The 2017-2018 boat introduces the use of a Raspberry Pi 3 B ("RPi") for increased processing capability for more complex navigation control. However, the onboard Arduino Mega ("Mega") board has been retained for its many IO pins and flexibility. In the current setup, the Mega controls access to all the sensors, servos, the RC and the XBee module. The RPi taps into the Mega's sensors to perform calculations required for autonomy and provides instruction back to the Mega. The XBee module behaves as a wireless serial line, allowing land-based monitoring and control of the system. The Mega is thus required to choreograph the transmission of messages between the RPi and XBee in addition to executing and responding to requests by both parties. This document will describe the physical wiring configuration of the three systems (Mega, RPi and XBee) and the common communication format for sending and requesting data and status and controlling devices.
The RPi GPIO 14 is connected to the Mega RX2. RPi GPIO 15 is connected to Mega TX2. The connection uses 9600 baud, 8 data bits, no parity, and 1 stop bit. Because the RPi GPIO pins operate at 3.3V (not 5V tolerant) and the Mega pins at 5.0V, a voltage divider is placed between GPIO 15 and TX2. The Mega registers 3.3V as HIGH so no level conversion is needed.
The Mega TX1/RX1 are connected to the XBee over serial TTL at 115200 baud.
Example: XXMESSAGE;
The devices share a common communication format for simplicity. Each message consists of a string terminated by a semicolon. Do not use line break characters or message contents may be invalidated.
The first two characters of the string represent the subject of the message. A full list of subjects and their corresponding meanings/sensor can be found below, along with the expected response. The remainder of the string up to the semicolon is the message block.
The message portion cannot start with ? unless the transmission is changing sensor update frequency
Any transmissions that are corrupted or malformed are ignored. Currently, there is no way to confirm receipt and proper execution of a command (outside of querying), which may be a problem with remote, XBee based transmissions. Error correction and reporting may be added in the future.
The following section describes the meaning of different message subjects and how the device should respond if it receives that kind of string. Where XBee is indicated, it is assumed that a land-based control device will receive and respond as expected.
By default, the Mega will transmit the message 00X; every 3 seconds to the RPi and XBee (described below), along with all sensor data. To receive data faster or slower than this from a specific sensor, the RPi or XBee can send the message XX?x;, where XX is the subject code for the sensor and x is the time between each transmission in milliseconds. If x is 0, data from that sensor will not be transmitted to the requesting device.
The Mega will send GP1; if the module is online and searching for a GPS fix and GP0; if the module is unresponsive. The Mega will send coordinates in decimal format. For example, GP44.225279,-76.497329; corresponding to 44°13'31.0"N 76°29'50.4"W. Six decimal points will be reported (%.6f).
The Mega will send CPX; with X being a degree (0-359) representing the boat's bow relative to magnetic north.
The Mega will send TMX; with X the box temperature in degrees Celsius.
The Mega will send WVX; with X being a degree (0-359) representing the current wind direction relative to the boat's bow.
The Mega will send PX0 if no object is detected. The Mega will send PXX; with X being a section (1-5) representing the objects location in the Pixy's view with 1 being the far left section and 5 being the far right section. Not yet implemented.
Not yet implemented, specification unknown.
The Mega sends 00X; with X indicating the device state (0 = Error/Disabled, 1 = Remote Control (Default), 2 = Autopiloted by RPi) to the RPi and XBee every 3 seconds while on.
00?; is used to check if the RPi/Xbee is online. Once received, the device should respond with 001;. The Mega will send 00?; to check RPi/Xbee response every 5 seconds initially and if the device misses a response. Once the connection is initially established, the Mega will send 00?; every 10 seconds to verify that the device is still alive.
| Received by | Action/Response |
|---|---|
| Mega | Saves the last response time of the RPi/XBee. The Mega ignores 00?; messages. |
| RPi | Responds to the Mega's 00?; with 001; |
| XBee | Responds to the Mega's 00?; with 001; |
011; is sent by the Mega to the RPi/XBee as it is powering on.
| Received by | Action/Response |
|---|---|
| Mega | No action/response (May change in the future) |
| RPi | Pauses, resubscribes to data and resumes navigation if Mega lost power while in autopilot (brownout). |
| XBee | Updates GUI |
When 02X; is received by the Mega it will blink the LED strip in a pattern corresponding to X. Currently only 021; is implemented.
| Received by | Action/Response |
|---|---|
| Mega | Blink LEDs |
031; to enable and 030; to disable RC control of rudders and sails.
| Received by | Action/Response |
|---|---|
| Mega | Enable/disable remote control |
Sent by the RPi to the Mega and relayed or by the Mega directly to the XBee to be displayed in the console and saved in the log.
