For Mechatronics, Spring 2024

Last updated by Anway Pimpalkar, March 27 2024

Latest sample code, PCB Layouts and STEP files available at https://github.com/anwaypimpalkar/jhockey-tracking

For the hockey robots you are constructing for your final project, it is essential to know the robot's exact location at any given time to determine the correct orientation and movements for approaching and shooting towards the goal. To simplify your project, we have constructed the tracking system for you. This system tracks ArUco tags on top of each robot using a computer vision algorithm and sends each robot its coordinates using a ZigBee protocol.

Each robot is assigned a unique ArUco tag and a corresponding alphabetical ID (A, B, C, D, etc.). These tags are tracked using a Computer Vision (CV) algorithm which captures the X and Y coordinates of the robot in centimeters. The tracking system runs on a JeVois Smart Machine Vision Camera connected to a Raspberry Pi.

The unique IDs are specific to each ArUco tag and XBee module; therefore, you must keep them paired together.

You can learn more about ArUco tags and tracking algorithms at https://docs.opencv.org/4.x/d5/dae/tutorial_aruco_detection.html

The coordinates found by the CV algorithm are relayed to your robot using ZigBee.

ZigBee is a high-level communication protocol used to create personal area networks with small, low-power digital radios. It is particularly suited for applications where power consumption is a critical factor, such as in wireless sensor networks and home automation devices. ZigBee operates on the IEEE 802.15.4 specification and is designed to provide secure and reliable wireless data transmission at low data rates.

You can check out this resource for a more detailed overview of ZigBee: https://www.geeksforgeeks.org/introduction-of-zigbee/

ZigBee modules (called XBees) can mesh together to form a network where each node can transmit and relay data to other nodes. This meshing capability allows for extended range and redundancy, as data can find multiple paths to reach its destination. The dynamic nature of ZigBee mesh networks ensures that if one node fails, data can be rerouted through other nodes, enhancing network reliability.

In our setup, the CV algorithm supplies the robot tracking information to the Raspberry Pi, which then broadcasts the match information via a XBee 3 Tx module to all receiver nodes on the same Personal Area Network (PAN) channel. At a time, a maximum of 15 bots can receive information at once.

The locations of all the robots on the arena are sent within the same packet. The match information consists of the following information:

• > Start (1 byte).
• Match Byte Binary (0/1) indicating whether a match is ongoing (1 byte).
• Time Match time in seconds, represented as 4 bytes.
• Robot #1 ID Alphabet corresponding to the first robot's ArUco tag (1 byte).
• Robot #1 X First robot's X coordinate in centimeters (3 bytes).
• Robot #1 Y First robot's Y coordinate in centimeters (3 bytes).
• Robot #2 ID Alphabet corresponding to the second robot's ArUco tag (1 byte).
• Robot #2 X Second robot's X coordinate in centimeters (3 bytes).
• Robot #2 Y Second robot's Y coordinate in centimeters (3 bytes).
• ...
• Checksum Checksum (2 bytes).
• ; End (1 byte).

This information is sent to all the XBee 3 modules as a string, as shown in the figure above.

XBee 3 Module Datasheet: https://www.digi.com/resources/documentation/digidocs/pdfs/90001543.pdf

The XBee modules have an inbuilt processor on them, capable of running Micropython scripts. We have flashed a script which processes the match information, and only returns selected information back to your Arduino-based robot. The script has a predefined Robot ID, and parses the string for information pertaining to that particular ID.

The output from the module after parsing the string is a comma-delimited string, which looks like M,TTTT,XXX,YYY.

where,

• M is the Match Byte, which is a binary (0/1) digit indicating whether a match is ongoing or not.
• TTTT is the match time in seconds. If a match is not ongoing, it will be set to 0000.
• XXX is your bot's X coordinate with respect to the arena in centimeters.
• YYY is your bot's Y coordinate with respect to the arena in centimeters.

• M,TTTT,XXX,YYY Ideal output.
• M,TTTT,---,--- If your bot’s information is not sent across by the Tx (camera has not detected it) OR the tracking system is not turned on.
• ?,????,---,--- If the XBee module is not receiving any information from the Tx for more than 1 second.
• /,////,---,--- If the checksums are not matching.

The Arduino and XBee module communicate via a UART protocol. On an Arduino Mega, four hardware serial lines are available for UART communication. One of these is used to establish communication with our computer, and a second one is utilized for XBee communication.

XBee modules operate with 3.3V digital logic, while the Arduino Mega operates at 5V digital logic. Therefore, it is necessary to convert the logic voltage levels between these devices to ensure proper transmission. For this task, we use a bidirectional level converter, as illustrated in the figure below.

Bidirectional Level Converter Link: https://www.sparkfun.com/products/12009

Bidirectional Level Converter Datasheet: https://cdn.sparkfun.com/datasheets/BreakoutBoards/BSS138.pdf

The circuitry for this has been condensed into a printed circuit board (PCB) for your convenience, on which you will only need to mount the respective components. Below, you will find a sample connection diagram that corresponds to the provided sample code. If necessary, you can change the hardware serial ports.

PCB files available at https://github.com/anwaypimpalkar/jhockey-tracking/tree/main/pcb

The DOUT and DIN labels on the PCB corresponds to the respective pins on the XBee module. You can learn more about them in the XBee 3 Datasheet.

The XBee module communicates with the Arduino via UART. To send a location query, you must transmit a ? character to the XBee module, which will then respond with the latest coordinate information in the comma-delimited format previously mentioned.

Sample code available at https://github.com/anwaypimpalkar/jhockey-tracking/tree/main/xbee_sample_code