Xmera Process and Task Creation

Source Code

The Python code shown below can be downloaded here.

To execute modules within a simulation, their evaluation is managed through Xmera’s Process and Task containers. A BSK Task contains one or more modules that are executed at the same update rate. A Process is a group of tasks that often represent a related functional unit.

For example, in a multi-satellite simulation, you could create a Process for each satellite to keep the associated tasks and modules organized. The diagram below shows an example layout where the dynamics of the bskSat satellite are added to a separate process from the flight software (FSW) algorithms executed on the satellite. Within each process, multiple tasks are created and executed at their respective update rates.

Simulation Setup

The Python code below shows a typical BSK simulation setup. Process and Task creation are handled using the utility package SimulationBaseClass, which provides the core simulation functionality:

• Creating processes and tasks

• Adding modules to tasks

• Initializing the simulation

• Executing the simulation

The macros utility module is also used to provide convenient helper functions.

A method named run() is created to execute the simulation. The name is arbitrary. A SimBaseClass() instance named scSim serves as the simulation container to which components are added. A process is created using CreateNewProcess(name), where name is a string representing the process name.

    #  create the simulation process
    dynProcess = scSim.CreateNewProcess("dynamicsProcess")
    fswProcess = scSim.CreateNewProcess("fswProcess")

    # create the dynamics task and specify the integration update time
    dynProcess.addTask(scSim.CreateNewTask("dynamicsTask", macros.sec2nano(5.)))
    dynProcess.addTask(scSim.CreateNewTask("sensorTask", macros.sec2nano(10.)))
    fswProcess.addTask(scSim.CreateNewTask("fswTask", macros.sec2nano(10.)))

    #  initialize Simulation:
    scSim.InitializeSimulation()

    #   configure a simulation stop time and execute the simulation run
    scSim.ConfigureStopTime(macros.sec2nano(20.0))
    scSim.ExecuteSimulation()

    return


if __name__ == "__main__":
    run()

Creating Processes

In this example, three BSK processes are created: two for dynamics and one for FSW. The order of process execution is determined by the order in which they are created. You can override this by assigning a priority using:

dynProcess = scSim.CreateNewProcess("name", priority)

A process with a higher priority value is executed before those with lower priority values. The default priority is -1, meaning the process runs after all prioritized processes and in the order they were created.

Creating Tasks

Next, task lists are created within each process. Each task requires a name (string) and an update rate (in nanoseconds). The helper function macros.sec2nano() is commonly used to convert seconds to nanoseconds. Xmera uses nanoseconds as its smallest time unit, chosen to be small enough for dynamics, sensors, and algorithms, yet large enough that a 64-bit unsigned integer can simulate mission lifetimes up to 584 years.

Like processes, tasks have a default priority of -1. You can assign a priority explicitly as follows:

dynProcess.addTask(scSim.CreateNewTask("name", updateRate, priority))

Delayed Task Start

You can delay the start of a task using the optional FirstStart parameter (in nanoseconds). The task will first execute at FirstStart, and continue at the regular updateRate thereafter. For example:

dynProcess.addTask(scSim.CreateNewTask("name", updateRate, priority, FirstStart=delayStartTime))

If delayStartTime is 1s and updateRate is 5s, then the task executes at 1s, 6s, 11s, 16s, etc.

Simulation Execution

To initialize all processes and tasks, call InitializeSimulation(), which sets up all modules and ensures their internal states are reset.

Next, set the simulation stop time using ConfigureStopTime(stopTime), where stopTime is given in nanoseconds. This defines the absolute stop time of the simulation. If you run for 5 seconds, modify parameters, and want to continue for another 5 seconds, you must set the new stop time to 10 seconds total. For example:

scSim.ConfigureStopTime(macros.sec2nano(5.0))
scSim.ExecuteSimulation()
scSim.ConfigureStopTime(macros.sec2nano(10.0))
scSim.ExecuteSimulation()