| Received by | Action/Response |
|---|---|
| Mega | Relay message from the RPi to the XBee. |
| XBee | Update GUI, show error notification |
Sent by the RPi to the Mega and relayed or by the Mega directly to the XBee to display a error message to the land-based operator.
| Received by | Action/Response |
|---|---|
| Mega | Relay message from the RPi to the XBee. |
| XBee | Update GUI, show error notification |
081; is sent by the Mega to the RPi/XBee if there is a critical failure that does not allow it to continue. Not currently implemented.
| Received by | Action/Response |
|---|---|
| RPi | Stops autopilot |
| XBee | Updates GUI, show error notification |
The Mega sends 09X; with X indicating the RPi device state (0 = Offline, 1 = Online, 2 = Error) to the XBee every 3 seconds. RPi status is determined through 00?; (device online) pings.
| Received by | Action/Response |
|---|---|
| XBee | Updates GUI, show error notification |
Many of these messages are sent via the XBee to the Mega and then relayed to the RPi. For all of the following subjects, the XBee can query the current state/stored variable for a subject by sending AX?; or something similar, as described below. It is therefore important that the RPi also responds to these requests by responding appropriately with an answer or 0 if empty.
A01; to enable and A00; to disable autopilot.
Warning: Enabling autopilot by sending
A01;to the Mega via XBee does not guarantee the system will switch into autopilot. If it does, however, RC control will be disabled.
If the XBee sends A01; to enter autopilot, the Mega will send A01; to the RPi. If the RPi has all the information needed to enter autopilot, it send A01; back and take over control. However, if the RPi is missing information (waypoints, GPS, compass data), no actions will occur.
| Received by | Action/Response |
|---|---|
| Mega | If sent by the RPi, the Mega will change it's internal variables relating to it's current mode, enable/disable remote control and relay A0X; back to the XBee. If sent by the XBee, it will relay the request to the RPi. |
| RPi | Assess readiness for autopilot and responds with A01; if ready, then takes over control. |
| XBee | Update GUI. |
The XBee will send a message in the format A1X; with X corresponding to the autopilot mode.
| Mode (X) | Description |
|---|---|
| 0 | Basic navigation. Navigate to each waypoint and disable autopilot when the last waypoint is reached. |
| 1 | Looping navigation. Navigate to each waypoint, going to the first when the last waypoint is reached. |
| 2 | Stationkeeping. Follow the challenge description for the Sailbot Competition. |
| 2 | Search. Follow the challenge description for the Sailbot Competition. |
| Received by | Action/Response |
|---|---|
| Mega | Relay the request to the RPi if received from XBee. Relay the mode to the XBee if received from the RPi. |
| RPi | Appropriately adjust autopilot mode. Respond with current mode in format A1X; if A1?; is received. |
| XBee | Update GUI. |
The XBee will send a message in the format A2XX,LAT,LONG; with X corresponding to the waypoint to overwrite (01-10) or A2?XX; with X corresponding to the waypoint to read (01-10).
| Received by | Action/Response |
|---|---|
| Mega | Relay the request to the RPi if received from XBee. Relay fulfilled read requests from the RPi to the XBee. |
| RPi | Read/write waypoint as appropriate. If it is a read request, respond with waypoint in format A2XX,LAT,LONG;. |
| XBee | Update GUI. |
In certain navigation challenges, not all the defined waypoints are to be navigated to. For example, maybe navigation should stop after navigating through waypoints 01-03.
The XBee will send a message in the format A3SSFF; with SS corresponding to the waypoint to start on and FF to the waypoint to end on (01-10). The XBee can also send or A3?; to request the active waypoint range.
| Received by | Action/Response |
|---|---|
| Mega | Relay the request to the RPi if received from XBee. Relay fulfilled read requests from the RPi to the XBee. |
| RPi | Read/write waypoint range as appropriate. If it is a read request, respond with waypoint range in format A3SSFF;. |
| XBee | Update GUI. |
Sent by the RPi to the Mega to the XBee in the format A8XX; where XX is the index of the current waypoint being navigated to.
| Received by | Action/Response |
|---|---|
| Mega | Relay message from the RPi to the XBee. |
| XBee | Update GUI. |
Sent by the RPi to the Mega to the XBee to display as a status message.
| Received by | Action/Response |
|---|---|
| Mega | Relay message from the RPi to the XBee. |
| XBee | Update GUI. |
Can be sent by the RPi or XBee to the Mega to actuate the servos.
00XXX; is used to move the winch where 0 is fully sheeted in and 100 is fully sheeted out. Do not pad number with zeros.
00XXX; is used to move the rudder between 0 to 180 degrees where 0 corresponds with fully turned clockwise, 180 fully turned counterclockwise and 90 being the "zero" rudder position. Do not pad number with zeros.