Kyle L. Carithers
REU 2006
Design
The design of the MiniRanger centered on the five sonar sensors and the camera. Positioning the camera at the very front was necessary for a clear view and to allow for tracking objects close to the vehicle. This positioning also allows for the possibility of road following with the camera tilted downward. Building the platform to allow for multiple sonar modules was challenging. The placement requirements were that the sonar be able to obtain a complete picture of its environment and that the sound waves from the sonar could not reflect directly off the vehicle even with the suspension fully compressed. Additionally, the platform allows for a 30° rotation of the sonar for better performance in different types of missions.
Steering is accomplished using a method most similar to recirculating-ball steering, commonly used in trucks and SUVs, where a gearbox translates the rotation of the steering wheel into rotation of an arm. This arm connects to rods that connect to the wheels and pull one side of one of the wheels into the car and do the opposite to the other wheel. The main difference between recirculating-ball steering and the robot’s steering is that a servo takes in a digital signal and rotates the arm based on that signal instead of a gearbox and steering wheel. MiniRanger has a turning radius of approximately three feet.
MiniRanger’s actuation is very similar to a rear wheel drive car. A single motor applies torque to a differential that applies an equal torque to both wheels. The R/C car that MiniRanger is built on originally used a four-wheel drive system. With this system, a universal drive shaft connects each wheel to the differential. Universal drive shafts translate rotation from one axis to another. They are similar to flexible shafts. Universal shafts work well when the angle between axes is minimal. With a larger angle, the shaft becomes inefficient, requiring greater, fluctuating torque. Front wheel steering causes the angle to increase significantly when the robot turns. The universal drive shafts caused oscillations in the robot’s speed when turning and prevented the wheels from spinning when a sharp turn was attempted. Four-wheel drive was not necessary for this robot. Therefore, the front universal drive shafts were removed. Doing so improved performance and decreased the turning radius.
The camera pan and swivel system consists of two pieces of polycarbonate and two servos. The tilt of the camera is accomplished using what is essentially a four bar mechanism. A servo, one of the two Hitec HS-322HD is mounted to the rear of what is called the base. A servo arm is mounted to the servo and a long rod connects to the end of the arm. The opposite side of the rod connects to another piece of polycarbonate. This piece is connected to the base using metal hinges. The base of the four bar mechanism is mounted to a servo, the Hitec HS-225MG, for panning capabilities and this servo is mounted to the front of the lower level of the platform.