RCwiproc
Contents
Wireless Protocol
The host will send each robot velocity, direction, and shoot directives, at a rate of about 40-50 Hz. (This data rate still needs to be confirmed with testing) In such a way the enitre system (host, camera, and robot) will behave as a real-time system. The robot will send its current error status, battery voltage, wheel velocity, unique ID, and whether or not it has the ball to the host but at the much slower rate of 10Hz.
To Do
- Research protocols
UDP (User Datagram Protocol)- Dropping UDP as a choice in protocol. Would need a module that implements it alreadyProtocol Book
- Determine the rate at which to update robots - Will need to find theoretical base for this and confirm with test.
Specify a custom protocol- Remeber to do Visio graph of packet structureEvaluate and compare UDP to a custom protocol
Protocol Structure
What is a Protocol?
A protocol is a set of rules to define how two or more entities comunicate. It contains 5 basic elements:
- A service definition - What information will the protocol provide the entities that utilize it?
- A set of assumptions - What is assumed to in the protocol? What isn't?
- A vocabulary - What will be the forms of data that I will use to communicate with this protocol
- A format/encoding - How will my vocabulary coherantly fit together to form larger msgs, and cmds
- A set of procedural rules - How will I intiate communication? How are the entities using this protocol indentied? What are thier roles?
Other considerations such as the medium are not specifically stated in the protocol but influence how the protocol is formed. Consider the difference between Wi-Fi where one router can talk to multiple devices versus communication between a wired video game controller and a console.
As an example of a protocol consider the english language in its vocal form. The service it provides is a means of conveying thoughts thruogh the use of sound. It assumes that all the parties involved can speak and understand english, that the medium is such that the transmission of sound waves is possible, and that meaning of the vocabulary used is the same among all parties involved. Its vocabulary is made up of 26 letters that by themselves or in groups represent a set of vocal patterns generated and recieved by all the parties involved. In this abstract defintion of vocabulary, it is not words but rather letters and the sounds they make that form the protocol. The basic format of the english language is such that the vocabulary of letters is formed into words, which each carry the meaning of the thought being conveyed. The procedural rules change based on the situation. Sometimes one person is communicating with a group of passive listeners, at other times several people are communicating with one person at once. There are even times (eavesdropping) where one or more parties are listening to a speaker not directly communicating to them.
Our Protocol
Packet Structure
A vital part of our robocup team is its communication system. Therefore an effective means of sending information across this system will need to be realized with a protocol of some sort. In general our protocol needs to allow the host to command movements and actions to the robots, while allowing the robots to provide status information back to the host. The medium will be RF signals in the 900mHz range. This means that we can assume an appreciable dropped packet rate, and interfereance from other teams, and devices. Binary data clocked at 57Kbps will be our vocabulary, and the msg format will be in packets of varying length. Specifically each packet will be divided in bytes, with the first byte consisting of a know header, followed by bytes containg the msg ID, its length, who its intended for, the data being sent, and a checksum. Below in the figure is a a detail of what each packet will look like.
