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What does it take to build a Line Follower Car?

I am currently having my internship with RS Components. One of my main functions is to work on a project that is related to the Renesas Micon Car Rally.

The aim is to simplify the procedure of assembling the Micon car kit. In the process provide related materials to the concepts for building a line following car that is easy to understand.

As a start, I learnt and explored two of the DesignSpark (DS) software tools - DS Mechanical and DS PCB. Both of the powerful software tools were easy to learn and totally free. With the knowledge and skills of the two softwares, I did modifications to the car kit, to simplify the assembly of the car.

Overview

The Renesas Micon car rally is a competition which competes against the time, where self-driven cars which are controlled by Renesas microcontrollers takes up the challenge of the recommended course. The company have produced a car kit that includes the necessary items for competitors to assemble.

The main items found in the Micon Car Kit are:

- 1 servo,

- 2 DC motor,

- 1 piece of universal plate,

- 4 pieces of leg structures,

- sensor board,

- driver board,

- Renesas RX62T microcontroller.


Block diagram of the car circuit

Sensor

There are two main methods of photo sensing. The first method is by the detection of visible light. The second method is by detection of infrared transmission. Both technologies are based on the same working principle. Light is reflected from the surface then converted to an electrical current that is proportional to the amount of reflected light through the photosensor. The amount of light or infrared that is reflected depends on the colour of the surface - darker surfaces absorbs more while lighter surfaces reflects more.

Sensor Board

The sensor board from the kit is equipped with a infrared detection method. It has a faster response rate compared to the detection of visible light which has a better sensitivity. For this project, the changes in the course gives a higher priority to the car than accurate sensing because of the different reflection intensities. As the course is made of only black and white surfaces, the amount of reflected light makes a huge difference between the two colours which is easily distinguishable by the infrared sensors.

Sensor Board BOM List can be found at the bottom of the article.

Driver

The driver board from the car kit is required in order to control and power the motors that are configured by the microcontroller. As the microcontroller is unable to deliver the current that is required to operate the motors, the driver board is in charge of operating them instead by using a much larger current. This is received from the microcontroller’s motor configuration signals.

The speed of the DC motors are controlled by the microcontroller through Pulse Width Modulation where a higher duty cycle results in a higher speed. The mode in which the DC motor operates is controlled by the H-bridges that is found on the driver board in the form of four Field-Effect Transistors (FETs). The FETs operates just like a switch, where it is able to control the flow of the current through the motor by configuring them in an H-bridge circuit with the use of two P-channel and two N-channel FETs. With the use of the H-bridge, the DC motor is able to operate in four different modes, forward, reverse, free and brake. This is configured by the different switching configurations of the four FETs.

The servo motor has an additional pin compared to the DC motor. This is for a PWM signal by the microcontroller to configure the servo angle. A different duty cycle would mean different servo angles of the servo motor which is used to steer the car’s direction.

The driver board also comes equipped with three LEDs and one push button. Two of the LEDs and the push button are programmable by the microcontroller and the remaining LED is a power indication for the driver board.

Driver Board BOM List can be found at the bottom of the article.

 MCU 

The microcontroller board from the kit uses Renesas RMC-RX62T which is developed specifically for the Renesas Micon car rally. The microcontroller board is the brain of the system. It is programmed to control the system that is connected to both the sensor and driver boards. It sends and receives signals from the boards. With information received from the sensor board, the microcontroller configures the motor to respond to the information by sending command signals to the driver board. In order to configure and program the microcontroller, it requires Renesas Integrated Development Environment software for the building of the program code for the microcontroller and Flash Development Toolkit software to upload the program code onto the microcontroller.

The development kit of the Renesas microcontroller that is used can be found HERE.

Body Structure

The universal plate and structure legs found in the assembly kit requires multiple sawing and drilling of the universal plates to various dimensions for creating the different plates of the car’s body. This process might take a long time for users that are not familiar with using saws and drills. The equipment required for the process might not be found easily and can be dangerous. I will be creating a template within the next few weeks which can be printed using a 3D printer instead of sawing and drilling of the universal plate. This can be dangerous and time consuming.

                               Universal Plate                                          Plate Cutting Pattern

Finally, after assembling the full car kit, this is what the car will look like. It took me three days to assemble the car and connect up the circuit. Lots of time was spent on cutting and drilling to create the different car plates. I will be creating a template for the different car plates using DS Mechanical and will be posting it in the following article, stay tuned for the updates.

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