PROJECT IDEA
The basic idea of our project stemmed from a rather playful interest in using an MCU to program a robot to perform some common everyday function autonomously. Given our lack of mechanical engineering expertise, an RC car was chosen as the hardware to be programmed since it provides all the components and infrastructure necessary for a simple mechanical system, all at a low cost with an easy-to-use interface. We decided to design the car so that it would automatically navigate a track made of parallel lines which were meant to mimic the roads that actual full-sized cars encounter. Given that there has been some societal interest in the development of automatic driving car technology, we figured that designing a low complexity system with this functionality would be an exciting and practical project to pursue. Perusing through past final projects helped give us tips on how to approach our own project. The Line-Following Car of Spring 2007 uses photo sensors to guide the vehicle along a single line track. Our original concept intended to use a low resolution camera as the input tracking device instead. Given the similarities in our projects, we were even more inspired to give our RC car the functionality of detecting a two line track and stay within the lane, which requires a more complex algorithm than a single line track follower. The Autonomous Self-Parking Car of Spring 2009 introduced us to a cheap, tractable IR distance sensor component that we later incorporated into our own design as a way to detect forward objects and avoid colliding with them.
THEORY:
All computations and processing are handled by the ATMega644 MCU. The MCU receives inputs from two peripherals the low resolution CMOS image sensor(camera), and IR distance sensor. Image input in the form of a 128x123 pixel array is processed in the MCU using the internal analog comparator, turning pixels that were originally represented by bytes of data into pixels now represented only by bits.
Since our RC car only has to detect the image of a road line in contrast to the rest of its environment, this binary pixel representation is sufficient for this purpose and saves immense amounts of computation time. The voltage threshold for the analog comparator had to be adjusted to a proper value for a given road line color which we chose to be black. Processing of this image input from the camera is done by the MCU to generate
signals that indicate whether the car should turn left or right or go straight. These command signals eventually control the servo motor on the RC car that allows it to turn left or right, but must first go through an H-Bridge circuit so that polar voltages can be created to drive the servo motor in both directions (since the MCU cannot generate negative voltages). The MCU also receives an analog input from the distance sensor which is processed with the MCUs internal ADC into an 8-bit value. This digital value can be mapped to an actual distance, but is not done in software since it would be unnecessary and would only add more computation time to an already time sensitive system. The 8-bit digital distance value is used in a simple control algorithm that determines the duty cycle of the PWM coming out of Timer2. The PWM controls the operation of the cars DC motor, supplying the motor with a higher voltage when its duty cycle is higher. The PWM is sent to a motor control circuit that is implemented using the same design as the control circuit from Lab 4 which uses an optoisolator to separate the MCU circuitry from the motor circuitry. The optoisolator is necessary so that excessive current draws from the motor do not damage the MCU hardware. We decided to only have the car move forward; therefore, a circuit like an HBridge was not required to operate the DC motor and only had to be driven in one direction. To power all of our hardware, several power sources had to be used. The target board for the MCU required a 9V battery for proper operation. Combined with the 5V regulator on the board this provided a constant 5V Vcc to the MCU. The camera was powered using the 5V supply on the MCU and since it required a minimal amount of current, this setup posed no problem to the system. The distance sensor is powered from a different battery source a battery pack consisting of three AA batteries in series at 1.5V each. The grounds of both this 4.5V battery pack and the MCU were tied together so that the analog voltage output of the distance sensor had a ground reference. The rest of the circuitry is properly isolated from the MCU and is powered from the cars battery setup which uses five AA batteries rated at 1.5V each for a 7.5V total supply. This served to be too much for the H-Bridge circuit which requires less of a voltage. To remedy that, diodes were placed in series between the positive terminal and the H-Bridge Vcc pin to drop the voltage instead of using an additional battery source.
