Student investigation 2: Robot. Jake Coppinger. Year

Transcription

1 Student investigation 2: Robot Jake Coppinger Year

2 Chassis /

3 Jake Coppinger / Jake Coppinger / Jake Coppinger 2014 To provide excellent maneuverability, four continuous rotation servos are mounted to the chassis. to move the robot. Servos were chosen for the drivetrain rather than Lego motors or a geared motor system because they are cheap (clones of Futaba S3003 servos can be bought for $4 online), easy to mount to a flat surface, and contain integrated drive electronics. With a wheel mounted on each servo I could use differential steering rather than a mechanical steering system making the robot more reliable. To enable to servos to continuously rotate, I first positioned the servo at 90 degrees with the servo. write() function and opened the case. I then glued the position feedback potentiometer at that

4 position, to fool the control electronics into thinking the cog wasn t moving. I then drilled out the plastic cross in the bottom of the primary drive cog so that it wouldn t cause the potentiometer to turn, and filed off the plastic stopper to allow the cog to move continuously. After reconstructing the servo, I could control the speed forwards and backwards with Servo.write(). As servos are modular components, upgrading to more powerful models does not require modification to the chassis. The chassis for the robot is laser cut out of one piece of acrylic plastic. I chose laser cutting for the base rather than 3D printing because it would be more durable. Four holes were included in the laser cutting design to accommodate for the Arduino Uno mounting screws. As the electronics are mounted on an Arduino shield, they can be upgraded or replaced by attaching more shields. This is an extremely convenient, modular, expandable electronics system. A laser cut acrylic mount is provided for the HC-SR04 ultrasonic sensor, with a brace to ensure durability in harsh conditions. Buffers were designed into the main chassis so that a front impact would dissipate the shock through the chassis rather than breaking the ultrasonic sensor. If the whole robot is tipped over the wheels will protect the electronics.

5 Electronic Hardware /

6 The electronics of the robot are based on an Arduino Uno with an ATmega 328p microcontroller. To interface with the various systems on the board an Arduino shield was designed and etched with a CNC router. This Arduino Uno and shield system has numerous advantages over a simple PCB with a soldered Atmega microprocessor: The Arduino can be upgraded to any microcontroller board that has the same form factor. This encompasses literally hundreds of different boards, including ATmega microprocessors, ARM SOC systems as well as 32 bit Pentium class processors such as Intel Galileo. Upgrading the microprocessor in a hard wired circuit board is simply impossible. The standardized Arduino shield system allows expandability of the electronics with simple plug-and-play compatibility. This allows for rapid prototyping and integration of additional electronic sensors, transmitters and receivers, data logging features, Global Position System systems, LED displays, LCD displays, and over 300 commercially available shields ( shieldlist.org/). Expanding a traditional circuit board with these features would be impossible without developing a completely new circuit board with another microprocessor embedded, increasing costs. By designing a custom shield for the Arduino an efficient interface for the robots electronics is achieved, with the opportunity for expansion in the future. An FTDI or equivalent upload system is already integrated into the Arduino Uno, reducing the number of external components required. If the microcontroller board is upgraded the means

7 of communicating with that board is included. As an Arduino Uno and custom-made shield setup can be upgraded, the long term cost will be cheaper. Arduino Uno clones can be purchased for under $9 on ebay, which includes a power supply regulator and FTDI or equivalent upload system. The robot is powered through four 1.5 volt AA batteries which provide the necessary supply power for the servo motors while also powering the Arduino through its inbuilt 5v regulator. Additional digital pins are broken out into headers for prototyping or expansion of the robots features. Two electrolytic capacitors smooth ripples in the supply voltage on the input voltage and regulated 5v lines to ensure smooth and reliable operation. For debugging and testing purposes a NICTA ed1 Arduino board can be connected through the UART serial header provided on the circuit board, allowing a way of controlling the robot when it is not line following. By attaching an XBee shield to the Arduino this could be achieved wirelessly. Controlling the robot has been achieved by requesting data from the I2C accelerometer on the ed1, sending it via serial to the Arduino Uno, and mapping the x and y tilt values to drive the robot through differential steering.

8 Line following Hardware /

9 To sense the line and green squares the robot with four LED and LDR pairs incorporated which sense the line through the intensity of the reflected light measured by the Light Dependent Resistors. The components are mounted on stripboard that is attached to the main chassis with Meccano, allowing convenient detachment for upgrades or repairs, and for unlimited adjustability. To adjust the field of view heatshrink can be formed around each LED/LDR pair, which satisfies all the requirements for an ambient light shield: adjustable field of view, adjustable hight, replacement and adjustment possibilities. The sensors are mounted towards of the back of the stripboard that is mounted behind the front of the robot to shield incoming ambient light sources. A cable with a removable socket connected the circuitry to the electronics mounted on the Arduino shield. (Stripboard diagram a rough representation of circuit - header was placed at the side of the board which the tracks not highlighted utilised)

10 Jake Coppinger / 2014