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Gus Jansson / BrickFest2007 | |
| Crate/Great Ball Contraption at BrickFest2007 As part of BrickFest2007 in Portland, I put up a new Crate Contraption/Great Ball Contraption. Traditionally this would have been a SMART collaborative display but this year it was pretty much a solo exhibit. David and a couple of others did provide GBC modules, which were included, but all the robots were my own. Here is a good overview photo of the whole display: 
Here is a compilation video on YouTube that covers all the robots and devices in an order that roughly reflects the flow of balls through the contraption: 
http://www.youtube.com/watch?v=bGmOAYqKSaU
Note: none of these photos are my own. I was so busy during the event, and I noticed so many cameras everywhere, that I didn’t bother to take out my own camera. Thank you to all the fine folks who took pictures and posted them on Brickshelf and flickr. Video clips are courtesy of David Schilling. Thank you David! Note: Video files are encoded using DivX. Go to www.divx.com and download the free codec if you have problems viewing the files. The display had some things old and some things new. My trusty old RCX ForkLift and DumpTruck were back as was my old ball contraptions the sorter, the Wave and the Ball Pump. New for this event was an NXT Tower Crane, an NXT Crate Dump Truck, and an NXT Front Loader (which also got 2nd place in the Easter Egg competition!). Overview Photos:
Overview Video Clips:
The Tower Crane This fully robotic crane is designed to pick-up and drop-off standard crates. It is capable of auto-balancing so there is never excessive stress on the tower. The traveler is equipped with a light sensor and a custom jumbo LED that makes it possible to see a reflective pad on the base surface. This makes it possible for the crane to wait until a crate is available for pick-up or to wait until a crate below has been taken away before putting down a crate. The grabber is a mechanical mechanism that does not require an additional motor.
Auto-balancing: The counter weight consists of eight LEGO weight elements as well as a 6AA 9v battery box. The battery power goes through a polarity switch that is connected with a linkage so that whenever the boom is out-of-balance, the counter weight moves to bring it back into balance. The boom angle is amplified through the linkage so that the boom only needs to be a few degrees off-level to trigger the counter weight to move. The counter weight motor also has a RCX type rotation sensor that is connected through an adaptor wire to the NXT. That way the NXT can tell when the counter weight is moving so that it can wait until it is back to level before continuing any lifting or traveling. One key aspect to making this system work is the boom hangs from an axle at the top of the boom. That way the whole boom is an inherently stable system. If the boom was balancing from a pivot point below, then it would be like an inverted pendulum and would require a much more sophisticated balancing system. Mechanical grabbing mechanism: The three outputs of the NXT are used for the obvious crane movements: rotation, travel, and lift. This means I don’t have an NXT output available for controlling the grabbing and releasing. My solution is a mechanical grabbing mechanism that alternates between grabbing and releasing every time it comes down onto a crate or with a crate onto a surface. Here are some close ups of the grabbing mechanism that were taken during the grabbing mechanisms talk:
Jumbo LED: The led in the standard light sensor is only useful for illumination for a few inches. Beyond that it is much too dim and also makes more of a ring than spot illumination. I needed the LED to illuminate a reflective pad that is as much as 24” away. To make this possible I dissected an NXT touch sensor and made a simple circuit that included a jumbo LED that I bought at Radio Shack. I intercept the sensor wire into a little junction box and pass the digital signal as well a v+ and gnd to the jumbo LED case. When examining a crate pad, the NXT pulses the LED and does a comparison if light sensor values to see if reflector is visible or not. I also use a LEGO mini-fig magnifying glass with the light sensor to increase sensitivity. In this display the crane would pick up empty crates from the elevated platform, turn about 180 degrees to a ball pump on the base level, lower the crate to get filled. Once filled, the crane would pick up the now full crate and bring it back up the platform for the forklift to take care of. The crates were filled by this ball pump:
The crane proved itself to be reliable but very slow. This was primarily caused by the counter weight motor being geared down too much. Both the lifting and traveling were primarily slowed down due to waiting on the counter weight to balance the boom. Since coming back from BrickFest, I have redesigned the counter weight and it is now approximatly three times faster. Video:
NXT Crate Dump Truck This new robot had the responsibility of getting the balls from the end of the GBC chain and bringing them back to the Crate Contraption. At the end of the GBC chain there was a crate that would get filled up by my wave module which was regulated by an RCX to not overfill and to suspend filling when the truck was busy dumping the contents of the crate. I don’t have a lot of great pictures (actually I have none, I’m relying entirely on the good will of others) of the new Crate Dump Truck but I did find these:
In addition to being able to load up on balls from crates by picking up and dumping the crates into the bed, garbage truck style, the bed is a side dumper. Like my old RCX based dump truck, this truck is an edge tracking steer-bot, But unlike the RCX counter-part, this truck occasionally uses dead-reckoning to get around. For example, when the truck does an 180 degree u-turn to get to the end of the GBC chain, the truck does the whole turn using DR. At the other end of the display after dumping the contents of the bed, the truck does a 3-point turn using DR. This robot actually worked really well. It went back and forth doing its job so reliably that it hardly attracted any attention. I believe that is one reason why there are so few photos of it. Hopefully some video will be available soon. Video:
NXT Front-End Loader This robot, unlike the Crate Dump Truck, got lots of attention, mostly because of how unreliable it was. This is mostly due to the fact that I spent so little time getting it ready for the show. I built the essential part of the front loader months ago as a proof of concept but then it stayed in a box until a week before the show when I decided that I really needed another robot in the display. Out of the box it came and in a frenzy I managed to convert the line tracking code from the truck to this articulated driving base. This was mostly a matter of changing the constants in PID code to account for the huge amount of mass associated with the steering. That done, it actually edge tracked quite nicely. Unfortunately, putting a bit of a load into the scoop tended to mess up the momentum of the steering and then the line tracking and DR were not so accurate. Oh well, given the amount of time that went into the robot, it worked okay. The crowd sure seemed to enjoy watching the loader drive to the edge and dump the scoop full of balls over the side.
In this display the robot would pick up basketballs from the sorter, turn around towards the old RCX DumpTruck, and wait for it to arrive. Once the DumpTruck stopped in front of the Front Loader, the FL would dump its scoopful of balls into the truck. Video:
RCX Fork Lift and Dump Truck These robots have been around for years and received nothing but a new script for this event. The Fork Lift did its work on top of the elevated platform, picking up full crates deposited by the crane, drove over to the corner chute where it dumped the crate, then deposited the empty crate for the crane to pick-up again. 
In addition to the forklift, the other RCX on top of the platform was used seeing crates coming and going from the crane and sending messages to the ForkLift so it would know when it when a full crate was available and when the empty crate pad was vacant. This was done with a couple of LED lights that were positioned to shine accross the platfrom and light sensors to detect the crates by breaking the light beam. Here is my trust old DumpTruck. Kind of slow, but worked like a champ. 
Video:
Sorter Unchanged from previous displays. I set it up so basketballs went to the Front End Loader while soccer balls went out to the GBC chain. I never bothered to calibrated it so it was only about 90% accurate. Still, a crowd pleaser. 
Video:
Wave I set up my Wave module to be the end of the GBC chain. I made it RCX controlled and added a light sensor with a small fiber-optic loop so it could count balls as it was filling the crates. The RCX limited the crates to about 40 balls and also suspended filling when the Crate Dump Truck was loading the balls from the crate. 
Video:
GBC video clips
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