Design and Development of Maisie the Micromouse

C.Kanesalingam

INTRODUCTION

A mouse, measuring approximately 15cmx15cmx15cm was built to demonstrate the guidance matrix path planning algorithm in a real environment. The environment is an unknown, static, structured maze of 256 linked square cells.

This mouse is driven by two stepping motors. Six down looking infrared sensors were used to map the environment and to detect any navigation error (due to wheel slippage). Interfaces were built to aquire the sensor data, control the stepping motors, and to produce a user interface. Software was written in Forth and assembly language for the TDS2020 microcontroller to perform the following tasks:

1. Control the sensors and sense the environment.
2. Build up the environmental map.
3. Plan the path for collision-free navigation.
4. Drive the stepper motors according to the planned trajectory.
5. Sense any navigation error and correct it.

MECHANICAL DESIGN

As shown above, an arrangement with two independently driven wheels and a ball bearing at the front was chosen. Streering is accomplished by driving the wheels at different speeds or in different directions.

• Stepping Motors: The stepping motors used in this vehicle are uni-polar type. The specification is as follows:-
  • Step Angle: 1.8°; Supply Voltage: 11 Volts; Current per phase: 0.32A; Holding Torque: 9 N cm
  • Dimensions : 42mm x 42mm x 34mm Shaft : diameter = 5mm and length = 20mm
• Wheels: The wheel dimensions are:- Tyre diameter = 50mm; Axle diameter = 5mm
• Ball bearing (Stock No: 687-679 from RS Components Ltd) diameter = 12.5mm
• IR Sensors: There are six infra-red proximity detectors (Type : Sharp GP2A22). The sensors were mounted on the robot in pairs. This arrangement enables the system to detect any mis-alignment.
• Chassis: The chassis was made out of components from a Meccano set.

INTERFACE CIRCUIT DESIGN

The interface circuit consists of four main sections

1. Voltage regulator
2. User interface (switches and LEDs)
3. Stepping Motor Drivers
4. IR sensor data acquisition module.

Voltage regulator: Some of the electronic components used in the circuit required +5 volts supply. A voltage regulator (7805CT) was used to convert the battery voltage (Type: 12Volt 1.2Ah) to +5 volts. Maximum output current of this regulator is 3 amps.

• Warning messages (LED7 & LED8): In addition to the six LEDs connected across the output of the IR sensors, two LEDs were connected to the microprocessor output port. This enables the microprocessor to provide diagnostic information to the operator.
• Input switches: There are two input switches. One is a push button switch (Acknowledge / Interrupt switch 1) which acts as a start and emergency stop switch. It has been designed in a way that the output of this switch can either be connected to the input port of the microprocessor or to the interrupt terminal of the microprocessor using jumper (J1). The second switch (Acknowledge Switch 2) is a dual-in-line switch and currently has no effect.
• Main switch: This is a SPST switch which isolates the battery from the interface circuit and the microprocessor.
• Stepping motor driver: Each motor is driven by UCN5804B driver.
• IR sensor data acquisition system: The robot is equipped with six infra-red (IR) proximity sensor modules in order to detect the walls and hence map the maze. These IR sensors are Optical Integrated Circuit (OPIC) proximity sensors. This OPIC sensor includes a photodiode, a signal processing circuit and a voltage regulator onto a single chip. In addition, the sensors used are Light Modulated type, thus they ar relatively free from external disturbing light. The detecting range of these sensors is between 9mm and 15mm.

SOFTWARE DESIGN

Most of the software modules for this demonstrator were written in Forth. Some modules which perform repetitive and time consuming processes were written in H8/532 assembly language.