Jamie Hyneman, Grant Imahara and Adam Savage, cohosts of the popular MythBusters TV show on the Discovery Channel, have many years of experience designing animatronic robots used in commercials, science fiction movies and other popular media. This team also has broad experience in radio control and hobby robotics. We are proud to include their review of Radio Shack’s new VEXTM Robotics Design System in the inaugural issue of ROBOT. Visit www.botmag.com/issue1 for additional photos, a detailed, photo-rich “build report” by Grant Imahara and video clips of the MythBusters commenting on their VEX PackBot system.
—the editors
PROJECT OVERVIEW
Jamie Hyneman: As a holder of some patents and a designer and builder of dozens of prototypes and devices, I must admit I was expecting to turn up my nose at a do-it yourself robotics kit from Radio Shack. But guess what? The VEX System kicks butt. In a total of about 12 person-hours, Adam Savage and Grant Imahara (my cohosts on MythBusters) and I were able to build a functional, if somewhat basic, prototype equivalent of an iRobot’s PackBot.
The iRobot
PackBot (see www.irobot.com) inspired the MythBusters’
first VEX robot
project.
Grant Imahara: For those unfamiliar with the PackBot, it’s a military UGV (unmanned ground vehicle) used for scouting and surveillance. This modular robot is equipped with a unique tank tread system (called “QuickFlip”) that uses a small pair of tracks (called the “flippers”) that pivot off the front edge of the chassis/payload bay.
JH: The key mechanical challenge here is that this propulsion system has four independent tread assemblies: two main treads and two flippers. The flippers raise and lower to allow the robot to climb over objects and debris.
Adam Savage: Our goal was to see if we could get it to climb stairs. As far as I know, there are few (if any) toys that will climb stairs; and only a few high-end robots like the PackBot and the humanoid Honda robots that can perform this task.
JH: Before we tell you how our prototype did, let us explain what you get with the VEX System. The basic kit is about $300 and includes an erector set style hardware kit with pre-formed plates and angles riddled with holes so that no drilling or machining is necessary (a real time saver). You have nuts, screws, and a couple of small wrenches to attach everything together. Note that the screws are button-head Allen type, which is what most serious prototypers use - no amateur-hour slotted or phillips heads here. Also, all main drive components (motors, wheels, shafts) are square drive, so you don’t have to deal with slipping set screws. There’s also a selection of plastic fixtures that serve as bearing blocks, as well as other standard mechanical items such as gears and shaft collars.
GI: As far as the electronics go, the kit includes a radio control transmitter and receiver, a servo, three drive motors and a few sensor switches. Not to fear radio control flyers - the transmitters are equipped to communicate on ground frequencies, so there should be no potentially dangerous interference with planes. For power, you can use regular alkaline batteries; but for repeated testing, you should really purchase the rechargeable battery setup (sold separately) which includes a charger. Also available is a programming module that will allow you to hook your robot up to a computer and download a program for adding autonomous capabilities. You can use the remote control only, let it operate autonomously, or have some combination of the two.
AS: Where the fun starts for us is when you start adding accessories. You can buy extras of all of the components in the kit as well as more specialized sensors, chain and sprockets, different gears, a tracked (caterpillar) system, and different wheels. They have pretty thoroughly supplied everything you need to build at least a basic version of most small robotic assemblies.
OUR THOUGHTS ON THE SYSTEM
JH: The thing that really got me is that everything is integrated so well. You don’t need to do anything but design and assemble. Unless you’re used to building by doing everything in CAD/CAM (Computer-Aided Design/Computer-Aided Machining), this set will allow you to make relatively complex robotic machines in hours instead of weeks or months.
AS: And, if you made a mistake, you just unbolt it, move something or change a gear and you try out the change—continuing to improve it until it works. In prototyping, these changes can take days or weeks to implement. With the VEX System, we were making changes in minutes on our PackBot. Motor not powerful enough? Grab another gear or two and shaft the motor to a different hole. EVERYTHING fits. I love this system and the engineers did a great job.
GI: During our brainstorming/design discussions, I found myself saying, “Let’s just try it.” With the VEX System, it’s not like you’re sacrificing a lot of time and energy just to try an idea. It’s quick and easy, and it promotes discovery through trial and error, which is what engineering is all about. That’s a fantastic way to learn.
JH: I‘m not criticizing the VEX System, but we do have some additions or changes we’d like to see. I’d like to see more selection in hardware. We built according to the shapes and sizes available. While kids can play with it as a toy, it is a powerful enough system for a professional to use for prototype work, problem solving and even low volume, light duty robotic applications. So with that in mind, a wider selection of sizes and varieties of hardware would be useful—bigger, smaller, etc. I’d also like to see reduction gears: a large gear and a small one on the same hub.
GI: It would be great to have some Y-connectors to gang multiple servos on the same control channel, more programmable mixing positions on the transmitter, and both larger and smaller motors and servos as well as premium, compact, high-output servos.
JH: High power servos are available from vendors like Hitec that have over ten times the power of the one in the kit. If VEX does not produce these, somebody should make adapter kits. I would also really like to see someone start making components that are standardized. I just want to build, and incompatibility slows me down and puts us back in the “machine it yourself” mode. This kit lets you avoid that to some degree, but unless they give you every last thing that you could ever use, which they probably won’t, we need adapters or standards.
AS: They should have at least an aluminum accessory hardware kit, which will make your robots lighter. I know steel is cheaper, but give us an accessory kit in aluminum.
GI: All in all, a very cool system. But let’s get on to the fun stuff: our PackBot!
