed to create a construction where the car could make sure that it was at the correct location before dropping a brick. We first thought about an open-loop control program [2], powering the motors for a certain amount of time. We discarded the idea since we could build a far more robust solution if we used feedback from the environment, which could be sensed by light sensors, touch sensors or tachocounters. We therefore choose to implement a closed-loop control program [2] where we would use only tachocounters, simplifying the construction considerably compared to adding touch sensors or light sensors. We decided to use toothed bars and gearwheels as wheels to drive the car around to give us maximum traction.
5. Conclusion
We settled on constructing a platform around the normal Lego Trays seen in Figure 3, which are already being used in the Lego Lab. To do this we need to construct a car that can move on the platform and when given a brick can place it in the correct tray compartment.
6. Next lab
We need to make sure that we can build a car that can move precisely on a toothed bar platform. We also need to make the tray holding mechanism for the car. In the next lab we will start with that.
7. References
[1] “Embedded Systems, Embodied Agents - Lecture slides week 2 - Slide 7-17.” [Online]. Available: http://legolab.cs.au.dk/DigitalControl.dir/NXT/Lectures/Lecture2/Lecture.pdf. [Accessed: 05-Jun-2013].
[2] “Embedded Systems, Embodied Agents - Lecture slides week 2 - Slide 3.” [Online]. Available: http://legolab.cs.au.dk/DigitalControl.dir/NXT/Lectures/Lecture2/Lecture.pdf. [Accessed: 05-Jun-2013].
