Mythbusters Test The Vex Robotics Design System
by Jamie Hyneman, Grant Imahara and Adam Savage page 2 return to page 1
PHOTOS BY CRIS ROCHA AND GRANT IMAHARA
Optical shaft encoder mounting system.
STRIPPED GEAR BLUES
Programming the robot to read the ultrasonic sensor and raise the front tracks was a snap using the easyC® programming environment that comes with the VEX System. Building from the included sample routines, we were able to quickly write a program to test the ultrasonic sensors and download it to the microcontroller. Though the
optical encoder had not yet been installed, we went ahead and
tested the program. When a hand was placed in front of the ultrasonic sensor to simulate an approaching barrier, the robot responded by raising the tracks as expected. We used a short “WAIT” statement to run the lifting motors for a brief period of time, so that the tracks wouldn’t slam into the body and strip the gears.
Unfortunately, we didn’t count on the robot interpreting multiple triggers; and the tracks continued to rise, slamming into the body and stripping the 60-tooth gear on the first stage, which is a situation we had hoped to avoid.
Failure of the square hole in the gear with shaft.
Upon removal, we found that the square hole in the gear had failed, allowing the gear to spin freely. Just goes to show that even experienced builders need to be careful. Deciding that this was indeed an excellent situation for a limit switch, we wasted no time in installing one as well as the shaft encoder, so the robot would know exactly where its front track was at all times.
PROGRAMMING AUTOMATED CLIMBING
With our sensors in place, we were able to continue programming with position feedback, giving the robot an accurate idea of the elevation angle of the tracks. The limit switch was used to calibrate the track position each time upon startup, giving us a home position. This absolute home reference is required because the optical shaft encoder only provides relative motion information in the form of pulses each time the shaft rotates.
Our goal was to give the robot some ability to make autonomous decisions for the driver, especially in conditions where he or she may not be able to see upcoming debris. In our test program, the driver has control of the robot under normal conditions, but the robot continually scans for obstacles directly ahead using the ultrasonic sensor. When an object is sensed, the robot takes over and initiates an automated climbing sequence, shown in the sequence, where it climbs over a large obstacle. After the sequence is complete, the robot returns control to the driver. Several versions of the program were tested to adjust timing and track angles.
HITEC HIGH-TORQUE SERVOS
by Jamie Hyneman
The HS-5645MG digital servo provides nearly half the torque of the HSR-5995TG robot servo and costs only $60.
The main reason our PackBot design was not capable of climbing stairs was power; the motors and servos supplied are the standard ones used for RC control and while we ganged them to lift the front section of the bot, it was impractical for our design to climb a full-size staircase. If you are looking for extra “muscle” to achieve this, the Hitec HSR-5995TG digital servo can easily handle the above task. It provides 333 oz.-in. of torque at 6 volts, and over 400 oz.-in. at 7.4 volts! That’s over ten times the torque of a standard analog servo at about the same physical size!
The HSR-5995TG robot servo has a small boss at its base that can serve as a pivot point for articulated action.
The HSR-5995TG digital servo even incorporates titanium gears to handle that massive torque. Some minor modifications would be necessary to make the shaft outputs compatible with the VEX hardware, but this is easily done. Because these servos are more expensive than standard RC servos (street price of about $125), it is unlikely their equivalent would be included in the VEX starter kit. Also note that Hitec offers 168 oz.-in. servos, the HS-5645MG, for about $60 - that may be plenty of torque for a hop-up at a more affordable price. Check out www.hitecrcd.com.
Even so, it is amazing that the PackBot we built in a day or so could go right over an apple box autonomously! If the kit doesn’t include a few high performance gadgets, it can’t be faulted.
Obstacle-climbing sequence.
FUTURE PLANS
Though we only had two days to build our PackBot, various expeditions around our shop at M5 have proven highly entertaining, as you can see in the online video clips.
