FlyByWireCpp/FlyByWire.hpp

#ifndef DEF_FlyByWire
#define DEF_FlyByWire

#include <cmath>

typedef double Real;

#define HUGE_VALUE_REAL Real(1e20)

// =============================================================================
// General fly-by-wire helper functions
// =============================================================================

namespace FlyByWire
{

/// Computes the heading error using the shortest route from hdg to tgt, both expressed in radians.
Real Hdg_err(Real tgt, Real hdg);

/// Computes the heading error using the shortest route from hdg to tgt, both expressed in degrees.
Real Hdg_err_deg(Real tgt, Real hdg);

/// Computes the median of the triplet (a, b, c). Useful to reject an invalid measurement from a triplet of sensors, or to mix 3 control laws in a continuous fashion (C0 continuity).
/// Internally, it is implemented as a sorting network. The middle value is the median of the triplet.
Real Vote(Real a, Real b, Real c);

/// Copies the input as long as it's not within +/- a from the origin. The output is then 0.
Real Deadzone(Real x, Real a);

/// Saturation function. Ensures that a <= x <= b.
Real Sat1(Real x, Real a, Real b);

/// Limits the rate of the signal x(t) : [dx/dt](t) to the interval [dy_dt_min, dy_dt_max].
/// Use like a recursive filter.
///
/// \variable x_n             : Input value at step n
/// \variable y_n_1            : Output value at step n-1
/// \variable dy_dt_min        : Min derivative
/// \variable dy_dt_max        : Max derivative
/// \variable dt               : Time step
/// \return y[n] rate-limited.
Real Ratelim(Real x_n, Real y_n_1, Real dy_dt_min, Real dy_dt_max, Real dt);

/// Rate limiter implemented as a self-contained recursive filter.
class RateLimiter
{
    public:
        /// Creates the RateLimiter object with the specified limits, and initial value. By default, y0 = 0.
        RateLimiter(Real dy_dt_min, Real dy_dt_max, Real dt, Real y_ = Real(0.));

        /// Applies the rate limiter filter to the input x[n] (parameter x_n) and returns the result y[n].
        Real Filter(Real x_n);

    public:
        Real dy_dt_min; //< Min rate.
        Real dy_dt_max; //< Max rate.
        Real dt;        //< Time step.
        Real y_n_1;     //< Previous filter value.
};

/// Heaviside function. 1 when x >= 0, 0 otherwise.
Real Heaviside(Real x);

// =============================================================================
// IRR Filters from the continuous Laplace transfer function.
// =============================================================================

/// 1st order integrator implemented using the trapezoïdal method.
/// G(s) = 1/s
/// The integrator can also be limited using the lower_bound and upper_bound parameters.
/// By default, the integrator is not bounded.
///
/// The class is to be used like so :
/// flt = Integrator1(Ts)
///
/// while control/filtering loop:
///     y_n = flt.Filter(x_n)
class Integrator1
{
    public:
        /// Creates an integrator object. By default, y0 = 0, and the limits are +/- HUGE_VALUE_REAL.
        Integrator1(Real Ts_, Real y_ = Real(0.), Real lower_bound_ = -HUGE_VALUE_REAL, Real upper_bound_ = HUGE_VALUE_REAL);

        /// Sets the state of the filter so that it can be initialized at any value.
        /// By default, the states are initialized to 0.
        void SetState(Real y_ = Real(0.), Real x_ = Real(0.));

        /// Computes the next output value of the filter y[n] from the next input value x[n].
        Real Filter(Real x_n);

    public:
        Real Ts;//< Time step.
        Real lower_bound;//< Time step.
        Real upper_bound;//< Time step.
        Real x_n_1;//< x[n-1].
        Real y_n_1;//< y[n-1].
};

}

#endif