10/29/07

What's a Micromouse?

Micromouse is a small, autonomous vehicle designed to get to the centre of a maze in the shortest possible time.

PathFinder I

PathFinder I wins 3rd place in Micromouse Competition,
Yantriki 2005.

Micromouse Competition Video, Yantriki 2005
p1.mpg (5.4 MB)

PathFinder I created history on 24th January 2004 by becoming the "FIRST MICROMOUSE IN INDIA TO REACH THE CENTER OF A  MAZE !" and secured 2nd position in the competition.

Micromouse Competition Video, Yantriki 2004 (Coming Soon!)

Performance and Specifications

* Batteries in pictures are different

Mechanical Design

    PathFinder I has a pair of drive wheels in a wheelchair configuration.  The wheels are driven by a pair of stepper motors.  Steering is achieved by running the motors at different speeds.  The wheels are made of wood, and have a hard rubber grip over them.  A castor wheel, made from a small toy motor, balances the structure.
    There is no chassis in PathFinder I.  The stepper motors are glued together, batteries glued to the motors and the castor wheel glued to the batteries!  Two long plastic IC carriers glued to the motors support the circuit board and the sensor wings!  The circuit board and the sensor wings lie above height of the maze walls.  So the effective in-cell dimensions of the Micromouse are only 8.5 cm x 9.5 cm.
The two motors and the batteries form a major component of the total weight of the Micromouse.

Batteries

    9 Ni-MH 1.2V 1000mAh rechargeable batteries power PathFinder I.  The voltage of the battery pack ranges from about 13V (fully charged) to about 10v (near discharge).  A 15V step-up switching regulator powers the stepper motors and a 5V step-down switching regulator powers rest of the electronics.  Both regulators are > 90% efficient, so PathFinder I is pretty efficient on power consumption.  PathFinder I can run more than an hour after a full charge!  The step-up regulator ensures that a constant 15V is available to the motors even when the batteries are nearing discharge.  This ensures that the torque provided by the motors does not deteriorate with the decreasing battery voltage.

Motors

    The two stepper motors are 12V type.  They are overdriven at 15V in half-stepping Unipolar mode (A-AB-B-BC-C-CD-D-DA).  The driving mode gives the maximum precision possible with a stepper motor.  But in this mode the torque at higher stepping frequency deteriorates due to the time required to attain maximum current in the coil after each step.
    PathFinder I now runs on dual coil Unipolar excitation mode (AB-BC-CD-DA).  The mode provides maximum torque from the stepper motors and allows PathFinder I to gain even higher speeds.  Although the precision in driving the motors in dual-coil excitation is half as compared to driving the motors in half-stepping mode, the speed advantage gains over precision here.
    The stepper motors expect a continuous smooth pulse train.  Once the pulse train messes up at speed, the motors are likely to just stop turning rather than slowing down.  The result will be a Micromouse that suddenly swings around one wheel to examine the wall a bit more closely.  So the motor control software is written in such a way that it never changes the frequency of stepping instantaneously.  When starting, it slowly accelerates the motors and when stopping it slowly decelerates them.

Sensors

    The sensors are an array of fifteen IR transmitter-receiver pairs which are placed at about 1 cm above the top wall height.  These sensors live on wings and look down at the walls from above.  There are seven transmitter-receiver pairs each on the left and right sides to detect side walls and one transmitter-receiver pair in front to detect front wall.  The distance between adjacent transmitter-receiver pairs on the side wings is about 1 cm. The position of the sensor wings is such that when the Micromouse is in the center of a cell, the maze walls are below the 3rd sensor on each side.  The total wing span is 25 cm and the outermost two sensors on both sides detect walls in adjacent cells which are perpendicular to direction of travel.
    The transmitters are IR diodes and receivers are IR photodiodes with a purple IR filter.  If a wall is present under a sensor, IR is reflected from the transmitter to the receiver and the IR photodiode starts conducting.  Ambient light - particularly sunlight, which is high on IR may also cause the sensors to respond.  So care is taken to properly shield the IR photodiodes from ambient light.
    The IR photodiodes form a voltage divider, the output voltage of which increases with more IR incident on the IR photodiodes.  The output of this voltage divider goes to a comparator, which compares it with a reference voltage.  The reference voltage is set so as to take care of the ambient light conditions.  The 15 sensors forming 15 voltage dividers interface to 15 comparators.  The outputs of the comparators go to the microcontroller through a 16-to-1 multiplexer.  The multiplexer is used to reduce the number of I/O pins required on the microcontroller.

Controller and other Electronics

The brain of PathFinder I 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.
    PathFinder I circuit is laid out on two boards.  The small board near the sensors is the power supply and motor driver board. The large board is the controller board.  This board hosts the 90S8515 microcontroller, 32K external memory, 16-to-1 multiplexer, 16 comparators for sensor inputs, a potentiometer to set the sensor reference voltage, 5-pin programmer connector, 3-pin serial connector, 4-pin connector for motor driver board, 2-pin power supply connector, a power ON/OFF switch, two press switches for start/stop/reset, three red LEDs to display sensor data, a blue LED to display searching/fast run status, a green LED as power indicator.

Software

The software does the following tasks:

	Reading / Analyzing sensor data
	Storing wall information
	Controlling the motors
	Aligning the Micromouse, if it drifts from the centre of a cell while moving
	Finding and following the shortest path