Vex Worlds

So this Tuesday I am going to the Vex Robotics World Championships in Dallas, and wanted to share what this year's worlds bot is. It has three key features. First is the 6 wheeled tank drive, which drives the bot. The wheels are all chained together, so every wheel receives power even if one motor breaks. Also, if one or all of the chain breaks, because the chain is plastic, we don't lose power to all of the wheels. The drive changes from chained drive to direct 4-wheel drive. This makes the wheels run at different speeds, but the driver can compensate. Next, is our tower with a four bar linkage. This is where four bars are connected to two points. The bars are exactly the same length, so when they rotate around the two points, the object being held maintains the same angle no matter what. Because we can then have a vertical angle when depositing cubes, gravity assists us, and doesnt fight our progress. Finally the BHM or block handling mechanism in the front has a small gear reduction connected to tank tread. This tread along with carefully placed zip-ties picks up cubes while maintaining the same speed, even with the friction of the cubes against the tread.

Thanks for reading,
Mark.

Phone Books

Phone books are extremely thick, but one would hardly think that two phone interleaved phone books could be extremely difficult to pull apart. The idea came to me while I was doing the physics review sheet because of its massive size. Though it is extremely difficult to measure the amount of Fs-max of two interleaved phone books. For starters normal force on each page is proportional to the number of pages above it. But you wouldn't want to do that because not every page puts its full weight on the page below it, and when you pull on the books, the pages compress and there is a force of pressure created holding the pages together. Even a miniscule amount of friction force on each page, multiplied by each page and each side causes massive friction. This friction is so great, that the pages will rip appart before the pages come appart. Its a little like a Chinese finger trap, when you pull, the force of friction increases due to more pressure on each page, but when you push together the pages come appart b/c the force of pressure is outwards not inwards. I really want to try this, but I didnt have 2 phone books or 3 hours to interleave them.

Thanks for reading,
Mark.

AutoDesk Inventor

So this long weekend I spend using AutoDesk Inventor (similar to pro/Engineer) building the chassis of our six wheeled skid-steered Vex Robot. The basis behind the omni-wheels in the back is that they reduce friction on horizontal motion. This reduced friction moves the turning radius from the center more towards the front. This is extremely desirable for the competition because we pick up cubes and deliver them near the front of the robot. This allows us to turn about the grabber moving our back out of obstacles and making smaller adjustments where it matters, in the front of our robot. The VEX robot chassis uses chains to ensure that each wheel moves at the same speed, and each side moves at a constant rate. Using encoders, which count 90 times the number of revolutions, a PID can be implemented (well a P due to lack of calculus) which equates the speed side, even though the amount of voltage being passed to the motor is different. This creates straight driving (though is quite hard to test). Both the programming aspects and the physical aspects of the robot have been well tested, but unfortunately holonomic drive has been scrapped.

Thanks for reading,
Mark.

Baseball Shooter

Continuing on the idea of FRC the next component of our robot was the baseball shooter, which we used to launch the orbit balls to set distances. Normally when used on footballs and baseballs, the wheels on the shooters compress and form around the balls to transfer more linear speed, because of greater friction. This compression is necessary for a successful launch. Unfortunately, the orbit ball is made up of six plastic strips woven together, so the ball would give before the wheels. We used this to our advantage by making a path which was two inches smaller than the ball. This created massive compresssion and created a larger transfer rate of linear speed to speed of wheel. Unfortunately, unlike baseball shooters, this type of shooter would not be able to fire a ball at anything higher than 30mph because the momentum of the wheels is not great enough to sustain top speed when throwing a ball. It begins to slow down as balls enter, because of the conservation of momentum...

I dont really have much more to say, but thanks for reading.
Mark.