10/29/07

Cruiser

Cruiser is my line following robot.

Cruiser can follow either a dark line in bright background or a bright line in a dark background.

Cruiser competed in Racerbots, the Autonomous Robot Competition in Techkriti 2003, organized by Indian Institute of Technology, Kanpur and secured 2nd position.  The objective of the competition was to traverse a track in shape of an 8 with a black line painted in centre in minimum possible time.

A very remarkable fact about Cruiser is that it was made in 3 days only!

Click here for IITK, Techkriti 2003 Photo Gallery

Mechanical Design

    A pair of stepper motors drive a pair of wheels in a wheelchair type configuration.  Balance is achieved by a castor wheel at rear.  The chassis is made of thick Aluminum sheets screwed to the stepper motors.  The chassis provides sufficient space for 12 AA size batteries, six in front of the motors and six in rear.  Two wooden beams mounted on the chassis support the circuit boards.  The small sensor board is mounted on the front lower region of the chassis.
 

Batteries

    12 Ni-MH 1.2V 1000mAh rechargeable AA size batteries power Cruiser.  The voltage of the battery pack ranges from about 16V (fully charged) to about 12V (near discharge).  The batteries directly power the stepper motors.  A linear 5V regulator powers rest of the electronics.
    As the competition allowed use of external power supply, the batteries were removed from Cruiser during the competition to reduce its weight and allow it to run at a higher speed.

Motors

    The two stepper motors are 12V floppy drive motors.  They are driven in dual coil unipolar excitation mode (AB-BC-CD-DA).  The mode provides maximum torque from the stepper motors.  The motors are driven directly from the battery supply.

Sensors

    Cruiser has three IR transmitter-receiver pairs mounted on the front lower region of the chassis.  Distance between adjacent transmitter-receiver pairs is about 2 cm.  Whenever the reflecting (non-reflecting) line comes below any sensor, the corresponding sensor turns ON (OFF).  The position of the sensors is such that the middle sensor is exactly at the central half of the robot and the other two sensors are on left and right sides.

    At any given time, at most two sensors can be ON/OFF.  The relative displacement of the robot with respect to its center can be calculated from the adjoining table.  With this information the software reduces the stepping frequency of the corresponding opposite motor by a factor proportional to the relative displacement.
i.e. if displacement is towards LEFT reduce the speed of RIGHT motor and vice-versa.
 

Controller and other Electronics

The brain of Cruiser is the Atmel AVR 90S8515 microcontroller with the following features:

	Up to 8 MIPS throughput at 8 MHz
	8K In-System Serial Programmable Flash Memory
	512 Bytes SRAM + 512 Bytes EEPROM
	8-bit Timer/Counter
	16-bit Timer/Counter - Dual PWM
	Serial UART
	Master/Slave SPI Interface

    A big advantage of using this microcontroller is the In-System Serial programming which means that there is no need to pull out the microcontroller to program it, just plug in the 5-pin programmer socket of the serial programmer (connected to a computer serial port) and you are done!  This is especially useful during development and testing.
    Cruiser circuit is laid out on two boards.  The power supply + motor driver board and the controller board.  The controller board hosts the 90S8515 microcontroller, 4 comparators for sensor inputs, a potentiometer to set the sensor reference voltage, 4-way DIP Switch to set the speed, two press switches for start/stop/reset, 5-pin programmer connector, 4-pin connector for motor driver board, 2-pin power supply connector, three red LEDs to display sensor data, a green LED as power indicator.

Limitations and Future Developments

    As Cruiser was made on a very short notice, it was made with the components in hand at the moment and thus the design is not very optimum.  Firstly its speed is severely limited because of the use of stepper motors.  DC Motors controlled by PWM will allow it to go at higher speeds.  Instead of using 12 batteries to provide sufficient voltage to driver the motors, less number of batteries with a step-up switching regulator can be used to reduce its weight and allow it to attain higher speeds.  Lastly instead of using three sensors with digital outputs, two sensors with A/D converters will offer higher precision in measuring displacement with respect to the track and thus would result in a smoother run.