Building the CART, or Cougar Autonomous Robotic Transport
As mentioned in the challenges slide on the home page, my role in the CART project was to integrate the autopilot into the CART system and to upgrade hardware installed by the previous team.
The autopilot system we used was a Pixhawk 1.5 flight controller running ArduRover firmware. We used MissionPlanner for our ground station software. Integrating the Pixhawk with the onboard actuator controllers required tuning four different PID control loops to give the autopilot seamless driving control; two to convert the autopilot's desired speed/turn rate into desired throttle/steering strengths (i.e. 35% of max signal strength), and two more to convert these signal strengths into desired actuator positions. This resulted in smooth and effective autonomous drive control as can be seen in the video below. (Note the parking lot image is a satellite image from months before testing; it was vacant during testing.)
The two main hardware upgrades required were increasing the brake actuator output and adding an odometer. The brake actuator is a pneumatic piston driven by an air compressor feeding a pressurized line. The pressure is set by an inline regulator, and the system is activated with an electro-pneumatic solenoid. The upgrade boosted the tank pressure of the compressor, added an additional regulator, line, and solenoid, and two one way valves to direct the flow. The upgrade successfully increased performance to meet our safe stopping distance requirements. The odometer was necessary to reduce the low-speed error of the GPS based estimate internal to the Pixhawk. The sensor was composed of a Hall-effect sensor, secured with a custom 3D printed mount, and an array of permanent magnets distributed evenly around the steel rim of the front wheel. This sensor showed an average error of half that of our design requirements, demonstrating its effectiveness in generating a reliable low-speed speed estimate to. This estimate was fused with the GPS based estimate onboard the Pixhawk.
QUICK ID
Phone