THE VEX PACKBOT: BUILD SUMMARY
by Grant Imahara
To explore the capabilities of the VEX Robotics System, we chose to build our very simplified version of iRobot’s PackBot. The goal was for the robot to be able to climb stairs and small piles of debris using only VEX components. Sounds like a tall order, right? You bet. Fortunately, our friends at Radio Shack were kind enough to allow us to preview some of the exciting accessories planned for the VEX System, including tank treads, a programming kit, ultrasonic rangefinder sensors, and optical shaft encoders. ROBOT magazine provided us with a wireless remote camera (available from Shulman Aviation) to complete our PackBot’s control package. I will highlight a few of the challenges we encountered and our solutions in this article. For a detailed discussion with lots of large photos, please visit www.botmag.com/issue1.
Although the basic VEX kit contains several different wheels, the PackBot is a tracked vehicle, which gave us a great opportunity to try out the new VEX tank tread kit. The kit consists of two long tracks comprised of snap-together pieces. These plastic pieces allow the builder to make the track longer or shorter simply by adding or removing links. Also included are two pairs of custom toothed rollers that can be used as idlers, or to transmit drive power from a motor to the track via the familiar square shafts used in the rest of the system. The rollers are positioned at the front and back of each side, and a tensioner (also included) goes somewhere in the middle to pick up the slack.
The first step in our build was to construct the tracks and connect them in pairs. We needed two pairs of tracks - one for the front (equivalent to the real PackBot’s “flippers”) and one for the back, or base of our PackBot. We dutifully followed the assembly instructions, opting for the longest tread we could make. One of the toothed rollers was connected to a motor, while the other was allowed to spin freely.
We knew that the iRobot PackBot’s front and back tracks were joined together with a common pivot, which was something we would also have to duplicate. The PackBot has the ability to lift its front tracks in order to climb over obstacles and debris. Our PackBot had to perform the same duties, so we immediately decided to use servos for their ability to hold a commanded position, which would be helpful in keeping a track elevated.
In initial drive tests, we noticed that sometimes the front tracks would stop turning, and sometimes the back tracks would. Upon closer inspection, we found that the single-supported square driveshafts were slipping out of the clutches in the drive motors. Easily enough, we added another rail running on the outside of each track, thus making all axes double-supported, and preventing any further slipping problems.
We also noticed a major problem with the pivot axis. Originally, we created two identical tracks of equal width: one for the front, and one for the back, with a pivot in the middle. When we attempted our first lift, we found that there was ample clearance to lift the front track, but when it was lowered (as in the case of climbing down off of a debris pile), the front and rear tracks crashed into each other, which was a major design flaw. Well, it’s back to the drawing board with a few changes. Fortunately, with the VEX System, major system changes like this only took an hour or two.
In order to remedy the crashing problem with our tracks, we decided to make the front tracks skinnier, so that they could fit within the rear tracks. As detailed in the online discussion, we ultimately designed a track-raising gear system with an idler. All of this engineering was accomplished using parts included in the VEX System.
DAY ONE: STAIR CLIMBING TEST
For the final step of day one, we took our PackBot to the main staircase at our headquarters, M5 Industries. We raised the tracks in anticipation of the first step, and the motors lifted them beautifully. As we advanced towards the step, we changed the angle of the front tracks to give us more leverage, which worked great for getting up onto the first step.
To get to the second step, however, we had to flex the body almost straight, which caused the plastic tracks to slip on the smooth front edges of the stairs. Quite a bummer, but we realized that the relative smoothness of the tracks gave almost no traction on the equally smooth step corners.
In a moderate debris pile test (i.e. driving over Jamie’s legs, see video clip at www.botmag.com/issue1), our PackBot performed well and had plenty of climbing power, as the front track provided leverage for the back track. While the stair climbing was a failure, the debris pile was a complete success; and this given only a single day’s work.
Camera mounting bracket.
DAY TWO: ADDING VISION AND SENSORS
We returned to the shop the next day to continue our explorations into the VEX System. So far, our PackBot is little more than a remote controlled vehicle. Out in the real world, these robots are sent into hazardous locations, usually without the benefit of a direct line of sight to the vehicle. Next, we added the wireless camera to the PackBot for remote vision capability, and a host of sensors to help with driving over debris and obstacles, since the driver won’t always be able to see what he or she is maneuvering around.
The miniature wireless camera was mounted to the frame using a bracket bent from a long plate. Power to the camera was provided by an onboard 9-volt battery. The receiver was connected to a standard video input for monitoring.
Radio Shack’s new VEX ultrasonic range-finder modules use high frequency sound waves to detect barriers. The robot emits a short burst of ultrasonic sound, and then listens for a return signal, which bounces off the object. The amount of time it takes to travel to the object and return to the robot can be used to approximate the distance. We mounted the ultrasonic modules on both front and back of the robot. The front module will sense upcoming objects, while the rear one will tell the robot when the back end is facing the ground, such as during a stair-climbing sequence, which requires a different set of movements compared to simply driving over a debris pile. Using these sensors together, the robot would be able to decide what type of climbing operation to perform.
An optical shaft encoder mounted to the shaft that drives the front track up and down was installed to help the robot keep track of the elevation angle of the front track. As the shaft rotates, it turns a perforated wheel inside the sensor. The slots in the wheel create pulses as they interrupt the light path between an infrared LED emitter-detector pair. By counting these pulses, the robot will know exactly how much the shaft has rotated. Since the shaft encoder is on the motor shaft, we made sure to compensate for the gear ratio when figuring the final angle of the front track.
