This package provides a specialized implementation of iRobot's Create Protocol Specification on an Atmel AVR micro-controller. This package is available from: https://github.com/ipa-josh/ipa_lcrob/tree/master/ipa_odroidx_open_interface.

Hardware Design

This implementation, while providing a framework for communication with the master using a variant of iRobot's Create Open Protocol Specification (http://www.irobot.com/filelibrary/pdfs/hrd/create/Create%20Open%20Interface_v2.pdf), has been focused to give its services as a:

• Dual Motor Controller.
• Motor Position Notifier.
• Battery Voltage Monitor.

This design has three UART communication ports. Communication with the master is done with the help of hardware UART port (using Peter Fleury's UARTLibrary http://homepage.hispeed.ch/peterfleury/avr-software.html#libs) where as the communication with the motors is done using two soft UART ports which have been implemented using a custom implementation of Thomas Martin's SOFTUART (http://www.siwawi.arubi.uni-kl.de/avr_projects/#softuart). Whereas for the battery voltage notifier an external voltage divider circuit has been connected to one of the ADC on the motor controller board.

Software Design

The tasks performed by the motor controller board can be distributed into:

• Initialization.
• Main Loop.

Initialization

Before the software can enter the main loop, all of the connected devices must be initialized. The baud rate between the controller board and the master is set to 115200 bps where as the the baud rate between the motor controller board and the motors is set to 9600 bps. Two timers are exclusively set up for soft UART communication. Initialization string "EN\n" is sent to both the motors to enable them and the message "HO\n" is sent to move the motors to home position. This ensures that the encoder values start from the initial position. In addition to that, a timer of 15 milliseconds is set up for polling the motor position from both the motors and also is used in case the "STREAM" command is used by the master.

Main Loop

The main loop is an infinite loop which actuates the parsing function and the polls the motors to update their positions and in case the "STREAM" command is enabled, it generates its response. It can be extended to poll the voltage reading from the battery.

Open Interface Protocol Implementation

Interaction with "turtlebot_node"

Since the motor controller board is required to interact with the turtlebot_node package (https://bitbucket.org/ipa-josh/turtlebot, branch: ipa_create) it is expected of it to implement certain protocol commands to ensure compatibility. The turtlebot_node first sets the the motor controller board to its required state (Off, Passive, Safe and Full) and expects the motor controller to save the state as specified as it uses it as a check that the board is communicating properly. Since currently there is no difference in any of the modes implemented on the motor controller board, only a variable is being used to store the state as requested by turtlebot_node. But for future work or if is required, the code for the motor controller board can be extended to implement this and also out in conditions for checking the current state of the board before any action is taken.

After initialization, depending on which turtlebot driver is being used, turtlebot_node repeatedly starts to send a "QUERY" command to the controller board. In case of this package, it is preferred that the ipa_create robot type be used since it introduces a custom query field "100" which responds with the all the information from QUERY command field "6" and also the encoder values from the motors attached. In case, the turtlebot_node senses a change in the "/cmd_vel" topic with respect to the velocity, it is required to generate a "DRIVE DIRECT" command for the controller board. The controller board should be able to handle this command.

Motor Actuation and Position Monitoring

On receiving a "DRIVE DIRECT" command from the turtlebot_node, the motor controller board forms a velocity message "VEL" for the motors. This message also contains the desired velocity as requested by the turtlebot_node. In addition to that, the motor controller board also also sends a "POS" message to the motors to poll the current position value. Its response is read and a position variable for each motor is updated. Whenever the master requests for the position, the updated position values are sent.