Future refinements have been planned, including modifications to the tracks to improve their traction on stairs, and the addition of more drive motors for increased climbing power. Once we have the traction to climb stairs manually, we also plan to add a routine in the program to deal with autonomously climbing stairs, which requires a different sequence of events than climbing over a single obstacle. Finally, a pan/tilt system for the camera built with VEX servos will expand our ability to assess our PackBot’s immediate surroundings. The best part is that with the VEX System, modifications can be completed easily, and numerous versions can be tested quickly until the best configuration is found. The pieces are well designed and their ease of use promotes creativity in the problem solving process. We’ve found the system to be highly adaptable and accessible for the robotics enthusiast from beginners to advanced builders; and look forward to seeing what the rest of you can do with this versatile system.
BUMPER CAR SWARM-BOTS
by Jamie Hyneman
As we go to press, I am in the middle of a project that really demonstrates the beauty of the VEX System. I was hired by a large software company to create a “robotics” entertainment event at an annual corporate gathering at which company strategies and other things would be discussed. I was retained to put together something ‘robotic’ in nature, but the question was what? I came up with this idea: what if I were to make bumper cars like you see at a county fair, but with a twist? Rig the cars for remote control; so that you can drive your own car, but if you hit an override button, you will shut down your car and instead be driving someone else’s.
Above:
Bumper car chassis under construction and detail of VEX receiver, brain and battery on bumper car front panel.
Left:
Freshly
painted body.
We would set up two teams, each trying to get to a goal line to make a touchdown - but the players would never know whose car was on which team because at any time a player could override someone else and send them into another car or a wall. The client liked the idea and I was awarded the job to make ten of these cars. I decided to use an electric wheelchair chassis as the vehicle platform as it would cost less than scratch building. All we had to do is make an exterior with padding and install the control system. This is the interesting part: while I was considering which radio system to use, I realized the VEX System had all the necessary features: mixing (to give skid-steer control), FM (for a good signal), and reasonable price. Then it hit me; it also has a brain!
When you start to think about what is necessary to rig this ‘override’ capability (a somewhat complicated setup requiring a lot of frequencies, a combination of normally on and off relays), it starts to get confusing pretty fast. I also wanted to have an override time-out built in; because I was pretty sure that within a few minutes of play, the players would all be going right for the overrides and all ten cars would end up getting beat against a wall over and over; and while funny, the game wouldn’t go anywhere.
I called the guys at VEX and they immediately told me that the VEX System’s brain was capable of giving me what I wanted without any extra hardware - just a little programming! This solution saved me a ton of time and money. The VEX robot’s control system is actually very powerful - it does much more than enable a tinkerer to experiment with a robot that tracks lines or senses walls. The “brain” is something that can be programmed with very little effort to make the radio system much more dynamic and flexible.
The next time the company holds a convention, they can have these cars demonstrate “swarm-bot behavior,” for example, by going into automatic evasive patterns that are autonomously controlled by the VEX brains. The possibilities are endless, and all each car has in it is one transmitter, one receiver, one brain, and a couple of speed controllers. One of these days, you will be able to get into your car in New York, relax with a good book and end up in LA. A system no more complicated than the robot brain in the VEX kit will get you there, and the local traffic system will override your car (just like we are doing for fun) to integrate it with other traffic and put you at the correct speed. I’m serious. Of course, to do that you may need additional hardware and the cooperation of the local government and highway authorities. Look to a future issue of ROBOT for more details on the bumper car robots.
WATCH MYTHBUSTERS!
MythBusters is broadcast on Wednesday evenings on the Discovery Channel at 9:00 p.m. Eastern Time and 9:00 p.m. Pacific Time.
Don’t miss it!
The VEX Robotics Starter kit is available at your local Radio Shack for $299.99 (catalog No. 276-2151). Be sure and check out the wealth of accessories that can be used with the VEX kit. —the editors
For more information, please see the source guide in the magazine.
Click here to read the online extension to this article, in which Grant Imahara discusses engineering details of the
VEX "PackBot" project. This online exclusive includes many extra photos that wouldn't fit in the above magazine article.
