This repository contains the NOS3 Generic_imu Component. This includes flight software (FSW), ground software (GSW), simulation, and support directories.
This generic_imu component is a CAN device that accepts multiple commands, including requests for telemetry and data. The available FSW is for use in the core Flight System (cFS) while the GSW supports COSMOS. A NOS3 simulation is available which includes both generic_imu and 42 data providers.
An inertial measurement unit, or IMU, is an electrical unit which uses a combination of accelerometers and gyroscopes to report a vehicle's attitude, heading, and rotation. Upon a command, it provides information about the attitude, heading, and rotation of the vehicle. The available flight software is for use in the core Flight System (cFS) while the ground software supports COSMOS. A NOS3 simulation is available which uses a 42 data provider.
The protocol, commands, and responses of the component are captured below.
The protocol in use is CAN. The device is speak when spoken too.
All commands received by the device are echoed back to the sender to confirm receipt. Should commmands involve a reply, the device immediately sends the reply after the command echo. Device commands are all formatted in the same manner and are fixed in size:
- uint8, command header
- 0x80
- uint8, command identifier
- (0) Get Housekeeping
- (1) Get Generic_imu
Response formats are as follows:
- Housekeeping
- uint32, Command Counter
- Increments for each command received
- uint32, Configuration
- Internal configuration number in use by the device
- uint32, Status
- Self reported status of the component where zero is completely healthy and each bit represents different errors
- No means to clear / set status except for a power cycle to the device
- uint32, Command Counter
- Generic_imu
- uint32, Command Counter
- Increments for each command received
- uint16, Data X
- X component of generic_imu data
- uint16, Data Y
- X component of generic_imu data
- uint16, Data Z
- X component of generic_imu data
- uint32, Command Counter
The NOS3 generic inertial measurement unit communicates using the CAN protocol.
The data structure for communications is explained below:
- Character transmission (unsigned char): (Byte_0(LSB)) --> to communications channel
- Floating data transmission (float): Float codification according to IEEE 754-1985 standard for single-precision floating 32 bits: (sign|exponent|fraction) = (Byte_3(MSB)|Byte_2|Byte_1|Byte_0(LSB)) --> to communications channel
- Floating value = s x 2^e x m, where:
- Sign(s): 1 bit (0 = positive, 1 = negative)
- Exponent: 8 bits (e = Exponent - 127)
- Fraction: 23 bits (m = 1.Fraction)
- Floating value = s x 2^e x m, where:
This protocol uses two message types:
- Request message: from master to slave
- Response message: from slave to master
The master is the onboard computer of the satellite, or the master of the communication bus, and the slave is the generic inertial measurement unit.
The CAN Protocol is the message-passing format which will be used in the case of the generic inertial measurement unit. A CAN frame consists of a 16-bit header, a 4-bit data length variable, and then between 0 and 8 bytes of data. There is then a trailing unit of 28 bits. The following chart indicates the parts of the message:
| Field Name | Length (bits) | Purpose | Value (in this case) |
|---|---|---|---|
| Start of Frame | 1 | Denotes the start of frame transmission | 1 |
| Identifier | 11 | An identifier (message priority) | |
| Stuff Bit | 1 | An opposing bit to maintain polarity | |
| Remote Transmission Request | 1 | 0 for data frames; 1 for remote request frames | |
| Identifier Extension Bit | 1 | 0 for 11-bit identifier frames | 0 |
| Reserved bit | 1 | Reserved bit; must be dominant (0) | 0 |
| Data length | 4 | Defines the length of the following | |
| Data field | <64 | Data to be transmitted (0-8 bytes) | |
| CRC | 15 | Cyclic Redundancy Check | |
| CRC delimeter | 1 | Must be recessive (1) | 1 |
| ACK slot | 1 | Must be recessive (1) | 1 |
| ACK delimeter | 1 | Must be recessive (1) | 1 |
| End-of-Frame | 7 | Must be recessive (1) | 0000001 |
| Inter-frame spacing | 3 | Must be recessive (1) | 001 |
The above chart is primarily reproduced from Wikipedia.
