Control Plane

Version: 0.5.1
Date: 28 October 2019
Status: Draft
Type: Component Specification

Overview

The command and control (C2) plane is responsible for communicating with onboard systems such as the engine, and ensure they remain responsive and under operator control at all times. Control signals shall be transmitted over 915Mhz LoRa radio

I2C Protocol

I2C shall be used as the inter-board communication method of choice between the custom controllers in this system. The I2C protocol should follow the following

Device IDs

To get the current EC state, request a single byte over I2C.

EC State packet

The EC will accept the following single byte write to set modifiable state

EC Command Packeta

• Vent is a two bit number to contain all of its states, with MSB at Bit 2 and LSB at bit 1.

Since the PI serves as the bus master, it should check the Requested MV-G1 State and compare it to the last one sent to the Ground valve controller. If it is different it should construct a single byte command packet with value 1 and sent it to the Ground valve Controller.

Radio protocol

The Ground Station, Relay Box and Ignition computer shall all implement the C2 over 915Mhz LoRa radio. Each packet sent over the LoRa network shall follow the following format:

Bytes 0-1 are considered the “header” of the packet and primarily serve as metadata that allow receivers to choose how to handle packets with minimal overhead.

The sender/receiver ID shall be an 8 bit integer (uint8_t) with the high 4 bits representing the sender ID and the low 4 bits representing the receiver ID. That is:

0b11110000 will be split into
Sender ID = 0b1111 = 15
Receiver ID = 0b0000 = 0

These IDs must follow the table below:

NOTE: ID #15 is reserved for broadcast messages sent to all radio devices on the same network and thus the maximum devices that can be connected to the same network as of this spec is 15.

Thus, if the Ground Station were to send a packet to the Ignition Computer, it would send 0b00000001 in the 1 byte of its radio packet. This also means that the sender of a packet can be determined by parsing byte 1 of the packet.

Packet types must be one of the following:

Synchronization and Acknowledgemen

The C2 plane uses an acked radio protocol. This means that every command sent from the Grand Station to the rocket must be acknowledged by a corresponding packet from the rocket to be considered successfully transmitted. However, since only the newest state of the ground station needs to be carried out by the rocket, only the latest command packet will need to be acked.

repeat forever:
    if button state changed from last transmitted state:
        record state of GS
        assign sequence code to state so ack can be track
        replace current state pending ack
        send state to rocket
    otherwise:
       if state transmission is pending ack
             check for ack with matching sequence number from rocket, if received, mark state as acked
             if no response, and time is over ack timeout:
                 resend only if there are retries left, otherwise light up transmission error led

Heartbeat Request Packet Format:

ACK Packet Format:

The sequence code is a pseudo-unique code that matches request packets with their corresponding ACK packets. Sequence codes should be incremented after every ack’d packet transmission and should roll over at 256 (uint8_t will do this automatically). All ack’d packets transmitted from the same node should use and increment the same sequence node counter to ensure that all packets transmitted in a short time window from the node have different sequence codes. Packets that require acknowledgement should include a sequence code so the resulting ack can be tracked back to the originating request.

Valve Control

Valve Control Packet

The ignition computer receives valve control commands from the operator. Corresponding control signals are then sent to the propulsion system. Radio commands for each valve shall be represented by an integer value, should be of the int8_t type. As there are 3 valves, a radio command shall be an array containing 3 integers of the following order:

1. Open/Close of fuel-combustion ball valve
2. Open/Close of fueling solenoid valve
3. Open/Close of venting solenoid valve

The commands values used by each of the command is outlined in the following tables. Note, the values here are only used at software level, whilst the actual signals sent to hardware components need to be defined based on the particular needs.

Ball valve control

Venting solenoid valve control

Fuel solenoid valve control

One example of a command would be {1,-1,0}, denoting the unlikely command of forward action of ball valve, no change in action of venting valve, and the closing of fuel valve. For future expansion of the commands, it is recommended that any simple command should obtain a value ascending from 1, whilst complex sequence command should obtain a value descending from 127. For the valves used, Simple and sequence commands are defined as:

• Simple command is a command that is accomplished by #one# #continuous# and #unidirectional# action of the valve.
• Sequence commands is a command that is accomplished by #multiple# #discontinuous(e.g. with intervals)# and/or #multidirectional# action of the valve.