ardupilot/libraries/SITL/examples/JSON/MATLAB

MATLAB

The SITL_connector function can be used to simplify the connection process. SITL_connector.m uses the TCP/UDP/IP Toolbox 2.0.6 by Peter Rydesäter and is much (10x) faster than the MATLAB functions available with the instrument control toolbox (and free!). However this may require compilation of a mex file, prebuilt mex files are included in most cases they will work without the need to re-compile.

see Copter/SIM_multicopter.m for example usage.

The function is defined as:

    SITL_connector(state,init_function,physics_function,max_timestep)

• state: this is the persistent physics state of the vehicle its is a structure and must contain the following felids:

    state.gyro(roll, pitch, yaw) (radians/sec) body frame
    state.attitude(roll, pitch yaw) (radians)
    state.accel(x, y, z) (m/s^2) body frame
    state.velocity(north, east,down) (m/s) earth frame
    state.position(north, east, down) (m) earth frame 

the structure can have also any other felids required for the physics model

• init_function: function handle that will be called to init the physics model, this will be called on the first run and after a SITL reboot. It should take and return the state. function state init(state) init_function = @init;

• physics_function: function handle that will be called to update the physics model by a single time step. It should take in the state and array of 16 PWM inputs and return the state.

    function state = physics_step(pwm_in,state)
    physics_function = @physics_step;

• max_timestep: this is the maximum allowed time step size, the desired time step can be set with the SIM_RATE_HZ ArduPilot parameter.

The JSON SITL interface is lock-step scheduled. This allows matlab breakpoints to work as normal.

Using the connection it should be possible to achieve > 1500 fps, at this speed MATLAB code efficiency plays a important factor in the max frame rate. For a 400hz physics time step this gives a maximum speedup of 4 to 5 times, physics time step can be adjusted using the ArduPilot SIM_RATE parameter. For planes and rovers it should be possible to use a much larger physics time step resulting in a larger maximum speed up. Note that large speedups risk the GCS getting left behind.

Simulink

Simulink input and output blocks are provided in AP_Conector.slx. There is a receive block that receives the PWM inputs from SITL. These are output in a 16 element array. The output block takes the vehicle state as outlighned above. The input block also has a reset signal, this will go high if SITL is restarted. This could be used to reset the Simulink model to its initial conditions. Unlike the Matlab connector the time step used by Simulink cannot be adjusted by ArduPilot, it must eb set in Simulink. Variable size time steps are supported.

Note that the Simulink connector and examples were made using Matlab2020a.