| Byte 1 | Byte 2 | Byte 3 | Byte 4 | Bytes 5-19 | Last Byte |
| Header (1 byte) | ID (3 bits) | DLC (5 bits) | Recipient ID (1 byte) | Data (0 - 15 bytes) | Checksum (1 byte) |
|---|
The header consists of two transitions 0 to 1 and 1 to 0 spread over the time of one byte. The ID of the msg is a unique 3-bit value the identifies the type of msg being sent. In the table below is a listing of the different msgs and thier ID's. Each msg's length is described by a 5-bit value called the DLC or data length code. Who the msg is intended for is determined by which bit is set in the Recipient ID. Bits 1-5 correspond to robots 1-5, 6 corresponds to the host and 7 means the msg is meant for all robots. The format of the data section of the packet is based on the msg being sent. In most cases it is blank but for data msgs it has the following format:
| Robot 1 Data (3 bytes) | Robot 2 Data (3 bytes) | Robot 3 Data (3 bytes) | Robot 4 Data (3 bytes) | Robot 5 Data (3 bytes) |
|---|
With the format for the data for each robot being defined as:
| Byte 1 | Byte 2 | Byte 3 |
| Vx (1 byte) | Vy (3 bits) | Theta (5 bits) |
|---|
| MsgID | ID | ACK? | To | DLC | Data Format | Description |
|---|---|---|---|---|---|---|
| Wakeup | 0x01 | Yes | Individual robots | 0 | None | Host polls for each robot individually. When this msg is recieved by a robot it responds with a status msg indicating that it is ready for action. |
| Go/Start | 0x02 | Yes | Individual robots | 0 | None | Intiates main process on robots. |
| Vx Vy and Theta | 0x03 | No | All robots | 15 | See below | Streaming data sent to all robots in one msg. |
| Stop | 0x04 | No | All robots | 0 | None | Stops all robot motion. |
| Shoot Ball | 0x05 | Yes | Individual robots | 1 | Robot Angle | Commands robot to take the shot. May be folded into msg 0x04 |
| Robot Stop | 0x06 | Yes | Individual robots | 0 | None | Commands an indivdual robot to stop. |
Protocol Procedural Rules
When messages are being sent between the host and the team robots care must be taken to cause data collisions, message oscillations, and other errors. To prevent such events a set of procedural rules are implemented dictating when each msg is sent and what happens both before and after the messages transmission. Below is a detailed explanation of the procedural rules for each message and finally some flow charts for events such as initial startup and stopping.
Wakeup
As the title says this msg brings the robots out of a known "sleep" state and registers them the host controller. The motivaton for the message and the procedure it details is so that message transfer starts from a known safe state, this state being the "sleep" the state. The "sleep" state is entered once the robot is turned on and is written in firmware. In "sleep" state the robots will not talk to the host nor respond to any messages except wakeup. The procedure for waking up a team is as follows:
- First the human operator feeds in the unique ID of each robot on the field. These ID numbers range from 1-10 and in fact are the IDs included in each message.
- When the command is given the host sends out the wakeup msg to the robot whose ID is first in the list of IDs fed in the the operator.
- If the robot with that ID is on the field it will immediately respond back to the host with a status message giving information on its health, and its unique serial number.
- When the host recieved this status message it will move on the next ID in its list and transmit a wakeup message with that ID.
- If the host does not recieve a status message back or the health of the robot is such that its not ready it will alert the operator and will not continue the wakeup sequence.
- Once all the robots have been verified to be on the field and ready the wakeup sequence shall be considered complete and host is now ready to start a game.
You may wonder why the wakeup msg is sent to each robot one at a time. Even though our protocol is full-duplex it doesn't define an arbitration scheme. As work-around messages from the robots to the host are only sent when asked for by the host. In such a manner we can ensure that only one robot can talk to the host at a time, avoiding data collisions. Future implementations we see the addition of arbitration.
GO/Start
Once the robots on the field are found and in good order we can start a game. The GO/Start message signals the robots to begin thier main process and prepare to recieve data. The robots do not acknowledge this message since it is a broadcast message. Broadcast messages are messages sent to all robots at once, and makeup the majority of msgs sent on the wirless channel. The motivation for these message types is simplicity. Instead of sending most messages to each robot individually and flooding the wireless channel thereby increasing chances for a dropped packets we package the information in one big message. As a consequence there can be no acknowledgments to these messages due to the lack of an arbitration scheme.
Data
Game Stop
Shoot Ball
Robot Stop
Articles
- Linx article on protocol design
- Paper on the Cylical Redundancy Check error checking algorithm (CRC)
- Enitre book on protocol design, a bit dated though like 1990 dated
- Berkeley article on wireless protocol design. Gives a real world example of how to design a wireless protocol.
- Ohio University article. Details an intresting communications method
- Paper from Maxim on Manchester Encoding
- Berkeley PPT on errors and wireless
- Circuit Cellar discussion on Manchester Encoder/Decoder
- Cornell electrical design document