There will be one command required:
This command will direct the IMU to return the data on both linear and rotational acceleration in all three axes.
| Field Name | Length (bits) | Purpose | Value (in this case) |
|---|---|---|---|
| Start of Frame | 1 | Denotes the start of frame transmission | 1 |
| Identifier | 11 | An identifier (message priority) | 00000000001 |
| Stuff Bit | 1 | An opposing bit to maintain polarity | 0 |
| Remote Transmission Request | 1 | 0 for data frames; 1 for remote request frames | 1 |
| Identifier Extension Bit | 1 | 0 for 11-bit identifier frames | 0 |
| Reserved bit | 1 | Reserved bit; must be dominant (0) | 0 |
| Data length | 4 | Defines the length of the following | 0000 |
| Data field | 0 | Data to be transmitted | -- |
| CRC | 15 | Cyclic Redundancy Check | 000000000000000 |
| CRC delimeter | 1 | Must be recessive (1) | 1 |
| ACK slot | 1 | Must be recessive (1) | 1 |
| ACK delimeter | 1 | Must be recessive (1) | 1 |
| End-of-Frame | 7 | Must be recessive (1) | 0000001 |
| Inter-frame spacing | 3 | Must be recessive (1) | 001 |
Because of size constraints on the message itself, there will be three telemetry messages. All three will be sent upon receipt of the aforementioned command message.
This command will contain the telemetry from the IMU, including data on linear and rotational acceleration in all three axes.
| Field Name | Length (bits) | Purpose | Value (in this case) |
|---|---|---|---|
| Start of Frame | 1 | Denotes the start of frame transmission | 1 |
| Identifier | 11 | An identifier (message priority) | 00000000010 |
| Stuff Bit | 1 | An opposing bit to maintain polarity | 1 |
| Remote Transmission Request | 1 | 0 for data frames; 1 for remote request frames | 0 |
| Identifier Extension Bit | 1 | 0 for 11-bit identifier frames | 0 |
| Reserved bit | 1 | Reserved bit; must be dominant (0) | 0 |
| Data length | 4 | Defines the length of the following | 1000 |
| Data field | 64 | Data to be transmitted | 32 bit float denoting linear X acceleration, 32 bit float denoting angular X acceleration |
| CRC | 15 | Cyclic Redundancy Check | |
| CRC delimeter | 1 | Must be recessive (1) | 1 |
| ACK slot | 1 | Must be recessive (1) | 1 |
| ACK delimeter | 1 | Must be recessive (1) | 1 |
| End-of-Frame | 7 | Must be recessive (1) | 0000001 |
| Inter-frame spacing | 3 | Must be recessive (1) | 001 |
This command will be identical to the X telemetry message, except that it will had an identifier of 00000000011, a stuff bit of 0, and the data field will denote linear Y and angular Y accelerations, respectively.
This command will be identical to the X telemetry message, except that it will had an identifier of 00000000100, a stuff bit of 1, and the data field will denote linear Z and angular Z accelerations, respectively.
The various configuration parameters available for each portion of the component are captured below.
Refer to the file fsw/platform_inc/generic_imu_platform_cfg.h for the default configuration settings, as well as a summary on overriding parameters in mission-specific repositories.
The default configuration returns data that is X * 0.001, Y * 0.002, and Z * 0.003 the request count after conversions:
<simulator>
<name>generic_imu_sim</name>
<active>true</active>
<library>libgeneric_imu_sim.so</library>
<hardware-model>
<type>GENERIC_IMU</type>
<connections>
<connection><type>command</type>
<bus-name>command</bus-name>
<node-name>generic_imu-sim-command-node</node-name>
</connection>
<connection><type>usart</type>
<bus-name>usart_29</bus-name>
<node-port>29</node-port>
</connection>
</connections>
<data-provider>
<type>GENERIC_IMU_PROVIDER</type>
</data-provider>
</hardware-model>
</simulator>
Optionally the 42 data provider can be configured in the nos3-simulator.xml:
<data-provider>
<type>GENERIC_IMU_42_PROVIDER</type>
<hostname>localhost</hostname>
<port>4242</port>
<max-connection-attempts>5</max-connection-attempts>
<retry-wait-seconds>5</retry-wait-seconds>
<spacecraft>0</spacecraft>
</data-provider>
If this generic_imu application had an ICD and/or test procedure, they would be linked here.
We use SemVer for versioning. For the versions available, see the tags on this repository.
- v1.0.0 - X/Y/Z
- Updated to be a component repository including FSW, GSW, Sim, and Standalone checkout
- v0.1.0 - 10/9/2021
- Initial release with version tagging