Starter and Breaks Sevo Mapping in FlightGear/JSBSim

ArduPlane Plane Simulation
1

I am working to get the C172p JSBSim model working in Auto Flight mode. The problem I have is that when the C172p model is setup, its engine is not started. I would like to be able to use the servo-to-function mapping to tie a servo to the starter/ignition function as well as brakes release and possible flap control.

I have spent a good deal of time looking at the SIM_JSBSim.cpp./.h files and I can see where the servo data is sent from ArduPlane to the JSBSim in void JSBSim::send_servos(const struct sitl_input &input). But I don’t understand how this works… or how to map it to set fcs functions… or even if this is the right place to do it.

From what I understand, if in mavproxy I type rc 8 1900 I am guessing that this sets servo_8 to max PWM and this should eventually get sent to the send_servos method in the JSBSim interface files? Is this information then contained in the sitl_input struct? Or should I perhaps be using a relay rather than a servo?

Any help in posting me in the right direction would be most appreciated. Or… maybe this level of fidelity is not possible with JSBSim and FG?

2

Update on this… after spending a good deal of time with the code, I understand the servo mapping now. However, I can’t seem to access other servo data outside of the first 5 channels.

I have created a tuned ArduPlane parameter file that successfully flies the full-size C172P in JSBSim in auto mission mode with a modified electric motor. In the parameter file, I have mapped SERVO5_FUNCTION to the auto flaps function and SERVO6_FUNCTION to ground steering. I then added the following code to JSBSim::send_servos method:

    // NOTE channel mapping begins at 1
    char *buf = nullptr;
    float aileron  = filtered_servo_angle(input, 0); // SERVO1_FUNCTION (ch1)
    float elevator = filtered_servo_angle(input, 1); // SERVO2_FUNCTION (ch2)
    float throttle = filtered_servo_range(input, 2); // SERVO3_FUNCTION (ch3)
    float rudder   = filtered_servo_angle(input, 3); // SERVO4_FUNCTION (ch4)

    //printf("Input ail: %f elv %f thr %f rud %f\n", aileron, elevator, throttle, rudder);

    float auto_flaps = filtered_servo_range(input, 4); // SERVO5_FUNCTION (ch5)
    printf("Input flaps: %f \n", auto_flaps);

    float auto_steer = filtered_servo_angle(input, 5); // SERVO6_FUNCTION (ch6)
    printf("Input steer: %f \n", auto_steer);

The new extracted values are auto_flaps and auto_steer. The flaps servo is at offset 4 within the sitl_input struct and it appears to correctly output a flaps setting while flying based the LAND_FLAP_PERCNT, TKOFF_FLAP_PERCNT, FLAP_X_PERCNT, FLAP_X_SPEED ArduPlane parameter settings.

The problem I have is that the auto_steer call to filtered_servo_angle generates an error – PANIC: Attempt to filter invalid servo at offset 5. It appears that only the first five channels exist in the sitl_input struct and there is no channel 6 (offset 5). The JSBSim::send_servos method is called from JSBSim::update which takes the sitl_input struct from its caller. This struct looks like this:

struct sitl_input {
    uint16_t servos[32];
    struct {
        float speed;      // m/s
        float direction;  // degrees 0..360
        float turbulence;
        float dir_z;	  //degrees -90..90
    } wind;

This struct is supposed to contain 32 values in the servos array. So it would seem that floats should exist at offsets 4 (ch5) and 5 (ch6).

I am not sure who calls JSBSim::update outside of the SIM_JSBSim.cpp file or where the sitl_input is populated. I am also not sure why the error says the servo value is “invalid” as opposed to some kind of access violation. What am I missing? Any help would be appreciated.