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Preparing for the Next ‘DARPA Grand Challenge’

By Terry Persun  12.30.2004 0

Team Overbot is a group of enthusiasts, engineers, and partners building an autonomous robot vehicle for the October 8, 2005 DARPA Grand Challenge. We're talking a trek close to 150 miles through all kinds of terrain. And at the end of the road is a $2,000,000 prize.

Last year, the course included a 142-mile trek from Barstow, CA to Primm, NV over well-traveled utility roads, switch-backs, severe elevation changes, blind turns, and sheer drops. None of the vehicles finished the course. Team Overbot dropped out before the race last year, but this year, they're ready.

The Contest
The DARPA (Defense Advanced Research Projects Agency) Grand Challenge was designed as a field test and as a tribute to the U.S heritage of innovation, risk-taking, and a sense of team spirit. It brings together individuals and organizations from industry, the R&D community, government, the armed services, academia, students, backyard inventors, and automotive enthusiasts in the pursuit of a great technological challenge. The competition's aim is also to accelerate R&D in autonomous ground vehicles that can be used to save lives on future battlefields. DARPA expects to qualify 20 vehicles to participate out of the more than 100 applicants.

Design Methodology
The Overbot vehicle is a commercial 6-wheel-drive all terrain Polaris Ranger Series 11 all-terrain vehicle manufactured by Polaris Industries (Minneapolis, MN). The front four wheels are on independent swing arm suspensions, and the rear axle is rigid, but on a swing assembly. The overall vehicle, fully-fueled, weighs approximately 1,900 pounds, with a maximum range of 350 miles.

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The front two wheels are steerable, four of the wheels are equipped with hydraulic brakes, and all wheels are driven when in 6WD mode. An electrical/vacuum system switches the vehicle from 2WD to 6WD. The vehicle uses servomotors to drive the steering, brake, transmission, and throttle. The engine choke is actuated using a solenoid. The laser range-finder atop the vehicle is tilted via a servomotor as well.

The Overbot vehicle computing systems include three small industrial control microcomputers for sensor and actuator control. Two larger computers are used for vision and navigation processing. All the computers use an IA-32 architecture and are interconnected via a 100baseT networking system.

Team Overbot developers chose a navigation approach that first measures terrain and obstacles using a variety of sensors. From the data collected, the system builds a local terrain map of the immediate vicinity of the vehicle. This map is probabilistic and contains uncertainty information. Next, an attractive/repulsive field type planner generates trajectories. In addition, a visual “road follower” attempts to recognize road surfaces and add them to the vicinity map, so that if a road is present and going in the desired direction, it will be used.

Out on the Challenge course, the vehicle is designed to head toward the next GPS waypoint, using a higher level processor, unless it has encountered an obstacle. According to John Nagle, project manager for Overbot, “Escaping from local minima is the job of the higher level processing system. Internally, we call this the backseat driver, because it has no direct authority over the control system.” This processor will then mark untraversable areas in its vicinity map and attempt to work around them, directing the Overbot to back up if necessary.

At a lower level, processing algorithms for individual sensors exert veto power over high-level decisions, which may result in the vehicle stopping suddenly if an obstacle is detected. “Our general approach is not to out-drive our stopping distance. We insist on good ground profiling data from the laser rangefinder out to our stopping distance. Pitch will be factored into the stopping distance computation, and rough ground will be covered at slower speed so that the vehicle sees shock levels well under 1G vertically. We will not exceed 40 mph at any time,” Nagle says.

Overbot sensing technology includes an anti-collision radar system fitted to detect impending collisions, and a laser range-finder mounted high on the vehicle on a semi-custom tilt head to profile the ground. Other sensors include: a digital camera used by the road-following vision system; ultrasonic sonars with overlapping sensing fields surrounding the vehicle for protection during low-speed operation; and sensors at two heights to detect when the vehicle has entered water.

Additional sensing includes the monitoring of all actuators with position and velocity feedback. Engine and driveshaft rpms are followed, along with some voltages and temperatures. A Doppler radar speedometer senses vehicle speed. A low-precision strap-down inertial navigation system (INS) and magnetic compass are provided for short-term acceleration, velocity, and position sensing.

To further control the vehicle, speed as measured by the radar speedometer is compared with the speed as measured at the driveshaft to detect slippage. Engine rpm is compared with throttle setting to check on engine load. Overheat conditions are detected and used as an indication to reduce speed. INS and GPS data are combined to maintain both a location relative to recent positions for local navigation, and an absolute position for global navigation.

The system determines its geolocation with respect to route waypoints through the use of a NovAtel ProPack LBHP GPS with Omnistar corrections, along with a Crossbow Technology inertial system. This combination provides location to within 20 cm. “With GPS information available, and in dead-reckoning mode, we expect to have drift rates of perhaps 1 degree per minute in heading,” Nagle notes.

Control Motor Drives
Because the Polaris engine is under the seat, there's considerable room in the compartment under the hood. That's one of the reasons the team chose the Polaris Ranger as their robot vehicle, and where the engineers placed the steering and brake hardware, as well as the controllers.

The steering actuator includes a geared brush-type dc servomotor from Maxon Precision Motor USA (Burlingame, CA). The motor is an F Series motor that uses ferrite magnets. Overbot engineers selected a Maxon motor for providing high torque within a reasonable size. This characteristic eliminated concern over how well the direct drive system operated. In fact, “These motors are powerful enough to turn the wheels even with the vehicle not moving,” Nagle says.

The drive actuator is connected to the steering box below via a rubber shock coupling. Peak motor current in normal operation is about 8A at 24V, which was more than the original motor controllers could reliably deliver. A new controller was found that could do the job. The new controller and its opto-isolator board are positioned to the left of the motor in the figure below. Previously, three of the older controllers were packaged and mounted in that same space. The replacement motor controllers have been permanently package, but in the final configuration, two controllers and the opto-isolator board will be packaged together in that space.

The break actuator is also a dc servomotor from Maxon. This motor drives a ball screw. Located below and behind the brake master cylinder, the high-speed actuator can start breaking within 200 milliseconds of actuation. A pressure sensor, monitored by the controller package, and a pressure limit switch, used to stop the brake actuator after an emergency stop, are attached to the front wheel brake line.

Lastly, other Overbot control drive components include a 62mm planetary gearhead with a 71:1 reduction ratio; an HP HEDL rotary encoder for feedback control; and a Maxon Model 4-Q-DC ADS 50/5 servo amplifier.

For more information and to follow team progress contact:
Silicon Valley Overland
Team Overbot
www.overbot.com

Maxon Precision Motor
838 Mitten Road
Burlingame, CA 94010
P: 650-697-9614
F: 650-697-2887
www.maxonmotorusa.com

Terry Persun is an electronics engineer and freelance technology writer.

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