Robocar: Unmanned Ground Robotics
The Association for Unmanned Vehicle Systems International (AUVSI) sponsors a yearly robotic vehicle contest. Schools from around the world gather to see whose vehicle will navigate an outdoor course the fastest and the farthest. Vehicles have no prior knowledge of the layout of the course.
The course track is defined by white or yellow lines painted on the grass, and the robot must stay within these lines. Obstacles of various sizes, sand traps, deep grades and sharp curves occur along the way to keep things exciting. The difficulty increases as the robot progresses down the course. In addition to navigating the pathways, the robot must carry a 20 pound load and cannot exceed a speed limit of five miles per hour. Each robot has three tries at navigating the course, and the winner is chosen based on the time used and distance traveled. Penalties are assessed for crossing lines and hitting obstacles. So far no team has successfully navigated the full length of the course.
This year's contest was held at Oakland University in Rochester, Michigan on May 31st, June 1st and June 2nd. Rules for the 1997 contest can be found on the Web at http://www.secs.oakland.edu/SECS_prof_orgs/PROF_AUVSI/rules97.html.
Robocar is the University of Colorado (CU) at Boulder's entry in this contest. Starting with a child's electric car, our team added basic sensing and control equipment as well as two computers running Linux and software specifically designed for the AUVSI contest. Over the four years CU has entered this contest, Robocar has changed significantly. This article documents the various systems of Robocar in this year's incarnation, its software architecture and navigation algorithms.
The basic frame of Robocar is a kid's vehicle, hacked apart—the kind of toy one or two children can sit in and drive. We ripped out the wimpy motors that came with the original toy and replaced them with big, beefy ones with chains and gearing to improve drive power. We substituted computer controls for the steering wheel and pedals and augmented its body structure. Not much remains of the original vehicle except some framework and the outer plastic shell.
An inverted metal “U” was welded to the car body above the former location of the windshield and two cameras were mounted to it. The original windshield was not used because it was too low to the ground and did not provide a large enough field of view. A metal box used for housing various circuits is also attached to the metal “U”.
Batteries fit snugly into the former seating area of the vehicle. Covering the batteries is a wooden board which holds the competition load. Likewise, a wooden platform has been added to the back of the car to carry our main computer, a conventional desktop machine with a big monitor. Additional equipment, including our smaller second computer, is stashed under the hood in place of the original battery.
Overall, the car measures 30 inches wide, 54 inches long and 62 inches high and weighs in excess of 200 pounds. Actually, the car's body has become a huge, heavy hack after three years of adding features. We'll probably rebuild it from scratch next year using lighter materials.
Robocar must withstand a fair amount of stress while traveling across the bumpy course (grass, sand, simulated pavement, wooden ramps). Vibrations need to be dampened to get clean, useful video—try driving down the road while looking through a jiggling video camera some time, and you'll get a fair approximation of Robocar's vision of the world. Besides, we don't want the hard drives bouncing around all over the place; therefore, all four wheels have shocks to help reduce the vibrations, and our mechanically fragile equipment is mounted on foam.
Finally, we replaced the slippery, stiff plastic wheels that came with the car with inflatable rubber tires to improve traction on the grass and the ramps. Robocar plows right through sand traps on these tires. Running with the tires slightly deflated helps absorb shock.
Figure 2. Picture of Robocar practicing. At this time we were using wall power to conserve batteries and a joystick to manually select the motor power. It really is following the lines by itself though. Our practice lines are impossible to miss compared to the more subtle effects of spray paint on grass.
This year, Robocar has large marine batteries that can easily power it for seven or more hours of operation. The batteries are used in pairs and power all the actuators, sensors and computers. These are deep cycle batteries; that is, they are designed to withstand numerous complete draining and recharging cycles. Each 12 volt battery can source 550 amps. Even though each battery lasts a long time, we keep two spare sets on the sidelines for backup. Unfortunately, the batteries are quite heavy—adding an extra 40 pounds each. The weight trade-off is well worth the power gain, which should enable us to climb the ramp that caused us so much grief in previous years.
We have two circuits in the car: a 12 volt circuit and a 24 volt circuit. Power for the noisy drive motors and CAN-AMP servo is provided on a separate circuit from the digital devices. Relays switch the drive motors from forward to reverse as well as cut power to the motors in emergencies. The diagram of the electrical system provides more information.
If Robocar should ever go wildly out of control, a quick slap on one of the two emergency stop buttons (one of which is a remote control) will quickly bring it to a halt by disconnecting the motors from the batteries. Even though the car is moving at a mere 5 miles an hour or less, it can still do a lot of damage to objects and people. I have scars to prove it.
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